olbie

Hello:

I was wondering if the temperature setting for the cabin and cargo hold has any effect on the fuel consumption?

My initial thought is that the cooler the selected temperature the higher the fuel burn but I really have no data to back this up...

Any ideas?

Thanks

Intruder

Depending on the aircraft, temperature regulation in the cargo hold requires switching packs to a higher flow setting. This can use up to 3% more fuel in the case of the 747 with both fwd and aft cargo holds temperature-regulated.

olbie

Thanks but in your case you are increasing the air flow supply, which inevitably increases fuel consumption.

For a given flow, do different temperature settings change the fuel burn?

Thanks

Intruder

The temperatures in the lower cargo holds are unregulated unless the lower hold switch is turned on. Turning on that switch will enable High Flow for 1 or 3 packs.

Green Guard

yes

if you set it to warmer temp you will spend LESS fuel, and v.v.

MarkerInbound

For a given flow no. You would be taking X amount of air from the bleed system that is warm. That air can go one of two paths and normally the air mix valves control how much air goes down each path. When you are trying to heat the cabin most of the air bypasses the packs and goes straight into the distribution lines. When you are trying to cool the plane most of the air goes through the packs and is cooled before it goes into the distribution system. But in the distribution lines you still have the same amount of air you bled off the engines.

Why do you think for a given flow cooler would affect fuel flow?

olbie

"in the distribution lines you still have the same amount of air you bled off the engines"

That's the key! This does make sense and means that there is no fuel consumption difference whether hot or cold is selected.

My initial thought was: cooler air means less hot bleed air required = lower fuel consumption but I can see now how this is wrong.

Thanks a lot for clearing it up!

Joetom

I am not sure if this is a simple question.

A few points of interest on this item.

Most engines are designed for cruise conditions, so bleed air is in the numbers.

747 using 2 packs instead of 3 saves fuel.

The pack inlet/outlet doors do different things according to various conditions/requests, the variations of these doors change drag and even lift due to changing airflow conditions.

Some aircraft packs can request different bleed air temp, lower I think, this can also use more fuel as a result.

Looking at the active fuel flow data of engines/apu and playing around with setting is a good way to see what goes on, an easy one is the apu with pack/packs on/off and genny on/off setting.

Also of interest, the 787 does not use bleed air off the engines/apu like older aircraft, the 787 engines don't use any fuel making air for these other systems.

TopBunk

Joe Tom saidHmmm, not according to an MEL I have access too:

Intruder

True for the 747 Classic, but not the 744. The 744 has the automatic high-flow mode that increases fuel consumption.

TopBunk

Intruder

My reply is for the B747-400 (RR engined) model