Starting Turbine Engines

Thu Jan 25, 2001 1:09 am

This is a thing that I have pondered for a long time. I understand how it works but not how you get it going. I have heard that air from the APU(small engine in the tail) is pumped directly into the high preassure combustion chamber. Is this true?. Also I have noticed that smaller turboprop aircraft have a battery hooked up to them on star up.......what does the battery do?, is there an electric motor type structure made of the main shaft and magnets around it inside the engine?.

RE: Starting Turbine Engines

Thu Jan 25, 2001 1:22 am

Large commercial turbofans are started by pneumatic pressure from the apc. On the accesory gearbox there is a small turbine starter that the APU air is pushed into. The air spins the turbine, which then turns the accesory gearshaft and therefore starts turning the spools of the engine.
Only really old engines (i.e. 1950's) pump the air directly into the engine.
Smaller engines, like turboprops, often use electric starters much like a car. Like the turbofans, the starter is on an accesory gearbox. The accesory gearbox also holds things like generators, hydraulic pumps, etc.
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Joined: Sat Oct 21, 2000 4:57 am

RE: Starting Turbine Engines

Thu Jan 25, 2001 1:53 am

On the JT8D-9 engine that is on the DC-9 I fly we use air bled off the APU to get the engine spinning. If the APU is inoperative, an access pannel located under the tailcone provides an attachment piont for a ground air cart.

Immediately before engine start the "below the line " checklist is completed.

Papers and logbook........................aboard
EPR and IAS bugs..........................set & crosschecked
shoulder harness............................on
airconditioning supply
fuel boost pumps...........................on

After the checklist is called complete the start switch is held on and the start valve open light illuminates. This allows the APU bleed air to turn the N2 section of the engine and rotation begins almost immediately. Oil pumps and Hydraulic pumps are geared off the N2 section of the engine so pressure for both begin to rise very early during the start.

N1 section of the engine begins to turn due to the airflow passing through the engine.

When N2 RPM reaches 20% the fuel controler is placed on. This does two things 1. introduces fuel into the engine and turns the ignitors on. Even though the ignition switch was turned on during the checklist, the ignitors do not operate until the fuel control lever is on as well.

Light off is confirmed by a rise in EGT (exhaust gas temp.) as well as engine acceleration. The EGT limits for engine start depend on the outside air temp, below 15C it is limited to 350C and above 15C it is 420C.

When the N2 rpm accelerates to 35% the start switch is released and the engine continues to accelerate to idle power, 50% N2.

With the engine running, electrical power is being supplied by the engine driven gen. insted of the APU or ground power.

Air conditioning is available from air bled off the engine insted of a ground A/C cart or the APU.

If ground icing conditions exist, the engine ice protection is turned on.

Once the other engine/s are started, the before taxi checklist is completed

electrical pannel..........set
pitot heat..................on
ice protection.............set
airconditioning supply...set
door lights..................out
hydraulic pumps..........high&on,3000psi,quant. checked

We also have a number of abnormal checklists if the engine doesn't start correctly.

Oil Pressure fails to rise
EGT exceeds limits (hot start)
N2 fails to accelerate to idle (hung start)
Start valve open light remains on
Oil strainer clogging light remains on after start

A turboprop I flew for a another airline had Starter/Generators. One unit functioned as both engine starter and generator. It was powered by the ships batteries when used as a starter and become a generator after engine start.

 Big thumbs up


RE: Starting Turbine Engines

Thu Jan 25, 2001 3:29 am


How do "Hot Starts" occur? Are they due to an error by the pilot at startup or are they due to a mechanical problem with the engine itself??

Thanks...your description of the engine startup procedure was excellent!!
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Joined: Sun Jan 23, 2000 9:07 pm

RE: Starting Turbine Engines

Thu Jan 25, 2001 7:08 am

Hot starts can occur for a number of reasons. First of all it can be a fault in the fuel scheduling. Too much fuel and therfore too much heat. Also putting the fuel on before the engine reaches the correct rotational speed. (differs for all engines).

second a hot start can rapidly develop from a hung start. The engine depends on cooling air that is sucked through it as it accelerates. If the engine fails to accelerate after light off, EGT tempertures can rise very quickly. Hung starts can be caused by a number of things, such as a weak starter/generator, weak bleed air pressure from the Apu, very cold tempertures make an engine much harder to rotate due to thick oil etc. If an engine does hang during the start it is always best to get out of the fuel before any damage is done and try again when the reason for the bad start is established.

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Joined: Wed Nov 03, 1999 3:54 pm

RE: Starting Turbine Engines

Thu Jan 25, 2001 12:18 pm

The two turboprop engines I have operated, the TPE331 and PT-6, both had 'starter generators, a single unit that started the engine then reverted to a DC generator.
The 331 is started by simply placing the RCS switch to 'run' and letting the 'speed switches' do their bit. The starter gen. starts turning the compressors and turbines (fixed shaft...both moved together), the speed switch schedules oil, ignition and fuel into the engine, then remove the ignition when the engine approached stable. The props are to be secured in fine pitch by mechanical locks to reduce engine drag for the starter generator.
The PT-6 was a little different. As it is a free turbine design the prop stayed in feather for the start until sufficient oil pressure drove it to fine. The start switch started the rotation of the compressor/turbine and ignition, and fuel was introduced with a fuel lever in the cockpit. The starter has to be manual disengaged once the engine approaches stable.
The B747-400 is even easier to start. Autostart allows us to start two engines at once, as the APU supplies sufficient pressure. The start switch on the over head panel is pulled and the fuel control switch placed to run, and the digital controls do the rest! If a hot or hung start occurs, the autostart aborts the start, then tries again.
Hope this helps!  Smile/happy/getting dizzy

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