Reducing idle fuel consumption

[el #]

Beside the drastic measure of shutting down engine, have researches been made with the objective of reduce fuel usage during descent ?

Could a "super idle-efficient" jet engine be invented ?

[FCS Explorer]

the fuel-flow during descend compared to cruise is (example for B738) roughly:
200kg for the 20 minutes of descend
versus
800kg for 20 minutes cruise.

sure a more-idle-descend could be invented.
but if you blow 5-10 tons during cruise saving 50kg once per leg during descend is nice but ....

[Rainboe]

Engine idle fuel flows are predicated on being able to get go-around power in a certain minimal time interval. Whilst you could quite happily tweak engines to get a lower idle speed, when you needed rapid power, it would be no good sitting there with a rotated aircraft and terrain rapidly approaching whilst the engines happily burbled away thinking about increasing N1 as the fuel flow appeared to be increasing. Remember the AA757 Cali. You must always be able to get high power quickly, so what decides idle speed and fuel flow is that engine types' response to power increases at those lower power settings.

[forget]

I thought the Cali engines responded as advertised. Wasn't it the Speed Brakes being left out that prevented them making the extra few feet needed to clear the ridge?

[411A]

I thought the Cali engines responded as advertised. Wasn't it the Speed Brakes being left out that prevented them making the extra few feet needed to clear the ridge?

The AA Cali B757 engines responded just fine.
Trying to climb away from rapidly rising terrain with the spoilers extended was the problem...that, and the crew not understanding the FMC inputs/display together with a rushed approach and ignoring raw data navigation information.

A ****-up of the highest order, and a classic example of how not to safely operate the airplane.

[Rainboe]

The Cali example was mentioned because if you had a lower idle than flight idle, they would not have got the response they needed from the engines when they banged on full power. The question was about cutting fuel flow to promote lower idle fuel consumption. You simply pour fuel into an engine, and it responds at the rate of fuel supply. I was explaining that trying to reduce flight idle speed would have a corresponding effect on throttle response.

Why the hell is there no 'Speedbrake out/throttles not in idle' warning on the 757? That would probably have saved them. The 737 has it.

[forget]

I was explaining that trying to reduce flight idle speed would have a corresponding effect on throttle response.

Point taken.

Why the hell is there no 'Speedbrake out/throttles not in idle' warning on the 757? That would probably have saved them. The 737 has it.

October 96.

Therefore, as a result of the investigation of this (Cali) accident, and with the concurrence of the Aeronautica Civil of Colombia, the National Transportation Safety Board recommends that the Federal Aviation Administration:

Evaluate the effects of automatically stowing the speedbrakes on existing airplanes when high power is commanded and determine the desirability of incorporating automatic speedbrake retraction on these airplanes for windshear and terrain escape maneuvers, or other situations demanding maximum thrust and climb capability.(Class II, Priority Action) (A-96-90)

Require that newly certified transport-category aircraft include automatic speedbrake retraction during windshear and ground proximity warning system escape maneuvers, or other situations demanding maximum thrust and climb capability. (Class II, Priority Action) (A-96-91)

Evaluate the Boeing Commercial Airplane Group procedure for guarding the speedbrake handle during periods of deployment, and require airlines to implement the procedure if it increases the speed of stowage or decreases the likelihood of forgetting to stow the speedbrakes in an emergency situation. (Class II, Priority Action) (A-96-92)

[veloo maniam]

Hello Aviators..I have a question. Does the fuel surcharge cover the the price
of the fuel carried on board? If it does, why all the request for track shortenings? I wonder how the costings are calculated.Replies appreciated.:ok:

[Rainboe]

Guarding the speedbrake handle is no good, you unfortunately need both hands, one on the controls, and one to keep free for the MSP and throttles. So it is not going to be back on the speedbrake lever. Auto retraction would be a big bonus, but just a tweeter like the 737 have would be a big safety feature.

The fuel surcharge is to account for the extra cost of the fuel. Pilots ask for directs to improve efficiency and save time. It saves costs which eventually saves airfares. You compare costs to last years operation...and find your eyes popping out! Somebody, somewhere along the line, is overdoing it financially. That is why surcharges exist. If your raw material (fuel) goes up astronomically, you cannot price your tickets at the same level.

[Jimmy Do Little]

The fuel surcharge is to account for the extra cost of the fuel. Pilots ask for directs to improve efficiency and save time. It saves costs which eventually saves airfares. You compare costs to last years operation...and find your eyes popping out! Somebody, somewhere along the line, is overdoing it financially. That is why surcharges exist. If your raw material (fuel) goes up astronomically, you cannot price your tickets at the same level.

Ummmm, Wow! Exactly correct!:D

[rubik101]

Rainboe, you write;
Guarding the speedbrake handle is no good, you unfortunately need both hands, one on the controls, and one to keep free for the MSP and throttles.

Why do you need to touch the throttles when the speedbrake is extended?

Other than aligning the HDG bug very occaisionally, why do you need to touch the MSP? All the other inputs can be made by your oppo in the seat next to you.

[Rainboe]

757 throttles move very very slowly, and I'm old enough not to trust automatics! I like to feel them moving with my hand, and sometimes prompt them on their way! When the AP is engaged, the handling pilot makes all MSP panel selections- heading, altitude, LNAV, VNAV, LOC and APP, so that hand is constantly flying up to the panel. That's SOPs.

[el #]

FCS, Rainboe,

thank you for explanation. For a coincidence, I was reading about AA in Cali few days ago.
I guess, if a compromise between spool time and fuel consumption could be found, that could be an area of improvement for short flights. Perhaps during descent, the engines could be kept spooling by using pressure from an air intake. Just crazy ideas, the really smart guys must have thought about that already.

[Rainboe]

They have! They already have air coming directly in to spool up the engine. They set the idle speed over a long time to get it right! The engine has to be working quite hard even at idle because it is also driving the alternators, engine driven hydraulic pumps and air is bled off for pressurisation. Idle power is not wasted power- it still goes towards propulsion for moving the aeroplane!

[SNS3Guppy]

In terms of efficiency, a turbojet (or turbofan) engine is most efficient at high RPM's. Typically about 90% of it's rated speed. Operating a turbine engine at very low RPM's is a very inefficient way to operate the engine.

The question really isn't about how to make a more efficient idle, but how to burn less fuel. This can only be accomplished by putting less fuel through the engine, which means a lower total fuel burn, lower temperature, lower flame pressure, lower thrust output, paradoxically lower efficiency, and a greater susceptability to flame-out, as well as longer response times, higher thermal transients, etc. Let's not forget that idle settings are also part of the design and measurement of thermal cycles on the engine, and tied not only to operational issues but to maintenance issues involving the cooling and expansion of internal components, to say nothing of the other primary issue noted by Rainboe, which is that the engine is still driving numerous accessories, as well as maintaining a minimum speed for the output of bleed air.

Simply idling it back farther than the manufacturer has originally intended isn't as easy as it seems, nor necessarily a good idea.

The reason we fly higher isn't just for an increase in true airspeed...it's because at higher altitudes we are able to push the power up farther and operate the engine at higher RPM's than at lower altitudes...putting the engine closer to it's most efficient range.

[barit1]

IIRC the 747 Classic would actually decel to ground idle at TOD with a clean configuration. However as soon as flaps (or gear???) were extended, the donks would spool up to "approach idle" (aka flight idle).

The objective was clearly lower thrust to permit greater vertical speed (descent angle), at the expense of a longer spoolup time. Once the ship was dirty, normal spoolup times applied.

[main_dog]

IIRC the 747 Classic would actually decel to ground idle at TOD with a clean configuration. However as soon as flaps (or gear???) were extended, the donks would spool up to "approach idle"

Selecting landing lap (25 or 30) with the aircraft in flight will activate flight idle on the 747-200. And just as well: the CF6s especially take ages to spool up.

However, reducing to idle thrust at high altitude will not get you as low an N1 as you can achieve at lower altitude... I noticed this on my previous A/C as well, as you descend, idle N1 appears to decrease. Anyone know what the technical explanation is (while I go search for my anorak:})?

MD

[SNS3Guppy]

The Classic employs a flight idle setting which is commanded when flaps are set at 25 or greater. Engine idle speed is automatically increased by 8% N2, as you indicated. Descent engine idle, however, remains at it's regular speed and power setting, per the engine requirements, during a normal flaps-up descent.

There ways to operate the engine more efficiently; operating it below it's idle speed isn't one of them, however.

[SNS3Guppy]

However, reducing to idle thrust at high altitude will not get you as low an N1 as you can achieve at lower altitude... I noticed this on my previous A/C as well, as you descend, idle N1 appears to decrease. Anyone know what the technical explanation is (while I go search for my anorak)?


If you're talking N1 speed, then you're talking fan speed, which isn't really related to the gas generator engine idle speed (N2 or N3, depending on your engine).

As you descend and drag increases, the fan speed will vary at idle...but it's not fan speed here that concerns us, but the gas generator speed, which is your power section (N2 for a two-spool engine).

[main_dog]

Interesting... so next time I'll watch for the N2 (or N3 if it's a "Roller") and see if that varies...

Thx.

MD

[SNS3Guppy]

Bear in mind while you're watching it that the engine doesn't simply use N2 as it's sole-source input for establishing idle speed. The idle power settings are established as part of the maintenance trim-in procedure, but the engine will set it's operating minimum based on multiple inputs...the speed isn't the only parameter it must consider.

Even on the ground, with the same engine on various given days, the idle speed may vary slightly, and for a given condition, the speed an engine must go to do what's required of it varies with wear and tear, compressor efficiency, engine load or demand, etc. Bleed demand loads, electrical and CSD loads, hydraulic pump loads and other accessory demands, etc, change the requirements on an engine to change the operating RPM, temperatures, etc, within a given set of environmental conditions...which also vary with airspeed (ram air input), affecting EPR, etc.

Regardless of the variances in the engine RPM within a given unit, or in comparison with several other engines on the wing, the bottom line is that the engine can be reduced in operating RPM only so far without compromising the job it must do (load), safety (flame-out), and integrity (flame propogation, compressor stall, etc). A slow turning turbine is not an efficient turbine.

[main_dog]

Fair enough.. now that I think of it, if one of the engines has a high-stage bleed locked out or CSD disconnected, for example, it will indeed idle slower (and have a much lower EGT)

:ooh:

[SNS3Guppy]

Your car is the same way, albeit a smaller, piston engine. Engage the air conditioning, and see how this affects your idle?