A deferred defect is one that has been assessed against the Minimum Equipment List, found acceptable to carry, and given a rectification interval: a hard deadline by which it must be fixed. The interval is not advisory. An aircraft that flies past it is not airworthy.
A defect has five stages, and most operations record them in five places
It is raised, usually by a crew member or engineer, at the aircraft, often at the end of a long day.
It is assessed. Does it ground the aircraft or can it be carried? That is answered against the Minimum Equipment List, and sometimes the Configuration Deviation List, which set out what may be inoperative, under what conditions, and for how long.
If it can be carried it is deferred, with a rectification interval and a category attached.
It is rectified, by someone qualified, with parts carrying their own traceability.
It is closed, with a signature somebody may be asked to produce at audit years later.
Five stages. In a typical operation they touch a paper log, a phone call to maintenance control, a planning spreadsheet, a parts system and a filing cabinet. The defect is real throughout. The record is scattered, and the thing most likely to fail is not the engineering, it is the handover between two of those stages. The cost of that gap is covered in the real cost of delayed defect reporting.
The deadline is the expensive part
Most attention goes to the assessment, because that is where the engineering judgement sits. The money leaks somewhere less interesting.
An expiring deferral that nobody is actively watching becomes an aircraft that cannot depart. Discovered at the gate, it is a disruption with passengers attached. Discovered three days earlier, it is a line on a plan.
The difference between those two outcomes is not competence. It is whether the countdown is running somewhere that a human has to remember to look at.
Why a spreadsheet cannot hold this
Most MEL intervals run in consecutive calendar days, counted from the day after discovery: three for a Category B item, ten for Category C, a hundred and twenty for Category D. A Category A item carries whatever limit the list sets against it, which may be a number of flights, flight hours or cycles instead. And a deferral rarely sits on its own: the directive thresholds, life-limited part positions and interim repair limits around it run in hours and cycles, and those accrue as the aircraft flies.
A spreadsheet does not know the aircraft flew. Somebody has to tell it, and that someone is working from a report that is itself a day old. So the countdown in the spreadsheet is a countdown to a date that was correct when it was typed. The wider version of this problem is set out in why spreadsheets are sabotaging your maintenance operation.
There is also the component problem. When a component moves between tails, the directives and limits that follow it move too. The spreadsheet does not know the rotation happened, so its assessment is accurate only until the next one. Keeping that reconciled against the live fleet configuration is the job of CAMO management software.
What tracking it properly actually means
The defect is one record, not five. It is raised at the aircraft and linked to the logbook entry it came from. It is assessed against the MEL with its interval attached. The countdown runs on its own, against actual hours, cycles and calendar time rather than a date somebody typed. It is closed with an attributed signature that stays with it.
Everyone who needs to see it sees it from the moment it is raised, rather than after a phone call.
See how defect management and electronic CRS sign-off work in one flow.
Common questions
What is a deferred defect?
A deferred defect is one that has been assessed against the Minimum Equipment List, found acceptable for the aircraft to continue operating with, and given a rectification interval by which it must be corrected. The interval is set by the category the item carries: three consecutive calendar days for Category B, ten for Category C, a hundred and twenty for Category D, each counted from the day after discovery, while a Category A item carries the specific limit written against it in the list, which may be expressed in flights, flight hours or cycles. It is a hard limit rather than a target: an aircraft operated beyond its rectification interval is not airworthy.
What is the difference between the MEL and the CDL?
The Minimum Equipment List sets out which items of equipment may be inoperative while the aircraft continues to operate, under what conditions and for how long. The Configuration Deviation List covers missing or damaged secondary airframe and engine parts rather than equipment. Both are approved documents specific to the operator and aircraft type, and both derive from manufacturer source documents. Neither is a judgement call made at the aircraft.
