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 be en 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.1.1 T YPES OF ABNORMAL OCCURRENCES
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:y y y y y y y y y y y
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 Tail strike Mercury spillage Dragged engine or e ngine seizure High-energy stop.
1.2 T YPES 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.
1.3 L IGHTNING 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. 1.3.1 Effects of a Lightning Strike Lightning strikes are likely to have two main effects on the aircraft:y 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 tailplane. The damage will usually be in the form of small circular holes, usually in clusters, and accompanied by burning or discoloration. y 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.
1.3.2 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:y Examine engine cowlings and engines for evidence of bur ning 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.
y Examine fuselage skin, particularly rivets for burning or pitting. y If the landing gear was extended, some damage may have occurred to the lower parts of the gear. Examine for signs of discharge. y 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. 1.4 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:y Examination of the External Surfaces for Lightning Strike y Examination of the internal Components for Ligh tning Strike y Inspection and Operational Check of the Radio and Navigation Systems.
1.4.1 Basic Protection The aircraft has all the necessary and known lightning strike protection measures. 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.
1.4.2 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 ski n trailing edge panels. wing-body fairing, antennas, vertical stabiliser, horizontal stabiliser, and along the wing trailing edge in Zone 2.
Zone 1. High Possibility of Strike Zone 2. Average Possibility of
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
1.4.3 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 burnin g 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 stru cture, can cause this magnetisation. 1.4.4 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. External components most likely to be hit are the:y y y y y y y y y y y
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
1.4.5 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 resista nt to lightning strikes but a strike of unusually high intensity can possibly damage such electrical system components as the:y y y y y
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. As the aircraft flies through the air, it can pick up a static charge fro m 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. 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 capac ity to carry only a few micro-Amps of current from the collected static energy. The approximate 200,000 Amps from a lightning strike will cause