Battery-Only at FL210: Load Shedding and the Diversion Decision
Night IFR in IMC, two pilots, pressurised twin turboprop level at FL210. Ten minutes after level-off the left generator trips and the left DC bus goes dead. The right generator takes the whole load and the battery ammeter reads +15 A for four minutes; it then swings to -20 A and the battery voltage decays from 28.0 V to 24.6 V over three minutes. The battery is 24 V, 34 Ah NiCd, rated at the C1 rate. After switching off windshield heat, radar, cabin lights and VHF 2, the essential load is still 20 A: No.1 attitude indicator, AHRS, VHF 1, ILS 1, transponder and the standby electric attitude indicator, with gear and flaps still to come. A single reset of the right generator was attempted with no change, and no GCU fault flag is displayed. Destination is 45 minutes away, RVR 1,800 m; a VMC aerodrome with a 2,000 m runway is 18 minutes away, CAVOK. ATC can offer any level or heading.
1. What do the indications tell you about the state of the electrical system, and what checks would you make before concluding?
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The initial +15 A after the left generator tripped is normal: the bus tie closed, the right generator picked up both buses and the battery was being charged.
The swing to -20 A with a falling battery voltage means the battery is now carrying the essential bus. That can only happen if the right generator's output is not reaching the bus, or the bus tie has opened:
- Right generator tripped (over/under-voltage, overcurrent, or a drive failure). The generator ammeter would fall to zero and the GCU would normally flag a fault.
- Bus tie contactor opened. The generator would still be healthy with near-zero generator ammeter and a dead opposite bus, but there would be no battery discharge on the live side.
- Generator online but not connected (contactor or wiring fault), giving a healthy generator reading with a discharging battery.
Checks: generator ammeter and loadmeter on the right side, individual bus voltages, GCU fault and field switch position, and whether the discharge began with a specific load selection. Do not keep resetting: a generator that trips twice in flight will not come back, and repeated resets can damage the GCU or the drive. Treat the aircraft as being on battery time and plan accordingly.
2. Calculate the realistic time available on the battery and state your load-shedding plan.
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Nominal arithmetic gives 34 Ah ÷ 20 A = 1.7 h. That figure is misleading and must not be used for planning:
- Capacity is quoted at the C1 rate at about 20 C. At a continuous 20 A discharge a NiCd delivers perhaps 60 to 70 per cent of nominal, so roughly 20 to 24 Ah, which is 1 h to 1 h 10 min to exhaustion.
- The battery must still have the energy to move the gear and flaps (peaks around 25 A) and to carry attitude and air data through the approach and landing.
- Voltage decay is non-linear. Below about 22 V avionics start dropping out, and an attitude indicator that dies at 300 ft is worse than an early diversion.
Plan on 30 minutes of comfortable systems time, not 60.
Load-shedding order: windshield heat and radar already off; then cabin lights, VHF 2, and No.2 attitude indicator if the standby AI is serviceable, keeping VHF 1, ILS 1, transponder, No.1 attitude indicator and the standby AI. Everything electric that is not needed to fly the approach and land comes off now, while the battery is still strong enough to make the decision orderly.
3. State your routing decision and how you would fly and communicate the arrival.
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Take the 18-minute CAVOK aerodrome. With about 30 minutes of planned battery endurance, a 45-minute flight leaves nothing for an approach, the gear cycle or a go-around, and the destination may not have electrical or engineering support. The VMC aerodrome permits a visual approach with no reliance on the ILS and a straight-in landing on a long runway.
Communicate: 'PAN PAN, electrical emergency, request diversion to (aerodrome), shortest routing.' State the possibility of radio failure and that you will squawk 7600 if contact is lost. Ask for the aerodrome weather and for fire cover on arrival.
Brief for the loss of No.1 attitude indicator: hand-fly if the autopilot is not on the essential bus, use the standby AI and the magnetic compass, standard-rate turns, no steep turns and a wide, stable circuit. Plan a flapless or reduced-flap landing if flap extension would overload the battery at a critical moment, accept a longer landing distance, and keep the landing light off until the final approach. Do not reset the generator again in flight.