Definition
An MEL deferral is a procedure with defined steps, and each step has to be completed for the deferral to be valid.
**Confirm the item is listed.** The operator's approved MEL is derived from the manufacturer's Master MEL and approved for that operator and configuration. An item absent from it cannot be deferred under it, however minor it seems.
**Check the number installed against the number required.** MEL entries are expressed as quantities — how many are fitted, how many must work for dispatch. Two fitted and one required means one may be inoperative; it does not mean the system may be wholly unserviceable.
**Apply the conditions.** These divide into an **(O) operational procedure**, performed or observed by the crew, and an **(M) maintenance procedure**, performed by maintenance staff. Deactivation, placarding, a performance penalty or an operating limitation may all attach. A deferral whose conditions are not yet applied is not in force.
**Placard.** The inoperative item is placarded so the crew accepting the aircraft can see it without reading the paperwork.
**Record and categorise.** The deferral is entered in the technical log with its category, and the rectification interval starts.
The **categories** set the clock. Category A carries an interval specified in the MEL entry itself rather than a standard period — sometimes expressed in flights or flight hours rather than days. Categories B, C and D correspond to fixed periods, conventionally three, ten and one hundred and twenty calendar days, with the count starting the day after the defect is recorded. Where an authority permits an extension it is generally limited, usually to one further interval of the same length for category B and C items, and it is a permission to be applied deliberately rather than an automatic grace period.
Two things are commonly misunderstood. An MEL is **relief from the certification requirement for dispatch**, not a statement that the aircraft is unaffected; the performance or handling penalty attached to an item is real. And the MEL applies at **dispatch**, not in flight — an item failing after dispatch is handled by the flight manual and the commander's judgement, not by consulting the MEL.
For flight training, the practical point is that most general aviation training aircraft have no approved MEL at all, so this framework does not apply to them and the narrower relief in 14 CFR 91.213(d) governs instead.
Why It Matters for Flight Schools
For an operator holding an MEL, the discipline is tracking intervals as dates rather than intentions. Category B is three days; a deferral raised on a Friday and forgotten over a weekend is close to expiry before anyone looks at it again.
The conditions are where deferrals most often fail an audit. An (M) procedure requires a qualified person to have performed and recorded it, and a deferral resting on an (M) procedure nobody carried out is invalid from the moment it was raised, not from the moment it was discovered.
For most flight schools, the more useful conclusion is that they are not operating under an MEL at all. Believing otherwise is its own risk: applying MEL-style reasoning, with intervals and categories, to an aircraft whose only relief is 91.213(d) produces deferrals with no legal basis.
How Aviatize Handles This
Aviatize tracks maintenance status per aircraft — instructions, Airworthiness Directives, Service Bulletins and open squawks — and limits an aircraft's availability to the schedule when its status requires it, so a condition attached to an aircraft is visible where bookings are made rather than only in the hangar paperwork.
The validation engine lets a school define what must be satisfied before a booking is confirmed, including maintenance conditions, and choose whether a failed check warns or blocks. Work arising from a defect is planned as work orders and work packs, with technicians recording time, parts and tools from a tablet as they carry it out. Aviatize records and surfaces these states; determining whether an item may be deferred under an MEL, applying its conditions and certifying the aircraft remain the operator's and the certifying engineer's responsibility.
Frequently Asked Questions
- What are MEL rectification interval categories?
- Category A carries the interval stated in its own MEL entry, sometimes counted in flights or flight hours rather than days. Categories B, C and D correspond to fixed periods, conventionally three, ten and one hundred and twenty calendar days, counted from the day after the defect is recorded. Authority-granted extensions, where available, are limited rather than automatic.
- What do (O) and (M) mean in an MEL entry?
- (O) marks an operational procedure carried out or observed by the crew; (M) marks a maintenance procedure carried out by maintenance staff. Both must be completed and recorded for the deferral to be valid. A deferral resting on an (M) procedure that nobody performed was invalid from the moment it was raised.
- Does an MEL apply after dispatch?
- No. The MEL governs whether the aircraft may be dispatched with an item inoperative. An item that fails after dispatch is handled by the flight manual and the commander's judgement. The MEL is also relief from the dispatch requirement rather than a statement that the aircraft is unaffected — any performance or handling penalty attached to the item is real.
- Do flight schools have a Minimum Equipment List?
- Most general aviation training aircraft do not have an approved MEL, so this framework does not apply to them. Relief instead comes from the narrower provisions of 14 CFR 91.213(d). Applying MEL-style reasoning — categories, intervals, deferral records — to an aircraft with no approved MEL produces deferrals that have no legal basis.