Aircraft Maintenance 101: All You Need To Know

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Why is Aviation Maintenance Performed?

From ultralights to high-performance fighters, proper aircraft maintenance is the lynchpin of safe flying. Ensuring the number of take-offs and landings stays the same is a science spread out under an umbrella that covers anything from changing a tire to a full tear-down and inspection.

The International Air Transport Association (IATA)’s Aviation Technical Reference Manual defines maintenance as “actions required for restoring or maintaining an aircraft, aircraft engine or aircraft component in an airworthy and serviceable condition, including repair, modification, overhaul, inspection, replacement, defect rectification and determination of condition.”

The main goal of maintenance is to ensure airworthiness; that is, to keep aircraft flying in a condition where they can perform as expected by the manufacturer without exposing those around them to danger. Other concerns immediately after that are availability (having aircraft in flyable condition when needed) and value (balancing acquisition, operation, and maintenance costs with the returns brought by the aircraft or fleet).

Defining Airworthiness

As the end goal of maintenance, it’s important to understand what airworthiness is and how it is achieved. The American Federal Aviation Administration (FAA) defines airworthiness as “when an aircraft or one of its component parts meets its type design and is in a condition for safe operation.” Not all aircraft are created equal, and it is up to regulatory agencies to evaluate the design, manufacturing, and maintenance of each type to ensure that both individual components and the airplane as a whole can comply with current regulations. Once these tests have been completed, the authority issues a type certificate and sets airworthiness guidelines that define the minimum acceptable condition for parts of the aircraft.

The purpose of airworthiness regulations is to reduce the number of preventable equipment failures in aviation and by association the incidents and accidents caused by them. Modern airworthiness guidelines and the maintenance procedures in place to achieve them have drastically reduced the number of equipment-induced accidents. The following table is sourced from the NTSB Annual Review of Aircraft Accident Data for General Aviation in 2006 and shows the primary causes of accidents.

Cause Accidents
Loss of Control (Air) 231
Loss of Control (Ground) 210
Collisions with Terrain/Water/Other 204
Loss of Engine Power (Nonmechanical) 129
Hard Landing 99
Airframe/Component/System Failure 97
Loss of Engine Power (Malfunction) 79
Total Caused by Equipment Failures 16.78%

A 2003 study by the IATA yields similar numbers – approximately 80% of crashes across all branches of aviation analyzed were attributed to human error, with a combination of equipment failures and weather making up the other 20%.

Despite current maintenance processes returning encouraging numbers, the drive to do better remains: the FAA aims to reduce accident rates by 10% each year, and regularly updates its regulations to match that.

Threats to Airworthiness

To understand aviation maintenance and airworthiness, it is important to identify the factors that could lead a plane or component to no longer be airworthy. Some of these issues are incidental, while others can be considered a natural part of an aircraft’s life cycle. The latter is accounted for in the manufacturer’s Maintenance Planning Document (MPD). Here are examples of common threats to airworthiness:

  • Operating outside parameters: Each aircraft is designed and certified to operate within certain conditions. The limits imposed may be due to immediate risks (e.g. high crosswind components due to aircraft controllability at slow speeds) or long-term risks (e.g. hot and high conditions that require exceeding allowed engine parameters for safe take-off and landing performance), and operating outside these guidelines for any reason incurs the risk of damage or loss of the aircraft.
  • Component degradation: Regardless of the above, the components that make up the aircraft are subject to certain stresses and wear factors, both when flying and when stored. Humidity, sea spray, landings, take-offs, go-arounds, gusts of wind, manoeuvres, rough handling, improper operation, the passage of time, and other items on a long list of factors contribute to component wear, corrosion, and fatigue.
  • Procedural issues: A more complex threat than the other examples is that of procedural issues. In layman’s terms, this is when what has been considered ‘the right way is, in fact, harmful to airworthiness. These problems can creep up at different levels. An aircraft may have been given a type certificate despite not meeting current airworthiness requirements, either through improper testing or rule changes between the bulk of its design and the certification campaign. A component manufacturer may not be taking the appropriate care during the making, storing, or shipping phases. Maintainers may be using procedures that lower turnaround times or costs but unintendedly cause harm in other ways.

While the best solution to airworthiness threats is preventive, eliminating them is impossible in practice, so efforts must be taken to identify which of the aircraft’s circumstances could require added caution and corrective care.

Types of Aircraft Maintenance

Under the maintenance umbrella, the context and purpose of each procedure land it in distinct categories. Most aircraft will undergo all of these during their lifetime.

The first divider of waters is the state of the unit in question: is it flying or in storage? Owning an aircraft is naturally more expensive when it takes to the air regularly, but even hangar queens need constant care. The Aircraft Maintenance Manual (AMM) has instructions on what preventive maintenance is needed when an airframe is not being flown and accounts for different storage scenarios including the expected time out of service, the environmental conditions at the storage area, and the state of the aircraft.

Storage maintenance is an expense, but it is better seen as an investment: planes that are properly cared for between operational periods are significantly cheaper and quicker to bring to an airworthy condition and do not depreciate as drastically over time.

Service Maintenance

Regular flying means regular care, and the complexity of this varies dramatically. A common aspect of most maintenance procedures is that it, and the problem leading to it if that is the case, are recorded on the aircraft’s technical log. Aircraft maintenance is usually divided into three tiers. The naming conventions depend on the country and operator, but the divisions stay the same.

Line Maintenance

As the name implies, line maintenance covers anything that can be performed by flight line personnel and, in some cases, the crew. It is the first and least intensive tier of aircraft maintenance, but also the one performed most often. Line maintenance procedures aim to meet conditions needed for the upcoming flight, and as such regular pre-flight and post-flight inspections and checks fall under it.

Some examples of line maintenance include visual inspection, replacement of components easily accessible, and troubleshooting issues identified on the ground or in flight. Most of the time this work can be performed quickly and the aircraft is made available on short notice unless deeper issues are uncovered.

Base Maintenance

This tier encompasses activities that work on a deeper level and are more time-consuming than line maintenance. In-depth inspections, overhauls, and modernization programs that require extensive reworks of the aircraft are carried at base level as they require facilities and personnel that are not always available at any flight line.

Due to the work sometimes required during them, corrective work dictated by Service Bulletins (SB) and Airworthiness Directives (AD) is often performed at the base level as well.

Depot Maintenance

The third and last tier of aircraft maintenance is the depot level, also known as workshop maintenance. In general aviation, this usually refers to component-specific work where the part in question is removed from the airport and transported to a specialized facility. One of the most common subjects of workshop maintenance is engines, which need regular overhauling to stay airworthy. Larger maintenance facilities may be able to perform both base and depot maintenance.

How Inspections and Intervals Work

National regulatory bodies have a set of mandatory preventive inspections, and guidelines for maintaining the aircraft airworthy between them. The timing and depth of these inspections were created based on established maintenance interval protocols, which are set by the manufacturer based on their engineering team’s estimations and may be adjusted over time based on practical experience.

Intervals

Maintenance intervals are divided into three categories: hard time, on-condition, and condition monitoring. The criteria for choosing which interval to use depend on the ability to detect component wear before it malfunctions or fails.

Hard Time

This maintenance interval is the most common in older aircraft and on components difficult to inspect without an extensive tear-down. A hard time interval is called such because it is ruled by a certain timeframe, either actual calendar time or a measurable limit such as several cycles or flight hours. As it is a preventive estimate, a hard time interval comes with a higher safety coefficient as it must guarantee that the aircraft will safely operate between two inspections. The downside of this is that equipment that is under-stressed may be replaced or overhauled sooner than it should, and the opposite is also true, which may lead to incidents and accidents if the interval is not adjusted accordingly.

On-condition

The purpose of on-condition intervals is to determine whether a component can remain in operation based on its present state. It is also a preventive method, performed by periodically inspecting or testing the item in question. Should this examination expose early signs of failure that could potentially lead to a problem before the next scheduled check, corrective action is taken. Due to the need for close inspection, on-condition intervals are most used for components that can be reached or verified without dismantling the aircraft.

Condition Monitoring

The condition monitoring interval is steadily becoming the new standard. The most common method for this is fully automated: sensors installed on aircraft components constantly generate reports on their status. This data is transmitted to a computer, sometimes built into the aircraft. A cumulative analysis of the component status and aircraft flight data is executed automatically, and if corrective action is needed, the appropriate warning is sent to the pilot or technician.

Inspections

The inspections required to maintain airworthiness are mostly designed around a hard time and on-condition intervals, as these are not automated. Exceptions to this rule are known deviations of expected parameters, like an exceptionally hard landing, an overload, or a collision, where an inspection is necessary to understand the state of the aircraft before determining corrective steps.

All aircraft need to undergo an annual inspection, performed within 12 calendar months of the last by a qualified A&P mechanic, a certified repair workshop, or the manufacturer itself. In case of commercial use, either with passengers, cargo, or for pilot training, an additional inspection must be performed after every 100 flight hours.

As always, the pilot in command (PIC) bears responsibility for the aircraft and its airworthiness for as long as they perform this role. It is the pilot’s duty to have the aircraft’s registration documents, a valid airworthiness certificate, and an approved document containing operational limitations (usually the aircraft’s pilot handbook or flight manual). They also are expected to be up to speed with current regulations, and have the authority to abort a flight in case issues are found or suspected during the pre-flight checks and inspections.

Airworthiness Directives and Service Bulletins

In an ideal scenario, the manufacturer’s estimates and tests are done to perfection, the design is future-proofed and stays in line with regulations for years to come, and the maintenance intervals find a perfect balance to catch problems before they spring up while also not replacing parts too soon. Unfortunately, the reality is not as kind, and what seemed like perfectly reasonable procedures can quickly be proven to be unsafe. To remedy such situations, two types of documents can be issued: airworthiness directives (AD) and service bulletins (SB).

An AD is published by the regulatory body, and its purpose is to warn owners and operators of unsafe conditions. The offending component found to be unsafe is listed, together with corrective recommendations. The AD may outline conditions, limitations, or inspections (either single or periodic) required for the aircraft to be considered airworthy again. The regulatory body will only allow it to fly after proof of compliance with the AD is submitted, including the method used and signature by the technician or certified company involved.

While regulatory bodies will issue an AD to warn of impending problems, manufacturers are allowed to publish an SB under certain circumstances. This document is also mandatory as it supplements aircraft manuals, and is based on findings through service experience or additional testing. Conditions for issuing an SB are when a product’s service life or performance can be extended after undergoing a certain maintenance procedure, the details of which are also contained in this document.

Who Performs Maintenance?

The specifics depend on the country of operation, but most situations are in line with the US FAA’s guidelines, where the purpose of the flight determines this. As per Federal Aviation Regulation (FAR) 61, a holder of a pilot’s license is allowed to perform preventive maintenance so long as they are the owner or operator, and the aircraft in question is not used for commercial passenger flights. If the latter is true, then only certified mechanics and companies can perform maintenance. All items listed under minor or major must also be signed off by a technician regardless of the aircraft’s purpose.

Special Cases

The Minimum Equipment List

A pilot might find themselves in a situation where they suffer an equipment failure halfway through a trip, or in a location that is not fit to rectify the issue in a suitable manner. Depending on the circumstances, the pilot may be allowed, at their discretion, to fly the aircraft until a convenient opportunity to perform the repairs is found.

The determining factor here is whether the systems currently operational match the minimum equipment list (MEL) in the aircraft’s manual. The MEL lists which systems and components must be available to still achieve safe operations, with certain restrictions, which shift the responsibility onto the pilot for accepting or rejecting the flight.

Caring for Older Aircraft

Airworthiness regulations and additional directives usually cover most cases, but owning older aircraft comes with an extra set of maintenance concerns. The two most common problems inherent to aging aircraft are electrical and structural wear.

Unlike its brethren fresh off the production line, aircraft with many years on their wings have accumulated a similar amount of metallic corrosion and structural fatigue. Load bearing elements must be inspected regularly for corrosion and cracks. Fortunately for owners and operators, these checks are becoming less intrusive over time, with lower costs and downtimes as a result. The degree of incidence of these issues varies wildly based on the service history of the aircraft, but they are by and largely unavoidable.

Electrical issues can be more difficult to manage depending on the aircraft’s design. Many older types did not have wiring replacements and detailed checks in mind during their conception, which makes identifying and fixing these issues extremely unpleasant and time-consuming for both technicians dealing with them and owners footing the bill and being without their aircraft. Certain modernization programs include wiring system overhauls as part of them, which make future maintenance more accessible, but these are often cost-prohibitive. It is down to the owner to assess whether such an option, if possible, is worth the investment.

Further Read:

FAQ

Question: Why is aviation maintenance performed?

Answer: Maintenance exists to ensure airworthiness and allow for long-term operations without safety risks.

Question: What can be done to make maintenance simpler?

Answer: Threats to airworthiness need to be avoided when possible, and accounted for when not. Acknowledging improper conditions and acting on their consequences is just as important as preventing them.

Question: Is there only one type of maintenance?

Answer: No, there are many branches of it. The most used division of aviation maintenance is into line, base, and depot maintenance, depending on the extent of the work required.

Question: When should aircraft be inspected?

Answer: Inspection and replacement intervals can be done on a hard time, on condition, or with condition monitoring. This is outlined in the aircraft’s maintenance manual.

Question: Can maintenance not originally outlined in the manual become mandatory?

Answer: Yes. If a threat to safe operations or a way to improve component performance is found after the aircraft is already operational, regulatory agencies and manufacturers can issue mandatory directives to ensure the safety and longevity of aircraft and their components.

Question: Who can perform maintenance besides certified technicians?

Answer: Pilots can perform preventive maintenance so long as no commercial activity is involved. Any other type is limited to certified personnel.

Question: What happens if you encounter failures away from your home airfield?

Answer: Exceptionally, aircraft may be flown with some systems inoperative so long as certain requirements are met. This list is available to the pilot as part of the standard flight documents.

Question: Is maintenance the same regardless of aircraft age?

Answer: No. Older aircraft require additional care to prevent failures of structural and electrical components, as wear via material fatigue and exposure to the elements builds up over time.

Sources:

IATA Aviation Technical Reference Manual http://www.aerohabitat.eu/uploads/media/29-07-2010_-_IATA_-_Aviation_Technical_Reference_Manual.pdf

Lycoming Basic Maintenance https://www.lycoming.com/content/basics-maintenance-general-aviation

Skybrary Aircraft Maintenance https://www.skybrary.aero/index.php/Aircraft_Maintenance#Further_Reading.2FReferences

AvStop.com What kind of Maintenance Can I do On My Aircraft?

http://avstop.com/technical/maintenance/maintenance.htm

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