APU turns off when Eng Start

[Apache600]

I noticed that if I start the APU before starting the engine, that when I hit the engine start switch, the APU switch automatically switches to off. I'm thinking this may be a bug; and can't, for the life of me, figure out why this would ever be the case if it wasn't.

 

- Apache600

[kreisch]

I can confirm this behavior.

 

To add something: Is it correct that you are able to START the engine without starting the APU first?

[OperatorJack]

Yes you can start the engine without needing to use the APU at all

[firmek]

Seems to be correct. From the NATOPS manual.

2.6 GAS TURBINE STARTER/AUXILIARY POWER UNIT

The gas turbine starter/auxiliary power unit is used to start the engine or drive the APU generator. It consists of a gas

generator, a free power turbine, reduction gear train, an ignition system, an electric starter motor, and electrical

circuits for automatic control. Fuel for the GTS is supplied by the aircraft fuel system. With the battery switch in

BATT, the electric starter motor is energized by placing the engine start switch to the electrically held ENG ST

position which engages the GTS output shaft to the engine. The GTS ignition and fuel control systems are automatic

and the GTS starts and accelerates to operating speed within 25 seconds. When the engine attains self--sustaining

speed, the GTS automatically disengages and the engine start switch returns to OFF. If the GTS does not reach

operating speed within 25 seconds or themain engine is not self--sustaining within 40 seconds, the GTS automatically

shuts down and the engine start switch returns to OFF. If the GTS is operating in the APU mode (APU generator

operating), engine start is accomplished by placing the engine start switch to ENG ST. In this condition, the APU

generator drops off the line, the APU switch automatically returns to OFF, the 40 second GTS shut down protection

circuit is activated and themain engine is automatically engaged for start. TheAPUadvisory light comes onwhenever

GTS/APU operation is selected and the APU is ready to accept an electrical load. For APU mode operation, refer

to APU, Electrical Power Supply System.

2.10.1.2 Auxiliary Power Unit

A6KVAemergency generator, referred to as theAPU, is installed as a backup for themain generator. TheAPUis

driven by the GTS, provided the GTS is operating in theAPUmode; that is, the GTS is not being used to start the

aircraftengine.WiththebatteryswitchtoBATTandtheenginestartswitchtoOFF,placingtheAPUgeneratorswitch

toONwill start theGTS to drive theAPU. TheAPUwill then power all of the buses in the electrical system, except

for the main 115 volt ac and themain 28 volt dc buses, provided the main generator is off the line. TheAPUcan be

operated in a standby mode by turning it on while on the ground or in the air with themain generator operating. If

themain generator then drops off the line the APU automatically comes on the line. If the main generator is then

restoredtotheline,theAPUwillreverttostandbystatus.IftheAPUisturnedonbeforetakeoffandthemaingenerator

isoperating,theAPUwillautomaticallyshutdownwhentheaircraftreaches325knots.IftheAPUisturnedonwhile

airborne, there is no automatic shutdown speed unless theWOWswitch is cycled (i.e., after a landing). If theAPU

modeis selected and the engine startmode is then selected, a translation startwill be made and theAPUmodewill

be terminated. The APUmode may be re--selected after the engine startmode is terminated. TheAPUmode may

be re--selected after the engine start mode is terminated. APU control circuits contain protection circuits which

preventtheAPUfromcomingonthelineinthepresenceofovervoltage,overfrequency,andunderfrequency.Should

afaultoccurandcausetheAPUtotripofftheline(theessentialacbuscontactorsdeenergizingandAPUGENcaution

lighton), theAPUcanbebroughtbackonthelineifthefaultclearsbyplacingtheAPUgeneratorswitchmomentarily

to the RESET position.

The APU is used during ground alert to recharge the battery. Before the dc voltmeter indicates 24.5 volts or below,

the APU is turned on by placing the battery switch from ALERT to BATT and then placing the APU GEN switch

to ON. After charging, placing the APU GEN switch back to OFF turns off the APU.

On radar aircraft, with weight--on--wheels, a load shed function decreases the power requirements when operating

on APU power. Systems not powered because of load shed are the radar warning receiver (RWR), TACAN, and

exterior lights (except taxi lights and side slip vane lights).

2.10.1.2.2 APU Generator Switch

The APU GEN switch on the electrical panel controls operation of the APU.

ON – With battery switch in BATT, GTS drives the APU provided it is not in the engine start mode.

RESET – Momentary position allows generator protective functions to reset.

OFF – Terminates GTS/APU operation.

If I get it correctly the GTS can provide the air to start the engine or be connected with a shaft which drives the APU but not both at the same time. Edited December 1, 2017 by firmek
Included more

[Zeus67]

That's normal behavior.

The APU is NOT used for engine start but to provide electric power when main generators are offline.

[=Pedro=]

That's normal behavior.

The APU is NOT used for engine start but to provide electric power when main generators are offline.

 

+1

Exact answer :thumbup:

[mvsgas]

I think it is just and abbreviation confusion. I think many of us (myself included) thought of the APU as an additional jet engine (all in a single unit), not literary a secondary power unit run by and GTS. It makes since now (to me anyway) that it does not turn off, just the GTS stops producing power through the APU and switches to turn the engine.

 

Thanks guys

[Apache600]

Well that's just pretty cool. Thanks for the clarification guys!