Without an effective inspection and maintenance program, no aircraft can withstand neglect and remain safe. An aircraft is subject to deterioration with age, such as fatigue, wear, and corrosion, as well as random failures such as tire bursts and excess structural loads.
To understand the various aspects of aircraft maintenance, it can be defined in multiple ways:
- “Those actions required for restoring or maintaining an item in a serviceable condition including servicing, repair, modification, overhaul, inspection and determination of condition” – World Airlines Technical Operations Glossary.
- “Maintenance is the action necessary to sustain or restore the integrity and performance of the airplane” – Hessburg, 2001.
- “Maintenance is the process of ensuring that a system continually performs its intended function at its designed-in level of reliability and safety” – Kinnison and Siddiqui, 2013.
Aircraft maintenance is a crucial aspect of aircraft technical activity that takes place in the line or base maintenance environment while the aircraft is still in service. Its main goal is to ensure the aircraft remains in a state that enables it to receive a certificate of release to service. Although a hangar environment is available, it is not always necessary.
According to Lam (2002), there are three primary reasons for carrying out aircraft maintenance: to ensure aircraft safety and airworthiness, to keep the aircraft in service and available to operators, and to maximize the value of the asset for the owner or lessor.
Aircraft maintenance comprises a mix of preventive and corrective work, including precautionary measures to detect any undetected chance failures. Inspection is carried out to monitor the progress of wear-out processes, and scheduled or preventive work is conducted to anticipate and prevent potential failures. Unscheduled work, such as repair and on-condition maintenance, is also necessary.
In general, for preventive maintenance to be effective, two conditions should be met: the item must be restored to its original reliability after maintenance, and the cost of maintenance should be less than the failure it aims to prevent.
Light or Line Maintenance
Typical line maintenance tasks include pre-flight and daily checks, fluids checks, and minor scheduled maintenance tasks, as well as failure rectification. According to EASA Part 145, AMC 145.A.10, line maintenance refers to any maintenance carried out before a flight to ensure the aircraft is fit for its intended journey. This may involve troubleshooting, defect rectification, and component replacement, including engines and propellers with the use of external test equipment as needed.
Scheduled maintenance and checks, including visual inspections to identify apparent failures that do not require extensive in-depth inspection, may also be included. This can include internal structures, systems, and powerplant items that are visible through quick opening access panels or doors. Additionally, minor repairs and modifications that do not require extensive disassembly and can be accomplished using simple means may also be carried out during line maintenance.
EASA Part 145, AMC 145.A.10 notes that the Quality Manager may accept base maintenance tasks to be performed by a line maintenance organization on a temporary or occasional basis, such as Airworthiness Directives (ADs) or Service Bulletins (SBs), provided all requirements are fulfilled as defined by the competent authority. Maintenance tasks that fall outside these criteria are considered to be Base Maintenance.
Base or Heavy Maintenance
Base maintenance is also known as heavy or depth maintenance, and it involves more comprehensive and long-lasting tasks compared to line maintenance. These tasks are performed less frequently, and specialized equipment and staff are required to carry out the maintenance. Operators may contract out base maintenance to an MRO (maintenance, repair, and overhaul) company that has large facilities.
The activities involved in base maintenance include performing C and D checks (also known as block checks), which check for the deterioration of the airframe, engines, and systems, such as corrosion and fatigue. Defects may also be removed during base maintenance through the implementation of Service Bulletins (SB) and Airworthiness Directives (AD), although this can also be done during line maintenance.
Additionally, technology upgrades such as fitting Terrain Avoidance and Warning Systems (TAWS) and Airborne Collision Avoidance Systems (ACAS) may be performed during base maintenance. Cabin reconfiguration, painting, and other aesthetic upgrades may also be carried out.
Shop or Component Maintenance
The third form of maintenance can be termed as “Workshop” or just Shop maintenance. This covers maintenance on components when removed from aircraft e.g. engines, APU, seats. Sometimes this is carried out within the same organisation as the Base Maintenance, but sometimes special companies carry out this work separately.
The intervals for aircraft maintenance are determined based on parameters set within the Approved Maintenance Schedule (AMS), which is derived from the Maintenance Planning Document (MPD). The intervals are established based on various criteria, mainly depending on how easily damage can be detected and the likelihood of failure predicted. [CAA, 2017].
- Hard time: maintenance carried out at fixed intervals of time in service, cycles, or landings.
- On-condition: inspection or testing carried out at specified intervals to determine whether an item can continue in service, with maintenance action taken as needed.
- Condition monitoring: automated monitoring and analysis of item data to determine whether corrective action is necessary.
The units used for maintenance intervals are:
- Flight hours (FH): for items that are in constant operation, such as fuel pumps and electric generators.
- Flight cycles (FC): for items operated once or twice per flight, such as landing gear, air starter, and brakes.
- Calendar time (Cal): for items exposed whether operated or not, such as fire extinguishers and corrosion to the structure.
- Operating hours: for items not operated every flight or otherwise independent of FH or FC, such as APU operation.