Alternator Failure at Night in IMC
Night IMC, a 28 V single-engine retractable is 45 NM from destination at FL080, 120 KTAS, in cloud. Forecast at destination: 1,500 ft overcast, light icing in cloud above 6,000 ft. The ALT FAIL caution illuminates and the ammeter shows a 25 A discharge. Cycling the alternator field, checking the field breaker and reducing load do not restore output. Battery bus voltage reads 24.4 V and is falling about 0.2 V every 5 minutes. Current loads are: pitot heat 10 A, nav and strobe lights 8 A, GPS/COM1 6 A, COM2 3 A, autopilot 5 A, landing light 10 A, cabin flood 2 A. The battery is a 24 V, 13 Ah unit; gear and flaps are electrically operated, and the gear warning and stall warning systems draw from the essential bus. MEA is 7,000 ft, there is no VFR below, and the nearest suitable airport with a precision approach is 25 NM away.
1. Estimate how long the battery will realistically last and explain the reasoning behind your figure.
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Nominal capacity is 13 Ah, but that figure assumes a slow discharge. At a 25 A load the battery is being discharged at roughly 2C, and at high rates the usable capacity falls markedly through internal losses and voltage depression - plan on perhaps 60–70% of nominal, i.e. about 8–9 Ah, which at 25 A is roughly 15–20 minutes to the point where bus voltage collapses and relays drop out. The observed 0.2 V per 5 minutes supports a similar order of magnitude; voltage falls slowly at first and then rapidly at the end, so the last minutes are not usable for planning.
At 120 KTAS the destination is 22.5 minutes away - beyond a realistic endurance at the present load. Load must be shed immediately and a closer diversion considered. Pitot heat cannot be dropped in icing conditions, which makes the electrical budget even tighter and strengthens the case for the 25 NM airport.
2. Set out your load-shedding priorities and the reasons for each choice.
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Keep what protects the aircraft from an immediate hazard and what is legally required: pitot heat (icing is forecast and a blocked pitot in IMC is a control hazard), nav lights (night, and they aid see-and-avoid), COM1/GPS (one communication and navigation source), and the essential-bus warning systems, which cannot be shed.
Secure COM2, the autopilot (it also fails abruptly when voltage collapses, so hand-fly while workload is manageable), the cabin flood and the landing light until short final. Strobes can go if the switch is separate from nav lights. This reduces the load from about 44 A to roughly 24 A, roughly halving the discharge rate and giving perhaps 30–40 minutes from 24.4 V.
If icing is encountered, pitot heat stays on and the diversion becomes mandatory rather than preferable. Advise ATC of the electrical failure and the intended diversion so that lost-communication procedures do not complicate the arrival.
3. How does the electrical failure change your approach and landing planning in this retractable aircraft?
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Gear and flaps are electrically driven and draw high current. The gear motor may take 15–20 A for several seconds, and a collapse of bus voltage during extension can leave the gear in transit or unlocked. Extend the gear early, well before the battery is depleted and at a point where a go-around is still possible, and check three greens before continuing. If the gear does not indicate down and locked, follow the emergency extension procedure rather than recycling the motor repeatedly, which drains the battery further.
Flap selection should be planned as partial or none, with a correspondingly higher approach speed and longer landing distance. The landing light is selected only on final. Expect the loss of some annunciators and possibly the gear warning horn, so fly a stabilised, fully briefed approach. Declare the electrical failure to ATC, request priority if the voltage is falling faster than planned, and be prepared for the loss of the transponder and radios at any point.