STUDENT GUIDE FOR T-1A AIRCRAFT SYSTEMS
Hydromechanical Metering Unit
The HMU controls primary fuel scheduling using high-pressure fuel from the engine-driven fuel
pump and is solely responsible for engine fuel flow during startup to approximately 38% N2.
Electronic Fuel Control Unit
The EFC augments the HMU and is provided to better establish fuel scheduling to the engines.
Failure of the EFC will result in a loss of scheduled fuel flow. Consequently, the maximum
thrust loss could be 40% depending on the ambient conditions and altitudes. This thrust loss
may be recovered by advancing the throttle(s), causing the HMU to deliver the required fuel
Safety Shutoff Valve
A safety shutoff valve is installed between the flow divider and fuel nozzles. The valve is
normally open, but is controlled by a sensing mechanism that is released by an aft motion of the
turbine rotor shaft (this would occur if a rotor shaft failed). This aft motion will close the safety
valve, isolating fuel flow to the nozzles and shutting down the engine.
308. ENGINE IGNITION SYSTEM
The engine ignition system is a high-energy type. The primary function is to provide a means of
initiating combustion in the combustion chambers during the start cycle.
Ignition Switches and Lights
The ignition is controlled via a three-position switch: ON - OFF - STBY. When the ignition
switch is placed to the ON position, 28-volt DC power is supplied to the exciter. The normal
position of the ignition switch is in the STBY position.
309. ENGINE STARTER SYSTEM
The engine starter system is the primary source of electrical power and functions as both a starter
and a generator. When the engine speed is approximately 35-40% N2, the speed sensor of the
starter/generator sends a signal to the Generator Control Unit (GCU) and removes the starter's
control power. The starter/generator then begins to operate as a generator.
310. ENGINE STARTER SYSTEM LIMITATIONS
Refer to Starter/Generator Limitations, Section V, T.O. 1T-1A-1, for specific starter system