L-1011 Engine: RPM Indication System

FAA regulations quire that all three engine rotating assemblies shall have an RPM indicator in the flight station. The three indicators are:

• N1 - LP (fan) shaft speed
• N2 - IP compressor shaft speed 
• N3 - HP compressor shaft speed

N1 and N2 shaft speed signals are generated by phonic wheel and magnetic probe systems. The N3 shaft speed is measured by a conventional tachometer generator.

Two identical probes are located inside the front bearing housing in line with each phonic wheel. One probe is connected to the indicator circuits; the second may be utilized if the first fails.