Q: Overhead console lights and EICAS caution/advisories? A: APU ON, APU ACCUM LOW, APU OIL TEMP HIGH, APU FAIL (overhead DC power only), same for EICAS with the addition of the APU GEN ON advisory and the APU GEN caution (need AC power).
Q: Power required to run the APU? A: The APU needs DC power for the start sequence and to keep the fuel valve open.
Q: APU fuel? A: The APU receives fuel from the right main cell. The APU fuel boost pump is an electrical pump that provides “boost” pressure to the system and positive flow for starting the APU (this pump also primes the engine if the PCLs are in lockout and the FSLs are in DIR or XFD).
Q: Electronic Sequencing Unit (ESU)? A: Controls and monitors APU start and steady state operation. During the start it controls fuel flow, ignition and starter disengagement. It monitors APU exhaust gas temp (EGT), RPM and oil pressure during start and steady state operation. If it detects a problem the ESU secures the APU and displays a fault on the BITE indicator. The ESU is located in the transition section (access door is on the right side of the aircraft).
Q: Clearing of BITE indications? A: BITE indications will be cleared if you cycle the APU switch to off or turn the BATT off.
Q: APU start system? A: The APU has a hydraulic start system powered by the APU accumulator. When the DC essential bus is powered and the APU control switch is turned to “ON” the ESU opens the start valve and the accumulator discharges and turns the starter.
Q: Recharging the accumulator? A: The accumulator can be recharged by the backup pump or the hand pump.
Q: Back up pump recharge? A: Operates for 90 seconds AND the ACCUM LOW is off.
Q: ACCUM LOW? A: Indication that the accumulator charge is less than 2650. Accumulator pressure can be viewed through a sight window on the aft cabin bulkhead.
Q: APU bleed air? A: The APU provides bleed air to the ECS and engine start system anytime the APU is running regardless of the air source switch position.
Q: APU AC power? A: Provided by the APU AC generator. If it is the only generator online it powers the #1 and #2 AC primary bus and the AC ESS bus. The monitor and secondary busses are not powered.
Q: APU GEN ON? A: Only illuminated when the APU GEN is the only gen providing power.
Q: APU GEN caution? A: Illuminated when the APU control switch is in the on position and the APU GEN is not yet up to operating speed, APU is on and the APU GEN switch is in the OFF position, the APU GEN has failed.
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