APU Testing - TM-55-4920-424-13-P0030

TM 55-4920-424-13&P

o. If APU shuts down because of high exhaust gas temperature, the HIGH EXH TEMP light will illuminate. The

reason for this shutdown may be due to loading APU prior to attaining 100 percent rated speed, residual fuel

remaining in combustor, a restriction in air inlet or exhaust outlet, or a failure in the engine (resulting in increased

drag). Correct these conditions accordingly. Also, the fuel control may be out of adjustment. Refer to

paragraph 3-10 for fuel control adjustment procedures.

p. If APU shuts down because of low oil pressure, the LOW OIL PRESS light will illuminate. This condition may be

caused by low oil level in the APU, malfunctioning oil pump, clogged oil filter, electrical discontinuity, or a

malfunctioning oil pressure switch. Refer to TM 55-2835-203-24 to correct these malfunctions.

Note

Rated speed (100 percent engine speed) corresponds to 4200 rpm of the

tachometer generator, 6000 rpm of the reduction drive output and 56,000 rpm of

the turbine.

3-13. APU Testing. After APU has automatically accelerated to rated speed, it is protected by three automatic safety

devices; a speed switch, a high exhaust temperature device, and a low oil pressure switch.

a. Speed Adjustment . After APU attains 100 percent rated speed (400-cycle ac) at no load, adjust fuel control rated

speed setting adjustment screw to obtain 102 percent engine speed. Run APU for approximately 10 minutes and

carefully check for fuel and oil leaks.

Note

Engine should shut down when rated speed is exceeded by 10 percent. The

ENGINE SPEED indicator registers in percent. The small hand makes one

revolution for each 10 percent.

b. Thermal Protective Device. It APU exceeds the temperature limit of 1120F (604.4C).

c. Low Oil Pressure Switch. The low oil pressure switch is set to shut down APU if decreasing oil pressure reaches

6 1 psig.

3-14. AC POWER OPERATION.

3-15. The mobile check stand may be used to furnish 115/200-volt, 400-cycle; ac power for aircraft preflight operations, or

other needs. A 50-foot power cable is provided with the check stand, and is equipped with a connector to engage the ac

power input receptacle on the aircraft.

3-9