Take-off power settings on commercial jets
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Take-off power settings on commercial jets
Hello PPRuNers
I have (yet another!) question about professional flying, if you don't mind.
I understand that when you start the take-off roll, you first advance the thrust levers to 70%* N1 to check the engines are spooling up simultaneously and are stable, before then hitting TO/GA (or selecting manually) your take-off thrust (say 90% for sake of argument?).
Sitting down the back, this two-step powering-up of the engines is always noticeable. In fact, I listen out for it. (Incidentally, it can be quite amusing when you occasionally see the look change on people's faces from 'ah, we're off' to 'oh, now we're off!') But the thing is, it always feel to me that it is the second increase in power (i.e. selection of T/O thrust) that has the real bite. That's the bit when you get pushed into the back of your seat.
So my question is: why is it that an addition of only 20% gives this effect? Is it because that second increase is more sudden?
As always, I do realise these figures vary from flight to flight. But I hope you understand what I'm driving at. Maybe you have experienced it too?
* 'Facts' based on this video.
I have (yet another!) question about professional flying, if you don't mind.
I understand that when you start the take-off roll, you first advance the thrust levers to 70%* N1 to check the engines are spooling up simultaneously and are stable, before then hitting TO/GA (or selecting manually) your take-off thrust (say 90% for sake of argument?).
Sitting down the back, this two-step powering-up of the engines is always noticeable. In fact, I listen out for it. (Incidentally, it can be quite amusing when you occasionally see the look change on people's faces from 'ah, we're off' to 'oh, now we're off!') But the thing is, it always feel to me that it is the second increase in power (i.e. selection of T/O thrust) that has the real bite. That's the bit when you get pushed into the back of your seat.
So my question is: why is it that an addition of only 20% gives this effect? Is it because that second increase is more sudden?
As always, I do realise these figures vary from flight to flight. But I hope you understand what I'm driving at. Maybe you have experienced it too?
* 'Facts' based on this video.
PPRuNe Handmaiden
The main reason is that the majority of the thrust from a turbofan engine is generated in the the top 10-20% N1 (or however you want to set thrust, EPR etc).
So, most of the thrust comes from 80-100% N1. It is not linear.
Hence the disproportionate feeling of "push".
So, most of the thrust comes from 80-100% N1. It is not linear.
Hence the disproportionate feeling of "push".
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It depends on the aircraft. On the A320, we set 50% N1, then stick the thrust levers in the FLX/MCT gate which gives the thrust as per the FLEX temperature that was a result of the performance calculations. So that will give you a feeling of greater initial acceleration.
The second point is that jet thrust is exponential i.e. movement of the thrust levers in the lower range does not necessarily correspond to the same thrust output at the upper range of the thrust lever movement. Edit: Just seen redsnail already mentioned this bit.
Some more info on it here: Jet power management [Archive] - PPRuNe Forums
The second point is that jet thrust is exponential i.e. movement of the thrust levers in the lower range does not necessarily correspond to the same thrust output at the upper range of the thrust lever movement. Edit: Just seen redsnail already mentioned this bit.
Some more info on it here: Jet power management [Archive] - PPRuNe Forums
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Depending on the runway and performance dictated takeoff required, the extra "push" you feel might also be due to the brakes being held in with power at 50% (which is more typical than 70%!). Then as the brakes are released more power is applied giving you the impression the power is doing it all.
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Depending on the runway and performance dictated takeoff required, the extra "push" you feel might also be due to the brakes being held in with power at 50% (which is more typical than 70%!). Then as the brakes are released more power is applied giving you the impression the power is doing it all.
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Depending on the runway and performance dictated takeoff required, the extra "push" you feel might also be due to the brakes being held in with power at 50% (which is more typical than 70%!). Then as the brakes are released more power is applied giving you the impression the power is doing it all.
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Or during the cold half of the year because it is a required procedure:
Do the normal Takeoff Procedure with the following modification:
When engine anti-ice is required and the OAT is 3°C or below, the takeoff must be preceded by a static engine run-up.
Use the following procedure:
Advance thrust to 40% N1 and let the engines momentarily stabilize. Then run-up to a minimum of 70% N1 and confirm stable engine operation before the start of the takeoff roll. A 30-second run-up is highly recommended whenever possible.
When engine anti-ice is required and the OAT is 3°C or below, the takeoff must be preceded by a static engine run-up.
Use the following procedure:
Advance thrust to 40% N1 and let the engines momentarily stabilize. Then run-up to a minimum of 70% N1 and confirm stable engine operation before the start of the takeoff roll. A 30-second run-up is highly recommended whenever possible.
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de facto - Denti gave you one good reason I will give you another two. There is a study (dont ask me where I read it) which claims most powerplant failures occure during initial power application below 70%N1 Many pilots run static power at 70% not only to test engines before roll out but also as a way to build power momentum especially on short runways.
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Denti,
Yes in the case when a ice run up is needed of course it MUST be done.
Apart fromthat case, Boeing clearly states that above 40 perc n1 and no fwd movement ,the engines start to ingest ground debris (especially with light crosswind)while the performance gained is closed to NIL.
CFM in its cold weather info advises to avoid thrust higher than 40perc or doing static run ups in areas where lose ice or snow accumulation are present.
Darius ,,you are dismissed.
Yes in the case when a ice run up is needed of course it MUST be done.
Apart fromthat case, Boeing clearly states that above 40 perc n1 and no fwd movement ,the engines start to ingest ground debris (especially with light crosswind)while the performance gained is closed to NIL.
CFM in its cold weather info advises to avoid thrust higher than 40perc or doing static run ups in areas where lose ice or snow accumulation are present.
Darius ,,you are dismissed.
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Indeed, especially the 737 is bad at ingesting stuff, even at idle thrust it will suck up water or snow from the ground.
Just wanted to present an example why sometimes it is advised to use static run ups, whereas of course you are correct in stating that usually it is not required nor advised.
Just wanted to present an example why sometimes it is advised to use static run ups, whereas of course you are correct in stating that usually it is not required nor advised.
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Typical initial power setting is 40-50%.
Typical thrust to get a/c moving, depending upon model and weight, is about 35%, so the initial power setting on T.O. is slightly above the first power application after pushback.
Derated T.O. thrust might be 95%. If they do a full power T.O. at 100% the last 5% is very noticeable.
Jet engines thrust curves are similar to F1 engines(for example). Idle at 11,000(50%), loafing at 17,000(85%), linear rush to 20,000 RPM(100%).
Typical thrust to get a/c moving, depending upon model and weight, is about 35%, so the initial power setting on T.O. is slightly above the first power application after pushback.
Derated T.O. thrust might be 95%. If they do a full power T.O. at 100% the last 5% is very noticeable.
Jet engines thrust curves are similar to F1 engines(for example). Idle at 11,000(50%), loafing at 17,000(85%), linear rush to 20,000 RPM(100%).
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Also...a few aircraft engines don't accellerate evenly from idle. So, by setting a part thrust prior to the final thrust level, any asymmetric thrust is at kept to a manageable level.
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above 40 perc n1 and no fwd movement ,the engines start to ingest ground debris (especially with light crosswind)
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Thank you for the replies, gents. Very much appreciated.
Yes, I did think 40-50% was more typical than 70%!
Denti: am I correct in interpreting the procedure you set out to mean that you would potentially hold the aircraft with brakes on with your final T/O power setting for 30 seconds? If so, crikey! That's about the same length of time as the take-off roll itself! Must be quite something to experience. Would you brief the passengers before push-back that you're going to do that?
Dariuszw: sod off out of this thread.
Yes, I did think 40-50% was more typical than 70%!
Denti: am I correct in interpreting the procedure you set out to mean that you would potentially hold the aircraft with brakes on with your final T/O power setting for 30 seconds? If so, crikey! That's about the same length of time as the take-off roll itself! Must be quite something to experience. Would you brief the passengers before push-back that you're going to do that?
Dariuszw: sod off out of this thread.
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N49 - not 'take-off power'. The 737 calls for "70% N1" for the procedure (designed to generate enough centrifugal [nitpickers please ignore
] force to allow ice to be shed from blades and spinner.) Well below take-off power which is typically 93% or more.
For others, I am wondering where all this FO stuff on the runway is coming from? What sort of airfields are you operating from?
] force to allow ice to be shed from blades and spinner.) Well below take-off power which is typically 93% or more.For others, I am wondering where all this FO stuff on the runway is coming from? What sort of airfields are you operating from?
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Right. Thanks BOAC. So when you use that procedure, do you spool up to 70% with the brakes on, hold it for 30 seconds, and then release the brakes and select T/O thrust (i.e. still only two power settings)? OR do you still go to 40% first, 70% second, then T/O thrust (i.e. three power settings)?
Are the brakes able to hold the aircraft completely stationery at 70% for 30 seconds?
And, would you brief the passengers that they are in for a 'fun take-off' (rephrased to sound professional and reassuring in a deep gravelly voice, of course
)?
Are the brakes able to hold the aircraft completely stationery at 70% for 30 seconds?
And, would you brief the passengers that they are in for a 'fun take-off' (rephrased to sound professional and reassuring in a deep gravelly voice, of course
)?
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Straight up to 70% for 30 seconds in severe icing conditions. No more. Just a few years ago a well known Brit airline crew in a 737 went to full power at Aberdeen (short runway) held on brakes. They ingested many pieces of tarmac sucked up in the not-insignificant time they had full power before releasing brakes. It damaged the runway, the engine and the tailplane (debris spattered at it). A warning was put out to everybody NOT to do it.
As for PA warnings in severe icing conditions, yes- if you are feeling generous and have time available.
As for PA warnings in severe icing conditions, yes- if you are feeling generous and have time available.
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I think this aberdeen incident was due mostly to the fact that the crew used a line up technique that was unecessarely close to the runway edge,along with a static take off not for ice shedding but supposely for a performance issue that was not existant.
The airline was reminded that a static take off doesnt do much for take off perf other than possibly damaging the aircraft the way it did.
Rolling take off (ie take off thrust with brakes released) is recommended.
If i have to ice shed, i will advise pax before push back or if conditions change ,before doing the procedure so they know what is going on.
It may worry first time and regular passengers for different reasons.
The airline was reminded that a static take off doesnt do much for take off perf other than possibly damaging the aircraft the way it did.
Rolling take off (ie take off thrust with brakes released) is recommended.
If i have to ice shed, i will advise pax before push back or if conditions change ,before doing the procedure so they know what is going on.
It may worry first time and regular passengers for different reasons.
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Difference between 70% N1 and take off power can be very small in some cases, we use N1s as low as 75%. With todays derate/assumed temperature stuff take off power can be very low indeed, especially on a long runway.
The brakes should be able to hold the aircraft at 70% N1, however on a slippery runway in a light aircraft that might not be the case in which case one does not do the whole 30 second runup, just up to 70%, check if engine indications are ok and off we go.
The brakes should be able to hold the aircraft at 70% N1, however on a slippery runway in a light aircraft that might not be the case in which case one does not do the whole 30 second runup, just up to 70%, check if engine indications are ok and off we go.
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Are the brakes able to hold the aircraft completely stationery at 70% for 30 seconds?
Denti - that is amazing - can you give me an example - OAT/Pressure Altitude/Assumed temp/Engine fit? You'll have SSG snapping at your heels with 75% N1.