Aviation Glossary – Acronyms and Abbreviations

“AD c/w @ EOL w/ 12564:34 a/c FH and 8457 FC, TLB# 245372” – that and similar uses of aviation acronyms and abbreviations are pretty common in the technical aviation business. People who deal with aircraft transitions and airworthiness management are engineers, not writers. They will attempt to shorten every possible common phrase. On the other hand, some of them will use their “secret” knowledge of aviation acronyms to show their worth and vast experience. Whatever the reason, we have to know our way around most common abbreviations and this is why I would like to provide you with a shortened aviation glossary.

7 1024x301 Aviation Glossary – Acronyms and Abbreviations

The list below is non-exhaustive, but it was my intention to provide you with some insight into the most commonly used aviation abbreviations and acronyms to allow you for an easier understanding of the technical talk which you may encounter at some point. For now, there are only short explanations of each acronym, but I hope to be able to write a short blog post about most of them (for which it actually makes sense) and in this way create a comprehensive glossary of aviation terms.

By the way, the sample sentence from the beginning means “An Airworthiness Directive has been complied with at the End of Lease check, which took place when the aircraft has accrued 12564:34 flight hours and 8457 flight cycles. The release to service of the airworthiness directive can be found in the Technical Log Book, page 245372.” I hope you agree that the original version was a bit shorter…


A/C        – Aircraft

ACFT     – Aircraft

ACMI    – Aircraft, Crew, Maintenance, Insurance – a form of operational wet lease of an aircraft. An operator may engage into ACMI leasing when they require an aircraft urgently and for a reasonably short time period. During an ACMI lease, the lessor must be an operator themselves and they provide a full service option to the lessee, meaning they provide a ready to fly aircraft with a full flight crew, maintenance support and flight insurance. This is a very expensive way to run an operation, but often necessary if an operator’s own aircraft goes AGO in the middle of a high peak season.

AD          – Airworthiness Directive – a document issued by an aviation authority, such as EASA or the FAA, in response to an unsafe condition of an aircraft type. An AD usually requires maintenance action to be performed on a fleet of aircraft in order to prevent a potentially unsafe condition.

ADD       – Allowable Deferred Defect – an aircraft defect, which has been discovered during routine maintenance, but which has not been rectified immediately. The deferral of such defects must be substantiated by proper documentation (such as the MEL, or special manufacturer or CAA permit). The reason for deferral may be the lack of parts, lack of ground time or the need to admit the aircraft to a maintenance facility at a different airport.

AFT        – The direction opposite to the direction of flight. For example an AFT galley is a galley located in the tail section of an aircraft, as opposed to a FWD (forward) galley, which would be located up front, in the proximity of the cockpit.

AGB       – Accessory Gearbox – a gearbox mounted on the engine of an aircraft, which transmits the shaft power to numerous mechanical devices, such as engine driven pumps. The gearbox reduces (or increases) the rotational speed of shafts needed to power such external devices.

ALI         – Airworthiness Limitation Item – a specific maintenance task, which has been categorized by the aircraft manufacturer (following an MRB evaluation) as being of special importance to flight safety. In practice, this often means that ALI task have no tolerance for maintenance intervals.

AMM    – Aircraft Maintenance Manual – the main maintenance manual issued by the aircraft manufacturer. It describes in practical detail and in steps all maintenance tasks which are required by the MPD and also others, related to defect rectification.

AMO     – Approved Maintenance Organization – an organization which has been approved by the aviation authority to perform maintenance on a given aircraft type. In Europe such organization need to have an EASA Part 145 approval. The same term may also be used towards organizations which have an approval from the aircraft manufacturer (for example a Cessna Authorized Maintenance Organization).

AMP      – Approved Maintenance Program – the maintenance program shows all maintenance tasks which need to be performed on an aircraft and specifies the thresholds and intervals for such tasks. The AMP is generally based on the MPD (Maintenance Planning Document) issued by the manufacturer, although airlines can and should evaluate the MPD requirements for their own operating conditions and apply certain changes. The final maintenance program is then approved by the appropriate aviation authority and constitutes the final maintenance planning document for a given operator / airline.

AOC       – Air Operator’s Certificate – a certificate permitting a company to engage in commercial air transport.

AOG      – Aircraft on Ground – an acronym used to describe any situation in which the aircraft cannot return to commercial service thereby causing large financial losses to the operator and problems for passengers. This term is almost like an “SOS” in aircraft maintenance. It is being used to expedite deliveries of parts, assure prompt reaction from the aircraft manufacturer, etc. It is very bad practice to misuse the term AOG for situation which are not critical.

AOM     – Aircraft Operating Manual – the name is pretty self-explanatory. Basically a “users guide” for the aircraft, present in the cockpit and used by the flight crew.

APU       – Auxiliary Power Unit – A small turbine engine, normally located in the tail section of an aircraft, used primarily to provide electrical power and air flow in an aircraft while it is on ground. A typical application of the APU would be during the embarkation of passengers, during which the aircraft engines are off, but electrical power and air conditioning is required. The APU may also be used in flight during emergency situation (like all engine failure) and is crucial for ETOPS operations.

ARC       – Airworthiness Review Certificate – a certificate used by EASA operators to show continuous airworthiness of the aircraft. The ARC is being issued once every three years, and each year its validity needs to be extended by a process called an airworthiness review. An aircraft without a valid ARC is not permitted to operate in Europe.

ATA       – Air Transport Association – this US Association has been renamed to Airlines for America. It is mentioned here, because it developed a standard for chapter numbers in aircraft manuals, which has since become widely used to describe in which parts of the documentation specific information can be found. For example ATA 29 is hydraulic power, whereas ATA 57 are wings.


BITE       – Built-In Test Equipment – a BITE test generally means, that a given piece of equipment (mainly avionics – electrical and electronic equipment) have test procedures built into them. In other words, the unit is able to test itself and provide the maintenance personnel information about defects and problems.

BOM     – Bill of Material – a document listing required material for a given maintenance task. Often used during major task, such as a large aircraft modification. A BOM shows exactly what part numbers and in what quantities are required to complete a modification.

BTB        – Back to Birth – A phrase used mainly in conjunction with the use of life limited parts. In order to be certain that a life limited part has not exceeded its life limit, it is necessary to have so-called full back to birth traceability. This means, that an operator must have full knowledge when the part has been manufacture and a chronological list of all the aircraft it has been installed in since, with flight times and flight cycles as required.


CAME   – Continuous Airworthiness Management Exposition – a manual, containing a set of procedures, for the operations of a CAMO. The CAME contains detailed and complete descriptions of all processes which take part in airworthiness management of aircraft. It also is a basis for quality assurance (or compliance monitoring) within the organization. The CAME needs to be approved by the airworthiness authority.

CAMO  – Continuous Airworthiness Management Organization – an organization which has received an approval for performing airworthiness management tasks on selected aircraft types. Every commercial operator needs to be a CAMO, whereas private operators and private aircraft may outsource CAMO tasks to third party providers. A CAMO may also have the privileges to extend the airworthiness review certificate (ARC) or issue a permit to fly (PTF).

CDCCL   – Critical Design Configuration Control Limitations – A set of maintenance tasks which require special attention due to the fact, that they affect more than one system at once and are critical to the overall design of the aircraft. At present, this acronym is connected solely to fuel tank safety, which requires maintenance personnel to be aware of heat and electricity sources in close proximity (or within) fuel tanks.

CDL        – Configuration Deviation List – A list of configuration changes in the aircraft. Those changes may be due to deferred defects or purposeful, temporary configuration changes of the aircraft (for example the removal of passenger seats). The purpose of the list is to inform the flight crew about what changes have been made to the aircraft.

CG          – Center of Gravity – the center of gravity of the aircraft is important for loading and balancing, and is determined during periodical weighing of the aircraft.

CMM    – Component Maintenance Manual – a maintenance manual for aircraft components. Under a typical Part 145 aircraft approval, CMM procedures may not be used. The CMM is used by specialized shops having a Part 145 approval for specific aircraft components. Therefore, the CMM contains maintenance tasks which must be done off wing. On wing maintenance procedures would be described in the AMM.

CNL        – Cancelled – everything can be cancelled. Typically, an incorrect entry in the technical log book or on a task card. It is common practice that cancelled entries must remain legible. They are being crossed out by a single line or two parallel lines, and the acronym CNL is used.

CofA      – Certificate of Airworthiness – a certificate issued by the aviation authority. It states that the aircraft, when first registered in a given country, has met all airworthiness requirements of that country. In Europe, this certificate has no expiry date, although it may be revoked if the authority determines that the aircraft has permanently become non-airworthy.

CofR      – Certificate of Registration – a certificate issued by the aviation authority. It shows the registration of a given aircraft and specifies the aircraft owner and the aircraft operator.

CP          – Conditions Precedent – a set of conditions, set out in an aircraft lease contract, which must take place before the complete contract becomes effective in full force. They may include the need to receive the initial payment, a confirmation of insurance from the lessee or similar documents.

CPCP     – Corrosion Prevention and Corrosion Protection – a set of maintenance actions, often referred to as a “program” which are aimed at preventing corrosion on an aircraft and ensuring that existing corrosion is properly removed. This program consists of maintenance tasks, which are mostly inspections of several parts of the aircraft.

CPT        – Captain – the commander or pilot in command of the aircraft. The big boss when the aircraft is flying.

CRS        – Certificate of Release to Service – a document stating that all maintenance activities have been successfully completed (or legally deferred) and the aircraft is ready for commercial service again.

CSN       – Cycles Since New – the number of flight cycles (number of landings) an aircraft or aircraft component has performed since manufacture.

CSO       – Cycles Since Overhaul – the number of flight cycles, which a component or a part thereof has performed since the last overhaul. Applicable particularly to landing gear and engine parts.

CSSV     – Cycles Since Shop Visit – as many components do not have strict overhaul times, this acronym is sometimes used to describe the number of cycles a component has performed since the last shop visit (major repair, restoration, etc.).

CVR       – Cockpit Voice Recorder – a device which records the sound from the cockpit. It’s main purpose is to be source of information after fatal aircraft accidents.


D&B       – Dent and Buckle Chart – a list of all external (visible) structural defects of an aircraft. It contains a detailed drawing of the fuselage, wings, empennage and engines and marks every allowable dent, scratch and other damage. It also make note of all visible repairs which have been performed on the aircraft. The chart is accompanied by a list which shows that the damage is in allowable limits. The purpose of carrying the D&B in the cockpit is to ensure that pilots and line station mechanics can determine whether a dent is new and requires evaluation or whether it has been already evaluated in the past.

DGR       – Dangerous Goods Regulations – regulations describing in what condition certain dangerous goods can be carried on board of an aircraft. This includes weapons, ammunition, pressurized containers, toxic substances, and many others.

DOA      – Design Organization Approval – an approval (in accordance with EASA Part 21) which needs to be held by organizations which design modifications or repair schemes for aircraft.

DOW     – Dry Operational Weight – the dry operational weight of the aircraft. Dry means with no fuel included.


EASA     – European Aviation Safety Agency – a European international body, which regulates aviation issues. EASA prepares drafts of new regulations, which are then passed by the European Commission. Once this happens, those regulations have to be applied by all European member states.

EDP        – Engine Driven Pump – a fuel or hydraulic pump which is operated directly by engine power, through the accessory gear box.

EEL         – Emergency Equipment List – a drawing (schematic) of the location of all emergency equipment on board of an aircraft. It specifies the location of equipment such as fire extinguishers, oxygen bottles, medical kits, ropes, etc. The drawing also contains a list of all the equipment, specifying part numbers to be used if a replacement is necessary.

EFB        – Electronic Flight Bag – an electronic system, being either a part of the aircraft’s system or as a standalone tablet, which contains all the flight paperwork in electronic format. This may be limited only to digital access to all crew manuals or may be significantly more advanced, in particular by including a digital version of the Technical Log Book. In this case, almost all paper documentation is removed from the flight deck.

EGT        – Exhaust Gas Temperature – the temperature of the gases leaving the engine (engine exhaust). This is a very basic parameter to determine the deterioration state of an engine.

ELT         – Emergency Location Transmitter – a homing device used during emergencies. Modern ELTs turn on automatically during a crash and start to transmit a signal both on 121,1 MHz and 406 MHz, which can be intercepted by satellites. The purpose of the ELT is to allow search and rescue teams to find the wreckage of an aircraft. Normally, there is a fixed ELT which is part of the aircraft and also one or two portable ELTs, which can be removed by survivors from the aircraft.

EO          – Engineering Order – a document used by airworthiness management organizations (CAMOs) to order specific work from a maintenance organization. Although the full purpose and shape of an EO is determined by internal CAMO procedures, EOs are generally used to order uncommon maintenance work, not included in the AMM or other regular work instructions,  such as a modification.

EOL        – End of Lease – often assumed to be the end of lease check. This is process of terminating an aircraft lease with one lessee and, typically, transferring the aircraft to a new lessee. This process often includes a heavy maintenance check, referred to in short as the EOL.

ESN        – Engine Serial Number – the serial number of a given engine, the only way to distinguish between different engines of the same type.

ETA        – Estimated Time of Arrival – the time at which we expect an aircraft to arrive.

ETD        – Estimated Time of Departure – the time at which we expect an aircraft to depart.

ETOPS   – Extended-range Twin-engine Operational Performance Standards – a set of standards which allow an aircraft to fly to a greater distance away from the nearest airport, in particular during flights over sea. A full ETOPS flight may operate as far as 180 minutes flight time away from nearest land. ETOPS is a full set of standards, which encompass the aircraft design, specific additional aircraft maintenance tasks, special crew training and specific operating procedures within the airline. An operator must be approved by the aviation authority in order to fly under ETOPS.

EW         – Empty Weight – An aircraft’s empty weight, with all technical fluids and all loose and additional equipment removed.

EWIS     – Electrical Wiring Interconnection System – an acronym used to describe the wiring system on an aircraft. EWIS training is mandatory for mechanics and technical management to ensure that particularly during zonal inspections, proper care is taken to inspect the wiring as a whole – including claps, braces, nicks on wires etc. – rather than focusing only on a single wire or set thereof.

EZAP     – Enhanced Zonal Analysis Procedure – the term is somewhat connected to EWIS. The idea of EZAP is to enhance the way zonal inspections are performed. It stresses the importance of all items / parts present in a given zone with particular attention to electrical wires and wire bundles, their attachments and connectors. The overall idea behind EZAP is to ensure that a zonal inspection is not just a “brief look” but a quite detailed process.


FA          – Flight Attendant

FAA       – Federal Aviation Administration – the US national aviation authority.

FADEC  – Full Authority Digital Engine Control – this is a digital device (a “computer” as some would say) which controls the operation of an engine. The FADEC is responsible that a simple input from the pilot or autopilot (such as a movement of the thrust lever) corresponds to appropriate engine behavior. In particular, the FADEC must adjust the engine fuel injection to current temperature, pressure and other environmental factors. It is very important and also very expensive aircraft component.

FAP        – Flight Attendant Panel – a panel, normally placed forward of the cabin, not far from the cockpit, which flight attendants use to control the cabin. This includes automated flight announcements, temperature control and cabin lighting. It also steers the inflight entertainment system.

FC           – Flight Cycle – a flight cycle is one airborne operation, which includes a take-off and a landing. Normally one flight is one cycle, but a training flight involving several “touch-and-go’s” may bring in several cycles. Cycles are used to measure the life of selected life limited components (for example parts of the landing gear) and constitute, together with flight hours, the life of an aircraft.

FCOM   – Flight Crew Operating Manual

FDM      – Flight Data Monitoring – the process of obtaining flight data for the purpose of technical analysis. Based on such data, it is possible to determine what problems exist or will shortly exist on an aircraft. It is also used by some airlines to monitor the way pilots operate the aircraft. The most basic flight data monitoring is done by modern flight data recorders, although some of them only keep records of the last flight. Modern FDM systems record data from a series of flights and allow for easy access to the data by means of a QAR (quick access recorder). The QAR allows to download the data to a portable drive or, in some cases, transmits it wirelessly to the airline technical quarters.

FDR        – Flight Data Recorder – the actual name of the infamous “black box” (which is actually orange rather than black). The FDR records a large set of flight parameters. Its purpose is to keep those parameters saved in case of an accident, particularly a crash. The FDR is designed in a way which should allow it to withstand the crash forces, immersion in water, etc. It is also equipped with an underwater locator beacon, which helps search and rescue teams to locate the unit under water.

FH          – Flight Hours – the hours an aircraft or component has spent flying. Flight time is calculated from take-off until touch down. This is different from so-called block time, which is calculated from the moment the aircraft starts the engines to the moment when they are shut down again. Flight Hours are the primary parameter used to calculate an aircraft’s life.

FL           – Flight Level – a description of altitude for aircraft flying in high altitude passage ways. During regular commercial flight, the altimeter pressure altitude is set to one value for all aircraft around the entire globe and flight levels, rather than feet, are being used. For example, an aircraft flying at FL280 is actually flying at 28000 feet at standard pressure altitude.

FMS       – Flight Management System – a digital system on board of the aircraft used for overall flight management. Some features of the FMS are also useful for maintenance staff and inform of issues which may appear in selected aircraft systems.

FO          – First Officer – the “second pilot” or “co-pilot”, sitting in the right seat of the aircraft. Typically, it is the first officer who is actually flying the aircraft, and not the captain.

FTS         – Fuel Tank Safety – a term closely related to CDCCL. A set of information about the maintenance and operating safety of aircraft fuel tanks. Fuel tank safety was given special attention after the disaster of flight TWA 800. Fuel tank safety training, and level 1 and 2, is required by maintenance technicians and maintenance managers alike.

FWD      – Forward – the opposite of aft, the front section of the aircraft.

GA         – General Aviation – the sector of aviation, which does not deal with commercial air transport. Typically, general aviation refers to small aircraft used for private purposes. However, a private jet used by a jet charter company is not considered to be general aviation.

GND      – Ground – the term is used on several occasions. By flight crew, this is the frequency of the ground controller of the airport. In maintenance, everything that is ground related.

GPU      – Ground Power Unit – an electric generator, typically a cart with a diesel engine, used to provide electrical power to the aircraft while it is on the ground. The GPU is connected via a GPU socket to the electrical system of the aircraft and provides required electrical power. On modern airports, the GPU is not a cart anymore but rather a cable coming from the airport’s electrical system.

GSE        – Ground Support Equipment – equipment which is used on the ground to assist in handling and maintenance actions on the aircraft. This can be a set of stairs, a crane, a jack or just about anything else.


HPC       – High Pressure Compressor – this is the compressor in a turbine engine, which takes in air already compressed by the previous, low pressure, compressor and compresses it even further. When looking at the cross section of an engine, this would be the compressor closest to the combustion chamber.

HPT        – High Pressure Turbine – the high pressure turbine is located directly behind the combustion chamber of a turbine engine. It is driven by high pressure gas created from the combustion of fuel in the engine and powers the high pressure compressor.

HT          – Hard Time – The term refers to hard time components. A hard time components requires repetitive maintenance actions performed at strictly defined intervals. This can be the need to reweigh a fire extinguisher bottle or recharge the aircraft batteries. Each maintenance event on a component with a fixed interval is a “hard time”.

HUD      – Head Up Display – a see-through display in the cockpit. It is placed in front of the pilots eyes and allows him or her to see through the windshield while, simultaneously, being able to read information from the display. In the past a HUD could be found only in fighter aircraft and nowadays is present in most modern jets.

HYD       – Hydraulic – either fluid, system, component or whatever else may be hydraulic.


IB            – Inboard – used to describe the location of selected items with respect to the aircraft fuselage. For example, on an aircraft with two engines under each wing, the engine closest to the fuselage would be called an “inboard” engine.

IAS         – Indicated Air Speed – the air speed indicated by the pressure airspeed indicator.

IATA      – International Air Transport Association – an international body, which derives standards for the aviation industry worldwide. IATA aims more at commercial, rather than technical, aspects of the air transport business.

IAW       – In Accordance With… – an acronym used very often in aviation, as all maintenance tasks need to be performed in accordance with specific instructions.

ICA         – Instructions for Continued Airworthiness – a supplement to “regular” airworthiness instructions such as the MPD or AMM. An ICA is often found as an appendix to a Part 21 modification. The purpose of the ICA is to describe additional airworthiness requirements caused by the modification, such as additional scheduled inspection requirements.

ICAO     – International Civil Aviation Organization – an international aviation organization, which works under the cover of the United Nations. ICAO was established in 1944 and its goal is to pursue unified aviation regulations, in particular concerned with aviation safety, throughout the world. ICAO publishes its Annexes, which are a set of guidelines (similar in form and matter to national regulations) for several aspects of air transport. The regulations from national aviation authorities, such as the FAA and EASA, are based largely on ICAO standards.

IFE          – In-Flight Entertainment (System) – the IFE constitutes everything in an aircraft which has been put in place to provide entertainment to the passenger. In simple systems, this would mean only the audio system and some extendable screens, whereas in sophisticated systems this is the audio-visual system in each seat and all the features and perks of a first class suite seat.

IFSD       – In Flight Shutdown – a situation in which an aircraft engine needs to be shut down in flight. This shutdown may be self-inflicted (the engine just stops to work) or induced by the flight crew (shut down of an engine due to fire). This is, of course, a critical situation and treated very seriously by airlines and oversight institutions.

IOSA      – IATA Operational Safety Audit – and audit of an airline the purpose of which is to confirm that the operator confirms to all IATA requirements. Although the audit is not mandatory (IATA is not a regulatory body), most major airlines conform to those standards and also require their subcontractors (such as code share or franchise airlines) to conform as well. The IOSA is performed by a third party company, and ensures that all manuals and procedures within an airline meet the strict standards of IATA.

IPC         – Illustrated Parts Catalogue – a publication, created by the aircraft manufacturer, which lists all the parts which can be found on the aircraft. The listing is divided into ATA chapters and provides part numbers for every part and component which may be replaced during regular aircraft maintenance. In most cases, the IPC will also list the data of the component vendor, so that airlines can easily source a given part.


LCC        – Low Cost Carrier – an acronym used to describe no frills airlines, whose business model relies on the making the journey as cost efficient as possible.

LDND    – Last Done / Next Due – a list of all maintenance tasks applicable to a given aircraft outlining when any given task has been last performed and when it is next due. The LDND is based on the approved maintenance program of the airline and constitutes one of the most crucial documents, showing the current maintenance state of an aircraft.

LE           – Leading Edge – the forward (front) edge of a flight surface, such as a wing or horizontal and vertical stabilizer.

LG          – Landing Gear – the set of components which constitute an aircraft’s landing gear.

LLP         – Life Limited Part – a part or component, for which a life limit has been set by the manufacturer. A life limit, unlike a hard time requirement, means the necessity to scrap (destroy) a given part. In modern aircraft, there are only a few life limited parts installed – mainly in the engines and the landing gear. However, the life limits are sacred and may not be, in any circumstance, overflown. Determining the actual life value for components of older aircraft is often a big issue and is related to large amounts of money paid for components, for which such a determination could not be successfully made.

LOI         – Letter of Intent – the first document which is signed prior to an aircraft entering into an operating lease with a new lessee. The LOI specifies vaguely the condition of the aircraft and the financial requirements, but also determines that the final lease will take place once all assumptions have been met. The LOI is a simple document, but it constitutes the basis for the final lease agreement.

LOPA     – Layout of Passenger Accommodations – a drawing, which shows the exact location of seats, galleys, lavatories etc. of an aircraft. A LOPA has to be Part 21 approved and cannot be diverted from without an approved modification. It also lists the part numbers and manufacturers of the seats, galleys and lavatories.

LPC        – Low Pressure Compressor – a part of a turbine engine, the low pressure compressor takes in air at outside air pressure and compresses it to an initial value. This value is normally too small for a turbine engine to operate efficiently and is then passed into the high pressure compressor. The low pressure compressor is located at the very front of a turbine engine (in case of a turbofan engine, just behind the fan).

LPT         – Low Pressure Turbine – a part of a turbine engine, the low pressure turbine is located at the very end of the gas path of an engine. It is being run by the exhaust gas, and is placed after the high pressure turbine. The low pressure turbine transmits its torque via a shaft to the low pressure compressor.

LRU        – Line Replaceable Unit – any component of an aircraft or engine which can be replaced during line maintenance. This constitutes complete components, like avionic units, whole engines or engine components like the FADEC.

LW         – Landing Weight – the weight at which an aircraft can land (dependent on the payable load and the amount of fuel in the tanks). The LW cannot be exceeded, which is why, in emergency situations, aircraft need to drop fuel before landing.


MAC      – Mean Aerodynamic Chord – an aerodynamics value. It constitutes something like the average chord of the airfoil (the cross-section shape of the wing). The MAC is a very important value for the aerodynamic behavior of an aircraft. Also, the center of gravity location is often provided as a percentage of the mean aerodynamic chord.

MDDR   – Maintenance Deferred Defect Record – a task card or another form of technical record which lists deferred maintenance tasks, explains why the tasks have been deferred and for how long. Depending on the airline’s record keeping system, an MDDR may also contain the rectification information once the deferred defect has been cleared.

MEL       – Minimum Equipment List – a list of aircraft components or systems which may be inoperative during flight. The list shows what may be defective and also a set of limitations which are imposed on a flight due to the defect. The list is derived from a master minimum equipment list, which is published by the aircraft manufacturer, but it can and should be updated by an airline and must be approved by the national aviation authority.

MMEL   – Master Minimum Equipment List – a generic version of the MEL, published by the aircraft manufacturer. Every MEL will base on the MMEL.

MFD      – Manufacture Date – the date an aircraft, engine or component has been manufactured. Typically, the date of the C of A or the export C of A is used to determine the actual manufacture date of an aircraft.

MOD     – Modification – any technical modification done to an aircraft. Minor modifications may be only Part 21 approved, whereas major modification normally require a supplemental type certificate.

MOE      – Maintenance Organization Exposition – a manual outlining all the procedures implemented by a maintenance organization (in EASA countries – approved in accordance with Part 145). The MOE contains detailed description of how the organization intends to comply with all relevant regulations and perform its work. The MOE is a basis for Part 145 certification.

MPD      – Maintenance Planning Document – a document issued by the aircraft manufacturer, outlining all maintenance tasks and maintenance intervals applicable to a given aircraft. The MPD is a basis for an airline’s approved maintenance program (AMP), although airlines can change some of the MPD requirements based on their own user experience and own operating conditions. Such changes must be approved by the local aviation authority.

MRB      – Maintenance Review Board – a board of experts which works on approving and creating methodologies for the creation of MPDs (Maintenance Planning Documents). Those groups define intervals for maintenance tasks based on their experience and research data provided by the manufacturers.

MRO     – Maintenance and Repair Organization – an organization which is certified to perform maintenance and repair on an aircraft, engine or aircraft component. In Europe, such organizations must be approved in accordance with Part 145.

MSN      – Manufacturer’s Serial Number – the serial number of an aircraft or aircraft component. The most used parameter to define a specific aircraft in a fleet or a part in stock .

MTOW – Maximum Take-Off Weight – the maximum weight of an aircraft allowable for take-off. An important parameter for flight safety, but also for operational planning. Aspects such as landing fees are often based on the MTOW of an aircraft.


NDT       – Non Destructive Testing – maintenance testing techniques which aim to discover defects in a material (metal or composite) without destroying the material. Those include tap testing, eddy current and ultrasonic inspections or x-ray pictures of aircraft structure.

NIS         – Non Incident Statement – a statement issued by operators after they have finished using an aircraft or major component (such as an engine or landing gear) to proof that the aircraft or component has not been involved in any major incident or accident, has not been subject to fire, immersed in salt water or used in military operation. A NIS is required for back to birth traceability of life limited parts and also required by insurance companies as proof that a given aircraft or component has not been originally damaged.

NRC       – Non-Routine Card – a task card issued during a maintenance check for non-routine work. NRCs are issued mainly when a schedule maintenance task (for example a zonal inspection) reveals a defect in a selected area, which cannot be rectified right away due to its complexity, lack of material, etc. The check package should contain a list of all non-routine cards raised in order to ensure that all maintenance tasks, including unscheduled maintenance, have been fully completed or correctly deferred before the aircraft is released back to service.


OB          – OutBoard – description of location with respect to the aircraft’s fuselage. An outboard engine is the engine furthest away from the fuselage.

OCCM   – On Condition / Condition Monitored components – all components other than hard time or life limited components. An OCCM component does not have any scheduled maintenance interval and its failure does not pose a threat to flight safety. OCCM components are replaced once they fail, and their condition is monitored by regular system tests and inspections.

OHM     – OverHaul Manual – a manual used by maintenance organizations (maintenance “shops”) during the overhaul of components. In particular large and complex components, such as engines, APU and landing gear, will have their own, dedicated overhaul manual. For less complex components, overhaul instructions may be included in the component maintenance manual (CMM).

OEM      – Original Equipment Manufacturer – the original manufacturer of an aircraft, engine or component. The OEM in aviation will be a company which holds the type certificate (TC) for the aircraft or engine. An OEM has several responsibilities, including the publication and updating of all required manuals, approving repair designs, issuing service bulletins, etc.


PAX       – Passenger – the word “passenger” seems to be too long for aviation personnel, and therefore the abbreviation PAX is being used.

PIC         – Pilot In Command – the captain of the aircraft. He or She has full authority on board during flight.

PIREP    – Pilot Report – Information provided by the flight crew during or after a flight. This may include operational information (for example about severe turbulence on the way) or, from a maintenance perspective, information about the technical state of the aircraft. Technical information is entered by the flight crew into the technical flight log, which is then reviewed by maintenance staff.

POA       – Power of Attorney – a legal document stating, that a given person has authority to act on behalf of a company in a given matter. This is required for example when lease agreements or acceptance certificates are being signed. It is crucial to know that the person signing such a document has sufficient authority to do that.

POH       – Pilot’s Operating Handbook – an operating manual carried on board of the aircraft, containing inflight procedures for the flight crew.

PSU       – Passenger Service Unit – the unit located above a passenger’s head in an aircraft. The unit is generally comprised of oxygen masks with a chemical oxygen generator, air outflow sockets, flight attendant call button and reading lights.

PTF        – Permit to Fly – a document issued by the national airworthiness authority or a CAMO holding a specific approval, which allows an aircraft to be flown despite a defect or other state which normally renders the aircraft not airworthy. A PTF can be used if an aircraft defect arises at a location, at which there is no maintenance facility available to rectify it. In this case, if the aircraft cannot be flown based on available manuals and data, an application for a permit to fly can be made in order to reposition the aircraft to an airport with a proper maintenance facility.


QA         – Quality Assurance – a set of tasks performed to ensure that all processes within an organizations are carried out in accordance with the aviation regulations and internal company procedures. Today, quality assurance or quality monitoring is often referred to as compliance monitoring.

QAR       – Quick Access Recorder – a unit used for flight data monitoring. It records several parameters of each flight and makes them accessible for maintenance staff. “Quick access” means that the data can easily be accessed without removing the unit from the aircraft either by means of a memory card, a USB connection to a laptop, or – in more modern units – wirelessly.

QM        – Quality Manual – a manual present in most certified organization, both Part M and Part 145 approved. The Quality Manual describes quality procedures present in a company and shows how quality assurance (nowadays referred to as compliance assurance) takes place. It’s somewhat similar to quality manuals known from ISO certified companies, but must comply to specific aviation regulations.

QRH       – Quick Reference Handbook – a manual carried on board and easily accessible to the flight crew. The QRH contains procedures for regular and emergency situations during flight. It lists them in a form and manner which is very easy to use and very clear. This is done so that the flight crew can easily find what they need even in a very stressful situation like a flight emergency.

QTY        – Quantity – just an abbreviation used for the word “quantity”. Can be found in many situations, particularly with respect to aircraft parts.


RAT        – Ram Air Turbine – a small turbine which can be extended during flight in order to provide electrical power in case of emergency. In practice, this is a small fan which is made to rotate by the air flowing past the aircraft’s fuselage. It powers a small generator which provides some electrical power to the aircraft’s systems thereby powering most crucial avionic components.

REV        – Revision – shortened version of the word revision. All aviation manuals, both operational and technical, are being regularly updated by the authorities who issue them. Special care must always be taken that all new revisions are properly distributed and that each person is using the latest revision of each document at any given time.

RTS        – Return to Service – the process of returning an aircraft back to service after maintenance activities. During a heavy check this includes completing the check work package, ensuring that all tasks have been completed and properly signed off and issuing the final certificate of release to service (CRS).


SATIS    – Satisfactory – an acronym used often by maintenance staff after performing a test which has been passed. For example “Tire pressure check performed. SATIS.”.

SB           – Service Bulletin – a technical document issued by the manufacturer (type certificate holder) of an aircraft or by a Part 21 design organization. An SB contains information on how to modify an aircraft or aircraft maintenance schedule. The modification can be safety related, but it may also be for the purpose of fixing a known, but not safety related problem or simply to satisfy the need of an airline to make a modification to their fleet (such as change of livery). A service bulletin is not mandatory. Those SBs which are safety related are often called up by an airworthiness directive, therefore becoming mandatory for a given fleet.

SN          – Serial Number – self-explanatory. The serial number serializes (defines) specific components of a given part number.

STC        – Supplemental Type Certificate – an STC supplements the original Type Certificate (TC) issued by the manufacturer of an aircraft or engine. The supplemental type certificate is issued after a major modification of an aircraft, which normally significantly changes the aircraft’s operational characteristics. A good example of an STC would be the installation of winglets on an aircraft which originally was not winglet equipped. STCs are issued by Part 21 organizations and the aircraft OEM and approved by EASA directly.

SV          – Shop Visit – a maintenance event on an aircraft component, such as an engine, landing gear or any other, less complex part. After a shop visit, the component is released back to service by means of a release certificate such as an EASA Form One.

SV          – Serviceable – the term “serviceable” or SV is used with regards to aircraft parts or components which are considered airworthy and ready to be installed on an aircraft. Those parts are normally accompanied by a serviceable tag, which is customary green in color and clearly shows the letters SV.

SVR        – Shop Visit Report – a report issued after a shop visit by the maintenance facility. The shop report details all work performed on a component. It is not mandatory to issue a shop visit report, although they are to be expected especially after work performed on complex components such as engines or landing gear.


TA          – Technical Acceptance – the process of accepting an aircraft. Either by a lessor (after a lease term with a previous lessee has come to an end) or by an airline starting a new lease. The TA, which is finished by signing a technical acceptance certificate, is crucial part of starting and/or terminating a lease.

TC           – Total Cycles – the number of total cycles (landings) accrued on the aircraft or an aircraft component.

TC           – Technical Consultant – a person, often a freelancer, hired by lease companies and operators to assist in the transition process of an aircraft (movement from one lessee to another).

TC           – Type Certificate – a certificate issued by the manufacturer of an aircraft or engine, which needs to be approved by an appropriate aviation authority. The Type Certificate details the technical and operating specifications of an aircraft and confirms that the aircraft confirms to all design requirements.

TE           – Trailing Edge – the aft (back) edge of flight surfaces (wings, horizontal and vertical stabilizers).

TLB         – Technical Log Book – The TLB has two main meanings. The most literate meaning is a log book, which is used to record details of every flight performed on an aircraft together with maintenance activities which took place after each flight. A technical log book will contain details of the flight (departure and arrival airports, flight crew names, flight time and cycles, date, flight number, etc.) and maintenance (defects found, specifics of the rectification actions, all scheduled maintenance, oil and fuel uplift). Another meaning of the Technical Log Book is a system of documents, which includes the log book mentioned above, but also several other documents such as the MEL, CDL, HIL and others. In common language, the first meaning is prevalent.

TR           – Thrust Reverser – a part of a turbine engine, which changes the passage route of the engine exhaust in a way which reverses the thrust of the engine. Thrust reversers act as “brakes” for the aircraft – they are engaged after the aircraft touches down on a runway to rapidly slow it down. This is accompanied by a fairly loud sound and therefore some airports limit the ability to use thrust reversers in which case only aerodynamic and hydraulic brakes can be used.

TSN        – Time Since New – the amount of hours an aircraft or component has flown since manufacture.

TSO        – Time Since Overhaul – the amount of hours an aircraft component, such as an engine, has flown since the last major maintenance event, referred to as an overhaul.

TSSV      – Time Since Shop Visit – basically the same thing as TSO. Modern engines often don’t have a maintenance event referred to strictly as an overhaul. Rather, they have different shop visits known as performance restorations, the scope of which is not completely known beforehand. Therefore, sometimes the term TSSV is used instead of TSO.

TT           – Total Time – similar to TSN, this is the total amount of flight hours accrued on an aircraft or aircraft component. This is a main parameter used to describe the age of an aircraft.


ULB        – Underwater Locator Beacon – a device mounted to the flight data recorder (“Black box”) and the cockpit voice recorder. The ULB turns on when it comes in contact with water and sends a signal which can be tracked by search and rescue teams. The purpose of the ULB is to make the retrieval of the FDR and CVR from underwater locations possible.

US          – Unserviceable – a term used to describe aircraft components which for whatever reason cannot be installed on an aircraft. Those could be components which are defective or which are time expired – meaning they require specific maintenance actions to be performed before they can be installed on an aircraft. A component would typically be tagged with a red tag, clearly showing the letters US.

UTC       – Universal Time Coordinated – a notion of time used to ensure a steady timeframe in aviation without regard to different time zones and summer / winter time differences. UTC is often referred to as Z (“zulu”) time.


W&B     – Weight and Balance – a document, carried on board of an aircraft, which specifies the weight of the aircraft and the location of its center of gravity. The W&B chart is a basis for calculating the loading of an aircraft for each flight. It is derived (at least once every four years) by physically weighing the aircraft and needs to be updated after significant modifications of the aircraft (such as interior layout changes or aircraft repainting).

WX         – Weather – an acronym used for the term “weather”.


XPDR     – Transponder – a digital unit on board of an aircraft used to transmit information about the flight (like position, heading, airspeed, etc.) to air traffic control. During each flight, an aircraft is asked by the ATC to enter a transponder code (referred to as Squawk) which allows the ATC to track which signal is coming from what aircraft.

X-REF    – Cross Reference – a broad term used to describe the cross reference between different information. For example, this could be table showing which Part M regulations are described in which chapters or pages of the CAME.


The glossary above has been prepared by me, based on my experience. If you miss any acronym which you would like to see in this glossary or believe that I may have gotten something wrong – please comment or drop me a line. I would like to keep this aviation glossary as current as possible, but this will not be easy without your help.

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