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 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 µAbnormal 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.

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

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 µnoise¶. 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 damage to the discharge wick or make it totally unserviceable. 1.4.6 Examination of External Surface Examine the Zone 1 surface areas for signs of lightning strike damage. Do the examinations that follow: y y

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 s urface starts.

y y y y

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. 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 delamination, it will produce a sound that is different to the sound of a solid bonded area.

y

y y y y y

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 bon ding 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.

1.4.7 Functional Tests Functional tests will need to be done as follows: y y

y y y

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.

y y y

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.

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

y

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: 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

y y y y y y y y y y

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.

1.4.9 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.
1.5 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 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. 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 ma terials 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 pro tection is maintained to a high level by a defined maintenance programme. 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. 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.

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

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 tha t 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. 1.5.1 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. The Low-frequency Loop Impedance testing method compleme nts 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: y y y y y

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 reliabilit y 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. µRaceway¶ conduits are separate conduits containing individual cables to th e various aircraft systems while µRF gaskets¶ have conducting properties.

Table 1 HIRF PROTECTION MEASURES
Protection Type Description Cable Shielding Over braid shield, critical individual cable shield Metallic conduit braid Corrosion, damage Visually inspect and measure cable shielding or bonding Applicable Maintenance Tasks for HIRF Protection Measures Aircraft Structure Shielding Raceway conduits RF gasket Shield for nonconductive surfaces Conductive coatings Structural bonding Circuit Protection Devices HIRF protection devices

Examples

Raceway conduits

Removable Panels

Contact bonds, rivet joints Corrosion, damage Visually inspect and measure bonding

Bonding lead/straps, pigtails Corrosion, damage, security of attachment Visually inspect for corrosion, attachment and condition, measure bonding

Degradation or failure modes Maintenance operations

Corrosion, damage Visually inspect and measure bonding

Corrosion, damage, deformation Visually inspect gaskets, bonding leads and straps

Damage, erosion Visually inspect and measure shielding effectiveness

Resistors, Zener diodes, EMI filters & filter pins Short circuit, open circuit Check at test/repair facility iaw maintenance or surveillance plans

1.5.2 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: y y y y

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

1.6 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. Over -stressing 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. Some aircraft are provided with heavy landing indicators, which give a visual indication that specified µg¶ forces have been exceeded. Long aircraft may have a tail scrape indicator fitted, as a scrape is more likely. In all instances of sus pect 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. Primary damage, that may be expected following a heavy lan ding, 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. 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. 1.6.1 Example of Post Heavy Landing Inspection The following items give an example of a typical post heavy landing inspection : Landing Gear y Examine tyres for creep, damage, and cuts. y Examine wheels and brakes for cracks and other damage. y Examine axles, struts and stays for distortion. y Check landing gear legs for leaks, scoring and abnormal extension. y Examine gear attachments for signs of cracks, damage or movement. Some aircraft require the removal of critical bolts and pins for NDT checks. y Examine structure in vicinity of gear attachment points. y Examine doors and fairings for damage. y Carry out retraction and nose wheel steeri ng tests

Mainplanes y Examine the upper and lower skins for wrinkles and pulled rivets, particularly if the engines are mounted on the wings. y Check for fuel leaks. y Check the root attachments and fairings for cracks. y Function the flying controls for freedom of movement. y Examine wing spars. Fuselage y Check skin for damage and wrinkles. y Examine pressure bulkheads for damage. y Check all supporting structures of heavy components like galleys, batteries, water tanks and APUs. y Ensure no inertia switches have tripped . y Check instruments and their panels are functional. y Ensure pipes and ducts for security. y Check all doors and panels fit correctly. Engines y Check controls for freedom of movement. y Examine all mountings and pylons for damage and distortion. y Check turbine engines for freedom of rotation. y Examine all cowlings for wrinkling and distortion. y Check all fluid lines, filters and chip detectors. y On propeller installations, check for shock -loading, propeller attachments and counterweight installations. Tail Unit y Check flying controls for freedom of movement. y Examine all hinges for distortion or cracks especially near balance weights. y 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.

1.7 FLIGHT THROUGH SEVERE T URBULENCE

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. 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. As with a µheavy landing¶ report, further inspection, involving dismantling of some major structural components, may be necessary if external damage is found during the initial inspection fo llowing flight through turbulence. .

2 MAINTENANCE PROCEDURES
An aircraft has to receive regular maintenance, of varying depths, to remain fully airworthy at all times. This is achieved in most circumstances by making various checks, at intervals, throughout the life of the aircraft. These intervals can be stated in quantities of flying hours, calendar time or combinations of the two systems. 2.1 MAINTENANCE PLANNING The periods of maintenance can be small or large. The aircraft can be in for a short period of maintenance over-night (or perhaps no longer than two days), whilst, on a large maintenance period, the aircraft might be in the hangar for a week or two, depending on the type of aircraft. It is normal to apply what is known as a µback-stop¶ to each period for safety. For example, if the frequency of each maintenance action is every 100 flying hours, then there will probably be a calendar µback-stop¶ of one month. This means that if the aircraft is only flown for 25 hours during one month, then it will have its maintenance done on the last day of that month, regardless if its low hours. Equally, if the aircraft is intensively flown day-and-night, it might reach its 100 hours after 19 days. It will then receive its maintenance at that time, as a result of its intensive flying. The decision as to the frequency and depth of this maintenance is controlled by the µType Design Organisation¶, the organisation which designed the aircraft. The maintenance programme contains a list of the most significant items and recommendations as to the maintenance actions, recommended frequencies and sampling/inspection points. It will also contain a programme that monitors engine critical parts and the inspections to be done on those parts. All aircraft have a list of critical parts, with which it cannot fly without them being serviceable, or which can be dispensed with, providing other parts can cover for the missing part.

2.2 MODIFICATION PROCEDURES Modifications are changes made to a particular aircraft, including all its components, engines, propellers, radio apparatus, accessories, instruments, equipment and their respective installations.

With the exception of modifications which the CAA agree to be of such a minor nature that airworthiness is unaffected, all modifications must be approved in accordance with the relevant parts of JAR OPS. The modifications are approved by the CAA or by the µApproved Organisation¶ carrying out the modification programme. Modifications must be such that the design of the aircraft, when modified, complies at least with the requirements which applied when the aircraft was originally certified. When a modification is being designed, a decision has to be made as to whether the modification is to be classified as µMinor¶ or µMajor¶. The installing of a new type of engine would most definitely be a major modification, whilst changing the type of clips holding cables together would be a minor one. It is somewhere in the middle when the decision as to the grading of a modification has to be decided by the CAA. 2.2.1 Major Modifications The organisation sends a form, AD282 to the CAA and, when approved, an approval note is returned to the organisation. This allows the modification to be embodied. 2.2.2 Minor Modifications The organisation writes to the CAA, requesting permission to embody the modification and, when approved, the CAA sends a form, AD261 back, to permit embodiment. If the organisation has CAA approval, it is permitted to approve its own modifications. All the organisation has to do is to keep full records of the design and embodiment of the modification. All modifications are recorded in the aircraft documentation, either inside the Airframe Log Book, if the aircraft weighs less than 2730 kg, or in a separate Modification Record Book if the aircraft weighs more than 2730 kg.

2.3 STORES PROCEDURES All aircraft and component manufacturing and maintenance establishments will have a stores department, whose object is twofold. Its purpose, firstly, is to ensure that all materials, parts, components etc. used on aircraft are to the correct specification. The second purpose of the stores is to enable the history of any important part to be traced back to its original manufacture and its raw materials. All stores transactions use the same forms throughout the JAA system as well as the USA and Canada. This system ensures that a store in one part of this country will receive a component from within the UK, all JAA countries or North America on the same form. This is known throughout the JAA system as the JAA Form 1. Stores that operate within an organisation that is approved by the CAA to operate, with little control or supervision from the CAA, is known as an µApproved Stores¶.

An µApprovedµ Store will contain three main departments: y y y A quarantine store, which accepts items from other companies and checks that they are satisfactory. A bonded store which takes items from the quarantine store, after approval, and, when requested, issues those components to the servicing technicians. An office or administration centre, which keeps adequate files and records, to enable cross-checking of any transaction through the store system.

2.4 CERTIFICATION AND RELEASE PROCEDURES Any maintenance done on an aircraft that has a Certificate of Airworthiness (C of A), has to be certified by the technician(s) doing the work. Depending on the company they work for, the technicians can have either personal certification or approval by their own company. The legal requirement is quoted as: µAn aircraft shall not fly unless there is in force a Certificate of Release to Service issued in respect of any overhauls, modifications, repairs or maintenance to the aircraft or its equipment¶. Normally the work is either certified by an approved engineer or, completed by a nonapproved engineer and certified by another, approved engineer. This certification is known as a Certificate of Release to Service. The wording on the document for signature is to a standard format and certifies that the work has been done in accordance with JAR 145 and that the aircraft is fit for release back to service.

The certification should also contain particulars of the work done or the inspection completed and the organisation and place at which the work was done. It is also required that the aircraft type and registration or component type, part and serial number shall be recorded as applicable. There are a number of minor maintenance operations that do not require certification/ release to service. This can include minor maintenance, done by the pilot, on a small private aircraft. 2.4.1 Interface with Aircraft Operation There are many links between aircraft maintenance and the flying done by both commercial and private operations. These links, or interfaces, include the legislation that dictates how the two operations are to work together. For the larger commercial companies, all the legislation is currently laid down under JAR-OPS, produced by the JAA as an approximate replacement for the publication CAP 360 which was the method by which commercial flying companies obtained their µAir Operators¶ Certificate¶. JAR-OPS controls many facets of commercial flying. This can include how the company maintains its aircraft, (or how it sub-contracts the work elsewhere); how the documentation and publications record all the information needed for both the engineers and the flight crew and how the quality of the whole operation is kept to an acceptable standard. The communication of information between maintenance and flying personnel is normally via a number of different publications such as: y y y

The Technical Log Book (Tech. Log) The Log Books (Aircraft, Engine and Propeller) The Modification Records.

The Tech. Log contains all details of the sector by sector flight operations, such as flight times, defects, fuel (on arrival and uplifted), other ground maintenance and replenishments. The Log Books are usually kept within the records department, but they are a long term record of not only the total flying hours, but of the life remaining on engines and propellers and the maintenance checks done on the aircraft. The Modification Records allow all to see what changes, (modifications), have been embodied to the aircraft. These changes might require different flight operations or maintenance actions than prior to their embodiment.

Other publications that can be used by both sections include the Minimum Equipment List (MEL) and Configuration Deviation List (CDL). These publications inform both the crews and the engineers which components and parts can be unserviceable, and yet allow the aircraft to be dispatched. There are minor, but no less important, systems in place to allow the same form of communication with smaller, private aircraft. They also have Log Books and records of modifications but, because of their lower utilisation and private ownership, most work is done during their annual and three-yearly Certificate of Airworthiness by approved and licensed engineers. 2.5 MAINTENANCE INSPECTION/ QUALITY CONTROL AND ASSURANCE All maintenance done on the aircraft, from the Pre-Departure Inspection (made before every flight); to the heavy Check µD¶ inspection (done every four to six years), is controlled from the Maintenance Schedule. This publication is produced by the aircraft manufacturer, and dictates the depth and frequency of work at which each inspection is completed. On light aircraft, the maintenance is normally done in accordance with a Schedule produced by the CAA, called the Light Aircraft Maintenance Schedule, (LAMS). This is a simple schedule, common to all private aircraft below 2730kg, which divides the maintenance into 50 and 150 flying hour, annual and tri-annual inspections. The personnel who do any of the inspections have to be either licensed by the CAA or µapproved¶ by their own company, (if the company is itself approved by the CAA). The types of aircraft being serviced, and their use, will control which type of qualification they require. If a company has CAA approval under JAR-145, it is permitted to control all of the maintenance it does as well as, in some instances (with the additional approval under JAR-147), the µin house¶ training of its own engineers. An approved company has to introduce a Quality Assurance Department, to the strict rules laid down in JAR-145. This department controls the standards of the company from the lowliest worker on the hangar floor to the Accountable Manager, usually the managing director. It is responsible for all of the engineers and their approvals. It also examines engineers and trainees, prior to their examination by the CAA. The Quality department also makes µaudits¶ throughout the company, at intervals, to ensure all the procedures, laid down in the company manuals, are being followed.

When certain operations are being done on an aircraft, whereby there might be catastrophic consequences to the aircraft if the work was not done correctly, then a duplicate inspection is required. This involves two engineers; one of whom completes the work while the second (who has had nothing to do with the task), checks the work and signs that it has been completed correctly. 2.6 ADDITIONAL MAINTENANCE PROCEDURES Apart from the regular maintenance checks, listed in the Maintenance Manual, there are a number of additional maintenance procedures that are done at irregular intervals. These could include washing the aircraft, de-icing it in the winter, weighing it after certain operations and painting it when its condition warrants it. The information and the correct procedures will probably be found in the maintenance manuals. (under Washing, De-icing, Weighing and Painting). Other work done, in addition to the normal regular maintenance, might include an ongoing sampling programme or condition monitoring, which is done during the normal day-to-day operation of the aircraft. These tasks would probably be organised at the request of the local CAA office, to comply with an airworthiness request from the manufacturer. 2.7 CONTROL OF LIFE-LIMITED COMPONENTS On almost any aircraft, there will be a number of components that have a stated µlife¶, usually quoted in flying hours, cycles, calendar time or operating hours. The correct terminology for µlife¶ is Mandatory Life Limitation. The components will have been given a life for various reasons. For example, a fatigue life on a structural component in flying hours; the landing gear legs due for retirement after 10,000 landings, the batteries due for replacement after 3 or 4 months and a retirement life on an APU measured in hours running time. The control of the replacement of components, on completion of their lives, rests with the Technical Control/Records department, which monitors all of the aircraft documents. When an item is due for replacement, the work is often synchronised with a scheduled maintenance check, so that the aircraft is out of service for the minimum amount of time. It is normal, however, for small items such as batteries, to be changed on the flight line, often at the end of the day¶s flying, with the battery replacement being done at the same time as the daily inspection.

The notification of the correct day for the replacement will be annotated on a document called the Maintenance Statement, which gives all items due for replacement, in between scheduled maintenance checks. In the front of the Maintenance Manual is a chapter, variously entitled µRetirement Lives¶; µLong Life Items¶ or µFatigue Lives¶. This chapter lists the retirement lives of many components and parts with long lives, which can include such items as engine µhot-end¶ components, landing gear legs and major structural items that have retirement lives in the thousands of flying hours/cycles. This list will be monitored by the Technical Records department, and the aircraft documents will be annotated and the work cards etc., raised when the task is required to be done.