Originally Posted by
RandomPerson8008
The confusing part about that statement in the POH is that the emergency procedure for losing the tach includes "use governor", which in electrical failure you will not have either.
It sounds circular but it isn't.
- If both tach's fail, you can still perfectly rely in the governor to control engine RPM.
- If in a fire you decide to switch master OFF, you don't have a governor, but you can control RPM manually. Use tachs (that still work despite master OFF) to set for the right RPM
If after an ALT failure you run down your battery, you have NIL power, so neither governor nor tachs will work. If you are really good, you can still safely fly, or autorotate her to the ground. All others better land before the battery is finished.
How long will the battery last once the ALT failure light comes on? It depends. I'd say between 15 min and 60 min. It says, switch off non-essential equipment to preserve battery charge. RHC doesn't list what is non-essential, but don't forget to switch off the air conditioner, as its (electric) fans draw a lot of current.
Anybody have a recipe how to judge when the battery is about to die, before it dies? With only the Amp meter, and without a Volt meter, that's really difficult to tell. An idea would be to mentally note the Amp meter negative deflection when switching on, say, landing lights, just for a second. Then test this every 5 min or so. Theoretically, with an almost empty battery, the Amps the battery can provide to the landing lights will go to zero (i.e., lesser negative deflection of the Amp meter).
I don't know how reliably this works in practice. I have tried it once with an ALT failure, but managed to land before the battery died; all the time the battery provided sufficient Amps to the landing lights when I flicked the switch for test purposes. But I haven't seen for myself how the system behaves closer to the end...