PMN
Topic Author
Posts: 547
Joined: Sun Mar 06, 2005 7:44 am

Starting Turbofan Engines And APU's

Wed Sep 28, 2005 9:27 pm

Hi All,

Just a few engine and APU questions!

I'm curious to know how exactly turbofan engines are started. Are all engines turned initially by compressed air or do some use an electric motor to get the engine rotating to start speed? Does the power (be it air or electric) to start the engines always come from the APU or do airports have facilities to start airliner engines without the APU running? Is this even possible? Are all engines (in the case of a 747 for example) started in the same way, or does one running engine supply power to start the other three? Are turboprops started in a similar way?

Also, if an engine shuts down in flight, how fast does the aircraft need to be travelling to re-light the engine? (This question stems from the 1989 Kegworth disaster when the only working engine was mistakenly shut down, but the aircraft was gliding too slow to re-light it).

I have a few APU questions as well. Firstly, how is the APU started? If it's a small jet engine presumably the starting procedure is similar to that of the main engines. Are they electrically turned by a battery driven motor? Why are the APU's on some airliners not startable in flight?

Secondly, how many of the aircrafts systems can the APU run? Is it able to run ALL the electrics or just the critical components (i.e. Not running the galleys, all the cabin lighting, etc).

And thirdly, am I correct in thinking the APU on a twin jet (777, A330, F100) has to be both startable in flight and be able to run all the aircrafts hydraulic systems? If this is the case for twins, can the APU on 3 or 4 engined aircraft also power the hydraulics (assuming it can be started in flight).

If anyone could possibly take the time to answer these questions, or point me in the direction of a website that could be of some use I'd be most grateful!

Thanks
Paul
Edith in his bed, a plane in the rain is humming, the wires in the walls are humming some song - some mysterious song
 
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longhauler
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RE: Starting Turbofan Engines And APU's

Wed Sep 28, 2005 10:22 pm

The process of starting a jet engine begins with the starter, it is pneumatically powered (usually) and start the engine stage two N2, turning. At about 19% N2, the fan (N1) also starts to turn, the ignition is added, then at about 24% N2, the fuel is added. It is done in this sequence to make sure the air is flowing in the proper direction before adding fuel. This is done manually in older aircraft, and in the A320 I fly, it is all controlled by the FADEC with the movement of the engine master switch. Any start faults like no light, or hung start again are handled manually in older aircraft and automatically in the A320.

An inflight start can either be unassisted if above approximately 230 knots, or APU assisted if available, or slower.

The APU is started in the same way, as it is simply a small jet engine. The main difference is that the starter is electrically / DC powered. Either by external power if available, or on the battery if not.

What the APU can power depends on the aircraft. On the A320, the one APU generator can power all systems. However, the Galley and Customer Service (IFE) is shed as a precaution. The APU can be used at all altitudes for electrics and lower altitudes for pneumatics. If already started you may let it run at any altitude, however, if not running, the chances of it starting greatly increase at lower altitudes.

Hydraulics depend on the aircraft. Most have an engine pump / electric pump mix. Some aircraft, also have a Ram Air Turbine which powers some hydraulics and limited electrics in an emergency.

Hope this helps.
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Tornado82
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RE: Starting Turbofan Engines And APU's

Wed Sep 28, 2005 10:23 pm

Quoting PMN (Thread starter):
Does the power (be it air or electric) to start the engines always come from the APU or do airports have facilities to start airliner engines without the APU running? Is this even possible?

I was on an ERJ which had an extremely long boarding and wait (handicapped pax, an equipment switch which caused the boarding passes to put everyone in the front rows of the aircraft because the computer thought we were getting an ERJ-135 not an ERJ-145.. and left the back rows of the -145 empty.. leading to a balance problem, and then waiting out a brief thunderstorm) on a day which was about 97 degrees with a dew point in the 70's. At startup after pushback, the the aircraft got completely silent, and we were then pulled back to the gate. The captain told us that our APU had overheated, and we'd be starting with an air start cart, then made reassuring comments for nervous pax about APU's are unnecessary for safe flight, etc. After about 15 minutes the engines were lit, and away we went.
 
avioniker
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RE: Starting Turbofan Engines And APU's

Wed Sep 28, 2005 10:30 pm

Here's a good starting place
http://www.b737.org.uk/

In short
The APU normally is an electrically started turbine jet engine with a generator which will power the vast majority of aircraft electrical systems. It normally has a system to bleed air from the engine compressor to power aircraft systems such as the main engine starters and air conditioning/pressurization.

The main engines are most often started using air bled from the APU to drive a starter which is coupled through a gearbox to turn the turbine section of the engine. It really isn't too different from your car except that the starter uses air instead of electricity.

Airports (with rare exceptions) have high volume air available in the event that the APU fails or company policy dictates that you use external air for starting or on-the-ground air conditioning pack use.

The air needed to start an aircraft main engine is more high volume than pressure. A typical commercial aircraft requires approximately 18 to 26 psi of continuously delivered pressure under load. Most APU's will be regulated to deliver about 38 to 40 psi with no load.

I'll leave the rest of your questions to others. Gotta get back to work.

Happy days
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777wt
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RE: Starting Turbofan Engines And APU's

Thu Sep 29, 2005 3:02 am

Quoting PMN (Thread starter):
And thirdly, am I correct in thinking the APU on a twin jet (777, A330, F100) has to be both startable in flight and be able to run all the aircrafts hydraulic systems? If this is the case for twins, can the APU on 3 or 4 engined aircraft also power the hydraulics (assuming it can be started in flight).

Yes it needs to be startable in flight, and the #'s of auto shutdowns must be recorded in order to comply with ETOPS prodecure.

The APU uses fuel in the main supply tanks, if there are fuel, it will run. If not, it ain't worth anything.

Very few APU has it's own hyd pump mounted on it. Most of them power the electrical system which then powers the electric hyd pumps.

On the 747, with all engines off in flight, the windmilling engines are more than enough to keep hyd pressure up.
On the 777, there's the RAT (Ram Air Turbine) that keeps the electrical and hyd system running in event of a fuel out, no engines running and APU InOp.

Some aircrafts can't start APU in flight or above a specified speed because they do not have the air pressure stablizier in front of the APU intake. Without it, the APU will not run right in flight.
On 737's, the 777's and 757, 767 you can see this by noticing a door open in front of the APU intake. On the 737, it's just a wedge shaped object.

Here's an APU door of a 737:

View Large View Medium
Click here for bigger photo!

Photo © Marlo Plate - Iberian Spotters



And here's the APU being serviced with the APU door visable:

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Click here for bigger photo!

Photo © Paulo Herren - Jumpseat Team



Here's the intake of a 757 APU:

View Large View Medium
Click here for bigger photo!

Photo © Søren Geertsen



Turbofans can be started via the APU, the ground cart or crossbleed start.

Say your at a remote area, and the APU is InOp. You can use a ground cart to start one of it's engines but it barely has the power, the weaker models that is. Once the engine gets started, the ground cart may crap out soon from trying so hard.

After that, the running engine is powered up to 60% N1 and the crossbleed air is opened, this provides bleed air from running engine to a non running engine that is about to be started. Once the running engine is started, the next engine is started in order.

This happened to a DC-8 on a remote island. The airport found a old aircart that hasn't been used for years. The starting battery was dead, so they managed to get like 8 or 10 car batteries and hook some of them up in parrell and some in series to get the power needed to start the aircart. Then one of the DC-8's engine was started. Then crossbleed start was done to start the rest of the engines.
 
PMN
Topic Author
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RE: Starting Turbofan Engines And APU's

Thu Sep 29, 2005 6:49 am

Thanks for your time and informative responses guys. I thought this forum was as good a place as any to find out some answers!

Cheers
Paul
Edith in his bed, a plane in the rain is humming, the wires in the walls are humming some song - some mysterious song
 
mrocktor
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RE: Starting Turbofan Engines And APU's

Thu Sep 29, 2005 10:46 pm

Quoting LongHauler (Reply 1):
On the A320, the one APU generator can power all systems.

I find this surprising, they have an installed power capability equal to three times the system loads (assuming each engine's generator can supply all systems as well)?

mrocktor
 
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longhauler
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RE: Starting Turbofan Engines And APU's

Thu Sep 29, 2005 11:03 pm

Quoting Mrocktor (Reply 6):
I find this surprising, they have an installed power capability equal to three times the system loads (assuming each engine's generator can supply all systems as well)?

mrocktor

This concept is the same in all Airbus aircraft I have flown. All three generators (Engine and APU) are the same, with the same output, and any one of the three can power all buses. (That's electrical buses, not Airbuses!)

The only difference I have noticed, is that on the widebodies, there is no restriction. On the A320 series, the Galley and "Commercial", ie. IFE are shed, if only one generator is powering everything. Then comes back on line with the addition of another generator.

This contrasts with the Boeings I have flown, where that is not the case, and quite a bit is lost when being powered by only one generator.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!
 
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Jetlagged
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RE: Starting Turbofan Engines And APU's

Sat Oct 01, 2005 1:05 am

The procedure Longhauler describes in reply 1 is specific to the A320. The N2 values will differ between engine types. E.g. on the CF6-50, fuel can be turned on at 15% N2, although company SOPs may dictate otherwise. On a RR RB211 or Trent, the starter will turn N3. On non-FADEC engines, fuel and ignition are usually initiated simultaneously by the start lever/fuel switch.

In flight assisted starts can be assisted by another running engine, as well as the APU.

Regarding turboprops, these are usually started by an electric motor, but I'm sure there are exceptions.

Quoting LongHauler (Reply 7):
This contrasts with the Boeings I have flown, where that is not the case, and quite a bit is lost when being powered by only one generator.

Not on a 747. One generator will power the whole aircraft, though galley power might be turned off to reduce the load. I think the 757, 767 and 777 are similarly capable. The 737 is a different story, but that's as much to do with the simplified design of the electrical system, as the generator capacity.
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