Aoa 737ngx Groundwork Tcas Handout

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7

TCAS

The material covered in this document is based off information obtained from the original manufacturers’ Pilot and Maintenance manuals. It is to be used for simulation purposes only.

Copyright © 2011 by Angle of Attack Productions, LLC All rights reserved Page 7-1

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Table of Contents What is TCAS? What is a Transponder? How Does TCAS Work? Advisories and Displays “Monitor Vertical Speed” “Maintain Vertical Speed, Maintain” “Maintain Vertical Speed, Crossing Maintain” “Climb, Climb” “Descend, Descend” “Climb, Crossing Climb”  Increase Climb, Increase Climb “Adjust Vertical Speed, Adjust”

TCAS Mode Control

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Table of Illustrations Figure 3-1. Navigation Display Indicating TCAS OFF Figure 3-2. Four Classes of TCAS Symbol Figure 3-3. Example Scenario for “Monitor Vert...” Figure 3-4. Example Scenario for “Maintain Vert...” Figure 3-5. Example Scenario for “Climb, Climb” Figure 3-6. Example Scenario for “Descend, Descend” Figure 3-7. Example Scenario for “Climb, Climb Now” Figure 3-8. Example Scenario for “Adjust Vert...” Figure 3-9. Example Scenario for “Clear of Conflict” Figure 3-10. Transponder Panel Figure 3-11. Example Scenario for “Adjust Vert...”

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TCAS

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What is TCAS? The FAA finally made TCAS mandatory in US airspace in 1993 following the 1986 collision between a DC-9 and a Piper Archer over Cerritos, California which resulted in the loss of 82 lives.

TCAS stands for Traffic Alert and Collision Avoidance System. TCAS is a system designed to identify and reduce the risk of midair collisions between aircraft. It provides traffic displays, alerts and resolution advisories to the crew to improve their situational awareness and assist them in resolving a conflict.

There are two principle versions of TCAS currently in commercial service: TCAS I and TCAS II. TCAS I is designed primarily for general aviation and small regional airliners. It provides a traffic display and traffic advisories only.

Aviation has had a long and troubling history of midair collisions, dating back to its earliest days. During the 1950s, air traffic was continuing to grow at an ever-increasing rate, and so was the risk of mid-air collisions. As concerns grew over the rising risk, research was conducted into proximity warning devices that might reduce it.

TCAS II is used on the majority of commercial airliners, and adds several improvements over TCAS I, most notably the ability to issue resolution advisories. The FAA mandates TCAS II on aircraft with more than 30 seats, or weighing more than 15,000kg.

The 1956 collision between a Lockheed Super Constellation and a DC-7 over the Grand Canyon outlined the need for both a reformed ATC system and accelerated development of a proximity warning device. A continuing trend of midair collisions continued throughout the 1960s and 70s however, with each accident continuing to catalyze development of collision avoidance systems.

The rules are more stringent in Europe under EASA. Currently operational on most aircraft TCAS II Version 7. Version 7.1 has been projected for mandate by EASA on new build aircraft in March 2012, and for retrofit by December 2015. Version 7.1 introduces improvements designed to

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What is TCAS? (Cont.) overcome weaknesses that failed to avert the 2002 Überlingen disaster.

Notes

Studies conducted for Eurocontrol, the European Organisation for the Safety of Air Navigation, indicate that the current probability of a mid-air collision in European Airspace is 2.7 x 10-8. This equates to one mid-air collision every 3 years. Implementation of TCAS II version 7.1 should reduce this by a factor of 4. TCAS II Version 7.0 is our main focus here however, as that is what is installed on our 737. TCAS continuously interrogates operating transponders in other aircraft to predict their positions and flight paths. It then provides traffic displays and advisories to the flight crew.

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What is a Transponder? Mode A and C transponders, but add many more features to the mix.

A transponder is a device that automatically transmits a coded signal when interrogated by ATC ground radar or TCAS.

Mode S adds improved accuracy, improved interrogation capabilities and most significantly: communication datalink. This datalink is used by TCAS II to coordinate resolution advisories between two aircraft. A Mode S transponder is a requirement for a TCAS II installation.

The term ‘transponder’ is actually short for ‘transmitterresponder’, which describes its basic functionality well. Key to the content of the signal sent out by a transponder is the altitude of the aircraft.

We will talk further about the other transponder modes and their effects on TCAS later in the lesson.

There are three main types of transponders in operation in civil aviation; Mode A transponders, Mode C transponders and Mode S transponders. When Mode A transponders are interrogated by ATC ground radar or TCAS, they transmit a four digit squawk code. This squawk code is used by ATC to identify the aircraft. Mode C transponders also transmit a four digit squawk code when interrogated, but also include altitude information. This altitude information is available both to ATC and to TCAS in other aircraft. Mode S transponders include all of the functionality of Page 7-5

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How Does TCAS Work? TCAS and the transponder are inherently linked. TCAS interrogates operating transponders in other aircraft, analyzes the transponder replies, and predicts the position and flight paths of those aircraft.

TCAS issues an RA both aurally and in the form of visual cues on the Primary Flight Display.

Directional radio antennas on the top and bottom of the fuselage determine the range, bearing and speed of the other aircraft. TCAS then computes the position and projected flight path of those aircraft.

The advisory capability of TCAS is dependent on the transponder mode of the conflicting aircraft. If switched to OFF or to STANDBY, that traffic will be invisible to TCAS.

We will elaborate further on both TAs and RAs later.

TCAS only sends out Mode C and Mode S interrogations. A ‘Mode A only’ capable transponder will not reply to TCAS interrogations, and will therefore be invisible to TCAS.

Altitude information may also be received from the other aircraft’s transponder if available, and vertical speed may then be calculated.

A Mode C transponder with altitude reporting switched OFF will reply to TCAS interrogations. TCAS Traffic Symbols and Traffic Advisories will be available, however Resolution Advisories will not be available due to the lack of altitude data. Additionally, Traffic Symbols on the Navigation Display will not display altitude data.

TCAS issues Traffic Advisories and Resolution Advisories as appropriate to avoid mid-air collisions. A Traffic Advisory, or TA, is an aural and visual alert to draw the flight crew’s attention to closing traffic. TA’s assist the pilot in the visual search for the intruder aircraft, and prepare the pilot for a potential Resolution Advisory.

A Mode C transponder with altitude reporting switched ON will reply to TCAS interrogations and supply altitude information. Traffic Symbols on the Navigation Display will display altitude data, and Traffic and Resolution Advisories will be available. Coordinated Resolution Advisories will not

A Resolution Advisory, or RA, is a recommended maneuver that will either increase or maintain the existing vertical separation from the intruder aircraft. Page 7-6

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How Does TCAS Work? (Cont.) be available however, as this feature requires the Mode S datalink and TCAS II installation in the other aircraft.

is approximately 40 seconds from the Closest Point of Approach.

If the other aircraft has a Mode S transponder and TCAS II installed, then the two TCAS systems may establish communications and coordinate Resolution Advisories. This ensures that the two Resolution Advisories given will be compatible, and not worsen the situation.

A Resolution Advisory is generated when the other aircraft is approximately 25 seconds from the Closest Point of Approach.

To provide advisories, TCAS constructs a three dimensional airspace around the aircraft where a high likelihood of traffic conflict exists. The dimensions of this airspace vary depending on the closure rate with the conflicting traffic. Traffic Advisories and Resolution Advisories are generated based on a ‘Closest Point of Approach’, or CPA. The CPA is the point at which the separation between the two aircraft will reach a minimum value. TCAS calculates the time to reach the Closest Point of Approach by dividing the distance by the closure rate. This is effectively a distance/speed/time calculation. Traffic Advisories and Resolution Advisories are issued based on this time. A Traffic Advisory is generated when the other aircraft Page 7-7

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Advisories and Displays TCAS issues aural advisories and visual displays to the flight crew. Visual indications are present both on the Primary Flight Display and the Navigation Display.

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The Navigation Display displays TCAS Traffic Symbols, TCAS Annunciations and other TCAS information. TCAS Annunciations are displayed in the lower left of the ND. These indicate the status of the system. An amber TCAS OFF annunciation is self-explanatory – it indicates that TCAS is switched off. A cyan TA ONLY annunciation indicates that TCAS is set to Traffic Advisory Only mode, so Resolution Advisories are disabled and will not be generated.

TCAS TA ONLY OFF

TFC ‘traffic’ indicates that the Traffic switch on the EFIS Control Panel has been selected on, and that the ND TCAS display is active.

Figure 3-1. Navigation Display Indicating TCAS OFF

TCAS TEST indicates that the TCAS is operating in test mode, and that corresponding indications are active on the display. We will be revisiting these four annunciations later. Page 7-8

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Advisories and Displays (Cont.) An amber TCAS FAIL annunciation may also appear, and indicates that the TCAS system has failed.

+10

TCAS Traffic Symbols may be displayed on the ND using TFC ‘traffic’ switch on the EFIS Control Panel. When Traffic is selected on, the cyan TFC ‘traffic’ annunciation appears.

+00

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TA ONLY TFC

TA ONLY TFC

Traffic may be displayed on the ND by a variety of symbols. The symbols may be white, yellow or red depending on the severity of the situation. There are four main classes of symbol that TCAS will generate on the Navigation Display (Figure 3-2): ●● Open white diamond ●● Filled white diamond ●● Filled amber circle ●● Solid red square

-2

TA ONLY TFC

The symbol may have an associated altitude tag attached which displays:

+2

TA ONLY TFC

Figure 3-2. Four Classes of TCAS Symbol

●● Relative altitude to your aircraft in hundreds of feet ●● Whether the aircraft is above or below you ●● Whether the aircraft is climbing, flying level or descending.

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Advisories and Displays (Cont.) The altitude trend arrow only displays when the traffic is climbing or descending at rates greater than or equal to 500 feet per minute.

sound only once, and will then be reset for another, future Traffic Advisory. Never try to self-separate from a Traffic Advisory.

This altitude information is dependent on the other aircraft’s transponder reporting altitude. If altitude information is not being transmitted, then the symbol will not have an altitude tag and Resolution Advisories will not be generated.

When the traffic is approximately 25 seconds from the Closest Point of Approach, the symbol will change to a solid red square, and a Resolution Advisory will be issued. A typical TCAS II Resolution Advisory requires a response within 5 seconds, and G-forces of plus/minus 0.25G. Because of this G-Force requirement and the immediacy of the response required, the autopilot must be disconnected prior to responding to the RA. It is too slow to rely on.

An open white diamond indicates non-threat traffic that is greater than 6 nautical miles away or greater than plus or minus 1200 feet vertically. This is non-threat traffic referred to as ‘other traffic’.

Bear in mind also that as we said earlier, TCAS II is capable of coordinating Resolution Advisories via the Mode S datalink. This is dependent on the intruder aircraft also having an operational TCAS II installation, but is effective in ensuring that the Resolution Advisories are compatible and will not worsen the situation.

A filled white diamond indicates traffic that is less than 6 nautical miles away, AND within 1200 feet vertically. This traffic is still not considered a threat, and is referred to as ‘proximate traffic’. A symbol change to a filled amber circle indicates that the intruding aircraft is now considered to be potentially hazardous. This will occur when the intruding aircraft is approximately 40 seconds from the Closest Point of Approach. This is called a Traffic Advisory, and is indicated aurally by a “Traffic, Traffic” message. The aural alert will Page 7-10

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There are many Resolution Advisory aurals, and visual commands on the PFD vary depending on the type of advisory triggered.

TCAS

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Advisories and Displays (Cont.) Resolution Advisories may be broken down into two types – Preventative Advisories and Corrective Advisories. Corrective Advisories require that the flight crew take positive action to maneuver away from the other aircraft. Preventative Advisories require that the flight path of the aircraft be maintained.

ARM

250 300

280

There are three aural Preventative Advisories.

MCP SPD

LNAV

CMD FD

276

ALT HOLD VNAV

306

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10400

10200 TRAFFIC

260

1 250 9

1

2

240

“Monitor Vertical Speed”

220

This RA requires that the pilot avoid the pitch areas highlighted on the attitude indicator. (Figure 3-3)

+05 +10

9800

-10 -05

200

276

A TCAS vertical speed tape on the vertical speed indicator, or VSI, is a customer option on the 737NG. The red areas should be avoided.

9600

TFC TCAS TEST

+2

-2

Figure 3-3. Example Scenario for “Monitor Vertical Speed”

A Monitor Vertical Speed RA rarely requires a change in vertical speed. You might expect this RA if targets are approaching from both above and below the aircraft, and vertical speed must be monitored closely to maintain separation. Page 7-11

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Advisories and Displays (Cont.) “Maintain Vertical Speed, Maintain”

This RA requires that the pilot maintain a rate of climb or descent in accordance with the indications on the PFD attitude indicator and VSI. (Figure 3-4) The pilot should always follow the guidance given on the PFD in this case, rather than trying to chase the exact vertical speed that was current when the RA was issued. Following the guidance ensures that the resultant vertical speed is sufficient to satisfy the RA.

ARM ARM

250 300

280

LNAV

FD

276

VNAV

306

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10400

10200 TRAFFIC

260

1 250 9

1

2

240

“Maintain Vertical Speed, Crossing Maintain”

220

This RA should be flown in the same way as “Maintain Vertical Speed, Maintain”.

+10

9800

-10 -05

200

276

The inclusion of ‘crossing’ in the aural message is merely for the pilot’s information, and indicates that his aircraft will pass through the level of the intruder aircraft while the RA is followed.

9600

TFC TCAS TEST

+2

-2

Figure 3-4. Example Scenario for “Maintain Vertical Speed, Maintain”

The rest of the Resolution Advisory aurals are Corrective Advisories, and require positive action by the pilot to maneuver the aircraft.

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Advisories and Displays (Cont.) “Climb, Climb”

This RA requires that the pilot climb to avoid the pitch regions displayed on the PFD. (Figure 3-5)

250

MCP SPD

280

In this case, the clear, safe region will be towards the top of the attitude indicator, prompting the pilot to pitch up.

306

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10400

CMD

300

276

ALT HOLD

10200 TRAFFIC

260

1 250 9

1

2

240 9800

220

Don’t forget that TCAS is expecting you to respond within 5 seconds and achieve a load factor of at least 1.25G in the pitch up to climb. The autopilot must be disconnected for this type of RA.

-02

200

TFC

9600

276

Figure 3-5. Example Scenario for “Climb, Climb”

“Descend, Descend”

This RA requires that the pilot descend away from the red pitch regions on the PFD. (Figure 3-6) 250

Descend RAs are inhibited below approximately 1100 feet radio altitude due to the lack of safe terrain clearance.

300

280

MCP SPD

276

ALT HOLD

CMD

306

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10400

10200 TRAFFIC

260

1 250 9

“Climb, Crossing Climb”

1

2

240

Or the reversed advisory “Descend, Crossing Descend”.

220

9800 +02

200

Crossing climb and crossing descend RAs are also issued where the intruder aircraft will pass through your level during the maneuver. Page 7-13

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9600

TFC

276

Figure 3-6. Example Scenario for “Descend, Descend”

TCAS

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Advisories and Displays (Cont.) The technique to follow these is exactly the same as for standard climb and descend Resolution Advisories. TCAS may sometimes reverse its initial Resolution Advisory command. Where the initial command was to descend, a reversal would result in the aural callout “Climb, Climb Now”. (Figure

ARM

276

250 300

3-7)

280

FD

306

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10400

10200 TRAFFIC

260

1 250 9

Where the initial command was to climb, a reversal would result in the aural callout “Descend, Descend Now”.

1

2

240

220

9800 +01

200

The aircraft must be pitched without delay to the new revised attitude. Follow the PFD guidance to achieve this. The reversal should be made within 2.5 seconds with additional load factor at plus/minus 0.35G.

9600

TFC

276

Figure 3-7. Example Scenario for “Climb, Climb Now”

Further Resolution Advisory aurals may be issued where the current rate of climb or descent is insufficient to resolve the conflict. Increase Climb, Increase Climb

Or the reversed advisory “Increase Descent, Increase Descent”. Page 7-14

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Advisories and Displays (Cont.) Guidance will be given for both of these commands on the PFD. ‘Increase descent’ RAs are inhibited below approximately 1,500 feet radio altitude so that pilots are not encouraged to descend too quickly close to terrain. 250

280

276

ALT HOLD

CMD

300

TCAS may also issue aural messages to reduce vertical speed.

MCP SPD

306

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10400

10200 TRAFFIC

260

1 250 9

1

2

240

“Adjust Vertical Speed, Adjust”

9800

220

This RA requires that the pilot reduce vertical speed. It may be triggered both as an initial Resolution Advisory and as a weakening Resolution Advisory. (Figure 3-8)

-05

200

9600

TFC

276

Figure 3-8. Example Scenario for “Adjust Vertical Speed, Adjust”

As an initial RA, ‘adjust vertical speed’ is commanded to maintain separation between two aircraft and avoid a conflict. It may also be issued following a climb or descend RA where the current rate of climb or descent is greater than needed. This minimizes ATC disruption.

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Advisories and Displays (Cont.) When any Traffic or Resolution Advisory occurs, a TRAFFIC message will appear on the Navigation Display. This will be amber for a Traffic Advisory, and red for a Resolution Advisory. If the Navigation Display range is too close, and TA or RA traffic is not displayed as a result, an OFFSCALE message will appear.

ARM ARM

250 300

280

If the bearing of traffic cannot be determined, it cannot be displayed as a traffic symbol on the ND.

LNAV

FD

276

VNAV

306

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10400

10200 TRAFFIC

260

1 250 9

1

2

240

220

If that traffic qualifies for a TA or RA, a No-Bearing Message will appear. This provides range, relative altitude and a vertical motion arrow.

+10

9800

-10 +2

200

276

9600

TFC TCAS TEST

-2 -06

Figure 3-9. Example Scenario for “Clear of Conflict”

All Resolution Advisories are inhibited below approximately 1000 feet radio altitude. Aural Traffic Advisories are inhibited below approximately 500 feet radio altitude. After a Resolution Advisory traffic conflict has ended, and traffic separation is increasing again, a “Clear of Conflict” aural message will sound, and all corresponding indications will disappear. (Figure 3-9)

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TCAS Mode Control TCAS mode control is conducted from the Transponder Panel, located on the Aft Electronics Panel. (Figure 3-10) The 737NG has two transponders which are individually selectable using the Transponder Selector at the top left of the panel. Altitude data for Mode C and Mode S reporting can be taken from either of the two Air Data Computers in the ADIRUs. The Altitude Source selector allows the crew to manually select the data source for altitude reporting. When flying into RVSM airspace, the source selected should correspond to the primary altimeter referenced by the autopilot in use.

ALT SOURCE

Figure 3-10. Transponder Panel

For Autopilot A, this is in Air Data Computer #1, and for Autopilot B it’s #2. The two code selectors allow pilots to enter the transponder code assigned to them into the code indicator window. An identification signal may be transmitted manually by pushing the ‘IDENT’ button. This is done in response to ATC Page 7-17

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TCAS Mode Control (Cont.) requests to ident. The Transponder Fail light indicates a transponder malfunction when lit. The control on this panel most relevant to TCAS is the Transponder Mode Selector. This is a five position selector with a pushbutton mounted on it.

ARM LNAV

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VNAV

FD

The pushbutton is a test switch that starts a test of the TCAS system. Symbols appear on the Navigation Display, along with the TCAS TEST annunciation (Figure 3-11). An aural annunciation signals the outcome of the test. PASS also appears in the transponder window, and the Transponder Fail light illuminates to indicate that the selected transponder is operational.

TRAFFIC

+10 -10 +2

TCAS TEST

-2

Figure 3-11. Example Scenario for “Adjust Vertical Speed, Adjust”

The Transponder Mode Selector may be rotated to select one of five transponder operating modes. The STBY position places the transponder and TCAS system in standby. In this mode, the transponder will not transmit. When set to the ALT RPTG OFF ‘Altitude Reporting Off’ Page 7-18

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TCAS Mode Control (Cont.) position, the transponder operates but without altitude reporting.

Notes

The XPNDR position enables altitude reporting, but Traffic and Resolution advisories are still disabled. The TA ONLY position enables TCAS Traffic Advisories, and the TA/RA position enables Resolution Advisories on top of that. Depending on airline Standard Operating Procedure, the TA ONLY mode may sometimes be selected following an engine shutdown. TCAS does not take into account aircraft performance capability when issuing Resolution Advisories. A 737 with an engine out will have inadequate climb performance to fully comply with climb Resolution Advisories. Selecting TA ONLY avoids the issuance of RAs that the aircraft would be unable to comply with.

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