Derated engines
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Typically engines are more fuel efficient when the power is pulled back and you get closer to some efficient L/D....
Most engines are so overbuilt for the job at hand where it will put out X power, but they only give you Y(derated) and the operator only let's you push it to Z (reduced) trying to stretch the life of the engines out indefinately..
Is it possible that some engines are loping along as such reduced power they aren't efficient? Sure....which ones? I don't know...but comes to mind putting a 454 in a Volkswagen Bug is a pretty inefficient use of power, milage sucks, but the engine certainly won't break pushing that little car around...
Personally I would take a lighter engine, less overbuilt, running in a more efficient operating range to save some fuel...if the fuel savings were such it made sense...but not if engines were breaking left and right...
I guess the question is the model of how much over building of engines you need that once powered back you get to some calculation of reliabilty where the engine will last for x amount of hours/years ect....
Most engines are so overbuilt for the job at hand where it will put out X power, but they only give you Y(derated) and the operator only let's you push it to Z (reduced) trying to stretch the life of the engines out indefinately..
Is it possible that some engines are loping along as such reduced power they aren't efficient? Sure....which ones? I don't know...but comes to mind putting a 454 in a Volkswagen Bug is a pretty inefficient use of power, milage sucks, but the engine certainly won't break pushing that little car around...
Personally I would take a lighter engine, less overbuilt, running in a more efficient operating range to save some fuel...if the fuel savings were such it made sense...but not if engines were breaking left and right...
I guess the question is the model of how much over building of engines you need that once powered back you get to some calculation of reliabilty where the engine will last for x amount of hours/years ect....
Originally Posted by wangus
OK, then answer this, why does Boeing publish takeoff weights above Maximum Structural?
MTOW: 60t
Max RTOW: 63t
CDL penalty: 2t
Corrected Max: 61t
ie you can still take off at MTOW.
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Sillypeoples:
You are probably quite right regarding efficiency, but then the issue becomes logistics and commonality.
The CFM56-3 started out at the 3B1 rating, then with some hardware upgrades was "pushed" to 3B2 and 3C1 ratings - all within the same carcass, mountings, accessories, etc. If an operator selected the lower thrust rating for his fleet, it would do him little good to insist on a physically smaller engine, when that would cost more for special tooling, spare parts, etc.
Further, if the operator decided to trade up to a newer fleet some years later, the new buyer of the old planes would have the option of using the higher thrust rating if required on his route system.
Personally I would take a lighter engine, less overbuilt, running in a more efficient operating range to save some fuel...if the fuel savings were such it made sense...but not if engines were breaking left and right...
The CFM56-3 started out at the 3B1 rating, then with some hardware upgrades was "pushed" to 3B2 and 3C1 ratings - all within the same carcass, mountings, accessories, etc. If an operator selected the lower thrust rating for his fleet, it would do him little good to insist on a physically smaller engine, when that would cost more for special tooling, spare parts, etc.
Further, if the operator decided to trade up to a newer fleet some years later, the new buyer of the old planes would have the option of using the higher thrust rating if required on his route system.
Last edited by barit1; 26th Nov 2012 at 00:58.
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We are advised to never go to full thrust(except for emergency) on a derated takeoff due to maximum certified thrust/controllability issues due to VMCG, whereas on an assumed temperature reduced thrust takeoff, there is no such restriction.
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Sillypeoples,
"Typically engines are more fuel efficient when the power is pulled back and you get closer to some efficient L/D".
Incorrect. Jet engines are at their most efficient when operating at highest speed/temperature. Selecting a derated climb thrust for example burns more fuel than going for full climb. However the ratio of engine life cost to fuel cost is what a company has to balance. Terms such as Cost index deal with this for climb crusie and descent. Take off is a different matter with the balance of commercial load against engine wear.
Remember the size of the engine and its larger than required thrust on day to day ops is down to the fact that it has to meet the performance requirements when the other engine stops working over a large atmospheric range !
p.s the B777-2 I fly has a MTOW of 347 tons
"Typically engines are more fuel efficient when the power is pulled back and you get closer to some efficient L/D".
Incorrect. Jet engines are at their most efficient when operating at highest speed/temperature. Selecting a derated climb thrust for example burns more fuel than going for full climb. However the ratio of engine life cost to fuel cost is what a company has to balance. Terms such as Cost index deal with this for climb crusie and descent. Take off is a different matter with the balance of commercial load against engine wear.
Remember the size of the engine and its larger than required thrust on day to day ops is down to the fact that it has to meet the performance requirements when the other engine stops working over a large atmospheric range !
p.s the B777-2 I fly has a MTOW of 347 tons
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Seems to be the standard way where I am as well, BUT.......would it not make more sense to do max assumed thrust first?
Regarding VMCG issues I am not sure what to answer. I don't have the RTOW charts here but would let's say a 26K assumed to 55 deg produced the same speeds as a full 22K? (Assuming they give the same RTOW)
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Vmcg can only be changed by using a derate. Not by using an assumed temperature. Which gives derate users quite an edge in winder operation as it allows higher take off weights due to lower Vmcg.
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Of course, frequently both get used together.
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why not use the assumed temperature method first instead of derated thrust?
If you are at a reasonable weight and a speed schedule well above Vmcg/Vmca, no reason why you can't use either approach to achieve the end thrust result.
However, at low weights (typically a ferry or short runway), the decision process involves certification considerations. The AFM pilot requirements are based on the installed rated thrust limits. In turn limiting speeds such as Vmcg and Vmca are based on the limit thrust.
If the pilot elects to depart with a reduced thrust schedule, there is nothing to prevent his/her pushing the power levers up to achieve rated thrust during the takeoff .. say, in the event of a failure. Mind you, if one intends to do that it is appropriate to do it CAREFULLY due to the risk of a thrust exceedance and, at low weights, the very real risk of getting into control problems.
Now, if one uses a DERATED (ie lower certificated) thrust setting, that is the same, conceptually, as unbolting the big engine and replacing it with the smaller one. The pilot cannot command higher than the DERATED thrust setting during the takeoff. The lower Vmcg/Vmca speeds will still apply.
In the real world, of course, we don't change the engines physically but call up a defacto smaller engine either by a fuel control setting preflight (which is not very convenient) or electronic cockpit selection. It is essential, in this case, that the pilot doesn't permit thrust levels to exceed the derate limits otherwise the derate Vmcg/Vmca speeds could catch the pilot out rather badly.
As a for instance, I was involved in the (fatal) accident investigation, quite some years ago, of a twin turboprop which suffered an uncommanded feathering but without an accompanying engine failure.
The crew, apparently, was quite confused by the whole thing and, during the latter stages of the takeoff, increased thrust settings in an attempt to improve the then existing abysmal performance.
End result (with very good spec engines) was that the realworld Vmca went through the roof (VERY rapidly) and it was yaw, roll, and all over, red rover. Fortunately only folks on board were killed but it would have been very much worse had it not been for the existence of the large ditch which took the aircraft out of contention.
While derate/flex wasn't involved, the same over thrust consideration killed them just as dead.
If you are at a reasonable weight and a speed schedule well above Vmcg/Vmca, no reason why you can't use either approach to achieve the end thrust result.
However, at low weights (typically a ferry or short runway), the decision process involves certification considerations. The AFM pilot requirements are based on the installed rated thrust limits. In turn limiting speeds such as Vmcg and Vmca are based on the limit thrust.
If the pilot elects to depart with a reduced thrust schedule, there is nothing to prevent his/her pushing the power levers up to achieve rated thrust during the takeoff .. say, in the event of a failure. Mind you, if one intends to do that it is appropriate to do it CAREFULLY due to the risk of a thrust exceedance and, at low weights, the very real risk of getting into control problems.
Now, if one uses a DERATED (ie lower certificated) thrust setting, that is the same, conceptually, as unbolting the big engine and replacing it with the smaller one. The pilot cannot command higher than the DERATED thrust setting during the takeoff. The lower Vmcg/Vmca speeds will still apply.
In the real world, of course, we don't change the engines physically but call up a defacto smaller engine either by a fuel control setting preflight (which is not very convenient) or electronic cockpit selection. It is essential, in this case, that the pilot doesn't permit thrust levels to exceed the derate limits otherwise the derate Vmcg/Vmca speeds could catch the pilot out rather badly.
As a for instance, I was involved in the (fatal) accident investigation, quite some years ago, of a twin turboprop which suffered an uncommanded feathering but without an accompanying engine failure.
The crew, apparently, was quite confused by the whole thing and, during the latter stages of the takeoff, increased thrust settings in an attempt to improve the then existing abysmal performance.
End result (with very good spec engines) was that the realworld Vmca went through the roof (VERY rapidly) and it was yaw, roll, and all over, red rover. Fortunately only folks on board were killed but it would have been very much worse had it not been for the existence of the large ditch which took the aircraft out of contention.
While derate/flex wasn't involved, the same over thrust consideration killed them just as dead.
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@JammedStab, of course that is sensible and quite often done. Although the Boeing performance software tries to go for lowest engine rating first. Anyway, on contaminated runways and those with a reduced braking action we are not allowed to use the ATM method, but we can of course use derates as derates will allow a better performance in these cases (lower possible V1). One has to be aware of the caveats as pointed out by John tough, it can get nasty very very fast if one advances the thrust at a V1 of 98kts just because one engine quit.
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Thanks for the replies.
I would think that if there is an engine failure at V1, the thrust most likely won't get advanced(at least I haven't seen it done in the sim and there are not many real world actual events to analyse).
But there may be other reasons to push the thrust way up. Maybe a feeling at higher speeds that acceleration is not normal, or a object on the runway or the end of the runway itself or windshear. Of course, using your emergency authority to advance the thrust is the obvious thing to do. The chance of having an engine failure at this time is extremely small, especially with EEC's that will prevent an overboost.
It just seems that after reading the warning that you are not to advance the thrust beyond the certified takeoff limit due to potential controllability issues could make some people hesitant to do so even when they should do so and therefore if max assumed temp thrust reduction is used first and most frequently then it could be possible to prevent a potential belief that you must not advance the thrust above the takeoff limit.
With overboost protection and all engines operating, I don't see much adverse risk to firewalling the thrust levers on normally operating engines when an urgent situation requires it.
So I wonder why the Boeing software does derate first. Maybe for displayng maximum payload capability in the quickest manner.
I would think that if there is an engine failure at V1, the thrust most likely won't get advanced(at least I haven't seen it done in the sim and there are not many real world actual events to analyse).
But there may be other reasons to push the thrust way up. Maybe a feeling at higher speeds that acceleration is not normal, or a object on the runway or the end of the runway itself or windshear. Of course, using your emergency authority to advance the thrust is the obvious thing to do. The chance of having an engine failure at this time is extremely small, especially with EEC's that will prevent an overboost.
It just seems that after reading the warning that you are not to advance the thrust beyond the certified takeoff limit due to potential controllability issues could make some people hesitant to do so even when they should do so and therefore if max assumed temp thrust reduction is used first and most frequently then it could be possible to prevent a potential belief that you must not advance the thrust above the takeoff limit.
With overboost protection and all engines operating, I don't see much adverse risk to firewalling the thrust levers on normally operating engines when an urgent situation requires it.
So I wonder why the Boeing software does derate first. Maybe for displayng maximum payload capability in the quickest manner.
Last edited by JammedStab; 28th Nov 2012 at 11:47.
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I wouldn't sweat too much on the Vmcg/Vmca consideration. Just be aware of it as part of your pretakeoff planning and general decision processes.
One needs to be low weight and using a minimum speed schedule to be exposed. That is, the weight range where you see the V speeds run constant with reducing weight.
However, be very wary, also, of crosswind operation at low weight. The declared V speeds will be based on nil wind (7kt for older UK certifications). In the real world, the V speeds will increase with crosswind from the failed side. Think of increases in the order of 0.5kt/kt for twins and in excess of 1.0kt/kt for quads ... if you have a high crosswind limit and you are at low weight with a min speed schedule .. the potential is there for a BIG surprise.
One area which you can control is that of empty ferries .. if the runway isn't short, why go off at the minimum speed schedule ? Maybe use a higher speed schedule appropriate to a higher weight ?
One needs to be low weight and using a minimum speed schedule to be exposed. That is, the weight range where you see the V speeds run constant with reducing weight.
However, be very wary, also, of crosswind operation at low weight. The declared V speeds will be based on nil wind (7kt for older UK certifications). In the real world, the V speeds will increase with crosswind from the failed side. Think of increases in the order of 0.5kt/kt for twins and in excess of 1.0kt/kt for quads ... if you have a high crosswind limit and you are at low weight with a min speed schedule .. the potential is there for a BIG surprise.
One area which you can control is that of empty ferries .. if the runway isn't short, why go off at the minimum speed schedule ? Maybe use a higher speed schedule appropriate to a higher weight ?
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You may also achieve a deeper reduction using the highest thrust as the start point if your aircraft automatically boost thrust to Rated or ever Higher (ATTCS/reserve/APR) as the thrust level post Vef is higher than if you had used a de-rate.
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We operate the largest in the sky, the ge90-115b derated to 110k lbs. an engine like the PW4062 is commonly derated to 60k. This is usually a program plug installed on the EEC.
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8che -
First off, 30 seconds at reduced thrust on a departure so the mechanic can walk out there and keep signing off blades that 'don't look that bad' is the reason why you put thousands of feet of runway behind you and climb out over the bushes every day is hardly a sales point for 'safety first'...
That said, as far as 'efficiency goes'...just because an engine can burn hot doesn't mean the fuel flow is going to make it to Hawaii.....
First off, 30 seconds at reduced thrust on a departure so the mechanic can walk out there and keep signing off blades that 'don't look that bad' is the reason why you put thousands of feet of runway behind you and climb out over the bushes every day is hardly a sales point for 'safety first'...
That said, as far as 'efficiency goes'...just because an engine can burn hot doesn't mean the fuel flow is going to make it to Hawaii.....
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Originally Posted by Sillypeoples
First off, 30 seconds at reduced thrust on a departure so the mechanic can walk out there and keep signing off blades that 'don't look that bad' is the reason why you put thousands of feet of runway behind you and climb out over the bushes every day is hardly a sales point for 'safety first'...
You will not find companies in this world (and especially in this economy) operating large numbers of medium or large engines (let's say 20K+ thrust) doing continuously only full rated thrust takeoff and then replacing engines at the first small sign of problem.
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Sillypeople
Firstly you dont de-rate to leave thousands of feet behind. The longer the runway used the more de-rate you can take. Yes ? Exactly what is unsafe about meeting the performance requirements while causing the least stress to the engine ?
Maybe the entire industry is wrong and you are right ?
Secondly we are talking about engine efficiency here not airspeed. Why do you think Jet engines are more economical at high altitude ?
Firstly you dont de-rate to leave thousands of feet behind. The longer the runway used the more de-rate you can take. Yes ? Exactly what is unsafe about meeting the performance requirements while causing the least stress to the engine ?
Maybe the entire industry is wrong and you are right ?
Secondly we are talking about engine efficiency here not airspeed. Why do you think Jet engines are more economical at high altitude ?
Last edited by 8che; 29th Nov 2012 at 10:12.
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8che -
Bottom line you can't fly all problems into the air at V1 like they would have you believe...so it's nice to have some runway ahead in the event you don't have a flyable plane.
If you don't get this then the chief pilot did his job...getting everyone to not think, a GO mentality, standardization is key, no lose cannons, no independent thinkers, and enough insurance to cover the Concords and GOAs that should have been stopped on the runway...but hey....that would have meant hiring pilots that could make a decision...nope we can't have that....better to crash a few planes that couldn't fly then hire guys that might stop a plane and put it into the weeds once in a while....or whatever they are thinking.....
Funny, when did max take off power become dangerous.?
The manufacturer has derated the engines so far anyway for normal ops to pretty much guarantee that they will go double typical standard overhaul times....
Anyway...watch what happens when the FAA stops 'on condition' determination of engine wear....all of a sudden you guys will flip flop around to 'better to get it off early'....just like you flip flopped on CRM, or hiring kids instead of pilots....
Always fun to come here and talk to guys that don't think....
Bottom line you can't fly all problems into the air at V1 like they would have you believe...so it's nice to have some runway ahead in the event you don't have a flyable plane.
If you don't get this then the chief pilot did his job...getting everyone to not think, a GO mentality, standardization is key, no lose cannons, no independent thinkers, and enough insurance to cover the Concords and GOAs that should have been stopped on the runway...but hey....that would have meant hiring pilots that could make a decision...nope we can't have that....better to crash a few planes that couldn't fly then hire guys that might stop a plane and put it into the weeds once in a while....or whatever they are thinking.....
Funny, when did max take off power become dangerous.?
The manufacturer has derated the engines so far anyway for normal ops to pretty much guarantee that they will go double typical standard overhaul times....
Anyway...watch what happens when the FAA stops 'on condition' determination of engine wear....all of a sudden you guys will flip flop around to 'better to get it off early'....just like you flip flopped on CRM, or hiring kids instead of pilots....
Always fun to come here and talk to guys that don't think....
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Originally Posted by Sillypeoples
Bottom line you can't fly all problems into the air at V1 like they would have you believe...so it's nice to have some runway ahead in the event you don't have a flyable plane.
But, being employed by beancounters who will sell the aircraft before engines have to go to overhaull and they regularly fill the aircraft so that TOM = PLTOM is the best job ever? In theory, if the s**t hits the fan at V1 (above Vef, just to be clear), in both cases you end up past the runway if you abort and the reversers don't deploy - the only difference is that with assumed temperature you can still get that extra performance you would normally have, should you decide to continue the takeoff.
Given that I myself fall into the group of people mistakenly thinking assumed temperature is a relatively safe and proven method of prolonging engine life, I have a suggestion. ALL airports involving operations of aircraft, certified under Part-25 (FAR/JAR/CS) shall be at -2000ft MSL, Siberian-like climate (max. -30°C in the summer), QNH shall never be below 1060 hPa and minimum runway length is 20 kilometers, so you can do a few touch and goes when you're deciding whether to really take the aircraft into the air.