On an R22 helicopter (I quote from the Pilot Operating Handbook):

Each tach, the governor, and the low rotor RPM warning horn are on separate circuits. Either the battery or the alternator can independently provide power to the tachs. A special circuit allows the battery to supply power to the tachs even if the master battery switch is off.

It is clear that the designers intend the the tachometers should work at all times, and there is no "off switch".

To falstro's point, the procedure for an electrical fire in flight includes turning off the alternator and battery master, and this is the precise moment when I am pleased to find that the tachs are still working, since the governor and RPM warning horn are not.

It is true that there is also a circuit breaker for the tachs, but there is no procedure, emergency or otherwise which demands for these to be pulled. The only time I have ever known these to be used is by instructors to simulate emergencies for training purposes.

The R22 is a pretty simple system and I don't know much about airliners but I imagine that it's impossible to access the flight data recorders in flight and I'm sure the designers try quite hard to make sure that they have power under all circumstances including catastrophic aircraft failure and hijack.

On an R22 helicopter (I quote from the Pilot Operating Handbook):

Each tach, the governor, and the low rotor RPM warning horn are on separate circuits. Either the battery or the alternator can independently provide power to the tachs. A special circuit allows the battery to supply power to the tachs even if the master battery switch is off.

It is clear that the designers intend the the tachometers should work at all times, and there is no "off switch".

To Falstro's point, the procedure for an electrical fire in flight includes turning off the alternator and battery master, and this is the precise moment when I am pleased to find that the tachometers are still working, since the governor and RPM warning horn are not.

It is true that there is also a circuit breaker for the tachometers, but there is no procedure, emergency or otherwise, which demands for these to be pulled. The only time I have ever known these to be used is by instructors to simulate emergencies for training purposes.

The R22 is a pretty simple system and I don't know much about airliners but I imagine that it's impossible to access the flight data recorders in flight and I'm sure the designers try quite hard to make sure that they have power under all circumstances including catastrophic aircraft failure and hijack.