Definition
Deferral is a permission, not a judgement call. The distinction between a deferred defect and an ignored one is whether an approved basis exists, whether it has been applied correctly, and whether the record says so.
The bases differ by regime and operation type.
The **Minimum Equipment List** is the principal route for operators who hold one. Derived from the manufacturer's Master MEL and approved for the specific operator, it lists items that may be inoperative, the conditions attached — placarding, an operational limitation, a maintenance procedure — and the rectification interval, conventionally categorised A through D, within which the repair must be completed.
A **Configuration Deviation List** covers missing secondary airframe parts rather than inoperative systems, with its own conditions and penalties.
For aircraft operated without an approved MEL — which includes most general aviation training aircraft — the route is different and considerably narrower. Under 14 CFR 91.213(d) an aircraft with inoperative instruments or equipment may fly only where the item is not required by the type certificate, by the applicable airworthiness regulations, by an Airworthiness Directive, or by the equipment list for the kind of operation, and the item must be removed or deactivated and placarded INOPERATIVE. That is a permission to fly with a defect, but it is not a deferral system: there is no interval, no category, and no approved list to point at.
Three obligations attach to every deferral regardless of basis.
**The conditions must actually be applied.** A deferral that requires a placard and an operational limitation is only valid once both exist. A placard promised and not fitted invalidates the deferral.
**The interval must be tracked.** A category B item deferred and forgotten becomes an unairworthy aircraft on a specific date, with nothing announcing the transition.
**The commander must know.** The point of recording a deferral in the technical log is that the next person to accept the aircraft understands what is inoperative and what limitations apply.
The cumulative risk is the one most often missed: several individually acceptable deferrals can combine into an aircraft that is legal on paper and materially degraded in practice, particularly where the items interact.
Why It Matters for Flight Schools
For a school, deferral is the mechanism that keeps a serviceable-enough aircraft earning rather than grounding it for a broken landing light. Used carelessly it becomes a way of postponing maintenance indefinitely, one justified decision at a time.
The practical risk is drift. Each individual deferral is defensible; the aircraft carrying six of them is not the aircraft the school believes it is operating, and students are being taught in it. Training aircraft make this worse, because the people accepting them are often the least experienced people on the field and least equipped to judge what a combination of deferred items means.
The administrative risk is the expiry date. A rectification interval that nobody is tracking converts a compliant deferral into an unairworthy aircraft silently, and the first indication is usually an audit finding rather than an alert.
How Aviatize Handles This
Aviatize tracks open squawks alongside Airworthiness Directives, Service Bulletins and maintenance instructions per aircraft, and limits that aircraft's availability to the schedule when its status requires it — so a defect raised against a tail is visible where bookings are made rather than living on a sheet at the hangar.
Defects can be reported from the mobile app at the aircraft, and the resulting work is planned as work orders and work packs with time, parts and tools recorded against it. The validation engine lets a school decide what a booking must satisfy before it is confirmed, including maintenance conditions, and whether a failed check warns or blocks — which is the control that stops an aircraft being dispatched against a condition the school meant to enforce. Aviatize records and surfaces these states; the airworthiness decision and the deferral basis remain the operator's and the certifying engineer's.
Frequently Asked Questions
- What is a deferred defect?
- A known fault that the operator is permitted to leave unrectified for a defined period while continuing to fly, under a documented basis such as an approved Minimum Equipment List, with the required conditions applied and a rectification interval set. Without such a basis and record, the defect is not deferred and the aircraft is unairworthy.
- Can a flight school defer defects without an MEL?
- Not in the same sense. Under 14 CFR 91.213(d) an aircraft may fly with inoperative equipment only where the item is not required by the type certificate, the airworthiness rules, an Airworthiness Directive or the equipment list for that operation, and the item is removed or deactivated and placarded INOPERATIVE. That permits flight with a defect but sets no interval and no category — it is not a deferral system.
- What are MEL rectification intervals?
- Categories, conventionally A to D, that set how long an item may remain inoperative before it must be repaired. Category A carries an interval specified in the MEL itself; B, C and D correspond to progressively longer fixed periods. The interval runs from the deferral and has to be tracked, because an expired deferral makes the aircraft unairworthy on a specific date with nothing announcing it.
- What is the risk of carrying several deferred defects?
- They combine. Each may be individually acceptable while the aircraft as a whole is materially degraded, especially where the items interact. In a training fleet the risk is sharper, because the pilots accepting the aircraft are often the least experienced people on the field and least able to judge what a combination of inoperative items means in practice.