Deferred Defect and Airworthiness Decision on a Charter Aircraft
A charter operator's aircraft is presented for a charter flight with a deferred defect against the cabin pressurisation controller, permitted under the operator's approved deferred defect list with a 10-flight limit. The aircraft has flown 7 flights since the deferral. The captain notes that the pressurisation has been slower to respond than usual on the last two flights, and the cabin altitude warning has not illuminated. The aircraft is otherwise serviceable, and the charter is a 3-hour flight over mountainous terrain with a planned cruise at 9,500 m. The operator's maintenance control is contactable but the flight is due to depart in 90 minutes.
1. Evaluate whether the deferred defect remains valid for this flight and what additional factors the captain must consider.
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The deferral is within its 10-flight limit at 7 flights, so on the face of it the defect is still valid. But validity is not the same as suitability for this particular flight. The captain must consider:
- Deterioration of the defect - slower response on the last two flights suggests the controller is degrading faster than the deferral assumed. A deferral is based on the system remaining at least minimally functional; a worsening trend undermines that assumption.
- Route and altitude - a 3-hour flight at 9,500 m over mountainous terrain leaves little diversion options and requires reliable pressurisation. The consequences of a pressurisation failure are far more serious here than on a short low-level flight, which is the kind of flight the deferral may have been written around.
- Warning system - the cabin altitude warning not illuminating is not reassuring on its own; it may simply not have been tested in the conditions that would trigger it. It should be tested before flight.
- Passenger considerations - passengers on a charter may include people for whom altitude is a risk, and the crew must be able to manage a depressurisation.
The deferral is technically valid but the trend and the route make it questionable. The captain should not treat the paperwork as a substitute for judgement.
2. Decide the course of action and justify it with reference to airworthiness and operational responsibilities.
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Decision: do not depart until maintenance control has reviewed the defect and the pressurisation trend. The deferral permits flight, but it does not require the captain to accept a flight that the evidence suggests is beyond the deferral's intent. The captain has a duty to refuse a flight when safety is in doubt, and the operator has a duty to support that decision.
Justification:
- The deferral was granted on the basis of a known, stable condition. Two flights of deteriorating response is new information that the deferral did not account for.
- The route over mountainous terrain at 9,500 m reduces the margin for a pressurisation failure and limits diversion options.
- The 90-minute departure window is a schedule pressure, not an airworthiness argument.
Course of action:
- Contact maintenance control immediately and report the trend, not just the deferral status.
- Have the pressurisation system tested on the ground, including the cabin altitude warning.
- If maintenance control cannot confirm the system is within limits for this flight, the flight is cancelled or the route is changed to one where pressurisation failure is manageable.
- Record the decision and the reasoning.
The key point is that a valid deferral is a permission, not a guarantee. The captain's judgement and the operator's support are what keep the decision safe.
3. Discuss how the operator's deferred defect process should handle a defect that is deteriorating.
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A deferred defect list works only if it distinguishes between a stable defect and one that is changing. The process should include:
- Clear limits on flights and calendar time, which this operator has.
- A requirement to report deterioration - any crew noting a change in the defect's behaviour should report it, and the report should trigger a review rather than a note in the log.
- Route and altitude restrictions tied to the deferral, so that a defect acceptable on a short low-level flight is not automatically acceptable on a long high-altitude one.
- A review point before the limit is reached, so the defect is not simply flown to the last permitted flight.
- Maintenance control authority to suspend a deferral when new information appears.
In this case, the process should have caught the trend on the first slow response. The fact that it reached the captain as a routine deferral suggests the reporting channel is weak. The operator should review why the deterioration was not flagged and consider adding a requirement for a functional test before each flight under the deferral.
The broader lesson is that deferrals are a controlled risk, and the control depends on the information flowing back from the crew.