to be drawn into the compressor by the power produced by the engine. The air is compressed and
directed into the combustor; fuel is introduced through six vaporizer tubes and is burned. During the
starting cycle, fuel from the start nozzle is ignited by a spark plug. When the APU reaches 90 percent
speed, a speed switch opens which closes the start fuel solenoid valve, shutting off fuel flow to the start
fuel nozzle. The hot expanding gas flows through a turbine nozzle and to the turbine assembly. Power
is extracted by the turbine rotor and is then transmitted to the reduction drive assembly.
6.4.
PROTECTIVE DEVICES FOR THE APU
The T62 is equipped with protective devices that shut the APU down if any of the operating
limitations are exceeded. If the oil pressure drops below approximately 6 psi, the low oil pressure switch
shuts off the fuel solenoid valve, which stops the flow of fuel to the engine. When this occurs the light
marked "Low Oil Press" will be illuminated on the instrument panel in the cockpit.
Overspeeding the APU is prevented by an overspeed switch, set at 110 percent. When this
limit is exceeded, the overspeed switch opens and shuts off the fuel flow to the engine. This causes the
overspeed (OVSP) light to illuminate on the instrument panel.
Hot starting of the APU is prevented by the high exhaust-temperature switch, which shuts
the engine off if the engine is too hot. Like the low oil pressure and overspeed switches, it stops the
fuel flow to the engine. When the operating temperature is exceeded, the high exhaust temperature
light, marked "HIGH EXH TEMP," in the cockpit will be illuminated.
If any of these conditions occur and the APU shuts down, the control switch in the cockpit
must be moved to the STOP position before attempting to restart the engine.
6.5.
REDUCTION DRIVE AND ACCESSORY DRIVE ASSEMBLIES
The major difference between the T62T-2A and the T62T-16A is the accessory drive
housing. A comparison can be made by looking at item 31 in figures 6.3 and 6.4. Reduction of high
engine rpm is accomplished by using a single stage planetary gear reduction. Three fixed-center
planetary gears are driven by the externally splined pinion of the rotor shaft, item 7. An internally
splined ring gear transmits the drive from the planetary gears to the output shaft. External gear teeth on
the output portion of the shaft drive an upper intermediate gear and oil pump gear. The intermediate
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