Technical Log, MEL and Deferred Defects

Every aircraft develops defects: a light that no longer works, a pack that trips, a cracked panel. The technical log is where they are written down, where the maintenance done on them is certified, and where the next crew finds out what state the aircraft is in. The minimum equipment list (MEL) decides which of those defects the aircraft can fly with, for how long and under what conditions.

Together they answer the question every commander asks before accepting an aircraft: is it airworthy for this flight? The rules below are those of EASA air operations and continuing airworthiness, with the FAA's differences. Most light aircraft have no MEL, and for them the answer is simpler and stricter (see required instruments and equipment).

The aircraft technical log

The aeroplane technical log, also called the aircraft technical log or tech log, and in FAA airline rules the aircraft maintenance log, is a record kept for each aircraft. Under Part-M (M.A.306) an operator's technical log system holds, for each aircraft:

• information about each flight needed for continued flight safety;
• the current certificate of release to service, signed by certifying staff after maintenance;
• the current maintenance statement, showing which scheduled maintenance is next due;
• all deferred defects that affect the operation of the aircraft;
• any guidance on maintenance support arrangements.

The technical log is carried on board with the other flight documents, and the operator keeps it for 36 months after the date of the last entry. It is different from the journey log, which records the flights themselves (see aircraft registration and documents).

The crew's side of it is reporting. Crew members report to the commander any fault, malfunction or defect that could affect safe operation, and defects found in flight are written into the technical log. The FAA's airline rule, 14 CFR 121.563, makes the pilot in command ensure that every mechanical irregularity occurring during flight time is entered in the maintenance log at the end of that flight time, and check before each flight the status of each irregularity entered at the end of the previous one. Events that may have damaged the aircraft, such as severe turbulence, a heavy landing, a bird strike or a lightning strike, are logged too, because they trigger inspections. A defect that is not written down cannot be fixed, deferred or tracked.

Pre-departure checks

Before accepting the aircraft the commander reads the technical log: a valid release to service after the last maintenance, no scheduled maintenance overdue, and every open or deferred defect with its limitations. Between flights the aircraft receives the checks the maintenance programme requires, such as daily and transit checks, and the crew's own walk-round (see pre-flight preparation and routine checks).

Extended-range twin operations add a formal step. Under the FAA's rule for two-engine ETOPS, 14 CFR 121.374, a pre-departure service check (PDSC), tailored to the operation, must be completed immediately before each ETOPS flight. It verifies the condition of all ETOPS-significant systems and the overall status of the aircraft from its maintenance records, and an authorised, ETOPS-qualified signatory certifies it before the flight may begin (see ETOPS and extended diversion time operations).

Deferred defects

A deferred defect is one that is not rectified before the next flight but carried forward under an approved procedure. It is entered in the technical log's deferred defect section with its reference, for example the MEL item and category, and its due date. A fault for which the MEL provides relief is sometimes called a dispatchable fault.

Deferral is not the crew's decision alone. The commander may accept an aircraft only within the relief that the approved documents give, and may still refuse one that is legally dispatchable if the combination of defects, weather and route makes the flight unsafe. Crews review the deferred items before each flight, because a deferred item is an anticipated threat in the sense of threat and error management: a deferred thrust reverser, for example, matters little on a long dry runway and a great deal on a short wet one.

MEL structure and no-go items

The master minimum equipment list (MMEL) is produced for the aircraft type by the manufacturer and accepted by the authority. The operator's MEL is based on it, may be more restrictive but never less, and needs the approval of the competent authority (ORO.MLR.105). It forms part of the operations manual, in Part B. Four rules follow:

• The MEL applies up to the commencement of flight, the point at which power is applied for take-off, so it covers taxi. A failure after that is handled with the flight manual and the QRH, and the MEL governs the next departure.
• An airworthiness directive overrides the MEL where they conflict.
• Anything not listed in the operator's MEL as deferrable must be serviceable, whatever the MMEL allows.
• Missing secondary airframe and engine parts, such as a fairing or a static wick, are not MEL items: they are covered by the configuration deviation list (CDL) in the flight manual, which states the performance penalties that apply.

Each MEL entry gives the item, its rectification category, the number installed, the number required for dispatch, and the remarks or exceptions under which it may be inoperative, with any (M) and (O) procedures. An item that must be serviceable for dispatch, either because it is not in the MEL or because the number required equals the number installed, is a no-go item. Relief can depend on the operation: the time to the ETOPS en-route alternate may be limited by the aircraft's MEL status, and an inoperative item can remove low-visibility landing capability (see low visibility operations).

Without an MEL, the European rule for non-commercial flights in other-than-complex aircraft (Part-NCO) is simple: a flight may not start with any instrument or equipment required for it inoperative. The FAA's 91.213(d) allows flight with an inoperative item if it is not required by the aircraft's type certificate for VFR day flight, by 91.205 or another rule for the kind of flight, by an airworthiness directive, or by the equipment list or kinds of operations equipment list.

MEL (M) and (O) procedures

Many MEL items carry procedures, flagged in the entry:

• (M), maintenance procedure: the work that maintenance must do before dispatch, such as deactivating the item, securing a valve closed, pulling and collaring a circuit breaker, or testing that a backup works.
• (O), operational procedure: what the crew must do or accept, such as a performance penalty, a limit on altitude or weather, extra fuel, or a check before or during flight.

Both must be completed before the aircraft flies, and they remain in force for as long as the item stays deferred. The (O) procedures often change the numbers: a deferred thrust reverser removes the credit for reverse thrust on a wet runway, a deferred spoiler panel increases the landing distance, and an inoperative anti-skid system forces a lower V1 and a lower take-off mass. Electronic flight bags increasingly carry the deferred items straight into the performance calculation, as the A320's does when MEL and CDL items are selected. Other items change the operation: with one A320 bleed system inoperative the crossbleed valve is opened so that one engine supplies both packs, and some items restrict altitude or passenger numbers.

Rectification intervals

Each deferred item has a MEL rectification interval, the time within which it must be repaired:

The day on which the defect was recorded is not counted, and the FAA uses the same categories for airline MELs. When the interval runs out, the aircraft is grounded until the item is fixed. Under ORO.MLR.105, an operator whose procedure the competent authority has approved may extend a category B, C or D interval once, by no more than the interval itself and within the scope of the MMEL, when events beyond its control have prevented rectification; the authority is notified, a plan to fix the item at the earliest opportunity is made, and the extension is never a routine way of managing defects.

Deactivation and INOP placards

An inoperative item is usually deactivated so that it cannot be used by mistake or cause further damage, then placarded. The INOP placard is a label fixed next to the control or indicator concerned, warning each crew that the item is out of service. The FAA writes the requirement into 91.213(d): an item that may be inoperative without an MEL must be removed, with a maintenance record entry, or deactivated and placarded "INOPERATIVE", and a pilot or certificated mechanic must decide that it is no hazard. MEL maintenance procedures specify the deactivation for each item.

More broadly, a placard is any sign in or on the aircraft that states a limitation or instruction, such as a maximum baggage load or a flap limiting speed. Placards are part of the operating limitations and are as binding as the flight manual.

Dispatch with inoperative flight recorders

Flight data recorders and cockpit voice recorders are required equipment, but their failure does not directly affect the handling of the flight, so short-term relief is possible. Dispatch with an inoperative flight data recorder or cockpit voice recorder is allowed only under the operator's MEL, never on the crew's own judgement, and the relief is short. The UK CAA gives the flight data recorder as an example of where master MELs differ: an FAA MMEL may allow a 10-day repair interval, whereas EASA's CS-MMEL permits only 72 hours and/or 8 flights, and the CAA expects an operator's MEL to use the more restrictive limitation.

The MEL entry for the type states the exact conditions, for example whether the other recorder must be working, and the time or number of flights allowed. See flight recorders for what the recorders capture and why they matter to an investigation.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.