Packs off fuel saving
One of the risks associated with a packs-off T/O is being distracted and not turning them off before going. I know, I’ve done it. OK, you’re only going to be compromised in the event of an engine failure but still...
We only do packs off if improved climb doesn’t get the TOPL up enough. As it’s normally hot’n’high to be facing that condition, I regard it as a last resort from a passenger comfort POV as well.
We only do packs off if improved climb doesn’t get the TOPL up enough. As it’s normally hot’n’high to be facing that condition, I regard it as a last resort from a passenger comfort POV as well.
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Originally Posted by despegue
Any additional task to accomplish in a high workload environment is adding to the risk of forgetting, omitting or incorrectly execution. With packs, this can be potentially very serious.
I am talking B737 here, not Airbus or better designed Boeing types.
I am talking B737 here, not Airbus or better designed Boeing types.
Just put the right engine bleed switch to ON when passing 1000ft and the aircraft will become pressurized in case of unpressurized takeoff or continue to become pressurized even if you shut down the APU before reconfiguring the bleeds.
Now, whether your company thinks this is more dangerous than postopning the entire C pattern until safe altitude - that's a different problem. As for passenger comfort goes, you always have the option to taxi with packs on the engines and reconfigure during line up - although I don't think this is a very good idea, safety-wise.
Back on topic: in my opinion, it is more safe to do FLEX + packs off than to depart with full TO thrust just for the sake of having bleeds/packs on for takeoff - if resulting PLTOM is the same. In case of full thrust + packs on you don't have any spare thrust margin to use, if you would really need it - except firewalling the thrust levers. If you depart with FLEX + packs off, you can still press TOGA and receive some extra thrust without overstressing the engines.
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If you work for a real airline its generally packs on (packs off if operationally necessary for perf reasons). El cheapo low cost outfits its packs off always. I've worked for both mobs.
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Reduce engine wear?!? IF you can find one, please post [a link to] even ONE engineering study that quantifies the reduction in engine wear.
Once you get about 10% below max rated thrust, incremental reduction of engine wear is miniscule. Engine speed and EGT which are the main drivers of wear -- are already significantly below max. In fact, on the 744 and 748 we too often take off with thrust set SIGNIFICANTLY below climb thrust! That is simply a beancounters' exercise to reduce engine lease payments, and has NO operational value.
Once you get about 10% below max rated thrust, incremental reduction of engine wear is miniscule. Engine speed and EGT which are the main drivers of wear -- are already significantly below max. In fact, on the 744 and 748 we too often take off with thrust set SIGNIFICANTLY below climb thrust! That is simply a beancounters' exercise to reduce engine lease payments, and has NO operational value.
The most efficient climb (zero-wind, ISA) is at maximum thrust straight op to optimum cruise, if you reduce thrust in the climb you burn more fuel.
The reason for flex is thus engine-wear, and at higher flex temps it's just a beancounter paperwork excercise that actually increases real term costs. But those costs are handed to the engine-lease companies, so the airline still saves money.
It's probably a bit more complicated though, as some engines can also be de-rated and then use flex temperature on top of that. So there must be a bit more to it then the simplified version.
Years ago we had an issue with a particular engine type - they suddenly had a rash of compressor failures (stators were breaking). Funny part was the engine failures were all occurring at a couple operators - the rest of the fleet was fine.
What we discovered was that the operators who were having the engine failures were doing derated climbs. Turns out that, during a derated climb the lower rotor speeds could cause a resonance in the compressor that fatigued the stators. Full rated climb avoided the rotor speed rangers that could cause the resonance. Talk about your false economy
(they did change the design to get rid of the resonance though).
It's counter-intuitive, but derated takeoffs can also increase noise - the engine is creating less noise but because the aircraft climbs slower, a larger area is exposed to the noise.
What we discovered was that the operators who were having the engine failures were doing derated climbs. Turns out that, during a derated climb the lower rotor speeds could cause a resonance in the compressor that fatigued the stators. Full rated climb avoided the rotor speed rangers that could cause the resonance. Talk about your false economy
(they did change the design to get rid of the resonance though).It's counter-intuitive, but derated takeoffs can also increase noise - the engine is creating less noise but because the aircraft climbs slower, a larger area is exposed to the noise.
I do not think it is surprising at all that the aeroplane sounds louder if it flies lower over your house. We do double derate takeoffs and the first time I used it I was asked by the tower if we had had an engine failure we were so much lower over the end of the runway.
Some airlines combine this with selecting full climb thrust (increasing noise) at 1000 feet. Double derate combined with improved climb on a long runway makes a huge difference to how high (read much lower) you are crossing the airfield boundary.
My local airport is having major trouble with opposition to them building a new runway.....I think there might just be some connection to the current approach to noise abatement.
Some airlines combine this with selecting full climb thrust (increasing noise) at 1000 feet. Double derate combined with improved climb on a long runway makes a huge difference to how high (read much lower) you are crossing the airfield boundary.
My local airport is having major trouble with opposition to them building a new runway.....I think there might just be some connection to the current approach to noise abatement.
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Yup, we do that on our (soon to be phased out) boeings. Double derate (derate plus ATM) and manually selected climb thrust, either one step up or full climb thrust. It saves money during take off dur to lower engine lease costs, and money during climb out as it gives the shortest possible tome from thrust increase altitude until top of climb.
The small busses dont have derates and so far i havent encountered a thrust increase at reduction.
The small busses dont have derates and so far i havent encountered a thrust increase at reduction.
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A packs OFF t/o does require the APU to be operating until the packs are returned to their required position for the flight which does increase the fuel burn along with APU maintenance....i guess it all depends on your taxi time
Not true. No requirement to have the APU running in order to take off with packs off.
Maybe you're thinking of running the packs from the APU to increase Go around gradient?
A packs OFF t/o does require the APU to be operating until the packs are returned to their required position for the flight which does increase the fuel burn along with APU maintenance....i guess it all depends on your taxi time
Not true. No requirement to have the APU running in order to take off with packs off.
Maybe you're thinking of running the packs from the APU to increase Go around gradient?
Quote:
Some airlines allow flex AND packs OFF ,some dont.
I don't get this: why would an airline not allow packs off and flex?
Some airlines allow flex AND packs OFF ,some dont.
I don't get this: why would an airline not allow packs off and flex?
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Hi,
I don't agree with your opinion. I don't think the engine wear is miniscule if the max rated thrust is decreased about 10%. When you reduce the max rated thrust 25% or 40% (assumed/flex and derate), the engine wear is reduced compared to the max rated thrust and 90% of max thrust (or 10% below max thrust that you mentioned). Please have a look to slides 6,7,8,8,10 of the this link:
Reduced Thrust Operations
It will be very helpful if you can post a link that shows when you get about 10% below max rated thrust, incremental reduction of engine wear is miniscule.
I thought when an airline purchased aircraft, the cost of the engines is included. If the aircraft are leased, the engines are leased. Are you saying that all engines used by airlines are leased?
Reduce engine wear?!? IF you can find one, please post [a link to] even ONE engineering study that quantifies the reduction in engine wear.
Once you get about 10% below max rated thrust, incremental reduction of engine wear is miniscule. Engine speed and EGT which are the main drivers of wear -- are already significantly below max. In fact, on the 744 and 748 we too often take off with thrust set SIGNIFICANTLY below climb thrust! That is simply a beancounters' exercise to reduce engine lease payments, and has NO operational value.
Once you get about 10% below max rated thrust, incremental reduction of engine wear is miniscule. Engine speed and EGT which are the main drivers of wear -- are already significantly below max. In fact, on the 744 and 748 we too often take off with thrust set SIGNIFICANTLY below climb thrust! That is simply a beancounters' exercise to reduce engine lease payments, and has NO operational value.
I agree, the first few degrees of flex save the most on engine-wear. After that it becomes a paper excercise
Reduced Thrust Operations
It will be very helpful if you can post a link that shows when you get about 10% below max rated thrust, incremental reduction of engine wear is miniscule.
I thought when an airline purchased aircraft, the cost of the engines is included. If the aircraft are leased, the engines are leased. Are you saying that all engines used by airlines are leased?
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There are no links to engineering studies of blade creep or erosion in that presentation.
Also, the entire premise of the presentation, while valid in a general sense, is not a strict scientific or engineering approach. For example, it discusses "average thrust reduction" for an airplane or fleet over time. It does not distinguish between a fleet where EVERY takeoff is done with a 20% reduction, and a fleet where 50% are done at max rated thrust, and 50% are done with a 40% reduction.
The real telling point is the comparison of the charts on pages 5 and 8. While both charts are generic, we find on page 8 that the right side of the chart is almost never used in reality, because the operating limits are set to avoid it completely.
Also, the "Maintenance Material Cost" shown on page 8 likely reflects the engine mfgrs' practice of pricing engine usage on a scale that reflects "average" thrust reduction, again with no regard for the distribution or frequency of max thrust events. I don't know how that graph can be taken as operational "truth" when a 40% takeoff thrust reduction results in a climb thrust significantly greater than takeoff thrust. The note on the bottom of page 8 supports my assessment.
My "about 10%" statement comes from information I found in a Materials Engineering course 30 years ago, regarding life cycle fatigue. I have no specific links, but the clear message was that most turbine wear happens at the very high end of the design limits [of RPM and temperature] scale, and that only "normal wear & tear" is expected when the actual cyclic limits are at a modest level below the stated limits.
I don't know how "all" airlines buy/lease engines and/or engine maintenance. MANY of them have separate lease terms for engines and airframes.
Also, the entire premise of the presentation, while valid in a general sense, is not a strict scientific or engineering approach. For example, it discusses "average thrust reduction" for an airplane or fleet over time. It does not distinguish between a fleet where EVERY takeoff is done with a 20% reduction, and a fleet where 50% are done at max rated thrust, and 50% are done with a 40% reduction.
The real telling point is the comparison of the charts on pages 5 and 8. While both charts are generic, we find on page 8 that the right side of the chart is almost never used in reality, because the operating limits are set to avoid it completely.
Also, the "Maintenance Material Cost" shown on page 8 likely reflects the engine mfgrs' practice of pricing engine usage on a scale that reflects "average" thrust reduction, again with no regard for the distribution or frequency of max thrust events. I don't know how that graph can be taken as operational "truth" when a 40% takeoff thrust reduction results in a climb thrust significantly greater than takeoff thrust. The note on the bottom of page 8 supports my assessment.
My "about 10%" statement comes from information I found in a Materials Engineering course 30 years ago, regarding life cycle fatigue. I have no specific links, but the clear message was that most turbine wear happens at the very high end of the design limits [of RPM and temperature] scale, and that only "normal wear & tear" is expected when the actual cyclic limits are at a modest level below the stated limits.
I don't know how "all" airlines buy/lease engines and/or engine maintenance. MANY of them have separate lease terms for engines and airframes.
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Complicated Calculation and...
IMHO, that's a legitimate, very smart question.
As an engineer with some experience, though not with this subject, my guess would be that it is a relatively simple exchange of energy. Knowing that I'm not an expert, try these ideas...
1) Take off power, at max, flex or any other standard is high. TO thrust comes first and packs second.
2) TO thrust + pack(s) requires more energy than TO thrust alone, thus placing a heftier demand on the engines.
3) Most take off procedures are performed at far less than Max Thrust, thus there is some wiggle room. That said, if it becomes necessary to use Max thrust - Firewall the levers (and pray), every ounce counts. Do you want to be flipping switches to shutdown packs and hope for the max possible motive energy?
It has been a while since I've flown on an energy-limited airplane, but some flyers still kill the packs, if briefly, during the TO roll. That may not be universal, but it remains common. One safely flying and starting to 'clean up' for the climb, the packs come back on. Even when shutdown for the TO roll and into a stable climb, the packs are off for 2-3-4 minutes. No one will suffocate, freeze or cook during those minutes.
I So NOT Know how much energy the AC Packs consume, as a percentage of energy produced by the engines. If only for engine health, shut the packs down during brief periods of high demand and return them to service when the energy needs are reduced. That's not flying procedure as dictated by the Chief Pilot's office, but simple energy conservation, common sense and of course, retaining the option of using 105% for thrust if necessary.
If I'm missing anything, please let me know. I am not criticizing, only suggesting that during those Critical Phases, the drivers should have 105% access to everything necessary. Darn good Question (even if I bent is a bit...)
As an engineer with some experience, though not with this subject, my guess would be that it is a relatively simple exchange of energy. Knowing that I'm not an expert, try these ideas...
1) Take off power, at max, flex or any other standard is high. TO thrust comes first and packs second.
2) TO thrust + pack(s) requires more energy than TO thrust alone, thus placing a heftier demand on the engines.
3) Most take off procedures are performed at far less than Max Thrust, thus there is some wiggle room. That said, if it becomes necessary to use Max thrust - Firewall the levers (and pray), every ounce counts. Do you want to be flipping switches to shutdown packs and hope for the max possible motive energy?
It has been a while since I've flown on an energy-limited airplane, but some flyers still kill the packs, if briefly, during the TO roll. That may not be universal, but it remains common. One safely flying and starting to 'clean up' for the climb, the packs come back on. Even when shutdown for the TO roll and into a stable climb, the packs are off for 2-3-4 minutes. No one will suffocate, freeze or cook during those minutes.
I So NOT Know how much energy the AC Packs consume, as a percentage of energy produced by the engines. If only for engine health, shut the packs down during brief periods of high demand and return them to service when the energy needs are reduced. That's not flying procedure as dictated by the Chief Pilot's office, but simple energy conservation, common sense and of course, retaining the option of using 105% for thrust if necessary.
If I'm missing anything, please let me know. I am not criticizing, only suggesting that during those Critical Phases, the drivers should have 105% access to everything necessary. Darn good Question (even if I bent is a bit...)
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Hi,
Thanks for the feedback.
How often the takeoff thrust is reduced below climb thrust? Does it apply to short/medium haul, long-haul (2 and 4 engine aircraft)?
About "10%" statement, just because it comes from a course 30 years ago, it does not mean it is right. You are taking it as absolute truth.
If you think it is absolute truth why Boeing and Airbus are not recommending or adopting this procedure (10%)? Boeing and Airbus have propulsion department with experts and engineers in this field.
Why airlines are not applying this procedure (10%)? Airlines have also propulsion department.
To my knowledge Boeing and Airbus are not engine leasing companies and they can't force airlines management (bean counters) to adopt reduced takeoff thrust (assumed and/or derate).
You don't have to decrease takeoff thrust to 40% all time. You can decrease thrust [B]up to 40% for certain conditions (40% is the limit). I think you can also use derate climb so the takeoff thrust is not reduced below climb thrust.
4 engine aircraft typically require a higher fraction of takeoff thrust in climb than twin-engine aircraft do. Climb operation may influence engine life on four-engine aircraft, but cruise altitude is reached earlier.
I think there is compromise between engine maintenance cost and the total fuel cost or the operating cost. Other factors can be considered noise abatement procedures, 4 engines aircraft vs 2 engine aircraft, medium haul vs long haul...etc.
Thanks for the feedback.
How often the takeoff thrust is reduced below climb thrust? Does it apply to short/medium haul, long-haul (2 and 4 engine aircraft)?
Once you get about 10% below max rated thrust, incremental reduction of engine wear is miniscule
If you think it is absolute truth why Boeing and Airbus are not recommending or adopting this procedure (10%)? Boeing and Airbus have propulsion department with experts and engineers in this field.
Why airlines are not applying this procedure (10%)? Airlines have also propulsion department.
To my knowledge Boeing and Airbus are not engine leasing companies and they can't force airlines management (bean counters) to adopt reduced takeoff thrust (assumed and/or derate).
You don't have to decrease takeoff thrust to 40% all time. You can decrease thrust [B]up to 40% for certain conditions (40% is the limit). I think you can also use derate climb so the takeoff thrust is not reduced below climb thrust.
4 engine aircraft typically require a higher fraction of takeoff thrust in climb than twin-engine aircraft do. Climb operation may influence engine life on four-engine aircraft, but cruise altitude is reached earlier.
I think there is compromise between engine maintenance cost and the total fuel cost or the operating cost. Other factors can be considered noise abatement procedures, 4 engines aircraft vs 2 engine aircraft, medium haul vs long haul...etc.
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First, I do not take ANYTHING as "absolute truth"! If I did, why would my assessment include the word "about"?
Second, I have seen takeoff thrust below climb thrust maybe 50% of the time in our 748 fleet. Just had it again today. Happens less in the 744 fleet, but still common (25%?). Since the B5F engines have higher standard derates than the B1F engines, Derate + Assumed Temp tends to push a more significant TO thrust reduction with the B5F engines in the 744.
As for your "why..." questions, the answer is relatively simple. First, Boeing and Airbus do not normally weigh in on engine operation as much as GE, P&W, CFM, and other ENGINE mfgrs. Second, those engine mfgrs want to make as much money as possible, so they will not likely recommend operations that decrease their bottom line. Third, the airlines are slaved to their engine lease/maintenance contract terms. When those terms are structured around an 'average TO thrust reduction', the airline will adapt their operating procedures to minimize their costs within those contracts. That is, they will strive to achieve the MAXIMUM average TO thrust reductions possible in order to pay the minimum for their engines/maintenance. In order to achieve those 'averages', they will dictate maximum derates + assumed temp reductions for as many flights as possible.
As for 2-engine vs 4-engine performance, remember that max engine thrust is predicated on specified performance with 1 engine inoperative. Therefore, a 2-engine airplane has to demonstrate performance on 50% total thrust, while a 4-engine airplane has to demonstrate the same performance on 75% total thrust. Therefore, each engine on a 2-engine airplane has relatively MUCH higher performance margins.
Finally (for now), basic physics has not changed much in 3 years. While advances in metallurgy and composit materials have allowed operations at much higher temperatures (or G forces), the relative stress-strain relationships at/near those limits have not changed so much. Plastic deformation still occurs at an exponentially increasing rate near the demonstrated limits of the material. Low-cycle fatigue is still a major factor in airframe and engine life; the materials allow higher stresses for the same lifetime, or the same stresses for a longer lifetime. The 50% material margin is still the benchmark, AFAIK.
Second, I have seen takeoff thrust below climb thrust maybe 50% of the time in our 748 fleet. Just had it again today. Happens less in the 744 fleet, but still common (25%?). Since the B5F engines have higher standard derates than the B1F engines, Derate + Assumed Temp tends to push a more significant TO thrust reduction with the B5F engines in the 744.
As for your "why..." questions, the answer is relatively simple. First, Boeing and Airbus do not normally weigh in on engine operation as much as GE, P&W, CFM, and other ENGINE mfgrs. Second, those engine mfgrs want to make as much money as possible, so they will not likely recommend operations that decrease their bottom line. Third, the airlines are slaved to their engine lease/maintenance contract terms. When those terms are structured around an 'average TO thrust reduction', the airline will adapt their operating procedures to minimize their costs within those contracts. That is, they will strive to achieve the MAXIMUM average TO thrust reductions possible in order to pay the minimum for their engines/maintenance. In order to achieve those 'averages', they will dictate maximum derates + assumed temp reductions for as many flights as possible.
As for 2-engine vs 4-engine performance, remember that max engine thrust is predicated on specified performance with 1 engine inoperative. Therefore, a 2-engine airplane has to demonstrate performance on 50% total thrust, while a 4-engine airplane has to demonstrate the same performance on 75% total thrust. Therefore, each engine on a 2-engine airplane has relatively MUCH higher performance margins.
Finally (for now), basic physics has not changed much in 3 years. While advances in metallurgy and composit materials have allowed operations at much higher temperatures (or G forces), the relative stress-strain relationships at/near those limits have not changed so much. Plastic deformation still occurs at an exponentially increasing rate near the demonstrated limits of the material. Low-cycle fatigue is still a major factor in airframe and engine life; the materials allow higher stresses for the same lifetime, or the same stresses for a longer lifetime. The 50% material margin is still the benchmark, AFAIK.
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Packs off
I agree with Field In Sight.
Over time the fuel saving is considerable, let alone the engine wear. Also, if you lease the engines you recover a premium from the company (who own the engines) for taking off in Flex and you may even incur a penalty for not using Flex in ideal Flex conditions.
The aircraft takes off very well in Flex and TOGA should be used in conditions only where increased performance is necessary.
Thats why the FLEX detent is there
Over time the fuel saving is considerable, let alone the engine wear. Also, if you lease the engines you recover a premium from the company (who own the engines) for taking off in Flex and you may even incur a penalty for not using Flex in ideal Flex conditions.
The aircraft takes off very well in Flex and TOGA should be used in conditions only where increased performance is necessary.
Thats why the FLEX detent is there
Sorry for the slight thread drift but there seems to be a lot of comments being banded about over Boeings, particularly the 737, packs off takeoffs possibly being problematic if you "forget" to turn them back on (understandably), however I find it surprising that it doesn't have a built in automatic system to turn the packs off for the take off and then back on in the climb.
I have never flown a 737 however I have experience on 717 (yes, some may say it is not a real Boeing given it is a derivative of the DC-9) and a packs off takeoff in it is a non-event. Simply select "Packs OFF" on the "Takeoff" page of the FMS and it does the rest. When you power up for takeoff the packs automatically shut down, when climbing through 3,000' AGL they automatically come back on. Would of thought this feature would be more common.
I have never flown a 737 however I have experience on 717 (yes, some may say it is not a real Boeing given it is a derivative of the DC-9) and a packs off takeoff in it is a non-event. Simply select "Packs OFF" on the "Takeoff" page of the FMS and it does the rest. When you power up for takeoff the packs automatically shut down, when climbing through 3,000' AGL they automatically come back on. Would of thought this feature would be more common.
