ExpressJet_ERJ
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APUs And GPUs

Fri Dec 26, 2003 4:26 am

When an aircraft like a B1900 or Dash 8 doesn't have an APU, they use a GPU, correct? But, when they fly into a more remote airport, what do they use? Do they just turn off all equipment after landing? Or use their battery? At CLE I see 1900s taxi in at night, and shut down. The plane will be completely dark, and they rarely connect a GPU.
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liamksa
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RE: APUs And GPUs

Fri Dec 26, 2003 7:38 am

Some (all?) of the ATR's have a prop brake feature which stops the prop from turning whilst the engine is running, so one of the engines is used for ground power.

I think the Dash 8 has this feature also plus the option of an APU.
 
EMBQA
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RE: APUs And GPUs

Fri Dec 26, 2003 8:52 am

Dash-8's have APU's as an option....and most do........

If an aircraft dosn't have an APU or Ground Power...you can always just do a battery start. You actually get a faster-better start with batterys.

[Edited 2003-12-26 00:56:56]
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jjbiv
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RE: APUs And GPUs

Fri Dec 26, 2003 11:06 am

B1900s rarely get a GPU at outstations in my experience. They seem built to start off of the ship's battery routinely.

Our Saab 340Bs, on the other hand, always get a GPU to minimize battery wear. In fact, we have an incentive program for rampers to always find a working GPU and connect it.

Another option on props is to do a single engine turn, wherein the number two engine is never shutdown. This assumes sufficient fuel and no need to service the R/H side of the aircraft.

joe
 
ExpressJet_ERJ
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RE: APUs And GPUs

Fri Dec 26, 2003 12:55 pm

So, Liamska...About them running power from the prop. In this photo, is that what they're doing, or do they still have the prop spinning?



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Photo © Daniel Wojdylo

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liamksa
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RE: APUs And GPUs

Fri Dec 26, 2003 1:39 pm

G'day ExpressJet_ERJ

I'm not the guy to ask. Just a PPL Cessna driver  Big grin

The prop brake controls for the ATR can be seen in these photos below the engine start panel.

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Photo © Dennis Bauer
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Photo © Thorbjørn Brunander Sund - Birdlike Multimedia


Cheers, Rob.
 
L-188
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RE: APUs And GPUs

Fri Dec 26, 2003 2:22 pm

Ok lets get some terms straight.

APU.....Auxiliary Power Unit. A generator mounted in the aircraft designed to provide electrical and maybe bleed air power to an aircraft.

These days an APU is universally a turbine but there have been examples of piston powered APU's. The B-17 had one that was a Briggs and Stratton single cylinder lawnmower engine in civilian life. Except in the APU it drove a generator.

GPU.....Ground Power Unit. A generator or inverter mounted on a trailer at the airport that is designed to provide electrical power to the aircraft.

GPU's typically supply two types of voltage, 24VDC and 115VAC. Smaller prop jobs typically have 24VDC hook ups and larger airlines use A/C voltages, though this should not be considered a hard and fast rule. For example a lot of single engines and light twins use a 12VDC hook up and Eastern set their Lockheed Electras up for 24VDC in addition to the standard 115VAC due to equipment constraints at some of their stations.



Those 19 seat aircraft usually try and use ground power for starting to take it easy on the batteries, but especially if they are already warm, the batteries can easily handle a start.

It is when they a first started and are cold....That it is much better to use a GPU for electrical power.
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airplay
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RE: APUs And GPUs

Sat Dec 27, 2003 1:59 am

The use of GPUs and/or APUs are pretty much based on the individual operator and how and where they operate.

If an aircraft needs to be self sufficient and operate in temperature extremes, an APU is the way to go. Many Dash-8s are fitted with one.

If the airplane will be operated from well serviced airports, then a GPU might be the way to go and leave the expensive APU at the manufacturer, like many Dash 8 operators do.

But...as in some airliners an APU increases dispatch reliability or acts as a secondary back-up power supply in case of failure, it may be required equipment.

Then again, aircraft like Beech 1900s really don't need an APU or a GPU. They are quite capable of starting and operating without either but you may need some lighting to comfortably load passengers at night and some pre-heating or pre-cooling when operating from extreme temperatures.

 
SlamClick
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RE: APUs And GPUs

Sat Dec 27, 2003 2:10 am

The big difference between the Beech 1900 and a 737 for example is what spins the starter. On the Beech the engine is tiny and the starter is electric. On a big jet engine the starter is an air motor. An electric motor big enough to spin up a CFM to start speeds would be enormous and would need more batteries to crank it than the plane could carry.

To keep a few cabin lights burning does not draw much power. Starting is the issue.
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avt007
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RE: APUs And GPUs

Sat Dec 27, 2003 5:22 am

Just a little clarification on the Dash8- the APU is an option, lots of fleets don`t have them, lots do. Personally I like them, but the weight penalty on the -100s is a bit large. You can do battery starts on the -8, no problem, seldom will you hit the 805C temp limit. The -8 does not have the prop brake as the ATR does. I don`t agree that battery starts are better or faster, a gpu or APU is much better in my experience. If you are getting better starts, then there is a problem with the GPU. When shutting down without extra power, the crews can just leave the batteries on for a little bit to power the cabin lights. More often, they will leave #2 running while they deplane the pax. This is what is being done in the first picture posted. The indicator that reads 253 on the digital part is the #2 prop rpm. In the feather position the prop is at 250 rpm, and the DC generator will be online. A better way is to move the #2 power lever to the "disc" position. In the photo, it is at flight idle. By going to disc, the ECU will govern engine rpm at 66%, slower and therefore quieter than the 72% at flight idle. It does help the noise levels for the pax outside.
 
atct
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RE: APUs And GPUs

Sat Dec 27, 2003 10:20 am

Well at Delta here at PIT we use GPU's (usually borrowed from Continental haha) when a Dornier comes in with an inop APU. The Md-80's and CRJ's connect directly to the jetway power unit (a GPU attached to the jetway basically via a long chord that hangs down right beside the end.) Air-start's we use when the aircraft also has an inop APU. These we have and use more frequently due to we only have three dornier's a day (FRJ/328Jet). As i said before, the mad dogs and the canadairs just connect to the jetway, and we'll air-start one engine after the jetway has been removed and then pushback the aircraft. They'll start the other engine once out on ramp control area. Hope this example helps Greg.

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MD11Engineer
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RE: APUs And GPUs

Sat Dec 27, 2003 11:05 am

Many airlines prefer to use GPUs instead of having the APU run all the time because APUs are real fuel guzzlers (B737 APU 300 lbs/hour). Additionally many airports have noise restrictions, diesel GPUs are much quiter. Therefore the APU only gets started about 15 min prior to flight. The GPU the provides eklectric power on ground and if the weather is exceptionaly hot or cold, external airconditioning units or heaters can be connected to the plane.

Jan
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ExpressJet_ERJ
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RE: APUs And GPUs

Sat Dec 27, 2003 11:34 am

Thanks guys! That really helped. A bit more complicated than I originally thought.




- ExpressJet
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acidradio
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RE: APUs And GPUs

Tue Dec 30, 2003 4:05 pm

Where I work (XJ in MSP), the Saab 340s are almost always started with GPUs. This saves battery wear and makes it so the flight crew doesn't have to wait until the battery recharges to start the second engine. The Avro RJ85s have APUs and when an APU is inop (quite common) we can start the engines off the jetbridge's 115VAC 400Hz power. This is not all that much of a hassle if it has to be done as the engines have electric starters.

The 9E CRJ's that we also ground-handle have APUs in them, have the GPU (a Hobart unit on the jetway which is attached to local 115V 60Hz AC power and converts it to 115V 400Hz aircraft power) attached while on the ground and need the APU or a ground air start (either a big giant compressed air tank or what is essentially an APU on wheels) to start engines. When the APUs break on these things it is not enjoyable.

APUs are nice as they provide fairly instant heat and cooling to the cabin of the aircraft and we don't have to have another cable/wire/hose/etc. attached to the aircraft. But we do wait until we really need to use them as it is a waste of fuel if you don't really need it.
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Francoflier
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RE: APUs And GPUs

Wed Dec 31, 2003 1:27 pm

Most turboprops will start from their batteries. I don't say all of them, because I have a doubt about those bigger older ones like the electra, but I think it also will.

GPU is preffered because it saves the battery, it is cheap, and since on many Tprops a battery start calls for a warmer start it will also preserve the hot section.

Batteries only give you between 24 and 26V DC, while a GPU will give a continuous 28V and a much better response while starting (less voltage drop).

Starting for a turboprop is usually accomplished by the generator, which serves as an electric motor to spin the turbine. It is much easier to start a free turbine (like the PT-6) because you don't have to spin the prop. On a single-shaft turbine (Garrett, Dart etc...) the turbine directly drives the prop which makes it harder for the starter motor to turn the whole thing, thus more current drawn.

On some aircraft using the Garrett TPE-331, like the Jetstream 31/32, during a battery start, the 2 batteries will actually connect themselves in series, thus doubling the voltage. It Makes for a very fresh start, cooler than with a GPU...

Happy new year, all!

Franco.
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ExpressJet_ERJ
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RE: APUs And GPUs

Sat Jan 03, 2004 2:59 am

Where the APU should be, there is something else on this photo. Does it do the same as an APU? Anyone know what it is? It's the second down from the right fire handle, and has a green light. http://www.myaviation.net/search/photo_search.php?id=00031033
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avt007
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RE: APUs And GPUs

Sat Jan 03, 2004 3:23 am

I can`t read anything on that panel, but it definitely isn`t a standard deHavilland panel.
 
L-188
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RE: APUs And GPUs

Sat Jan 03, 2004 1:19 pm

On some aircraft using the Garrett TPE-331, like the Jetstream 31/32, during a battery start, the 2 batteries will actually connect themselves in series, thus doubling the voltage. It Makes for a very fresh start, cooler than with a GPU...


As an employee of a metro and casa 212 operating company, All I can say is not with a cold engine when it is near zero outside.

A GPU is almost mandatory. A 331 is a bitch to start when they are cold.
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Francoflier
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RE: APUs And GPUs

Tue Jan 06, 2004 12:53 pm

Thanks for the info L-188.

Actually I have never started a TPE 331 below 15 degrees Celsius OAT... (!)
(I love the caribean for that...  Smile/happy/getting dizzy)

All I know is that on the J-31, the battery start is "fresher" because the 2 batteries in series actually do give quite a "punch". It starts fresh because the RPM raises quite rapidly, thus maintaining a lower EGT, especially on a hot engine, a few minutes after shutdown on a 35 deg Celsius weather.

Starting up in cold weather calls for other problems; the battery output is lower, the ignition is harder to achieve, the oils are less fluid, etc...

I don't know wether the metro or casa 212 use this series-connected battery system. If they don't, then a GPU is definitely a better option.

As for the TPE 331 being a b**** to start in cold weather, I really didn't know, but I'll take your word for it!!  Wink/being sarcastic

Regards.
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