Aircraft Maintenance: Abnormal Events, Inspections, and Procedures

Posted on May 25, 2024 in Other subjects

Abnormal Events and Inspections

These are events that occur outside the normal care and use of aircraft that may affect airworthiness. Some of these abnormal events could be lightning strikes, hard or overweight landings, severe turbulence, fire damage, and flood damage.

Lightning Strikes: When lightning strikes an aircraft, the electrical current must be conducted through the structure and dissipated in controlled places such as static dischargers. A visual inspection of the structure for evidence of composite resin degradation, burning, or erosion is necessary on all affected structures, electrical connection straps, static discharge wicks, and null field arresters. Also, non-destructive testing may be necessary. In the event of a lightning strike in an area where materials are good conductors such as aluminum or steel, the damage will probably be minimal. On the contrary, if it happens in an area of non-metallic or poorly conductive material (composite materials), without electrical discharge connections, the damage due to burns or structural damage is greater. These inspections can be found and followed according to the aircraft maintenance manual in chapter 5.

Bonding and Grounding

The system to which bonding and grounding belong is EWIS. Grounding is the process of electrically connecting conductive objects to either a conductive structure or some other conductive return path. A bad bonding or grounding can cause unreliable systems operation, electromagnetic interference (EMI), electrostatic discharge damage to sensitive electronic components, risk of electric shock to personnel, or lightning damage. Bonding is the electrical connecting of two or more conducting objects not otherwise adequately connected.

There are 3 different types of bonding:

  1. Equipment bonding: Low impedance paths to aircraft structure are normally required for electronic equipment to provide radio frequency return circuits and for most electrical equipment to facilitate reduction in EMI.
  2. Metallic surface bonding: All conducting objects on the exterior of the airframe must be electrically connected to the airframe through mechanical joints, conductive hinges, or bond straps capable of conducting static charges and lightning strikes.
  3. Static bonds: All isolated conducting parts inside and outside the aircraft, having an area greater than 3 square inches and a linear dimension over 3 inches.

The procedure of bonding can be found on chapter 20 in the aircraft maintenance manual.

Aircraft Jacking

The aeronautical technician must lift the plane for certain maintenance and repair operations, such as changing wheels, landing gears, or structural repairs of the aircraft. Airplanes have several jacking points provided by the manufacturer depending on the center of gravity of the aircraft to keep it stable and balanced during lift. These jacking points are attached to structural areas with sufficient strength to support the weight of the aircraft. The process of lifting an airplane on jacks is dangerous and requires maximum attention. Before starting, the perimeter must be secured and any platform or object that could impact the aircraft must be removed. The circuit breakers provided in the manual will be turned off, the parking brake will be disengaged and the shims will be removed.

To lift it, the jacks will be placed as required by the maintenance task to be performed. The aircraft will then be lifted in a balanced and equal manner at all points to maintain its stability. Before lowering the airplane on jacks, make sure that the landing gear locking pins are installed, the gear lever is in the down position and the flap position is in the same position as the flaps. The procedure aircraft jacking can be found in the aircraft maintenance manual on chapter 7. For some tasks such as replacing a wheel or changing the nose landing gear it is not necessary to jack up the airplane completely. For changing wheels, a single jacking point is enough to accomplish the task.

Riveting in Aircraft Construction

The solid shank rivet is the most common rivet used in aircraft construction. Used for joining structures, they are relatively low cost, permanently installed fasteners. They are quicker to install than nuts and bolts as they fit well.

  • Rivet spacing: The measured distance between the center lines of rivets in the same row.
  • Edge distance: The minimum distance of a rivet to the edge of the part or repair.
  • Rivet pitch: The distance between the centers of neighboring rivets in the same row.

A variety of hand tools are used in the normal course of driving and driving rivets. They include rivet cutters, rebar, hand riveters, countersinks, and dimple tools. The most common power tools used for riveting are the pneumatic rivet gun, rivet squeezer, and micro shaver.

The procedure for removing riveting with a universal or protruding head is:

  1. File a flat area on the head of the rivet and punch the flat surface with a center drill punch.
  2. Use a drill bit one size smaller than the rivet shank to drill the rivet head.
  3. Drill the rivet to the depth of its head, while holding the drill at a 90° angle. Do not drill too deeply, as the rivet shank will rotate with the drill and tear the surrounding metal.
  4. If the rivet head does not release on its own, insert a punch into the hole and turn it slightly to either side until the head comes off.
  5. Punch out the remaining rivet shank with a punch slightly smaller than the shank diameter.

Aviation Fuel and Contamination

There are two types of aviation fuel, in general use: aviation gasoline (AVGAS), which is used in airplanes with reciprocating engines, and turbine fuel, also known as JET A fuel. Contamination is anything in the fuel that is not supposed to be there. In aviation fuel we can find water, solids, and microbial growth.

The water in aviation fuels will generally take two forms: dissolved (vapor) and free water. Dissolved water is not a major problem until, when the temperature drops, it becomes free water. This is a problem if ice crystals form and clog filters and other small holes.

The effects of microorganisms are:

  • Formation of silt or sludge that can foul filters, separators, or fuel controls.
  • Fuel emulsification.
  • Corrosive compounds that can attack the structure of the fuel tank.

The best way to prevent microbial growth is to keep the fuel free of water. Static electricity is a byproduct of the friction of one substance against another. Fuel flowing through a fuel line causes a certain amount of static electricity. The biggest concern about static electricity around airplanes is that during flight, the airplane moving through the air causes static electricity to build up on the airframe.

Non-Destructive Testing (NDT)

NDT are non-destructive tests where an inspection is carried out without altering its condition to determine if there is any damage. Examples of NDT are magnetic particle testing, liquid penetrant, radiography, ultrasound, eddy current, and acoustic emissions. The NDTs can be found in the aircraft SRM chapter 51. Before performing NDT, it is necessary to follow preparatory steps in accordance with the specific procedures for that type of inspection. Generally, the parts or areas should be thoroughly cleaned. Some parts must be removed from the aircraft or engine. Others may need to have their paint or protective coating removed.

Penetrant Inspection

Penetrant inspection is a non-destructive test to detect open surface defects in parts made of any non-porous material. Penetrant inspection will detect defects such as surface cracks or porosity. Briefly, the steps to perform a penetrant inspection are:

  1. Thorough cleaning of the metal surface.
  2. Apply penetrant.
  3. Remove penetrant with emulsifying remover or cleaner.
  4. Dry the piece.
  5. Apply the developer.
  6. Inspect and interpret the results.