Night VMC Departure with a MEL-Deferred Generator
You command a twin-turbine RBAC 135 passenger flight departing at 22:40 local from an aerodrome with no precision approach and no radar service. During preflight the No. 2 generator fails its voltage check and is placarded per the operator's approved MEL as a Category C item with a 10-day rectification interval. The aircraft has two generators; the MEL entry permits dispatch provided the APU generator is available and the battery is at full charge. Weather at destination, 55 minutes away, is forecast 1,800 ft broken, 4 km visibility, light rain, with a temporary condition of 900 ft scattered and 2 km. The first officer has 210 hours total time, 18 hours on type, and this is her third night flight on the aircraft. The company's operations manual requires an alternate for any destination forecast below 2,000 ft ceiling or 5 km visibility at the estimated time of arrival.
1. Evaluate whether the aircraft may legally be dispatched under RBAC 135 with the generator deferred, and identify what additional approval or limitation applies given the night, non-precision environment.
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Dispatch legality rests on three independent checks, all of which must pass.
First, the MEL itself: a Category C item allows dispatch for the stated interval only if every proviso in the entry is met. The APU generator must be confirmed serviceable by an actual check, not by assumption, and the battery must be verified at full charge - a battery that is merely 'not flat' does not satisfy the entry. If either proviso fails, the aircraft is not dispatchable regardless of the 10-day interval.
Second, the operating rule: RBAC 135 requires that, with a generator inoperative, the remaining electrical source be capable of carrying the essential loads for the intended route and the aircraft be operated under the conditions specified in the approved MEL, which typically include a limitation on operations in known icing and a requirement for the APU generator to be running or immediately available.
Third, the operator's own manual: the forecast ceiling of 1,800 ft and visibility of 4 km at ETA is below the 2,000 ft / 5 km threshold, so a destination alternate is mandatory. The alternate must itself meet the weather minima for the approach to be used, and the temporary 900 ft / 2 km condition must be treated as forecast weather for planning purposes.
If all three pass, dispatch is legal. If the APU generator cannot be verified, the correct decision is a maintenance delay, not a deviation.
2. The first officer's experience is limited. What crew-qualification and human-factors considerations should drive your decision on whether to accept this pairing for the flight?
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The pairing itself is legal - 18 hours on type and a third night flight do not breach any minimum - but legality is not the test here. The relevant question is whether the crew composition is appropriate for a night, non-precision departure with a deferred system, which is a higher-workload environment than the first officer has yet encountered on type.
Considerations to weigh:
- Recency and exposure. Three night flights is very little. Ask specifically what non-precision departures and what abnormal electrical configurations she has flown, not just total hours.
- Task allocation. With one generator deferred, the non-normal checklist and the electrical load management become an additional duty. Decide in advance who flies and who manages the checklist, and brief it.
- Threat and error management. Brief the specific threats: no radar, no precision approach at departure, degraded electrical redundancy, marginal destination weather, and a mandatory alternate.
- Authority gradient. A low-experience first officer paired with a captain in a non-standard configuration is a classic setup for a steep gradient. Explicitly invite challenge and state that a go-around or a diversion call will not be second-guessed.
- Fatigue. A 22:40 departure means the crew is operating in the window where circadian low begins. Confirm duty time and rest before accepting.
If the briefing cannot be completed to a satisfactory standard, the correct action is to delay to daylight or to reassign the pairing, not to proceed because the paperwork is in order.
3. During the departure climb the APU generator trips offline. What is the immediate operational priority and what does this mean for the flight?
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The immediate priority is to fly the aircraft and stabilize the flight path, then manage the electrical failure - not the reverse. The sequence should be:
- Aviate. Maintain the climb profile and assigned heading/altitude. Do not allow the electrical problem to pull attention off the flight path at low altitude at night.
- Confirm the state. Verify which sources remain: main generator, battery, and whether the APU generator can be reset once. Establish actual electrical load and shed non-essential load per the checklist.
- Assess against the MEL. The dispatch proviso that the APU generator be available is now violated. The aircraft is no longer in a configuration covered by the dispatch release, which means the flight must be replanned to the nearest suitable aerodrome where the discrepancy can be rectified.
- Communicate. Advise ATC of the electrical status and request priority and, if needed, radar vectors to the nearest suitable field. Advise the company.
- Replan weather. With reduced electrical redundancy, the destination's 1,800 ft / 4 km and the temporary 900 ft / 2 km become a much stronger argument for diverting to an aerodrome with a precision approach and better forecast conditions.
The decision rule is simple: once the aircraft is outside the configuration under which it was dispatched, the flight is a diversion candidate, and the burden shifts to justifying continued flight rather than to justifying the diversion.