R22 Corner

[Figure Of Merit]

I hope I can clear up some confusion particularly in relation to the Tachs and where they get their power from.

CrazyBS is right in that both tachs will continue to read with both the battery and the alternator off. It is wrong, though, to infer from this that the tachs are being POWERED by the magnetos. They are not.

Both tachs require power to work. They also need an input that tells them the speed. (The sensing inputs are 1. a hall effect magnetic sensor on the forward yoke of the forward flex coupling in the case of the rotor tach and 2. a connection to the right magneto in the case of the engine tach. To confuse us pilots further the right tach is on the left side of the helicopter. This is because the engine is mounted back to front compared with the way it is generally used in fixed wing applications.)

The power to the tachs with the alternator and battery master both off comes from a "tach bypass circuit". This is a direct link from the battery to the tachometer bus. So-- switch the alternator off and then switch the battery off and you'll still have power to the tachs. You WILL lose the governor and the Low RRPM warning horn.

This is handy if you've got an electrical fire in flight when you need to stop electrical power getting to the panel but need the tachs to get down.

To prevent the battery from going flat while the aircraft is sat in the hangar the tach bypass circuit runs through the clutch switch. With the clutch switch in the "disengaged" position the bypass circuit is switched off (isn't Mr Robinson clever?). A diode in the circuit prevents electrical current travelling from the Tach Bus back to the main bus.

So with your "electrical fire in flight" scenario, you switch off the Alternator and Master Batt but keep the tachs. Only if you were then to put the clutch switch to "disengage" (or pull the tach breakers) would you lose the tachs. (But you wouldn't want to do that would you)

All this is academic if you've lost the alternator and you've flattened the battery! Hence the warnings in the POH about jump starting a machine with a dead battery. If you jump start it and then go flying before the battery has charged you are one failure away from not having a clue about your RRPM. (Unless you've got perfect pitch to judge the engine note).

Back to the original question posed by jibboo. The battery is a 25 ampere hour battery. This THEORETICALLY means that it will supply 25 amps for an hour (or 5 amps for 5 hours...or 1 amp for 25 hours...etc). In reality the voltage will be plummetting long before that time is reached. It does give you some idea of the effect of using each bit of kit on battery duration. Have a look at the rating of each breaker for a rough idea of how much current each item consumes.

Hope this helps