AD Hour Limit, MEL Deferral and Repositioning from an Outstation
Aircraft on a domestic charter in Bangladesh. An Airworthiness Directive requires a repetitive inspection of the elevator trim tab drive every 500 flight hours. It was last accomplished at 3,842.0 FH; the airframe is now at 4,338.5 FH. The charter is four sectors totalling 11.0 FH (3.0, 2.5, 2.5 and 3.0). The aircraft is away from base where no approved facility or tooling is available. Separately, the No. 2 VHF comm has been unserviceable for 3 days and was deferred under the operator's MEL. The operator argues that the MEL is valid for 10 days, so the aircraft may continue, and proposes a normal passenger ferry back to base at the end of the charter.
1. Calculate the flight hours remaining before the AD is due and decide whether the charter can be operated as planned.
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Hours remaining on the AD
Next due = 3,842.0 + 500 = 4,342.0 FH Current = 4,338.5 FH Remaining = 4,342.0 - 4,338.5 = 3.5 FH
The charter
Sector 1 (3.0 FH) ends at 4,341.5 FH, still 0.5 FH inside the limit. Sector 2 (2.5 FH) would reach 4,344.0 FH, which is 2.0 FH past the limit, and the remaining sectors would take the aircraft 7.5 FH past it.
Decision
An AD is mandatory. The limit may not be exceeded in flight, so the aeroplane cannot be dispatched on this charter. Even flying the first sector alone is a bad operational decision: it strands the aircraft and passengers at an outstation with 0.5 FH left, which is worse than staying put.
The correct outcome is to comply with the AD before dispatch, or to move the aircraft under a specific approval (see below). The 3.5 FH remaining is not a buffer to be spent on revenue flying.
2. Can the operator rely on the MEL deferral to dispatch the aircraft? Explain the interaction between the MEL and the AD.
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No. The MEL does not help here, for two separate reasons.
- Subject matter. An MEL, derived from the master minimum equipment list, allows operation with specified equipment inoperative for a defined rectification interval. It has no authority over a continuing airworthiness directive. An AD cannot be deferred, extended or traded against a defect deferral; the AD must be complied with at its stated interval whatever the MEL status of unrelated equipment. The operator is comparing two unrelated things.
- The No. 2 VHF entry must itself be checked on its merits. A category C item typically allows 10 days of operation, but only if the entry's conditions are met: minimum number of serviceable comms required, the route or airspace requirements, and any operational or maintenance procedure required by the entry. The MEL entry and the configuration (how many comms are installed) must be checked before the deferral is used at all.
Judgement: dispatch is unlawful on the AD alone; the MEL argument is irrelevant to that. Accepting the operator's reasoning would be a false assurance built on two correct-looking facts that do not connect.
3. The aircraft must be moved to base. Set out the lawful options and the records required.
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Option 1 - comply away from base
Check whether the AD inspection can actually be accomplished at the outstation: approved personnel holding the correct type authorisation, the specified tooling and inspection aids, and a suitable environment. If yes, accomplish it, record it (date, airframe hours, AD number and revision, method, signature), and the aircraft is released normally.
Option 2 - special flight permit / ferry approval
If the AD cannot be met before flight, apply to the national authority (CAAB) for a special flight permit or ferry flight approval. That approval must come with operating limitations set by the authority, typically non-revenue, no passengers, a defined route, daylight VFR, and a maintenance programme for the transit. It is not a normal passenger ferry and the operator's proposal must be refused.
Records
- AD compliance entry in the aircraft logbook with hours, AD reference and revision.
- Defect entry for the No. 2 VHF, the MEL reference and the rectification interval due date.
- Ferry approval reference and the associated limitations.
- Every entry certified by an appropriately licensed engineer, with the CRS issued only in accordance with the approved maintenance organisation's procedures.