Module 7 (Maintenance Practices) Sub Module 7.19 (Abnormal Events).pdf

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Sub Module 7.19 – Abnormal Events

MODULE 7 Sub Module 7.19

ABNORMAL EVENTS

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Module 7 - MAINTENANCE PRACTICES Sub Module 7.19 – Abnormal Events

Contents ABNORMAL EVENTS --------------------------------------------------------------- 1 A)

ABNORMAL EVENTS ELECTRICAL ---------------------------------------- 2

LIGHTNING STRIKES ---------------------------------------------------------------- 2 HIGH INTENSITY RADIATED FIELDS (HIRF) PENETRATION ---------------- 9 B)

ABNORMAL EVENTS MECHANICAL -------------------------------------13

HEAVY LANDINGS -----------------------------------------------------------------13 FLIGHT THROUGH SEVERE TURBULENCE ------------------------------------15

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Sub Module 7.19 – Abnormal Events

ABNORMAL EVENTS All aircraft are designed to withstand the normal flight and landing loads expected during a typical flight cycle. These loads will include the normal manoeuvres the aircraft is expected to make. The designer will build in a safety factor to compensate for loads slightly larger than normal. Sometimes extreme circumstances occur which cause stresses outside the normal design limits. If the design limits are exceeded, then damage may occur to the aircraft. If it is known or suspected that the aircraft has been subjected to excessive loads, then an inspection should be made, to ascertain the nature of any damage that may have occurred. The manufacturer will normally have anticipated the nature of some of these occurrences and detailed special checks for these ‘Abnormal Occurrences’. Types of abnormal occurrences

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Tail strike Mercury spillage Dragged engine or engine seizure High-energy stop.

Types of damage It is not intended to describe the types of damage applicable to every type of occurrence. It is more important to understand that, often, the damage may be remote from the source of the occurrence. In many cases the inspection would be made in two stages. If no damage is found in the first stage then the second stage may not be necessary. If damage is found, then the second stage inspection is done. This is likely to be a more detailed examination.

The aircraft maintenance manual will normally list the types of abnormal occurrences needing special inspection. The list may vary, depending on the aircraft. The following items are a selection from a typical aircraft:       

Lightning strikes High-intensity radiated fields penetration Heavy or overweight landing Flight through severe turbulence Burst tyre Flap or slat over-speed Flight through volcanic ash

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A) ABNORMAL EVENTS ELECTRICAL

Sub Module 7.19 – Abnormal Events

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LIGHTNING STRIKES Both lightning strikes and high-intensity radiated fields (HIRF) are discussed in Module 5. Consideration is given in this topic to their effects and the inspections required in the event of their occurrence. Lightning, of course, is the discharge of electricity in the atmosphere, usually between highly charged cloud formations, or between a charged cloud and the ground. If an aircraft is flying in the vicinity of the discharge or it is on the ground, the lightning may strike the aircraft. This will result in very high voltages and currents passing through the structure. All separate parts of the aircraft are electrically bonded together, to provide a low-resistance path to conduct the lightning away from areas where damage may hazard the aircraft. Effects of a Lightning Strike

Inspection The maintenance schedule or maintenance manual should specify the inspections applicable to the aircraft but, in general, bonding straps and static discharge wicks should be inspected for damage. Damaged bonding straps on control surfaces may lead to tracking across control surface bearings, this in turn may cause burning, break up or seizure due to welding of the bearings. This type of damage may result in resistance to movement of the controls, which can be checked by doing a functional check of the controls. Additional checks may include: 

Examine engine cowlings and engines for evidence of burning or pitting. As in control bearings, tracking of the engine bearings may have occurred. Manufacturers may recommend checking the oil filters and chip detectors for signs of contamination. This check may need to be repeated for a specified number of running hours after the occurrence.

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Examine fuselage skin, particularly rivets for burning or pitting.

Lightning strikes are likely to have two main effects on the aircraft: 

Strike damage where the discharge enters the aircraft. This will normally be on the extremities of the aircraft, the wing tips, nose cone and tail cone and on the leading edge of the wings and tail plane. The damage will usually be in the form of small circular holes, usually in clusters, and accompanied by burning or discoloration.

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Static discharge damage at the wing tips, trailing edges and antenna. The damage will be in the form of local pitting and burning. Bonding strips and static wicks may also disintegrate, due to the high charges.

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If the landing gear was extended, some damage may have occurred to the lower parts of the gear. Examine for signs of discharge. After the structural examination it will be necessary to do functional checks of the radio, radar, instruments, compasses, electrical circuits and flying controls. A bonding resistance check should also be done.

Example of a Post Lightning Strike Procedure This procedure is an extract from the Boeing 757 Maintenance Manual. It is included to give an idea of a typical aircraft inspection procedure. Not all of the details have been supplied, but there is enough information to provide a general idea. The student will not be examined in detail on this procedure, but should be able to identify specific checks that highlight the previous notes. This procedure has these three tasks:   

Module 7 - MAINTENANCE PRACTICES Sub Module 7.19 – Abnormal Events

Most of the external parts of the aircraft are metal structure with sufficient thickness to be resistant to a lightning strike. This metal assembly is its basic protection. The thickness of the metal surface is sufficient to protect the internal spaces from a lightning strike. The metal skin also gives protection from the entrance of electromagnetic energy into the electrical wires of the aircraft. The metal skin does not prevent all electromagnetic energy from going into the electrical wiring; however, it does keep the energy to a satisfactory level. If lightning strikes the aircraft, then all of the aircraft must be fully examined, to find the areas of the lightning strike entrance and exit points. When looking at the areas of entrance and exit, this structure should be carefully examined to find all of the damage that has occurred.

Examination of the External Surfaces for Lightning Strike Examination of the internal Components for Lightning Strike Inspection and Operational Check of the Radio and Navigation Systems.

Basic Protection The aircraft has all the necessary and known lightning strike protection measures.

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Strike Areas Lightning strike entrance and exit points (refer to Fig. 1) are, usually, found in Zone 1, but also can occur in Zones 2 and 3. Lightning strikes can, however, occur to any part of the aircraft, including the fuselage, wing skin trailing edge panels. Wingbody fairing, antennas, vertical stabiliser, horizontal stabiliser, and along the wing trailing edge in Zone 2. A

A&B

Zone 1. High Possibility of Strike Zone 2. Average Possibility of Strike Zone 3. Low Possibility of Strike A = Aerials and Protrusions B = Sharp Corners of Fuselage and Control Surfaces

Risk Areas for Lightning Strikes Fig. 1

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Signs of Damage In metal structures, strike damage usually shows as pits, burn marks or small circular holes. These holes can be grouped in one location or divided around a large area. Burned or discoloured skin also shows lightning strike damage. In composite (non-metallic) structures, solid laminate or honeycomb damage shows as discoloured paint. It also shows as burned, punctured, or de-laminated skin plies. Hidden damage can also exist. This damage can extend around the visible area. Signs of arcing and burning can also occur around the attachments to the supporting structure. Aircraft components made of ferromagnetic material may become strongly magnetised when subjected to large currents. Large currents, flowing from the lightning strike in the aircraft structure, can cause this magnetisation.

External components most likely to be hit are the:           

Nose Radome Nacelles Wing Tips Horizontal Stabiliser Tips Elevators Vertical Fin Tips Ends of the Leading Edge Flaps Trailing Edge Flap Track Fairings Landing Gear Water Waste Drain Masts Pitot Probes

External Components at Risk A lightning strike usually attaches to the aircraft in Zone 1 and goes out a different Zone 1 area. Frequently, a lightning strike can enter the nose radome and go out of the aircraft at one of the horizontal stabiliser trailing edges.

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Electrical Components at Risk Lightning strikes can cause problems to the electrical power systems and the external light wiring. The electrical system is designed to be resistant to lightning strikes but a strike of unusually high intensity can possibly damage such electrical system components as the:     

Fuel valves Generators Power Feeders Electrical Distribution Systems Static Discharge Wicks

NOTE: Should inaccuracies in the standby compass be reported, after a lightning strike, then a check swing will be necessary. Frequently, a lightning strike is referred to as a static discharge. This is incorrect and may create the impression that the metal static discharge wicks, found on the external surfaces of the aircraft prevent lightning strikes. These static discharge wicks are for bleeding off static charge only; they have no lightning protection function.

The static discharge wicks help to bleed the static charge off in a way that prevents radio ‘noise’. The static discharge wicks are frequently hit by lightning. Some personnel think static dischargers are for lightning protection. The dischargers have the capacity to carry only a few microAmps of current from the collected static energy. The approximate 200,000 Amps from a lightning strike will cause damage to the discharge wick or make it totally unserviceable. Examination of External Surface Examine the Zone 1 surface areas for signs of lightning strike damage. Do the examinations that follow:   

 As the aircraft flies through the air, it can pick up a static charge from the air (or from dust/water particles in the air). This static charge can become large enough to bleed off the aircraft on its own. If the charge does not bleed off the aircraft on its own, it will usually result in noise on the VHF or HF radios.

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Examine the external surfaces carefully to find the entrance and exit points of lightning strike. Make sure to look in the areas where one surface stops and another surface starts. Examine the internal and external surfaces of the nose radome for burns, punctures, and pinholes in the composite honeycomb sandwich structure. Examine the metallic structure for holes or pits, burned or discoloured skin and rivets. Examine the external surfaces of the composite components for discoloured paint, burned, punctured, or de-laminated skin plies. Use instrumental NDI (NDT) methods or tap tests to find composite structure damage which is not visible. For Training Purpose Only Rev. 00 Mar 2014

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Note: Damage, such as de-lamination can extend to the areas around the damage area which is not visible. De-lamination can be detected by instrumental NDI methods or by a tap test. For a tap test, use a solid metal disc and tap the area adjacent to the damaged area lightly. If there is de-lamination, it will produce a sound that is different to the sound of a solid bonded area.

Sub Module 7.19 – Abnormal Events

Functional Tests Functional tests will need to be done as follows:  

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Examine the flight control surfaces for signs of strike damage. If the control surfaces show signs of damage, examine the surface hinges, bearings and bonding jumpers for signs of damage. If the ailerons show signs of a lightning strike, examine the surface hinges, bearings, and bonding jumpers for signs of damage. If the speed brakes show signs of a lightning strike, examine the surface hinges, bearings, and bonding jumpers for signs of damage. If the trailing edge flaps show signs of a lightning strike, examine the surface hinges, bearings, and bonding jumpers for signs of damage. If the leading edge flaps/slats show signs of a lightning strike, examine the surface hinges, bearings, and bonding jumpers for signs of damage. Examine the nose radome for pin-holes, punctures and chipped paint. Also ensure bonding straps are correctly attached. Examine the lightning diverter strips and repair or replace them if damaged. If there is radome damage, examine the WXR antenna and wave-guide for damage.

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Ensure the navigation lamps, rotary lights and landing lights operate. If the previously mentioned control examinations show signs of damage: Do an operational test of the rudder if there are signs of lightning strike damage to the rudder or vertical stabiliser. Do an operational test of the elevator if there are signs of lightning strike damage to the elevator or horizontal stabiliser. Do an operational test of the ailerons if there are signs of lightning strike damage to the ailerons. Do an operational test of the speed brakes if there are signs of lightning strike damage to the speed brake system. Do an operational test of the trailing edge flaps if there are signs of lightning strike damage to the trailing edge flaps. Do an operational test of the leading edge flap/slats if there are signs of lightning strike damage to the trailing edge flap/slats. If there are signs of strike damage to the landing gear doors, disengage the main gear door locks and manually move the doors to ensure they move smoothly. Visually examine the door linkage, hinges, bearings and bonding jumpers for strike damage. Ensure the proximity switch indication unit gives the correct indication.

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Examination of Internal Components If a lightning strike has caused a system malfunction, do a full examination of the system.   

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Do a check of the stand-by compass system if the flight crew reported a very large compass deviation. Make sure the fuel quantity system is accurate. This can be achieved by a BITE test. Examine the air data sensors for signs of strike damage. Do an operational test of the pitot system if there are signs of damage to the probes. Do a test of the static system if there are signs of damage near the static ports. Do an operational check of any of the following systems that did not operate following the strike, or if the flight crew reported a problem, or if there was any damage found near the system antenna:

If one or more of the previous systems have problems with their operational checks, examine and do a test of the coaxial cables and connectors. Return the Aircraft to Service After all areas have been inspected and lightning damage has been repaired, components replaced as necessary and tests completed if necessary, the aircraft may be returned to service.

HF communications system VHF communications system ILS navigation system Marker beacon system Radio altimeter system Weather radar system VOR system ATC system DME system Automatic Direction Finder (ADF) system

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Module 7 - MAINTENANCE PRACTICES Sub Module 7.19 – Abnormal Events

HIGH INTENSITY RADIATED FIELDS (HIRF) PENETRATION Module 5 discusses electromagnetic phenomena, in particular the problem of electromagnetic interference. HIRF may be generated by airborne transmitters such as high-powered radar or radio to commercial aircraft. Increased use of digital equipment has increased the problem.

HIRF can interfere with the operation of the aircraft’s electrical and electronic systems by coupling electromagnetic energy to the system wiring and components. This can cause problems relating to the control systems, both of the aircraft and its power- plants, the navigation equipment and instrumentation.

HIRF can be generated from an internal (within the aircraft and its systems) or external source (i.e. HIRF may be transmitted by military aircraft in close proximity). All of the systems which might cause, or be affected by, HIRF, must be suitably protected.

Design philosophies in the area of aircraft bonding for protection against HIRF can employ methods that may not have been encountered previously by maintenance personnel. Because of this, the HIRF protection in the aircraft can be unintentionally compromised during normal maintenance, repair and modification. It is critical that procedures, contained in the AMM/CMM, reflect reliable procedures, to detect any incorrect installations, which could degrade the HIRF protection features.

Electronic developments have yielded greater miniaturisation and complexity in integrated circuits (IC) and other electronic circuitry and assemblies, increasing the probability of electromagnetic interference. Rapid advances in technology and the increased use of composite materials and higher radio frequency (RF) energy levels, from radar, radio, and television transmitters, have substantially increased the concern for electromagnetic vulnerability of flight critical systems, relative to their exposure to HIRF. Environmental factors such as corrosion, mechanical vibrations, thermal cycling, damage and subsequent repair and modifications can potentially degrade electromagnetic protection. Continued airworthiness of these aircraft requires assurance that the electromagnetic protection is maintained to a high level by a defined maintenance programme. ISO 9001:2008 Certified PTC/CM/B1.1 Basic/M7/04

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There are three primary areas to be considered for aircraft operating in HIRF environments:   

Aircraft Structure (Airframe Skin and Frame). Electrical Wiring Installation Protection (Solid or Braided Shielding Connectors). Equipment Protection (LRU case, Electronics Input Output Protection).

Visual inspection is the first and generally most important step in HIRF maintenance. If errors have been made that do degrade the protection (paint over spray and incorrect assembly of connectors for example), then they should be found during inspections. Whilst the visual inspection may suffice for observation of the deterioration of the protective features, any time that this method is found to be insufficient or inefficient, then specific testing may be required. These techniques should make use of easy-to-apply, quick-look devices that can be readily integrated into the normal maintenance operations. Specific Testing – HIRF The milliohmmeter is often used to measure the path resistance of earthing straps or other bonding. This technique is limited to the indication of only single path resistance values.

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Sub Module 7.19 – Abnormal Events

The Low-frequency Loop Impedance testing method complements dc bonding testing and it can be used together with visual inspection. It can give good confidence in the integrity of the shielding. This loop impedance testing can be used to check that adequate bonding exists between braiding/conduits and the aircraft structure, especially where there are multiple earth paths, when the dc resistance system will not indicate which earth has failed. The frequency of any maintenance tasks selected for the HIRF protection features should be determined by considering the following criteria:     

Relevant operating experience gained. Exposure of the installation to any adverse environment. Susceptibility of the installation to damage. Criticality of each protective feature. (within the overall protection scheme) The reliability of protective devices fitted to equipment.

Table 1 gives some indication as to the maintenance tasks that may be applied to certain types of electromagnetic protection features. ‘Raceway’ conduits are separate conduits containing individual cables to the various aircraft systems while ‘RF gaskets’ have conducting properties.

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Protection against HIRF Interference The manufacturer will normally protect the aircraft against HIRF. Bonding, shielding and separation of critical components usually achieve this. It is difficult to know when the aircraft has been subjected to HIRF; consequently protection is best achieved by regular checks of:    

Bonding of the aircraft Correct crimping Screens correctly terminated and earthed All bonding terminals correctly torque loaded.

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B) ABNORMAL EVENTS MECHANICAL HEAVY LANDINGS A heavy or overweight landing can cause damage to the aircraft both visible and hidden. All damage found should be entered in the aircraft’s Technical Log. An aircraft landing gear is designed to withstand landing at a particular aircraft weight and rate of descent. If either of these parameters was exceeded during a landing, then it is probable that some damage has been caused to the landing gear, its supporting structure or elsewhere on the airframe. Overstressing may occur if the aircraft is not parallel to the runway when it lands or if the nose- or tail-wheel strikes the runway before the main wheels.

Different aircraft have their own heavy landing procedures. For example, some aircraft, which show no primary damage, need no further inspection, whilst others require that all inspections are made after every reported heavy landing. This is because some aircraft can have hidden damage in remote locations whilst the outside of the aircraft appears to be undamaged. Example of Post Heavy Landing Inspection The following items give an example of a typical post heavy landing inspection: Landing Gear

Some aircraft are provided with heavy landing indicators, which give a visual indication that specified ‘g’ forces have been exceeded. Long aircraft may have a tail scrape indicator fitted, as a scrape is more likely. In all instances of suspect heavy landings, the flight crew should be questioned for details of the aircraft’s weight, fuel distribution, landing conditions and whether any unusual noises were heard during the incident.

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Primary damage that may be expected following a heavy landing would normally be concentrated around the landing gear, its supporting structure in the wings or fuselage, the wing and tailplane attachments and the engine mountings. Secondary damage may be found on the fuselage upper and lower skins and on the wing skin and structure. ISO 9001:2008 Certified PTC/CM/B1.1 Basic/M7/04

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Examine tyres for creep, damage, and cuts. Examine wheels and brakes for cracks and other damage. Examine axles, struts and stays for distortion. Check landing gear legs for leaks, scoring and abnormal extension. Examine gear attachments for signs of cracks, damage or movement. Some aircraft require the removal of critical bolts and pins for NDT checks. Examine structure in vicinity of gear attachment points. Examine doors and fairings for damage.

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Mainplanes 

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Examine the upper and lower skins for wrinkles and pulled rivets, particularly if the engines are mounted on the wings. Check for fuel leaks. Check the root attachments and fairings for cracks. Function the flying controls for freedom of movement. Examine wing spars.

Fuselage       

Check skin for damage and wrinkles. Examine pressure bulkheads for damage. Check all supporting structures of heavy components like galleys, batteries, water tanks and APUs. Ensure no inertia switches have tripped. Check instruments and their panels are functional. Ensure pipes and ducts for security. Check all doors and panels fit correctly.

Sub Module 7.19 – Abnormal Events

Tail Unit   

Check flying controls for freedom of movement. Examine all hinges for distortion or cracks especially near balance weights. Examine attachments, fairings and mountings of screw jacks.

There are numerous other checks that need to be done, depending on the damage found (or not found), during the inspections. This can include engine runs and functional checks of all the aircraft systems. Signs of some damage and distortion could be a reason to do full rigging and symmetry checks of the airframe.

Engines      

Check controls for freedom of movement. Examine all mountings and pylons for damage and distortion. Check turbine engines for freedom of rotation. Examine all cowlings for wrinkling and distortion. Check all fluid lines, filters and chip detectors. On propeller installations, check for shock-loading, propeller attachments and counterweight installations.

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FLIGHT THROUGH SEVERE TURBULENCE If an aircraft has been flown through conditions of severe turbulence, the severity of the turbulence may be difficult to assess and report. For aircraft that utilise accelerometers, flight data recorders or fatigue meters, the records obtained can give an overall picture of the loads felt by the aircraft.

As with a ‘heavy landing’ report, further inspection, involving dismantling of some major structural components, may be necessary if external damage is found during the initial inspection following flight through turbulence.

They cannot, however, give a full picture and so must only be used for guidance. Turbulence can be too fleeting to record on some forms of load instrumentation. As a general guide only, loadings greater than – 0.5g and + 2.5g on transport aircraft could indicate some damage to the airframe and engines. Aircraft, which have no recording devices installed, must have reports of flight through severe turbulence thoroughly investigated. Severe turbulence may cause excessive vertical or lateral forces similar to those felt during a heavy landing. The forces felt may be increased by the inertia of heavy components such as engines, fuel and water tanks and cargo. Damage can be expected at similar points to those mentioned previously concerning heavy landings. It is also possible for damage to occur in those areas of the wings, fuselage, tail unit and flying controls where the greatest bending moment takes place. Pulled rivets, skin wrinkles or other similar structural faults may provide signs of damage. ISO 9001:2008 Certified PTC/CM/B1.1 Basic/M7/04

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