L-1011 Trainer Installation: %RPM N3 Power-Up and BITE

L-1011 Trainer - %RPM N3 Power-Up

The %RPM N3 indicators receive 115VAC, 400Hz power from the AC Standby Bus. So they are part of the essential engine instrumentation. The indicator has an Overlimit Painter that will record any overspeed condition. The pointer will remain at overspeed indication until 28VDC is supplied to the Reset Solenoid in the instrument. Current, we have not connected the reset circuit quite yet ... but it will be one of the next things.

BITE: Depressing the Test switch on the lower left hand side of the indicator face should cause the pointer and counter to read 64.0%

Below is a short video showing the indicator BITE in action:

%RPM N! and N3 from Delta L-1011 Pilot's Reference

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:

Original Flight Hardware: Engine Cluster

The Lockheed L-1011 has an awesome engine cluster. It is home to the EPR, N1, EGT, N3 and FF gauges. In the picture above, you actually see instruments from a Boeing 747. I chose the B747 instruments because the will be easier to automate and control with the flight simulator software.





Another deviation from the standard L-1011 engine cluster that you might have noticed by now is that I, for now, substituted N2 gauges for the L-1011 N3 gauge. The reason being that the N3 gauges are pretty difficult to find. However, it does not mean that I will not replace the B747 N2 gauges with N3 gauges that better fit the 3 compressor stages of the L-1011 engines.



As you can see from the picture above, there are quite a few things still missing from the center panel. I don't have any of the standby instrumentation ... which will hopefully come soon.

Panel Location


L-1011 flight station picture used with permission from Sam Chui. Retrieved from airliners.net