Reducing idle fuel consumption
Thread Starter
Join Date: May 2007
Location: Both Emispheres
Posts: 226
Likes: 0
Received 0 Likes
on
0 Posts
Reducing idle fuel consumption
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 ?
Could a "super idle-efficient" jet engine be invented ?
Join Date: Nov 2005
Location: right here
Posts: 342
Likes: 0
Received 0 Likes
on
0 Posts
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 ....
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 ....
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: Jul 1999
Location: 58-33N. 00-18W. Peterborough UK
Posts: 3,040
Likes: 0
Received 0 Likes
on
0 Posts
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?
Join Date: Mar 2000
Location: Arizona USA
Posts: 8,571
Likes: 0
Received 0 Likes
on
0 Posts
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?
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.
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jul 1999
Location: 58-33N. 00-18W. Peterborough UK
Posts: 3,040
Likes: 0
Received 0 Likes
on
0 Posts
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.
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)
Join Date: Aug 2004
Location: malaysia
Posts: 404
Likes: 0
Received 0 Likes
on
0 Posts
Fuel surcharge
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.
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.
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jul 2008
Location: Gone to my "Happy Place".
Posts: 157
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: Jul 2000
Location: Thailand
Posts: 942
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes
on
0 Posts
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.
Thread Starter
Join Date: May 2007
Location: Both Emispheres
Posts: 226
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes
on
0 Posts
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!
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Apr 2006
Location: Krug departure, Merlot transition
Posts: 658
Likes: 0
Received 0 Likes
on
0 Posts
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"
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
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
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.
There ways to operate the engine more efficiently; operating it below it's idle speed isn't one of them, however.
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
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)?
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).