MEL Penalty at a Mountain Airport
At 21:10 local you are scheduled to depart a mountain airport: elevation 5,900 ft, altimeter 29.92, runway 5,200 ft dry, wind calm, OAT 26°C, night IFR. Takeoff weight is 12,450 lb, 50 lb below MTOW, CG within limits. The left engine bleed valve is deferred under the MEL, which requires a 12% increase in all AFM takeoff distances and lowers the second-segment climb gradient from 3.1% to 2.4%; the departure obstacle requires 2.5%. AFM dry takeoff distance at 5,900 ft/26°C is 4,850 ft at 12,500 lb and 4,400 ft at 11,500 lb; at 11,500 lb the second-segment gradient is 2.8%. Fuel is 2,960 lb: trip 1,700, alternate 400, 45-minute reserve 700, taxi 60, contingency 100. Dispatch has released the flight as loaded.
1. Is the flight legal to depart as released? Show the controlling arithmetic.
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No. Two independent limits fail once the deferral is applied:
- Takeoff distance: 4,850 ft x 1.12 = 5,432 ft required against 5,200 ft available. Short by 232 ft.
- Second segment: the deferred configuration gives 2.4% against the 2.5% obstacle requirement. The airplane cannot meet the departure obstacle gradient.
The 50-lb MTOW margin is irrelevant, because MTOW is not the controlling limit here; the MEL performance penalty is. The release was built on unpenalized AFM data, so it does not represent the airplane as configured. The correct answer is not to depart and not to 'try it and see' at a 5,900-ft airport at night with an obstacle departure.
2. The captain proposes offloading 1,000 lb of fuel to reach 11,500 lb. Evaluate that plan.
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The performance side works: 4,400 ft x 1.12 = 4,928 ft, inside 5,200 ft, and the gradient improves to 2.8%, clearing the 2.5% obstacle requirement.
The fuel side does not work. Required fuel is 1,700 trip + 400 alternate + 700 reserve + 60 taxi = 2,860 lb before contingency. Offloading 1,000 lb leaves 1,960 lb, which is less than trip fuel plus reserve, let alone the alternate. The flight cannot be legally dispatched.
The better option is to offload payload: about 1,000 lb, roughly five passengers plus bags, reaching 11,500 lb or below. That restores both the distance and the gradient while preserving the fuel plan, then requires a new release and load sheet and a re-brief of V speeds for the lower weight.
3. Assuming the payload is offloaded and the flight departs, what must be verified about the deferral itself, and what changes on the departure?
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Verify four things before accepting the new numbers:
- The deferral applies to this airplane and configuration and has not expired.
- The MEL operational and maintenance procedures are accomplished: the affected bleed is closed, the associated pack is configured as required, and no takeoff is attempted with it open.
- The performance penalty was applied to the actual runway, slope, wind, and obstacle analysis, not just to a generic AFM distance.
- Any secondary effects are considered: single-engine service ceiling, go-around gradient, and enroute performance with the bleed unavailable.
On departure, the practical change is that V speeds and the rotation/target pitch are flown for 11,500 lb, not for the original 12,450 lb, and the crew should expect slightly different acceleration and climb numbers. The obstacle gradient of 2.8% is a planning number, not a cue to accept a sluggish climb; if the actual gradient looks worse than planned, the correct response is to level off, clean up, and reassess rather than to press the departure.