Seven technical and mechanical reasons flights get delayed are less about broken parts than about what the airline is allowed to do with the defect. The MEL repair category, the station’s repair capability, and crew duty-time limits set how long a delay runs.
The single most useful takeaway: a defect’s legal deferral window and where the aircraft is stuck matter more than which component failed.
This is not a list of parts that break. It is a list of the documented decision points that turn a technical defect into 40 minutes or four days. The same broken component can produce a short delay at a hub and a cancellation at an outstation.
Every item below traces to FAA advisory material, EUROCONTROL delay-code definitions, 14 CFR Part 117 duty rules, or the Van der Lans judgment. Nothing is ranked by faulty-part frequency, because no public carrier data supports that.
At a glance
- 1. The MEL category, not the broken part, sets the clock
- 2. The station’s repair capability decides whether you wait hours or days
- 3. The logbook and placard are the legal tripwire, not paperwork
- 4. IATA codes 41 and 42 split defects from late maintenance
- 5. A repair that runs long becomes a crew duty-time problem
- 6. In the EU, most mechanical delays are not extraordinary circumstances
- 7. A defect outside the MEL needs extra sign-off before it can fly
Seven factors that decide a mechanical delay
1. The MEL category, not the broken part, sets the clock
A Category D defect can legally wait 120 consecutive calendar days before repair.
The federally approved Minimum Equipment List (MEL) tells the carrier which components may be inoperative while the aircraft stays airworthy. That list assigns each deferrable item a category — A, B, C, or D — and the category, not the failed component, starts the operational clock.
The window for Category A is written case by case into the MEL entry. Category D is the longest of the four, so a minor cabin item can stay deferred far longer than a fail-critical system carrying a tighter window. The FAA’s advisory material sets the repair intervals, and the flight cannot legally dispatch beyond them.
| Code/Category | Scope | Maximum deferral window | Operative rule or definition |
|---|---|---|---|
| MEL Category A | Specific deferred equipment item in an operator’s approved MEL | Interval defined case-by-case in MEL entry; calendar-day intervals exclude day of discovery | Repaired within interval specified in MEL “Remarks or Exceptions”; clock starts at discovery or deferral point |
| MEL Category B | Deferred instruments or equipment listed as Category B | 3 consecutive calendar days (72 hours), excluding day of discovery | Repaired within three consecutive calendar days; cannot dispatch beyond window without authorized extension |
| MEL Category C | Deferred instruments or equipment listed as Category C | 10 consecutive calendar days (240 hours), excluding day of discovery | Repaired within 10 consecutive calendar days, excluding day recorded in maintenance log |
| MEL Category D | Deferred equipment judged minimal safety impact or classified optional | 120 consecutive calendar days (2,880 hours), excluding day of discovery | Repaired within 120 consecutive calendar days, excluding day of discovery; longest MEL deferral interval |
| IATA delay code 41 (TD) | Technical delay attributed to aircraft defects | No fixed time window | Applied when aircraft defects require repair before departure |
| IATA delay code 42 (TM) | Technical delay caused by scheduled maintenance running over | No fixed time window | Applied when planned maintenance releases aircraft late |
| Source: Federal Aviation Administration (FAA); EUROCONTROL Performance Review Unit | |||
2. The station’s repair capability decides whether you wait hours or days
No major airline publishes a standard rule for whether to fly in a part or reroute the aircraft when a defect appears at an outstation.
EUROCONTROL breaks its delay-cost figures out by phase of flight — at gate, taxiing, en route, arrival — and stops there. Airlines do not split out the cost or duration of a tire change versus an avionics fault.
The same MEL framework appears in Asia-Pacific regulators such as CASA and Oman, but those authorities publish no equipment-failure share comparable to the FAA’s roughly 10%. Predictive-maintenance simulations reported by the American Institute of Aeronautics and Astronautics (AIAA) can cut cost and Aircraft-on-Ground time, but no public statistic links them to fewer surprise mechanical delays.
3. The logbook and placard are the legal tripwire, not paperwork
Before a defect can be deferred, it must be logged and the affected control placarded INOP.
The approved MEL list is not enough by itself. The FAA requires every technical delay and maintenance action to be formally recorded in the aircraft technical logbook, and an INOP placard tells the flight crew the component is out of service.
This is an airworthiness step, not an admin nicety. If the entry and placard do not match the MEL listing, the aircraft cannot be dispatched with that defect.
4. IATA codes 41 and 42 split defects from late maintenance
IATA delay code 41 means aircraft defects; code 42 means scheduled maintenance released the aircraft late.
The table above shows the official definitions. The split matters because a passenger sees both as “maintenance,” but the airline’s reliability counting treats them as separate events.
5. A repair that runs long becomes a crew duty-time problem
Under 14 CFR Part 117, a maintenance problem that develops before departure can add as much as two hours to a crew’s maximum flight duty period, provided the pilot-in-command and the certificate holder both agree.
That extension is not bottomless. Cumulative duty limits remain absolute, so a repair that burns through the margin can make the crew illegal to fly. At that point a mechanical delay becomes a regulatory one, and the defect is no longer the only thing determining departure.
6. In the EU, most mechanical delays are not extraordinary circumstances
A technical problem that arrives unexpectedly and is not tied to poor maintenance generally does not qualify as an “extraordinary circumstance” under EU261.
The European Court of Justice set that line in Van der Lans v KLM, interpreting Article 5(3) of Regulation (EC) 261/2004.
In that case, a KLM passenger flying Quito to Amsterdam was delayed almost 24 hours by a fuel-pump failure; the replacement part had to be flown from Amsterdam. The Court still treated the delay as ordinary operational risk, so KLM could not escape its compensation duty.
7. A defect outside the MEL needs extra sign-off before it can fly
If a defect sits outside the MEL, the configuration deviation list, or the structural repair manual, deferral requires two separate approvals — one from the regulator, one from whoever holds the type certificate — before the aircraft can fly.
The approved MEL covers many known items, but not every possible failure. If a fault sits outside those lists, the operator cannot simply placard and go. The manufacturer and the regulator need to sign off on a one-off deferral, which adds negotiation time.
That extra step can turn a small defect into a long outstation delay, even when the part itself looks minor.
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How we picked
Each factor is anchored in FAA advisory material, EUROCONTROL‘s IATA delay-code definitions, 14 CFR Part 117 duty rules, and the Van der Lans judgment. Every item describes a documented decision point or reporting threshold rather than a ranked list of part failures. The most recent incident cited occurred August 17, 2026.
Key terms
- Minimum Equipment List (MEL)
- The Minimum Equipment List is the FAA-approved document that names which components may be inoperative while an aircraft is still allowed to fly. Each deferrable item carries a repair category — A, B, C or D — and that category sets the deadline, from a case-by-case interval for Category A to 3, 10 and 120 consecutive calendar days for B, C and D, with the day of discovery excluded from the count. In this article’s terms, the category is what decides whether a defect costs you an hour at the gate or keeps the aircraft out of service for months.
- EU261
- EU261 is shorthand for Regulation (EC) 261/2004, the European Union rule that sets when carriers owe passengers compensation for delays and cancellations. Article 5(3) of the regulation supplies the “extraordinary circumstances” defence, which lets an airline off the compensation hook when a disruption lies outside its normal control. The practical effect for a carrier is that a mechanical defect is rarely a way out of paying: the compensation duty generally survives the fault.
- 14 CFR Part 117
- 14 CFR Part 117 is the US rule that sets flight and duty limitations and rest requirements for flightcrew members. It caps how long a crew may be on duty, and it lets the pilot-in-command and the certificate holder add up to 2 hours to the maximum flight duty period when unforeseen circumstances such as a late-developing maintenance problem arise before takeoff. That extension is a margin rather than a reset, which is why airlines treat a long repair as a crew-scheduling problem as much as a maintenance one.
- Configuration deviation list
- The configuration deviation list is one of the approved documents — alongside the MEL and the structural repair manual — that an operator checks before deciding whether a defect can be deferred. A defect that falls outside all three cannot simply be placarded and flown; the type certificate holder and the regulator must both agree to a one-off deferral. Because that sign-off runs between the airline, the manufacturer and the authority rather than the line engineer, it moves at the speed of a negotiation.
- Aircraft-on-Ground
- Aircraft-on-Ground, or AOG, is the industry term for an aircraft that is out of service and cannot fly until a part or repair reaches it. Recovery usually means sourcing the part and moving it to the aircraft, and public accounts of that playbook describe the logistics without disclosing any standard carrier rule for choosing between flying a part in and rerouting the aircraft. For a passenger, AOG marks the point where the airline stops fixing and starts replanning.
Questions? Answers.
What does it mean when a plane has mechanical issues?
A technical defect or a scheduled-maintenance overrun is delaying departure. IATA delay code 41 covers aircraft defects requiring repair before departure, while code 42 covers scheduled maintenance that releases the aircraft late.
What are the common reasons for flight delays?
The confirmed fact base does not rank one mechanical cause above others. EUROCONTROL publishes per-minute delay cost by flight phase, not by individual technical cause, and the industry separates aircraft defects (code 41) from scheduled-maintenance overruns (code 42).
What is the most common reason flights get delayed?
There is no verified single most common reason in the source set. The one widely cited figure — the FAA’s roughly 10% share of delays attributed to equipment failures — is US-only, and no comparable percentage has been published for other regions.
