Battery Discharge with One Generator Online
On a twin turboprop with two 28 V, 200 A starter-generators and a 24 V, 35 Ah NiCd battery, generator 1 trips shortly after takeoff. Generator 2 picks up the load, but the loadmeter reads 185 A and the battery ammeter shows 10 A discharge. The essential bus draws 45 A. Non-essential loads currently on are: cabin heat 30 A, windshield heat 25 A, landing lights 20 A, cabin entertainment 10 A, and anti-collision lights 5 A. The generator 2 control unit is stable and its voltage is 28.2 V. The battery is at 24.5 V and its temperature is normal. The aircraft is at 9,000 ft, 40 minutes from destination and 15 minutes from a suitable alternate. The starter-generator 1 fault cannot be reset.
1. Explain why the battery is discharging despite generator 2 being online, and calculate a load-shedding plan.
Read the worked answer
Generator 2 is at its 200 A limit and is not covering the total load, so the battery supplies the deficit. A 185 A loadmeter reading plus a 10 A battery discharge means about 195 A demand, leaving no margin. Shed at least 40 A: cabin heat (30 A) and entertainment (10 A) are nonessential, which brings the load to about 145 A and allows the battery to recharge. Keep anti-collision lights. Windshield heat and landing lights should be managed by phase of flight; if icing is not present, windshield heat can also be shed. Do not attempt to reset generator 1 in flight. Monitor generator 2 load and battery current after shedding.
2. If generator 2 fails, estimate battery endurance for the essential bus and decide whether to continue or divert.
Read the worked answer
With a 35 Ah battery, the theoretical endurance at 45 A is 35/45 = 0.78 h, or about 47 minutes. NiCd batteries should not be run flat, so use perhaps 50-70% usable capacity: about 23-33 minutes at 45 A, less if other essential loads are added. The 40-minute destination is not safe; the 15-minute alternate is. Divert, shed all nonessential loads, and plan to land before the battery reaches its minimum voltage. If generator 2 also fails, the aircraft becomes battery-only and a landing should be made as soon as practicable.