Conditional Aircraft Inspections

• Conditional inspections are unscheduled and occur after specific over-limit or abnormal events.

Events Requiring Conditional Inspections

• Lightning strike
• Hard or heavy landings
• Over-stress conditions
• Severe turbulence
• Flight through volcanic ash
• Over-temperature conditions
• Exceeding placarded speeds of flaps or landing gear
• Bird strike
• Foreign object damage (FOD)

General Inspection Procedure

• Conduct inspections per the Aircraft Maintenance Manual (AMM) ATA Chapter 5, Section 50 (Unscheduled Inspections).
• Consult the AMM and Service Bulletins to determine necessary inspections and potential damage areas on similar aircraft.
• Look for distortion or twisting of the main structure before detailed inspections.

Stress Overloads

• Types of stress overloads include tension, compression, torsion, shear, and bending.
• Tension and compression are the two basic stresses that combine to form the other three.
• Overloading can cause failure or deformation, often with visual damage.

Turbulence

• After heavy or severe turbulence, check the entire structure for deformation or cracks.
• Inspect skins for waviness and rivets for tipping.

Heavy Landings

• Heavy landing inspections are generally divided into Phases 1 and 2.
• Phase 2 is required if Phase 1 reveals defects, and involves jacking the aircraft.

Heavy Landings Phase 1

• Inspect landing gear, wheels, and brakes for damage, cracks, and fluid leaks.
• Examine wheel bearings for smooth operation.
• Inspect the aircraft externally for damage like buckling, cracking, distortion, or loose fasteners.
• Inspect landing gear struts for damage and fluid leaks.

Heavy Landings Phase 2

• Requires jacking the aircraft.
• Inspect the entire landing gear for damage.
• Remove tires and use eddy current inspection on wheels, especially in the bead seat area.
• Check the inside of tires for broken chords.

Lightning Strikes

• Aircraft are inevitably struck by lightning; effects can be minimized.
• Entry and exit points usually have small structural damage, such as burn holes or melted rivets.
• Unscheduled inspections are performed to check for damaged structure at entry and exit points.
• Radio and navigation equipment are tested for proper operation.
• Damage typically appears as pitting or small, circular burned holes.
• Damage to solid, laminated, or honeycomb structures appears as discolored paint or burned, punctured, or delaminated skin plies.
• Arcing and burning can occur around support structure attachments.
• Diverter strips on fiberglass nose radomes conduct the charge away from the weather radar.

High-Intensity Radiated Fields (HIRF)

• HIRF can interfere with electronic equipment.
• Composite materials offer less shielding than metal structures.
• Intensity is similar to electrostatic charge: high voltage, low current flow.
• Visual indicators of HIRF penetration include incorrect radio compass readings with a correct magnetic compass and constant radio static.
• Flight crew may report HIRF-related problems in COM, transponder, NAV, VOR, and HF systems.

Heavy or Overweight Landings

• Landing gear is designed for specific weight and descent velocity.
• Over-stressing can occur from landing with drift or in an abnormal attitude.
• Consult the flight crew for details on aircraft weight, fuel distribution, landing conditions, and unusual noises.
• Damage is typically concentrated around the landing gear, supporting structure, wing and tailplane attachments, and engine mountings.
• Secondary damage may occur on the fuselage and wing skin and structure.

Preliminary Inspection Items

• Examine tires for creep, flats, bulges, cuts, pressure loss, growth, and balance weight security.
• Inspect wheels and brakes for cracks, damage, and fluid leaks.
• Examine axles, struts, and stays for distortion and other damage.
• Check shock struts for fluid leaks, scoring, and abnormal extension.
• Examine landing gear attachments for cracks, damage, or movement.
• Examine structure near landing gear attachments for cracks, distortion, rivet/bolt movement, and fluid leakage.
• Examine doors and fairings for damage and distortion.
• Jack the aircraft and test retraction and nose-wheel steering; check locks, lights, clearances, door fit, and leaks.

Wing Inspection

• Examine skin surfaces for wrinkling, pulled rivets, cracks, and joint movement.
• Check for fuel leaks and seepage from integral tanks.
• Examine wing root end fillets for cracks and movement.
• Check flight controls for freedom of movement.
• Check balance weights, powered flight control unit mountings, control surface hinges, and surfaces for cracks or buckling.
• Check wing spars for distortion, where possible.

Fuselage Inspection

• Examine fuselage skin for wrinkling or damage, especially at joints and near landing gear attachments.
• Examine pressure bulkheads for distortion and cracks.
• Examine supporting structure for heavy components.
• Check inertia switches for fire extinguishers and emergency lights.
• Check instruments and panels for damage and security.
• Check ducts and pipes for damage, security, and leaks.
• Check access doors, emergency exits, and surrounding areas for distortion and cracks.
• Check cargo container loading and restraint system condition.
• Check gyroscopic instruments for erection time, precession, and unusual noises.

Engine Inspection

• Check engine controls for full and free movement.
• Examine engine mountings and pylons for damage and distortion.
• Check tubular members for bow and cracks at welds.
• Check mounting bolts and attachments for damage and movement.
• Check turbine engines for freedom of rotating assemblies and piston engines for rotation without spark plugs.
• Examine engine cowlings for wrinkling, distortion, and fastener integrity.
• Check for oil, fuel, and hydraulic fluid leaks.
• Check propeller shaft for shock loading per the Maintenance Manual.
• Check propeller attachments and counterweight installations.
• Check oil system filters and chip detectors.

Tail Unit Inspection

• Check flight controls for freedom of movement.
• Examine rudder and elevator hinges for cracks, and control surfaces for cracks and distortion near balance weight fittings.
• Examine tailplane attachments and fairings, screw jacks, and mountings for distortion and movement.

Engine Runs

• Conduct engine runs per the Maintenance Manual if no major structural distortion is found.
• Check for system leaks and turbine engine run-down time.

Damaged Area Inspection

• Examine supporting structure for distortion, loose rivets, and cracks if superficial damage is found.
• Check rigging and symmetry.
• Cabin leak rate check on pressurized aircraft.

Tyre Failures

• Tyre failures can overload the companion tyre on the same axle.
• Bursting tyres can damage aircraft parts.
• Multiple wheel landing gears are less affected, but axles, bogies, torque links, or steering mechanisms may be strained.
• Tyre and wheel degradation can reduce braking performance.

Post Tyre Burst Inspection

• Examine undamaged wheels and tyres.
• Sometimes all tyres on the affected leg must be discarded or examined.
• Examine brake units on the affected leg.
• Check landing gear bay for damage and hydraulic fluid leaks.
• Examine the affected leg, including pipelines and operating jacks.
• Inspect the supporting structure and attachments for cracks, warped panels, and loose rivets.
• Examine adjacent fuselage or wing skinning and landing gear doors.
• Check rear-mounted engines for debris ingestion.

Tyre Explosion Cause

• Tyre carcass weakened by foreign object damage, scuffing, etc.
• Failures often occur after gear retraction due to brake heat transfer, internal tyre temperature, and differential pressure.
• Overheated tyres inflated with air can release volatile gases, leading to a chemical reaction and explosion.

Tyre Explosion Prevention

• Inflate tyres with nitrogen or another inert gas to limit oxygen content to 5% by volume.
• Inert gases reduce wheel corrosion, tyre fatigue, and fire risk from melted fusible plugs.

Tyre Inflation Procedures

• At airfields without inert gases, air can be used if a log entry is made.
• Tyres must be reinflated with inert gas at the earliest opportunity or within 15 flight hours.

CASA Airworthiness Directive AD/WHE/4 (Tyres)

• Applies to all aircraft over 5700 kg MTOW and those below with tires rated over 266 km/hr, except those with fixed, non-retracting undercarriages.
• Goal: Ensure tyres on braked wheels contain no more than 5% oxygen by volume.

AD/WHE/4 Compliance Options

• Install placards stating 'INFLATE TYRES WITH NITROGEN ONLY'.
• Incorporate procedures into the maintenance program:
  • Install only tyres inflated with dry nitrogen or other inert gases.
  • Tyres may be serviced with air at remote locations if oxygen content doesn't exceed 5% by volume.
  • Within 15 flight hours, the tyre must be purged and inflated with dry nitrogen.

Severe Turbulence Assessment

• Assess using accelerometers or fatigue meters, but readings may be exaggerated.
• Readings exceeding -0.5 g and +2.5 g on transport aircraft may indicate damage.
• Turbulence effects increased by inertia of heavy components.
• Damage may occur at wing-to-fuselage joints, tail-to-fuselage joints, and engine mountings.
• Skin wrinkles and pulled rivets may indicate damage.

Severe Turbulence Inspection

• Follow the procedures for Heavy or Overweight Landings, excluding landing gear checks in most cases.
• Dismantling and skin removal may be necessary to inspect supporting structure.

Aircraft Impact Damage

• Damage can be caused by other aircraft turning or taxiing nearby.
• Inspect flight control surfaces for distortion.
• Inspect for skin dents and cracked windscreens or windows.
• Examine engine and heat exchanger air intakes for debris.
• Inspect light aircraft for structural damage if moved by jet blast.

Other Events Triggering Inspections

• Exceeding flap limiting speed requires inspection of flaps, hinge brackets, and operating mechanism.
• This is an example where specific instructions may not exist, and experience with the aircraft type and knowledge of the structure and stress paths are important.

Lightning Strike Facts (CAAP 42L - 1 (0) Paragraphs 4.1 to 4.4)

• Lightning strikes occur on average once every 3000 flying hours on commercial aircraft.
• Aircraft manufacturers design under the assumption that all lightning strikes are high intensity.
• Lightning has at least two strike points, an entrance and an exit point.

Types of Lightning Damage

• Direct damage: metallic structures burned, melted, or distorted; non-metallic structures burned, punctured, or delaminated.
• Indirect damage: damaged or upset electrical/electronic systems, wires, shielding, and shield terminations from electrical transients.

Lightning Strike Locations

• Initial strike damage most commonly occurs:
  • Nose section and radome
  • Engine nacelles
  • Wing tips
  • Horizontal stabilizer and elevator tips
  • Vertical fin and rudder tip
• Initial strike damage does not normally occur:
  • Drain masts
  • Pitot probes
  • Blade antennas
  • Extended ends of leading-edge slats
  • Trailing edge flap track fairing tips
  • Landing gear

Lightning Damage Inspection - Procedure

• If the strike area is known, inspect for damage. If unknown, start with areas most prone to damage in a systematic inspection.
• Direct damage to metal structures typically appears as small, circular melt marks or pits (approximately 1/8 inch).
• Composite components may exhibit discolored paint or burned, punctured, or delaminated skin plies.
• Dis-bond and delamination damage may be more widespread than surface appearance suggests; use non-destructive testing.

Lightning Diverter Strips

• Lightning diverter strips are bonded (earthed) to the airframe to give them the same electrical potential
• Protect composite structure from catastrophic damage.

Lightning Strike - Repair

• Repair all lightning strike damage per the Structural Repair Manual. Temporary sealing may be used as a ‘get-you-home’ strategy.

Post Lightning Strike - Magnetic Interference

• Ferrous components can become magnetized, affecting magnetic instruments.
• Non-ferrous fasteners are used near sensitive equipment.
• Investigate any compass deviation after a lightning strike.

Post Lightning Strike - Moving Parts

• Lightning current across bearings can cause burning or melting, leading to stiff movement or seizure.
• Prevent this with bonding leads providing a low-resistance path.

Static Dischargers

• Static discharger wicks bleed off static charge to prevent radio interference.
• Static dischargers do not prevent lightning strikes.
• They May be installed for added protection to nearby electrical & electronic components.

Damage Inspection Checklist

• Metallic structure: holes, pits, burned/discolored skin or rivets.
• Nose radome: internal and external surfaces for burns, punctures, and pinholes; check bonding straps and diverter strips.
• Composite honeycomb components: discolored paint, burned/delaminated skin plies; use NDI or tap tests to find delamination.
• Locations where strikes don't normally occur: check areas where surfaces end and new surfaces begin.
• Controls (manual): Check Surface hinges, bearings and bonding leads
• Controls (powered): Test Surface hinges, bearings and bonding leads check actuators. - (fly-by-wire): Perform system tests
• High-lift devices: mechanisms, tracks, actuators, and bonding leads for damaged surfaces
• Static dischargers
  • Check attachment, condition, and breaks
  • Look for burns, rough coating, and damaged metal retainer bases
  • Check for bent, broken, or blunted tungsten pins
  • Perform a static discharger resistance check if damage is suspected

Internal Systems Checks

• Examine and test all external lights for damage and check associated wiring.
• Check and perform Stand-by Magnetic Compass Tow-Around Procedure (if outside tolerance).
• Determine fuel system accuracy (indicated vs logbook entry) and applicable tank units for inconsistencies.
• Fully functionally test both wire flight control and high-lift systems after lightning strike. Check electrical element with mechanically operated systems.
• Examine/test antennas
  • HF - High-Frequency radio antenna
  • VHF - Very High Frequency radio antenna
  • VOR - VHF Omnidirectional Rangefinder antenna
  • DME - Distance Measuring Equipment antenna
  • ATC - Air Traffic Control antenna
  • ILS - Instrument Landing System antenna
  • ADF - Automatic Direction Finder antenna
  • ELT - Emergency Locator Transmitter antenna
  • RA - Radio Altimeter antenna
  • ACARS - Aircraft Communications Addressing and Reporting System antenna
  • GPS - Global Positioning System antenna
  • SATCOM - Satellite Communications antenna
  • TCAS - Traffic Alert and Collision Avoidance System antenna
  • Marker Beacon Systems antenna
  • Airborne Telephone antenna
• Check Radar antenna and waveguide for damage if radome is damaged.

Required Tests after Lightning Strike

• Flight crew operational tests & additional inspection if malfunction found
• Look for system cable/connector burns, breakage, melting and other damage.
• Look for pin burns at contact point & extensive damage remote from the contact
• Trimming may be required to cut out damaged matrix around burnt areas
• Check all electrical paths and contacts.

Integrity of Protection Systems

• Bonding testers are used to check that the resistance of the system remains within limits
• Protection from these conditions is built into new aircraft through shielded enclosures and shielded wiring, which are grounded to aircraft structure.

High-Intensity Radiated Fields (HIRF) Overview

• HIRF are electromagnetic energy external to the aircraft.
• Can impose sudden, serious damage to critical aircraft systems affecting flight safety.
• Protection via shielded enclosures and wiring grounded to the aircraft structure.

HIRF Sources

• External radio frequency (RF) emissions (high-power radio, TV signals, radar).
• Similar electromagnetic fields to lightning.
• Suspected PEDs (Personal Electronic Devices)

HIRF Effects

• Interference with electrical and electronic systems.
• Potential problems with control systems, navigation, and instrumentation.

Mitigation Concerns

• Integration circuit miniaturization increases electromagnetic vulnerability.
• Increased use of composite materials.
• Higher RF energy levels from radar, radio, and television transmitters.
• Environmental factors

Visual Inspection steps

• Visual inspection is the first and generally most important step in HIRF maintenance.
• Degradation examples: corrosion, mechanical vibrations, thermal cycling, damage and subsequent repair and modifications.
• Critical that procedures contained in the AMM and Component Maintenance Manuals reflect reliable procedures to detect any incorrect installations which could degrade HIRF protection features.

Areas Affected by HIRF

• Aircraft structure (airframe skin and frame)
• Electrical wiring installation protection (solid or braided shielding connectors)
• Equipment protection (LRU case, electronics input/output protection)
• Cable shielding is essential in protecting critical aircraft systems.

HIRF Inspection Tool

• Portable loop resistance tester for testing cable shields and shield connections in aeroplane wire bundles.

Corrosion

• Occurs at junctions where metal-to-metal contact has air/water and allows oxide to form.
• Over time, resistance increases at the junction with corrosion, limiting electrical current & potentially creating open circuit.
• Designer should assume moisture is inevitable, must mandate corrosion materials/sealants

Shielding techniques

• Aircraft designers must base their techniques for shield grounding on the assumptions that moisture is inevitable and that the use of corrosion-resistant materials and sealants is mandatory. Using corrosion-resistant materials involves several compromises, however.
• Proper shield installation exhibit certain amount of resistance in shield circuit

Aircraft Maintenance

• Proper shield installation exhibit certain amount of resistance in shield circuit
• Aircraft designers must base their techniques for shield grounding on the assumptions that moisture is inevitable and that the use of corrosion-resistant materials and sealants is mandatory
• By monitoring for resistance, maintenance personnel can ensure the circuit's ability to protect internal wiring
• This allows for testing limits to be established for determining when corrective action must be taken

Rotor system - Post incident, special inspections are required

• Tail boom - Strike damage from rotor, cracks/symmetry
• Blades - Removed and check for twisting/distortion, cracks/damage & security
• Head - Disconnect pitch change rods/dampers, check for binding/roughness & check for damages
• Tail rotor - security and coning
• Skid Gear - wear, bows, integrity/security of fasteners

Helicopter - Over Speeding inspection

• Inspected for rotor blade distortion and damage and Rotor Head operation. If limits exceeded then additional inspection needed