Avionics Bending: Percent N3 Tachometer Indicator

L-1011 %N3 Tachometer - RB.211

For today's avionics bend I am going to show you the %N3 Tachometer. The RB.211 is a three spool engine and therefor there is a %N1, %N2 and %N3 tachometer present in the L-1011 cockpit. The %N1 and %N3 tachometers are part of the engine cluster on the center main instrument panel. The %N2 tachometer is part of the engine cluster on the Flight Engineers station. The %N3 tachometer works pretty much just like the %N1 and %N2 tachometers in that they take a frequency and coverts it to a needle position as well as a digital readout (bottom of the instrument). Here is the schematic for the %N3 instrument:

L-1011 %N3 Tachometer Schematic and Pinout

What makes the %N3 tachometer different from the %N1 and %N2 tachometer is the fact that the instrument plays a pretty critical role during engine start up. To be more exact, it controls the duration of bleed air being supplied to the air-start motor on the engine.

Below is a picture of the Ground Start Switchlight for Engine 1:

The magic inside the %N3 instrument (on the left) happens during the engine start up and once the 3rd compressor stage has spooled up to 52% N3 with the air-start motor. Here is what happens 

The GROUND START switchights are solenoid held, and are latched in to initiate the start . This gives a command to open the associated start control valve to allow air (from the APU, another engine, or a ground cart) and begin the rotation of the compressors. The green VALVE OPEN light illuminates to indicate that the associated start control valve is NOT CLOSED. Once the %N3 Indicator reaches 52% the Ground Start switch for that engine is released and the valve closes. At this point the engine runs on it's own.

Here is a demo video of the %N3 action and solenoid held switch release: