IDG disconnected fuel saving

[tony64peter]

Would anyone be able to advise the fuel saving with an IDG disconnected on -80C2 engine? Running APU burns about 90kg/hr, would this usage be offset by the reduced engine burn with IDG disconnected? EDTO ops.

[NSEU]

Wouldn't a decrease in fuel burn be offset by the increase in fuel burn on the other engine (due to a greater generator load)?

[tony64peter]

Thats why the APU is started to run a second generator.

[nitpicker330]

Jesus H.

What is your companies fuel policy!!:eek:

Is your company doing that badly it needs to disconnect IDG's !!:confused:

Remember once you push the disconnect button it cannot be reconnected in flight. :D

[frogone]

And if the APU fails? :ooh:

[tony64peter]

The reason for asking. On long range flights that are ETP1D critical. If after an IDG disconnect the APU is started to provide a second generator source. Is this APU fuel usage going to be offset by the engine fuel saving with IDG disconnected and prevent a fuel diversion. Has anyone any experience on this matter?

Delta Sierra's please do not respond!

[NSEU]

Are you talking about a Boeing 767? Are you talking about the application of an MEL prior to flight?

Rgds
NSEU

[Capn Bloggs]

If after an IDG disconnect the APU is started to provide a second generator source.
I'd be starting the APU first... :cool:

[dhardesthard]

The APU must burn more fuel than what could be saved from a disconnected IDG. The size of the APU turbine I am sure would consume more fuel than what could be saved from a disconnected IDG.

[nitpicker330]

Just remove the Pilot seats, that'll save weight and therefore fuel burn.:D

Disconnecting the IDG to save fuel!! Now I've heard it all:rolleyes:

[tony64peter]

Delta Sierras. IDG's are not disconnected to save fuel.

For an IDG fault that requires disconnection and APU started iaw QRH, this will eat into ETP1D minimums unless the reduced gearbox load on the engine provides sufficient fuel saving to offset this burn.

DOES ANYONE HAVE ANY INFO RE THIS SITUATION?

I say again Delta Sierras DON'T REPLY

[BOAC]

You do not specify type, but I would agree with dhardesthard post #9.

2 questions:

Is your scenario pre-EDTO entry? In which case I would have thought you were not EDTO 'capable'?

If post-EDTO entry, would you not require the APU running anyway for EDTO (twin assumed) in which case I would have thought the extra fuel would be pretty much negligible? There is always load-shedding to be had as well.

[compressor stall]

would you not require the APU running anyway for EDTO (twin assumed)

Not with Airbus 320 Family.

[8che]

tony64peter/BOAC

What does it matter if it affects you're crit fuel at an ETP ? There are a multitude of reasons why you may end up with less than crit fuel at your ETP. It happens regularly particularly in New York if your number 40 for departure.

This is fuel for DISPATCH with the requirment ending at "the commencement of flight". ETOPS entry points etc are irrelevant.

As long as you are confident you can still arrive at destination with the legally required reserve and the required ETOP's technical equipment is still servicable then carry on. The extra ETP fuel is to cover the worst failure at the worst point on the route at dispatch. Once airborne this is not controlling as the statistical chance of it happening at that point are remote to say the least.

Now if you dispatch with an IDG u/s then you would require extra fuel (the APU usage will burn more fuel) but you didnt ask that.

[glum]

I don't know that there are any figures freely available for generator fuel burn (if there are I wish I could find them).

However, if you consider that a typical generator consumes about 150-200hp at full power, and that a typical aircraft engine is making well over 50,000hp (depending how the number is arrived at - a whole other topic) you can see that the fuel used to turn the generator is really rather small, something in the region of 0.3% perhaps?

Then take into account that most generators are only running at 30-50% load during cruise, so essentially 0.15% of your fuel flow.

An APU is about 15-20% efficient at turning fuel into electricity, and gobbles up anything upwards of 120kg per hour for a 50kVA unit.

Maybe you can do the maths for your engine and work out the fuel flow required to spin the generator...

[NSEU]

Just trying to learn here... Hopefully, this post does not classify myself as a "Delta Sierra"... Whatever that is.

Since the QRH is specified, I assume you're in flight, tony64peter????
Again, you have not specified the aircraft, only a partial engine name, so you may be drawing flack because of the lack of information in your post.


The QRH does not specify the reason for starting the APU and my B767 Maintenance and Training Manuals don't go into enough detail to tell me what happens in flight if you disconnect the IDG and (then) start the APU. With the IDG disconnected (prior to APU start), the automatics have told the good generator to provide power to the whole aircraft. Does the act of just starting the APU cause the good generator BTB to trip and the APB to close? Or is the APU started just to provide a backup should the other generator fail?

Thanks.
Rgds
NSEU

[Cough]

Tony,

Does your MEL specify that you have to carry extra fuel for this configuration if you dispatch in this state (say non ETOPS) with your IDG disconnected and APU supplying elec? Have a look, I guess it will answer your question..

[thermostat]

Unfortunately the APU is NOT reliable. I once had a departure with #1 gen inop. (They didn't want to take a delay to change it). We were told to use the APU for the flight as the we only needed 2 generators (MEL). Well, on the return flight, at night, over a bit of water, the APU did an Auto Shutdown. So with only one gen working we made a landing short of destination which grounded the flight. Became a very expensive flight. Don't rely on the APU, ever.
T