Getting to Grips With Fuel Economy
Short Description
Getting to Grips With Fuel Economy...
Description
10 / 2001 - ISSUE 2
getting to grips with FUEL ECONOMY
getting to grips with F U E L ECONOMY
Flight Operations Support - Customer Services Directorate
Flight Operations Support & Line Assistance
GETTING TO GRIPS WITH FUEL ECONOMY MANAGING FLIGHT OPERATIONS WITH RECOMMENDATIONS ON FUEL CONSERVATION
A FLIGHT OPERATIONS VIEW
STL 945.7190/99
November 2001 Issue 2
Flight Operations Support & Line Assistance
STL 945.7190/99
November 2001 Issue 2
TABLE OF CONTENTS
1.
PREAMBLE...................................................................................................................... 11
2.
INTRODUCTION: BASIC PREMISES AND OUTLINE.................................................... 12
3.
PRE-FLIGHT PROCEDURES ......................................................................................... 14 3.1
Center of gravity...................................................................................................... 14 3.1.1 Preliminary ................................................................................................... 14 3.1.2 Automatic center of gravity management .................................................... 14 3.1.3 Influence on fuel consumption ..................................................................... 15 3.1.4 Summary...................................................................................................... 17
3.2
Excess weight ......................................................................................................... 18 3.2.1 Aircraft weight .............................................................................................. 18 3.2.2 Overload effect............................................................................................. 19 3.2.3 Means to diminish aircraft weight ................................................................ 22 a) Zero fuel weight..................................................................................... 22 b) Embarked fuel ....................................................................................... 23 Embarked fuel minimization .................................................................. 23 Fuel transportation ................................................................................ 25
3.3
A.P.U....................................................................................................................... 26 3.3.1 Preliminary ................................................................................................... 26 3.3.2 Fuel conservation and A.P.U. ...................................................................... 26 3.3.3 Optimisation procedures .............................................................................. 27 3.3.4 Summary...................................................................................................... 28
3.4
Taxiing..................................................................................................................... 28 3.4.1 Preliminary ................................................................................................... 28
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3.4.2 Engine-out taxi operation ............................................................................. 29 Taxiing out with one (or two) engine(s) shut down ...................................... 29 Taxiing in with one (or two) engine(s) shut down ........................................ 29 During taxi in and out, one (or two) engine(s) shut down ............................ 29 3.4.3 Summary...................................................................................................... 30 3.5 4.
Conclusion on pre-flight procedures ....................................................................... 30
WITHIN THE FLIGHT ENVELOPE.................................................................................. 31 4.1
Climb ....................................................................................................................... 31 4.1.1 Preliminary ................................................................................................... 31 4.1.2 Managed mode ............................................................................................ 32 a) A300-600, A310, A320 family, A330 ..................................................... 32 b) A340 family............................................................................................ 34 4.1.3 Selected mode ............................................................................................. 36 4.1.4 Crossover altitude versus optimum altitude................................................. 43
4.2
Step climb ............................................................................................................... 46 4.2.1 Preliminary ................................................................................................... 46 4.2.2 Trade-off between manoeuvrability and economy....................................... 46 4.2.3 Delays in altitude follow-up .......................................................................... 48
4.3
Cruise...................................................................................................................... 49 4.3.1 Preliminary ................................................................................................... 49 4.3.2 Managed mode ............................................................................................ 50 a) Economy Mach number ........................................................................ 50 b) Time/fuel relation................................................................................... 55 4.3.3 From Managed to Selected Mode ............................................................... 57
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4.3.4 Selected mode ............................................................................................. 59 a) Preliminary .........................................................................................…59 b) Flight at a given Mach number..........................................................….61 b.1 Optimum altitude. .......................................................................
61
b.2 Optimum altitude on short stage ..................................................... 68 c)
Flight at a given Flight level................................................................... 71
d) Wind influence....................................................................................... 74 4.4
Descent ................................................................................................................... 79 4.4.1 Preliminary ................................................................................................... 79 4.4.2 Managed mode ............................................................................................ 80 4.4.3 Selected mode ............................................................................................. 81 Fuel consumption with respect to speed ..................................................... 81 Premature descent....................................................................................... 87 Temperature influence ................................................................................. 88
4.5
Holding .................................................................................................................... 89 4.5.1 Preliminary ................................................................................................... 89 4.5.2 Various configuration / speed combinations ................................................ 90 4.5.3 Linear holding .............................................................................................. 94
4.6
Approach................................................................................................................. 97 4.6.1 Decelerated and stabilised approach .......................................................... 97 4.6.2 Premature landing gear extension............................................................. 100
4.7
Conclusion within the flight envelope.................................................................... 102 4.7.1 With regard to the climb phase: ................................................................. 102 4.7.2 With regard to the step climb phase: ......................................................... 102 4.7.3 With regard to the cruise phase: ................................................................ 102
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4.7.4 With regard to the descent phase:............................................................. 102 4.7.5 With regard to holding:............................................................................... 103 4.7.6 With regard to the approach phase: .......................................................... 103 5.
GENERAL CONCLUSION............................................................................................. 104
APPENDICES APPENDIX 1............................ ECONOMY MACH NUMBER ACCORDING TO COST INDEX .............................................................................................................................................
AND FL
APPENDIX 2......................................................................................... OPTIMUM ALTITUDES
116
APPENDIX 3......................................................OPTIMUM ALTITUDES ON SHORT STAGES
120
APPENDIX 4............................................................ LONG RANGE CRUISE MACH NUMBER
123
APPENDIX 5......................... WIND ALTITUDE TRADE FOR CONSTANT SPECIFIC RANGE
127
APPENDIX 6.............................................................................................................. DESCENT
128
APPENDIX 7............................................................................................................... HOLDING
132
List of Figures Figure 1:
Specific range variation for different center of gravity positions ........................ 15
Figure 2:
Specific range variation versus CG Variation – MO.82 ..................................... 16
Figure 3:
Fuel burn penalty. Specific range variations for the case of 1000 kg excess weight .................................................................................. 20
Figure 4:
Fuel burn penalty. Specific range variations for the case of 1000 kg excess weight .................................................................................. 20
Figure 5:
Fuel burn penalty. Specific range variations for the case of 1000 kg excess weight .................................................................................. 21
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Figure 6:
Fuel burn penalty. Specific range variations for the case of 1000 kg less excess weight........................................................................... 21
Figure 7:
Contingency fuel with respect to flight distance ................................................ 24
Figure 8:
Climb profiles ..................................................................................................... 31
Figure 9:
Climb laws ......................................................................................................... 32
Figure 10:
Profile used for the computation........................................................................ 37
Figure 11:
∆ consumption between the optimum climb law and different climb laws......... 37
Figure 12:
∆ time between the optimal climb law and different climb laws......................... 38
Figure 13:
∆ time between the optimal climb law and different climb laws....................... 398
Figure 14:
∆ consumption between the optimum climb law and different climb laws...... 389
Figure 15:
∆ time between the optimum climb law and different climb laws...................... 39
Figure 16:
∆ consumption between the optimum climb law and different climb laws......... 40
Figure 17:
Annual potential savings per aircraft if 10 kg fuel is saved for each climb ....... 42
Figure 18:
Climb law ........................................................................................................... 43
Figure 19:
Specific range variations for different weights and altitudes ............................. 46
Figure 20:
Optimum altitude ............................................................................................... 46
Figure 21:
Step climb profiles ............................................................................................. 47
Figure 22:
Economic cruise Mach number for various flight levels and cost indices ......... 50
Figure 23:
Economic cruise Mach number for various flight levels and cost indices ........ 51
Figure 24:
Economic cruise Mach number for various flight levels and cost indices ........ 51
Figure 25:
Economic cruise Mach number for various flight levels and cost indices ........ 52
Figure 26:
Economic cruise Mach number for various weights and cost indices ............... 52
Figure 27:
Economic cruise Mach number for various weights and cost indices ............... 53
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Figure 28:
Economic cruise Mach number for various weights and cost indices ............... 53
Figure 29:
Economic cruise Mach number for various weights and cost indices ............... 54
Figure 30:
Economic cruise Mach number for various weights and cost indices ............... 54
Figure 31:
Time/fuel relation for a typical stage length. Stage length: 3000 Nm, M 0.8..... 55
Figure 32:
Time/fuel relation for a typical stage length. Stage length: 2000 Nm, M 0.8..... 56
Figure 33:
Time/fuel relation for a typical stage length. Stage length: 4000 Nm, M 0.8..... 56
Figure 34:
Fuel consumption increment on a 2000 Nm stage length, when the pilot switches in selected mode and increases Mach number .................................. 57
Figure 35:
Fuel consumption increment on a 2000 Nm stage length, when the pilot switches in selected mode and increases Mach number ................................. 58
Figure 36:
Fuel consumption increment on a 2000 Nm stage length, when the pilot switches in selected mode and increases Mach number .................................. 58
Figure 37:
Fuel consumption and time for different flight levels and Mach numbers on a 3000 Nm trip. TOW=130t. ......................................................................... 60
Figure 38:
Fuel consumption and time for different flight levels and Mach numbers on a 2000 Nm trip. TOW=60t............................................................................. 60
Figure 39:
Fuel consumption and time for different flight levels and Mach numbers on a 3000 Nm trip. TOW=210t. ......................................................................... 61
Figure 40:
Fuel consumption increment when flying above optimum altitude. Stage length: 1000 Nm. M 0.78......................................................................... 62
Figure 41:
Fuel consumption increment when flying above optimum altitude. Stage length: 1000 Nm. M 0.78......................................................................... 62
Figure 42:
Fuel consumption increment when flying below optimum altitude. M 0.8 ......... 63
Figure 43:
Fuel increment when flying below optimum altitude. M 0.8............................... 63
Figure 44:
Fuel consumption increment when flying below optimum altitude. M 0.8 ......... 64
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Figure 45:
Optimum flight profile......................................................................................... 64
Figure 46:
Optimum altitude with respect to aircraft weight................................................ 65
Figure 47:
Optimum altitude with respect to aircraft weight................................................ 65
Figure 48:
Optimum altitude with respect to aircraft weight................................................ 66
Figure 49:
Optimum altitude with respect to aircraft weight................................................ 66
Figure 50:
Optimum altitude with respect to aircraft weight................................................ 67
Figure 51:
Optimum altitude with respect to aircraft weight................................................ 67
Figure 52:
Optimum altitude on short stage........................................................................ 69
Figure 53:
Optimum altitude on short stage........................................................................ 69
Figure 54:
Optimum altitude on short stage........................................................................ 70
Figure 55:
Optimum altitude on short stage........................................................................ 70
Figure 56:
LRC Versus MRC at Given altitude ................................................................... 71
Figure 57:
Long Range Cruise Mach number..................................................................... 72
Figure 58:
Long Range Cruise Mach number..................................................................... 72
Figure 59:
Long Range Cruise Mach number..................................................................... 73
Figure 60:
Long Range Cruise Mach number..................................................................... 73
Figure 61:
Long Range Cruise Mach number..................................................................... 74
Figure 62:
Fuel consumption and trip time for various Mach numbers and flight levels in windy conditions. TOW=130t. ........................................................................ 75
Figure 63:
Fuel consumption and trip time for various Mach numbers and flight levels in windy conditions. TOW=80t. .......................................................................... 75
Figure 64:
Fuel consumption and trip time for various Mach numbers and flight levels in windy conditions. TOW=250t. ........................................................................ 76
Figure 65:
Wind influence on specific range…………………………………………………...77
Figure 66:
Wind altitude trade for constant specific range ................................................. 78
Figure 67:
Descent profiles ................................................................................................. 79
Figure 68:
Descent consumption from the same point in cruise......................................... 82
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Figure 69:
Descent consumption from the same point in cruise......................................... 82
Figure 70:
Descent consumption from the same point in cruise......................................... 83
Figure 71:
Descent consumption from the same point in cruise......................................... 83
Figure 72:
Descent consumption from the same point in cruise......................................... 84
Figure 73:
Annual potential money savings for a 20 KT decrease in descent speed......... 85
Figure 74:
Profile of a too early descent ............................................................................. 87
Figure 75:
Green dot speed definition ................................................................................ 89
Figure 76:
Fuel flow with respect to holding altitude for several configurations ................. 90
Figure 77:
Fuel flow with respect to holding altitude for several configurations ................. 90
Figure 78:
Fuel flow with respect to holding altitude for several configurations ................. 91
Figure 79:
Fuel flow with respect to holding altitude for several configurations ................. 91
Figure 80:
Description of the holding options ..................................................................... 95
Figure 81:
The stabilized approach .................................................................................... 97
Figure 82:
The decelerated approach................................................................................. 98
Figure 83:
Money saved by the decelerated approach in comparison with the stabilized one..................................................................................................... 99
Figure 84:
Premature landing gear extension................................................................... 100
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List of Tables Table 1:
Fuel consumption variation between a 20% CG and a 35% CG. Distance: 1000 Nm. ........................................................................................... 17
Table 2:
Fuel consumption increment for 1000 kg excess weight................................... 22
Table 3:
Extra fuel for single engine use rather than the A.P.U. ..................................... 27
Table 4:
Climb parameters for different cost indices (climb to FL330) ............................ 33
Table 5:
Delta in fuel and in time between a high and a low cost index.......................... 34
Table 6:
Comparison between the previous climb speed laws and the new ones.......... 36
Table 7:
Delta in fuel and in time between the optimum climb law and the most unfavorable one ................................................................................................. 40
Table 8:
Annual fuel savings corresponding to 10 kg fuel savings.................................. 41
Table 9:
Recommended climb laws................................................................................. 42
Table 10:
Standard climb laws .......................................................................................... 44
Table 11:
Crossover altitude and first optimum altitude .................................................... 44
Table 12:
Delta in fuel and time when flying at crossover altitude instead of flying at optimum altitude. Stage length: 1000 Nm. Cruise Mach number.................. 45
Table 13:
Percentage of fuel increment and of time gain when flying at crossover altitude instead of flying at optimum altitude. Stage length: 1000 Nm. Cruise Mach number. ........................................................................................ 45
Table 14:
Delta between optimum FL and MAX FL........................................................... 48
Table 15:
Fuel increment for delayed climb to FL 370 at optimum weight ........................ 48
Table 16:
Fuel increment in percent for a delayed climb................................................... 49
Table 17:
Delta in fuel and time in the case of a 0.005 Mach increase............................. 59
Table 18:
Fuel consumption variation with head/tailwind .................................................. 76
Table 19:
Relevant parameters for a descent from FL 370 at different cost indices (standard conditions) ......................................................................................... 80
Table 20:
Delta time and fuel between a descent at a high and a low cost index (decent from FL 370, standard conditions)........................................................ 81 9 GETTING TO GRIPS WITH FUEL ECONOMY
Table 21:
Annual potential fuel savings for a 20 KT decrease in descent speed.............. 84
Table 22:
Comparison of descent A319/A320/A321 ......................................................... 85
Table 23:
Comparison of descent A330 GE/PW/RR ......................................................... 86
Table 24:
Comparison of descent A340 CFM ................................................................... 86
Table 25:
Comparison of descent A300-600 ..................................................................... 86
Table 26:
Comparison of descent A310 ............................................................................ 87
Table 27:
Fuel increment for a one minute early descent from flight level 350. Cruise Mach number ......................................................................................... 88
Table 28:
Recommended holding configurations .............................................................. 89
Table 29:
Percentage of fuel flow increment when holding at S speed in conf 1 instead of holding at green dot speed in clean conf. Flight level 100 ............................ 92
Table 30:
Fuel increment when holding at S speed in conf 1 instead of holding at green dot speed in clean conf for a period of 15 minutes. Flight level 100 ....... 93
Table 31:
Maximum recommended speeds ...................................................................... 94
Table 32:
Fuel saved by reduction from M 0.78 to green dot speed at FL 350................. 95
Table 33:
Fuel saved by reduction from M 0.82 to green dot speed at FL 390................. 95
Table 34:
Fuel saved by reduction from M 0.82 to green dot speed at FL 390................. 96
Table 35:
Fuel saved by reduction from M 0.8 to green dot speed at FL 350................... 96
Table 36:
Money and fuel saved thanks to the second option .......................................... 96
Table 37:
Fuel increment between a stabilized approach and a decelerated approach ... 98
Table 38:
Potential annual savings due to the decelerated approach............................... 99
Table 39:
Fuel increment in case of premature extension .............................................. 101
Table 40:
Potential annual savings.................................................................................. 101
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1.
PREAMBLE
The energy crisis of the 70's woke airlines up to the seriousness of fuel savings. Nowadays, this fear of a sudden or even gradual fuel price rise, coupled with a very competitive and deregulated aviation market forces commercial airlines once again into drastic measures to save fuel. Airlines try to reduce their operational costs in every facet of their business. Fuel conservation has become one of the major preoccupations for all airlines. Fuel bills indeed are representing a considerable part of overall aircraft operating costs. All ways and means to keep fuel costs under optimal control have to be rationally envisaged, safety being of course the number one priority in any airline operation. Some operational costs cannot be cut down without degrading safety and are therefore totally inflexible. The purpose of this document is to examine the influence of flight operations on fuel conservation with a view towards providing recommendations to enhance fuel economy. No dedicated attempt is made to identify the trade-off of fuel saved versus the other operating variables, such as cost or trip time. This is the scope of another brochure called "Getting to Grips with the Cost Index: Balancing Cost of Fuel and Cost of Time". The present brochure systematically reviews fuel conservation aspects relative to ground and flight performance. Whilst the former pertains to center of gravity position, excess weight, auxiliary power unit (A.P.U.) operations and taxiing, the latter details climb, step climb, cruise, descent, holding and approach. Wind/altitude trade effects are also reviewed to provide airline pilots, engineers, or managers with useful insights on operational factors. None of the information contained herein is intended to replace procedures or recommendations contained in the Flight Crew Operating Manuals (FCOM), the objective being rather to highlight the areas where flight crews can contribute significantly to fuel savings, circumstances permitting. In reviewing the fuel economy theme with regard to our whole fleet (A300-600, the A310, the A319/320/321, the A330 and A340) the major tool used was PEP for Windows, the Airbus' software enabling airlines' operation engineering departments to compute aircraft performance as a function of specified operational conditions.
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Would you please send your comments and remarks to the following contact point at Airbus. The topic of fuel conservation has been the subject of a lot of debate/controversy, action/inaction in recent years and we value your contributions very much. These will be taken into account in our follow-up with you as well as in the following issues to be edited.
Flight Operations Support & Line Assistance Customer Services Directorate 1, Rond Point Maurice Bellonte, BP 33 31707 BLAGNAC Cedex - FRANCE TELEX AIRBU 530526E SITA TLSBI7X TELEFAX 33/(0)5 61 93 29 68 or 33/(0)5 61 93 44 65
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2.
INTRODUCTION: BASIC PREMISES AND OUTLINE
This brochure considers the two flight management modes: the "managed" mode and the "selected" mode. The managed mode corresponds to flight management by means of a dedicated tool, the flight management system (FMS). Crews interface through the multipurpose control and display unit (MCDU) introducing basic flight variables such as weight, temperature, altitude, winds and the cost index. From these data, the FMS computes the various flight control parameters such as the climb law, step climbs, economic Mach number, optimum altitude, descent law. Hence, when activated, this mode enables almost automatic flight management. When in managed mode, aircraft performance data is extracted from the FMS database. These same databases were simplified to alleviate computation density and calculation operations in the FMS; results may therefore be less precise than reality but they constitute useful indications for experienced guidance. When in selected mode, crews conduct the flight and flight parameters such as speed, altitude and heading have to be manually introduced on the flight control unit (FCU). The databases used to compute aircraft performance in this configuration are used on ground-based mainframe computers; hence they are more complete and more precise than those of the FMS. For this reason, straight comparisons between performance results stemming from these two modes necessitates adjustments beyond the scope of this brochure. Calculations presented here were made taking into account average numbers of take-offs and landings for the year 1998 and for the following aircraft types: A300-600: 1300 take-offs and landings per year per aircraft A310: 1100 take-offs and landings per year per aircraft A319: 1800 take-offs and landings per year per aircraft A320: 1700 take-offs and landings per year per aircraft A321: 2000 take-offs and landings per year per aircraft A330: 1200 take-offs and landings per year per aircraft A340: 700 take-offs and landings per year per aircraft Assumed price of fuel: 1 US dollar per gallon
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3.
PRE-FLIGHT PROCEDURES
Operation of the aircraft starts with the aircraft on ground during aircraft preparation and loading. This section should highlight the impact of some ground operations on fuel consumption. Even if these operations enable only little savings in comparison with savings made during the cruise phase, ground staff have to be sensitive to these and should get adapted professional practices. This part is divided into four different sections: −
The first section is about the center of gravity position and its impact on fuel consumption.
−
The second one is about excess weight and fuel consumption.
−
The third section is about Auxiliary Power Unit consumption.
−
The fourth section is about fuel saving taxi practices.
3.1
Center of gravity
3.1.1 Preliminary The gross weight is the sum of the dry operating weight, payload and fuel. The resultant force acts through the center of gravity of the aircraft. The balance chart allows to determine the overall center of gravity of the airplane taking into account the center of gravity of the empty aircraft, the fuel distribution and the payload. The center of gravity must be checked to be within the allowable range referred to as the center of gravity envelope. In terms of fuel consumption, forward center of gravity needs a nose up pitching moment, which adds to the one created by weight and leads to an increase in fuel consumption because of induced drag. It is better to have the center of gravity as far aft as possible. However, such a center of gravity position deteriorates an aircraft's dynamic stability. 3.1.2 Automatic center of gravity management AIRBUS has developed a trim tank transfer system, which controls the center of gravity of the airplane. When the airplane is in cruise, the system optimizes the center of gravity position to save fuel by reducing the drag on the airplane. The system either transfers fuel to the trim tank (aft transfer) or from the trim tank (forward transfer). This movement of fuel changes the center of gravity position. The crew can also manually select forward fuel transfer.
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The Fuel Control and Management Computer (FCMC) calculates the center of gravity of the airplane according to the aerodynamic surfaces and compares the result to a target value. From this calculation, the FCMC determines the quantity of fuel to be moved aft or forward in flight (usually one aft fuel-transfer is carried out during each flight). 3.1.3 Influence on fuel consumption The following graphs show the gain or loss in fuel expressed in terms of specific range with a center of gravity of 20% and 35%, compared to the consumption for a center of gravity position of 27% at cruise Mach. For the other aircraft, all curves have a similar shape to these ones: Figure 1: Specific range variation for different center of gravity positions
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Figure 2: Specific range variation versus CG Variation – MO.82
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For the A300-600, A310, A330 and A340 types, the further aft the center of gravity, the more significant fuel savings will be. Furthermore, when flying above optimum altitudes, at high weights, the decrease or increase in fuel consumption as a function of the reference is significant. Thus, loading is very important especially for high weights and for aircraft having no automatic center of gravity management: we notice that the specific range variation can reach 2% for high weights and FL's above 350 for the A300-600, A310, A330 and A340 types. Contrary to the other aircraft, specific range variations with respect to the center of gravity position are random for the whole A320 family. This is due to a complex interaction of several aerodynamic effects. Whatever the influence of the center of gravity position on specific range, it can however be said on the A320 family, this influence is very small. As the specific range characterizes fuel consumption of aircraft at a given weight, it is quite difficult to quantify the impact of the center of gravity position on an entire stage length. On a 1000 NM stage length, the increases in fuel consumption when the center of gravity position is 20% with regard to the fuel consumption when the center of gravity position is 35% are summed up in the following table. The results characterize the worst cases, that is to say that an aircraft with a high weight and at a high flight level is considered. Table 1: Fuel consumption variation between a 20% CG and a 35% CG. Distance: 1000 Nm. Aircraft types
Fuel increment (kg)
A319/A320/A321
Negligible
A330
220
A340
380
A310
250
A300-600
230
3.1.4 Summary For better fuel consumption the center of gravity must be placed further aft, but aircraft stability must be the deciding factor. The aircraft must be so loaded that the center of gravity is still within the allowable range.
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3.2
Excess weight
Another way to save fuel is to avoid excess weight. 3.2.1 Aircraft weight Airlines must first of all correctly estimate real aircraft weights: an unrealistic weight may put aircraft at increasing risk during take-off. Airbus already reported the market tendency of increasing passenger weights, mainly because of carry-on baggage and duty-free articles. The J.A.A. and the F.A.A. also noticed this phenomenon and therefore reviewed regulations concerning passenger and baggage standard weights. The J.A.A. has produced JAR OPS (1.4). For the purpose of calculating the mass of an aircraft, the total masses of passengers, their hand baggage entered on the loadsheet shall be computed using: −
either the actual mass values to be weighed case-by-case;
−
or standard mass values such as: a) passengers including hand baggage: All flights except
84kg / 185lb
Holiday charters
76kg / 168lb
Children (2-12 years)
35kg / 77lb
b) checked baggage All flights except domestic and intercontinental flights
13kg / 25lb
Domestic flights
11kg / 24lb
Intercontinental flights
15kg / 33lb
A review of these weights will have to be performed every five years, and the loadsheet should always contain references to the weighing method adopted. Initially, the following standard mass values for males and females including hand baggage had been agreed upon:
Scheduled, medium/long-haul Schedules, European short-haul Non-scheduled
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Males
Female
86kg / 190lb 89kg / 196lb 84kg / 185lb
69kg / 152lb 71 kg / 157lb 69kg / 152lb
18
In the USA, the F.A.A. has issued an Advisory Circular to provide methods and procedures for developing weight and balance control. This also involves initial and periodic re-weighing of aircraft (every 3 years) to determine average empty and actual operating weight and CG position for a fleet group of the same model and configuration. In the past the following standard average weights had been adopted:
Summer (1/5 thru 31/10) Winter (1/11 thru 30/4) Carry-on baggage allowance
Males & Females
Children
73kg / 160lb 75kg / 165lb 4.5kg / 10lb
36kg / 80lb 36kg / 80lb 4.5kg / 10lb
AC 120-27B features a 10 Ib increase for adult weights: Summer (1/5 thru 31/10) Winter (1/11 thru 30/4) Carry-on baggage allowance
77kg / 170lb 80kg / 175lb 4.5kg / 10lb
3kg / 80lb 36kg / 80lb 4.5kg / 10lb
Similar to J.A.A., airlines will have to adopt standard weights unless they request different values, which would have to be proven by a survey at the risk of ending up with higher statistics. Harmonization between F.A.A. and J.A.A. is desirable, as it would eventually prompt all airlines to undergo the same penalty with minimal competitive detriment. 3.2.2 Overload effect The specific range, flying at given altitude, temperature and speed depends on weight. The heavier the aircraft, the higher the fuel consumption. In addition, fuel savings can be made during climb since the aircraft would reach its optimal flight level earlier if it were lighter. The effect of overloading with respect to in-flight weight is shown on the following graphs for 1000 kg excess load in cruise for four different aircraft. The characteristic curves for the other aircraft types have a similar shape.
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Figure 3:
Figure 4:
Fuel burn penalty. Specific range variation for the case of 1000 kg excess weight
Fuel burn penalty. Specific range variations for the case of 1000 kg excess weight
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Figure 5: Fuel burn penalty. Specific range variations for the case of 1000 kg excess weight
Figure 6: Fuel burn penalty. Specific range variations for the case of 1000 kg excess weight
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The increase in fuel consumption is more important at high flight levels and heavy weights. Indeed, specific range can be increased by 2% if aircraft weight is diminished by 1000 kg. Excess weight has a more important impact on the A310 and A300-600 as the increase in specific range for 1000 kg less varies from 0.15% up to 3%. The impact is also significant on A319, A320 and A321 types as the increase in specific range varies between 0.4% to 2%. Compared to that, the A330 and A340 types seem to be the least affected by excess weight in terms of fuel consumption. But as wide bodies fly longer segments, it represents a bigger amount of fuel: by way of example, the following table hints at fuel savings per 1000 kg for typical stage lengths at optimum altitude. Table 2: Fuel consumption increment for 1000 kg excess weight Aircraft types
Mach
Stage
Fuel penalty
A319 (55 000 kg)
0.78
1000 Nm
40 kg
A320 (65 000 kg)
0.78
1000 Nm
70 kg
A321 (75 000 kg)
0.78
1000 Nm
90 kg
A330 (200 000 kg)
0.82
4000 Nm
200 kg
A340 (260 000 kg)
0.82
6000 Nm
160 kg
A310 (140 000 kg)
0.8
2000 Nm
130 kg
A300-600 (160 000 kg)
0.8
2000 Nm
260 kg
This table shows that the increase in fuel consumption is significant just for the case of a single ton's excess weight. Ground staff must try to avoid this whenever possible. 3.2.3 Means to diminish aircraft weight In order to diminish aircraft weight, either the zero fuel weight or the embarked fuel can be reduced. a) Zero fuel weight One must be aware that zero fuel weight can increase substantially during an aircraft's life because of an accumulation of unnecessary catering equipment, supplies etc... Any empty cargo containers, interior and exterior dirt and rubbish must be removed in order to minimize aircraft zero fuel weight. Airline staff have to be sensitive to these issues and dedicated efforts are necessary to avoid excess weight.
GETTING TO GRIPS WITH FUEL ECONOMY
22
b) Embarked fuel Embarked fuel minimization In the same vein, unnecessary fuel weight must be resolutely avoided: flights must be planned very precisely to calculate the correct amount of fuel to be embarked. Trip fuel is estimated with a certain accuracy depending on theoretical aircraft performance. However, if real aircraft performance is far below the nominal one, the pilot will take more fuel than necessary (to ensure having enough fuel) which in itself will result in aircraft weight and fuel consumption increases. So flight planning should be based on aircraft performance monitoring by taking into account performance factors derived from specific range variations. According to the JAR OPS 1.255, contingency fuel is the higher amount of fuel between (a) and (b). (a) Either: •
5 % of the planned trip fuel or, in the event of in-flight replanning, 5 % of the trip fuel for the remainder of the flight; or
•
Contingency fuel can be less. In this case it can be no less than 3 % of the planned trip fuel, or in the event of in-flight replanning, 3 % of the trip fuel for the remainder of the flight provided that an en-route alternate is available. The enroute alternate should be located within a circle having a radius equal to 20 % of the total flight plan distance, the center of which lies on the planned route at a distance from the destination of 25 % of the total flight plan distance, or at 20 % of the total flight plan distance plus 50 NM, whichever is greater; or
•
It can also be the fuel necessary to fly 15 minutes at holding speed at 1500 ft above the destination airport in standard conditions, when an operator has established a program, approved by the Authority, to monitor fuel consumption on each individual route/airplane combination and uses this data for a statistical analysis to calculate contingency fuel for that route/airplane combination; or
•
Finally, if the airline has kept track of the consumption for each aircraft, the contingency fuel required is an amount of fuel sufficient for 20 minutes flying time based upon the planned trip fuel consumption, provided that the operator has established a fuel consumption monitoring program for individual airplanes and uses valid data determined by means of such a fuel calculation program.
(b) An amount to fly for 5 minutes at holding speed at 1500 ft above the destination aerodrome in standard conditions.
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23
What we can conclude is that depending on flight distance, there is a lowest contingency fuel. The following graphs show the different contingency fuel quantities for different distances. Figure 7: Contingency fuel with respect to flight distance
15 min holding 5 min holding
According to the Federal Aviation Administration, contingency fuel is the amount necessary to fly for 45 minutes at normal cruising speed. In this case, there is no way to influence the embarked fuel. In the same vein, in order to diminish the amount of embarked fuel, alternate airports should be chosen as near as possible to the destination so as to minimize the diversion fuel reserve. According to the FAA, the diversion fuel reserve is not necessary if: −
Part 97 of subchapter F prescribes a standard instrument approach procedure for the first airport of intended landing and
−
For at least one hour before and one hour after the estimated time of arrival at the airport, the weather reports or forecasts or any combination of them indicates: (i) the ceiling will be at least 2 000 feet above the airport elevation, and (ii) visibility will be at least 3 statute miles.
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According to the JAR OPS 1.295c, the diversion fuel reserve is not necessary if: (1) Both: •
The duration of the planned flight from take-off to landing does not exceed 6 hours; and
•
Two separate runways are available at destination and the meteorological conditions prevailing are such that, for the period from one hour before until one hour after the expected time of arrival at destination, the approach from the relevant minimum sector altitude and the landing can be made in VMC (see IEM OPS 1.295(c)(1)(ii)); or
(2) The destination is isolated and no adequate destination alternate exists. Fuel transportation Carrying extra fuel may be of value when a fuel price difference exists between two airports. However, since the extra fuel on board leads to an increase in fuel consumption the breakeven point must be carefully determined. K is the transport coefficient:
K=
∆TOW ∆LW
The addition (or the subtraction) of one ton to landing weight, means an addition (or a subtraction) of K tons to take-off weight. EXAMPLE: with K=1.3, if 1300 kg fuel is added at the departure, 1000 kg of this fuel amount will remain at destination. So carrying one ton fuel costs 300 kg more fuel. At the departure, if ∆MD tons of fuel are embarked, at destination
∆MA =
∆MD K
will remain. The extra-cost at departure is:
∆MD x Pd
With Pd: fuel price at departure. At arrival, the saving is:
∆MA x Pa
With Pa: fuel price at arrival. GETTING TO GRIPS WITH FUEL ECONOMY
25
A cost linked to a possible increase in flight time can be added:
∆T x Ph With Ph: one-hour flight price. It is profitable to carry extra fuel if:
∆MD x Pa − ∆MD x Pd − ∆T x Ph ≥ 0 K That is to say:
Pa ≥ K x Pd +
K x ∆T x Ph ∆MD
With ∆T=0, it is profitable to carry extra fuel if Pa ≥ K x Pd that is to say if the arrival fuel price to departure fuel price ratio is higher than the transport coefficient K. Thus carrying extra fuel may be of value when a fuel price differential exists between two airports. Graphs such as FCOM 2.05.70 on A320/A330/A340 assist in determining the optimum fuel quantity to be carried as a function of initial take-off weight (without fuel excess), stage length, cruise flight level and fuel price ratio. 3.3
A.P.U.
3.3.1 Preliminary The Auxiliary Power Unit (A.P.U.) is a self-contained unit, which makes the aircraft independent of external pneumatic and electrical power, supply. A.P.U. fuel consumption obviously represents very little in comparison with the amount of fuel for the whole aircraft mission. Nevertheless, operators have to be aware that adopting specific procedures on ramp operations can help save fuel and money. 3.3.2 Fuel conservation and A.P.U. On ground, at sea level, under ISA conditions, A.P.U. fuel consumption varies depending on A.P.U. types. It goes from 60 to 80 kg/h in no load conditions and from 110 to 160 kg/h for air conditioning + electric load and for main engine start operations. A.P.U. specific procedures to save fuel have to be defined by the operators: they have to choose between using ground equipment (Ground Power Unit, Ground Climatisation Unit, Air Start Unit) or A.P.U. It depends on several parameters: when the turn-around is quite long, or when the aircraft does a night-stop, the use of G.P.U. is well adapted, as time considerations are not prevailing. It enables to save both fuel and A.P.U. life.
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26
Remember that one extra minute of A.P.U. operation per flight at 180 kg/hr fuel flow, means an additional 3000 kg per year per aircraft. However, for short turn-around (45 minutes on average), the use of A.P.U. is advantageous. Moreover, to limit A.P.U. start cycles and improve reliability, we advise to keep the A.P.U. running, even if it is not fully used during the next 45 minutes. It is better to operate with A.P.U. at Ready To Load (RTL) than to shut it down and perform a new start cycle. So operators are advised to use ground equipment when this is of a good quality level, and therefore to try to conclude agreements with airport suppliers to get preferential prices. However, in some countries, ground operations are restricted by law. The use of the APU is limited to a defined time prior to departure time and after the arrival. Note: It is not really correct to compare the amount of fuel burnt by A.P.U. and ground equipment, as we also have to consider A.P.U. maintenance. 3.3.3 Optimisation procedures The disconnection of ground equipment supplies and the start of A.P.U. must be coordinated depending on A.T.C. A « one minute anticipation » in each A.P.U. start will lead to a significant amount of fuel savings at year's end (2000 to 4000 kg depending on A.P.U. types). The following table gives the amount of fuel saved per year and per flight which can be attributed to nothing more than good coordination. Engine start-up too should, if possible, be carefully planned in conjunction with A.T.C. If push-back is delayed, it is preferable to wait and use A.P.U. for air conditioning and electrical requirements. The following table shows extra fuel consumption per minute for using a single engine rather than the A.P.U.: Table 3: Extra fuel for single engine use rather than the A.P.U. A.P.U. types
One engine consumption (kg/h)
∆consumption (kg/h)
GTCP 36-300 A319/A320/A321
340
+210
APS 3200 A319/A320/A321
340
+225
GTCP 331-350 A330GE
550
+335
A330 PW
580
+365
A.P.U. types
One engine consumption
∆consumption (kg/h)
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(kg/h) A330 RR
820
+605
300
+80
GTCP 331-350 A340
3.3.4 Summary It is quite difficult to give advice for using A.P.U. rather than ground equipment because it depends on several parameters. Operators have to define the most economical solutions, depending on their own aircraft operations. 3.4
Taxiing
3.4.1 Preliminary Jet engine performance is optimized for flight conditions. Nevertheless, all aircraft spend significant amounts of time on ground for various operations. As regards taxiing conditions such as: •
congestion of ground traffic,
•
ramp to runway distance,
•
holding point with dozens possibly waiting for take-off,
all lead to a waste of precious time and fuel. One (or two) engine(s) taxi can help. But such procedures need to be discussed, and operators have to define their field of application. Airbus provides standard procedures to operators (FCOM 3.04.90). Regardless of these, operators have to keep in mind some specifics.
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Taxi is split into two distinct phases: •
taxi out: from ramp to runway
•
taxi in: from runway to ramp
3.4.2 Engine-out taxi operation Taxiing out with one (or two) engine(s) shut down a) No fire protection from ground staff is available when starting engine (s) away from the ramp. b) Potential loss of: − braking capability (brake accumulators are nevertheless operational), nose wheel steering (in case APU is not used), could lead to a taxiway excursion. c)
FCOM (3.04.90) requires not less than a defined time (from 2 to 5 minutes depending on the aircraft) to start the other engine(s) before take off. On engines with a high bypass ratio, warm-up and cool-down time prior to applying maximum take off thrust, is vital for engine safety and lifetime.
d) Mechanical problems can occur during start up of the other engine(s), requiring a gate return for maintenance and delaying departure time. Taxiing in with one (or two) engine(s) shut down a) FCOM requires APU start before shutting down the engine, to avoid an electrical transient (A319/A320/A321). b) FCOM (3.04.90) requires not less than a defined time before shutting down the other engine(s). On engines with a high bypass ratio, the cool-down time after reverse operation, prior to shut down is vital for the engine safety and lifetime. During taxi in and out, one (or two) engine(s) shut down a) Caution must be exercised when taxiing one (for twin engine) or two engine(s) shut down (A340) to avoid excessive jet blast and FOD. b) Slow and/or tight taxi turns in the direction of the operating engine may not be possible at high gross weight. c)
More thrust is necessary for breakaways and 180 degrees turn. Be aware of higher blast effect.
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29
d) For the A340, it is recommended to taxi with the outer engines to pressurize the green hydraulic system, so as to allow normal operation of braking and nose wheel steering. e) For the A319/A320/A321, it is preferable to use engine 1 for taxi to pressurize the green hydraulic system, and without using the PTU. 3.4.3 Summary Operators have to define their own taxiing policy depending on airport configurations (taxiways, runways, terminals and ramps,...) and crew training with an eye on FCOM prescriptions. 3.5
Conclusion on pre-flight procedures •
On all aircraft except the A320 family, it is advised to load the airplane so that its center of gravity is further aft, provided it is still within the allowable range.
•
It is advised to avoid excess weight by diminishing zero fuel weight and embarked fuel due to accurate flight planning.
•
Operators have to decide whether the use of A.P.U. is appropriate depending on turn around time, quality of ground equipment and airport specific procedures. However, whenever possible, the use of ground equipment is recommended to save both fuel and A.P.U. life.
•
Taxiing with one engine out saves fuel but has some drawbacks. Operators have to define their own taxiing policy depending on airport configurations (taxiways, runways, ramps…).
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30
4.
WITHIN THE FLIGHT ENVELOPE
As aircraft spend more time airborne than on ground, much fuel can be saved by disciplined flight crews. This part intends to give flight crew recommendations on how to save fuel during flight. It reviews the different flight phases, that is to say: •
Climb
•
Step climb
•
Cruise
•
Descent
•
Holding
•
Approach
4.1
Climb
4.1.1 Preliminary Depending on speed laws, climb profiles change. The higher the speed, the lower the climb path, the longer the climb distance. Figure 8: Climb profiles
TOC: top of climb. It is suggested to climb at "best rate of climb" on a constant IAS/Mach climb speed schedule. Climb is performed in three phases at max climb thrust: •
indicated air speed is maintained at 250 KT until flight level 100, then the aircraft accelerates to the chosen indicated air speed,
•
constant indicated air speed is maintained,
•
constant Mach number is maintained.
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31
The crossover altitude is the altitude where we switch from constant IAS climb to the constant Mach number climb. It only depends on the chosen IAS and Mach number, and does not depend on ISA variation. Climb can be schematized as below: Figure 9: Climb laws
TAS: True Air Speed IAS: Indicated Air Speed During climb, at constant IAS, both TAS and Mach number increase. Then, during climb at constant Mach number, both TAS and IAS decrease up to the tropopause. 4.1.2 Managed mode The Flight Management System computes the climb speed law taking into account cost index, wind, temperature, and take-off weight. It allows a climb schedule with a continuous evolution of speed during climb to be determined. To get conclusive information, climb and cruise flight must be viewed in relation to each other. A short climb distance for example extends the cruise distance, a low climb speed requires more acceleration to cruise speed at an unfavorable high altitude. One has therefore to consider sectors that cover acceleration to climb speed, climb, acceleration to cruise speed and a small portion of the cruise as depicted in figure 10. a) A300-600, A310, A320 family, A330 The following table gives the different relevant accurate climb parameters (time, fuel, and distance). The first column gives climb parameters relative to the climb part only, and the second column take into account the remark in § 4.1.1. These results come from theoretical aircraft models.
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Table 4: Climb parameters for different cost indices (climb to FL330)
Only climb segment Aircraft type
CI
Climb with cruise segment
Fuel (kg)
Time (min)
Dist (NM)
Fuel (kg)
Time (min)
CAS/Mach
RATE at TOC (ft/min)
A300-600
0
2891
17
115
2977
18
320/.777
869
(PW 4158)
30
2959
17.5
119
2993
17.8
325/.791
842
160 000 Kg
60
3004
17.8
122
3004
17.8
325/.800
810
0
2787
17.4
114
2922
19
302/.791
1037
30
2833
17.6
118
2929
18.7
311/.8
1024
60
2870
17.7
121
2938
18.5
320/.803
1009
100
2920
17.9
124
2952
18.3
330/.807
991
150
2942
18.1
125
2958
18.3
330/.811
968
200
2965
18.2
127
2965
18.2
330/.814
936
0
1757
22.4
150
1984
27.5
308/.765
584
20
1838
23.1
159
2009
26.9
321/.779
566
40
1897
23.7
165
2030
26.6
333/.783
550
60
1980
24.7
175
2056
26.3
340/.791
506
80
2044
25.6
183
2072
26.2
340/.797
461
100
2080
26.1
187
2080
26.1
340/.8
439
0
3568
19.1
122
3927
23
293/.761
963
50
3773
20
135
3984
22.2
309/.8
943
80
3886
20.5
141
4018
21.8
320/.812
917
100
3927
20.7
143
4031
21.8
320/.818
896
150
4005
21.3
148
4053
21.7
320/.827
837
200
4068
21.7
152
4068
21.7
320/.833
786
A310 (CF6-80) 140 000 Kg
A320 (CFM 56) 75 000 Kg
A330 (PW 4168) 200 000 Kg
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33
Since these values depend a lot on flight conditions (first assigned flight level, take-off weight, temperature, wind), the most representative values are delta in time, delta in fuel between high and low cost indices which are almost constant even with external conditions. The results are summed up in the following table: Table 5: Delta in fuel and in time between a high and a low cost index Aircraft types
Time gain
Fuel increment (kg)
A320
1min30s
100
A330
1min20s
140
A300-600
10s
30
A310
50s
40
Time to climb is only slightly affected by the cost index (less than one minute) for the A300-600 and A310 between low and high cost indices. Moreover, climbing at high cost indices is only valuable if time to climb is really essential since time differences between low and high cost index climb are very small. b) A340 family A340 family aircraft have a different climb behavior than twin engine aircraft. Indeed, twin engine aircraft have a higher thrust than four engine aircraft, as they must satisfy more stringent climb requirements with only one engine operative. Hence, twins climb faster than four engine aircraft and reach their allotted cruise flight level with higher vertical speed because of extra thrust. As the climb performance of the A340 was not fully satisfying, and as ECON climb was not fully optimized, another ECON climb law was implemented so as to reduce time to climb (FMGC L7).
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The following table gives the relevant parameter with regard to the previous climb laws for a climb towards flight level 330. Climb with cruise segment
Only climb segment Aircraft type
A340 (CFM 56) 250 000 Kg
CI
CAS/Mach
RATE at TOC (ft/min)
Fuel (kg)
Time (min)
Dist (NM)
Fuel (kg)
Time (min)
0
5363
25.4
168
5532
26.8
298/.793
503
50
5450
26
172
5551
26.7
298/.805
485
80
5492
26.2
174
5560
26.7
298/.810
475
100
5510
26.3
175
5563
26.7
298/.812
469
150
5547
26.5
177
5570
26.7
298/.816
457
200
5574
26.7
178
5574
26.7
298/.819
447
The following table gives the new ECON climb laws for various take-off weights and flight levels: CRUISE FL
TOW (1 000 kg) 280
260
240
220
270
315/.78
315/.78
315/.78
315/.78
313/.775
280
309/.78
309/.78
309/.78
309/.78
309/.78
290
302/.78
302/.78
302/.78
302/.78
302/.78
310
291/.784
289/.78
289/.78
289/.78
289/.78
290/.79
286/.78
286/.78
286/.78
293/.797
286/.78
286/.78
295/.803
286/.78
330 350 370 390
GETTING TO GRIPS WITH FUEL ECONOMY
200
294/.8
35
The following table compares the new ECON climb laws to the previous ones for a climb towards flight level 330 and a take-off weight of 250 000 kg: Table 6: Comparison between the previous climb speed laws and the new ones Cost index
FMGC L6
FMGC L7
0
298/.793
288/.78
50
298/.805
288/.78
80
298/.810
288/.78
100
298/.812
288/.78
150
298/.816
288/.78
200
298/.819
288/.78
ECON climb law is no longer function of the cost index. Mach number at the end of the climb is in most cases M0.78 and the acceleration to cruise Mach number is made in level flight. This new climb law enables to reach cruise flight level some 6 to 20 NM earlier than with previous climb laws, and rate of climb is also improved above 29 000 ft. One must remember than the A340 is a four engine aircraft and thus cannot have climb performance similar to that of a twin. 4.1.3 Selected mode Under many circumstances, the ideal scenario computed by the FMS cannot be sustained. A.T.C. may impose speed or altitude constraints, and crews will consequently have to perform climb in selected mode. The following charts give fuel consumption as well as time variations depending on different climb laws, taking into account the acceleration and the cruise to reach the furthest top of climb. As already explained in § 4.1.2 the fuel consumption for the climb segment was added to the fuel consumption to accelerate to the cruise Mach number and to the fuel consumption to cruise till the furthest top of climb.
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Figure 10: Profile used for the computation
The following graphs show delta in fuel and delta in time for different climb laws in comparison with the optimal one. As they may be not very clear, the following figure aims at explaining them. If you want to know the increase in fuel consumption when performing a climb with a certain speed law rather than of the optimum speed law, you will have to find the point which represents the speed law on the graph. You will then be able to read the corresponding fuel consumption increment (in percent) on the vertical axis. EXAMPLE: You decide to perform a climb at 250Kts/300Kts/0.78. Here the optimal climb law is: 250Kts/280Kts/0.76 (figure 12). To find the point which represents your climb law, you just have to find the intersection of the "isolAS 300Kts" and of the "isoMach 0.78", as shown on the graph: Figure 11: ∆consumption between the optimum climb law and different climb laws
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37
As can be noticed, the chosen climb law consumes 1 % more than the optimal one. The colored legend helps to directly appreciate the range of fuel increment. For example the blue color highlights the speed laws which lead to an increase in fuel consumption for the optimum climb law. The same graphs were plotted for time variations: Figure 12: ∆time between the optimal climb law and different climb laws
This particular document contains graphs characterizing the A319 CFM, the A320 CFM and the A330 GE. Figure 13: ∆time between the optimum climb law and different climb laws
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Figure 14: ∆ consumption between the optimal climb law and different climb laws
Figure 15: ∆ time between the optimum climb law and different climb laws
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Figure 16: ∆ consumption between the optimum climb law and different climb laws
Consumption curves are stretched for high climb laws, while time curves are quite regular. This means that climbing at high speed costs more in fuel for an identical time gain. We also notice that for slow climb laws, fuel curves are shrinking whereas time curves are regular. So slow climb saves less fuel for equivalent time loss. In other words, time variations are linear with respect to speed increase whereas fuel consumption variations are not. Fuel consumption increases a lot for high climb laws and after being at its best for an optimum climb law, increases for low speed laws. To conclude, it is neither profitable to climb at high climb laws except for time imperatives, nor to climb at very slow climb laws. The following table gives delta in fuel and in time between the optimum climb law and the most unfavorable one: Table 7: Delta in fuel and in time between the optimum climb law and the most unfavorable one Aircraft types
∆fuel (kg)
∆time (min)
A319 (60,000 kg)
110
2
A320 (70,000 kg)
110
3
A321 (78,000 kg)
120
3
A330 (190,000 kg)
170
2
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Aircraft types
∆fuel (kg)
∆time (min)
A340 (250,000 kg)
250
2
A300-600 (160,000 kg)
100
1.5
A310 (130,000 kg)
130
1.5
However, even if the adjustment of climb speed saves only 10 kg fuel per flight, the annual savings are bound to be somehow significant: Table 8: Annual fuel savings corresponding to 10 kg fuel savings Aircraft types
Annual fuel savings (kg)
A319
18 000
A320
17 000
A321
20 000
A330
12 000
A340
7 000
A300-600
13 000
A310
11 000
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41
The following figure gives the potential money per year per aircraft for a saving of 10 kg fuel. Figure 17: Annual potential savings per aircraft if 10 kg fuel is saved for each climb
Hence the optimal climb law enables significant savings. The optimal climb law mainly depends on the aircraft take-off weight. The following table gives the recommended climb laws with respect to the aircraft take-off weight: Table 9: Recommended climb laws Aircraft types
BELOW
ABOVE
TOW60t
250/260/0.78
250/280/0.78
TOW65t
250/260/0.78
250/280/0.78
TOW70t
250/260/0.78
250/280/0.78
A319
A320
A321
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42
Aircraft types
BELOW
ABOVE
TOW190t
250/280/0.8
250/300/0.8
TOW240t
250/300/0.78
250/320/0.78
TOW140t
250/300/0.8
250/320/0.8
TOW150t
250/300/0.78
250/320/0.78
A330
A340
A310
A300-600
4.1.4 Crossover altitude versus optimum altitude In managed mode, the crossover altitude varies with the cost index; in selected mode, it depends on the speed law set on the FCU. Figure 18: Climb law
This graph clearly shows that the TAS is maximum at the crossover altitude. One can wonder whether it is profitable to stay at this altitude, instead of climbing to the first optimum altitude. Considering standard climb laws, it is possible to know the crossover altitude and the first optimum flight level.
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The standard speed laws are summed up in the following table: Table 10: Standard climb laws Aircraft types
Speed law
A319/A320/A321
250kts/300kts/M0.78
A330
250kts/300kts/M0.8
A340
250kts/290kts/M0.78
A310
250kts/300kts/M0.8
A300-600
250kts/300kts/M0.78
The next table exhibits the crossover altitude and the first optimum flight level for an ISA variation below 10 degrees Celsius and for an aircraft weight near the maximum takeoff weight. Table 11: Crossover altitude and first optimum altitude Aircraft types
Crossover altitude
A319 A320
1st optimum FL (ISA
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