B747-400 Flaps acceleration height and THR Reduction

[havigdort 0]

Hello,I can't figure out what is the difference between the Flaps acceleration height (LSK 1L of the TAKEOFF REF page of the FMC) and the THR REDUCTION (LSK 3L).Could someone explain ?THks.

[downscc 1,327]

Please sign your posts in this forum.Acceleration height is AGL value that acceleration from V2+10 begins, thrust reduction is where thrust is reduced to CLB. See also FMC 12-40 in AOM.

[havigdort 0]

And how should we know that Flaps Acceleration Height is 1000 fts AGL and THR reduction to CLB should happen at flap 5 or any altitude? How to set the correct value?Other question: flaps acceleration and THR reduction shouldn't happen at the same time ?Etienne P.

[ykikamoocow 0]

And how should we know that Flaps Acceleration Height is 1000 fts AGL and THR reduction to CLB should happen at flap 5 or any altitude? How to set the correct value?Other question: flaps acceleration and THR reduction shouldn't happen at the same time ?Etienne P.

Etienne it is airline specific, for example the Qantas 744 SOP was a flap 20 takeoff with an acceleration height of 1500ft and I thinka flap 5 thrust reduction (this is what it was 10 years ago- may have changed since) cheersJon

[VHOJT 172]

Etienne it is airline specific, for example the Qantas 744 SOP was a flap 20 takeoff with an acceleration height of 1500ft and I thinka flap 5 thrust reduction (this is what it was 10 years ago- may have changed since) cheersJon

Hi there chapsIt is indeed airline specific, and airport specific to an extent.When I spoke to a Qantas captain a year or so ago, he said there was a number of procedures depending on airport and runway (all Flaps 20) He gave the following examples:- SYD (southbound, over water initially): 1000' AGL accel. height, thrust reduction at Flaps 5- SYD (northbound, over suburbs/city): 1500' AGL thrust reduction, acceleration at 3000' AGL- LHR: max. thrust takeoff (no assumed temp. or derate), thrust reduction at 1000' AGL, acceleration at 4000' AGL- MEL (I believe, can't remember exactly): same as SYD southboundI believe SYD northbound corresponds to the ICAO-A procedure: THR RED at 1500' AGL, ACCEL 3000' AGL. HKG uses this as well I think. There is also ICAO-B: THR RED and ACCEL at 1500' AGL. I think there are also NADP procedures around too, don't know where they are used or what they stand for. I would love to know what airlines tend to use at WSSS SIN Singapore Changi.Also, at least a few years ago, BA used to use (at LHR on 744):- flap 20 takeoff to 1000', THR RED at 1000' AGL to CLB, then open the MCP speed window to Flap 10 speed + 10, then flap 10 when past retraction speed, then climb to 4000' before closing the MCP speed window and beginning acceleration, when flaps up select CLB-1 or CLB-2At least that's how I remember it, I could be rusty.CheersRudy

[havigdort 0]

Jon, Rudy,Thanks for your answers.I think I will stick to preset settings in the FMC (1000 fts for acceleration height and Flap 5 for thrust reduction). Let me summarize my understanding.Acceleration height is when the aircraft should start quitting its takeoff configuration in terms of flaps, thus enabling it to gain airspeed.Thrust reduction, is when thrust reduces from TO (D-TO or whatever) to CLB.Etienne

[FlyChuck 3]

KLM: 1500' THR reduction to Climb mode, 3000' acceleration to retract the flapsTom

[Guest chrut]

Hi there chapsIt is indeed airline specific, and airport specific to an extent.When I spoke to a Qantas captain a year or so ago, he said there was a number of procedures depending on airport and runway (all Flaps 20) He gave the following examples:- SYD (southbound, over water initially): 1000' AGL accel. height, thrust reduction at Flaps 5- SYD (northbound, over suburbs/city): 1500' AGL thrust reduction, acceleration at 3000' AGL- LHR: max. thrust takeoff (no assumed temp. or derate), thrust reduction at 1000' AGL, acceleration at 4000' AGL- MEL (I believe, can't remember exactly): same as SYD southboundI believe SYD northbound corresponds to the ICAO-A procedure: THR RED at 1500' AGL, ACCEL 3000' AGL. HKG uses this as well I think. There is also ICAO-B: THR RED and ACCEL at 1500' AGL. I think there are also NADP procedures around too, don't know where they are used or what they stand for. I would love to know what airlines tend to use at WSSS SIN Singapore Changi.Also, at least a few years ago, BA used to use (at LHR on 744):- flap 20 takeoff to 1000', THR RED at 1000' AGL to CLB, then open the MCP speed window to Flap 10 speed + 10, then flap 10 when past retraction speed, then climb to 4000' before closing the MCP speed window and beginning acceleration, when flaps up select CLB-1 or CLB-2At least that's how I remember it, I could be rusty.CheersRudy

Agreed that it's airline specific, but also I believe that it IS airport specific (and not only to an extent). This is because certain airports have obstacles in the way and thus requiring aircrafts to climb in certain angles in order to be safe and/or the airport is surrounded by a city and therefore requires a specific noise abatement procedure.Could be VERY wrong, as I'm not only new here but I have no real flying experience whatsoever - but that's the empression I got when I read about these feauters in the PMDG manual. Noise abetment procedures I learned from Scandinavian airlines virtual airliner and is specific to them (published at their site under procedure ops), but these procedures can also be read in the Fly the maddog 2006 manual (link below). This is what the PMDG manual says about flap acceleration height (p. 365, FMC user's manual):"This height should take in consideration factors such as terrain elevation surrounding the departure airport, noise abetement requirements and the desiere to have atleast 1500 ft. of altitude above airport elevation before reducing the initial climb rate...". So I believe the airport in use is taken in consideration when the airliner begins writing these flap acc. height instructions for the pilots.What I learned in the Fly the maddog manual and from SAS VA's procedures, is that a normal noise abetment procedure begins with flying to 1500 ft AGL with best climb angle (derates or whatever are not only accepted, but recommended if weather permits), at 1500 ft AGL the change to CLB-thrust occurs (again, CLB1-2 is not prohibited) and when reaching 3000 ft the flap acceleration begins. Exactly like KLM according to post above by "etter".If someone doesn't know which value to use at a certain aiport/for a certain airliner, a tip is to read the charts that come with the aiport (usually in the introduction "text-chart") and/or check the takeoff procedures for the VA that you belong to.Anyway, I find this topic to be VERY interesting, seeing as these procedures adds quite a bit to the realism factor.Cheers!

[havigdort 0]

The link (if any) between ACCEL HT and THR REDUCTION still puzzling me. So I dig on the Internet and found this interesting piece of information from what appears to be Boeing official document : "What is Quiet Climb concept?<ANSWER> A Preprogrammed thrust reduction to give climb gradient just equal to minimum required, for noise abatement over noise-sensitive areas. Per AC 91-53A, the quiet climb thrust level is the level that would provide a minimum climb gradient of 1.7% with three engines operating. To reduce crew workload, instead of crew having to identify the proper point and pull the thrust back manually, it is preprogrammed into the FMC. The cutback from Takeoff to quiet climb thrust occurs at a pre-set altitude (THR REDUCTION on Takeoff Ref page). The airplane remains in takeoff flap configuration and maintains the takeoff climb speed (V2+10) until reaching the pre-set thrust restoration altitude (ACCEL HT on Takeoff Ref page)."Boeing Flight Operations SymposiumOctober 29

[downscc 1,327]

<<It seems that THR REDUCTION is only linked to noise consideration>>I would agree without the "only," since safety is also a consideration. Say there is no noise constraint, you still reduce thrust to climb once above ground high enough to no longer require the extra thrust for safety. The decrease in thrust results in a slight decrease in climb rate in a trade off for engine reliability and fuel consumption.

[Guest chrut]

<<It seems that THR REDUCTION is only linked to noise consideration>>I would agree without the "only," since safety is also a consideration. Say there is no noise constraint, you still reduce thrust to climb once above ground high enough to no longer require the extra thrust for safety. The decrease in thrust results in a slight decrease in climb rate in a trade off for engine reliability and fuel consumption.

Yes, agreed. I think this is how airliners and how "airport supervisors" that operate within noise-sensitve aeras think. Becuase, once you're "above ground high enough to no longer require the extra thrust for safety", you can "decrease [...] thrust [which] results in a slight decrease in climb rate in a trade off for engine reliability and fuel consumption". But with lower thrust the noise level is brought down automatically. This, along with a higher flap acc. height, would result in a greater climb out of the noise sensitve area than if flap acc height would occur at the same time as THR reduction.So when is it safe to allow THR reduction? PMDG manual says that the atleast 1500 ft AGL is desired (as long as there are no conflits such as obstacales etc.) and Fly the maddog (MD80-83) manual says the same. I can't speak for all aircrafts like MD11, B767 and so on but I think 1500 ft. is perhaps a standard? But why is 3000 ft AGL used as flap acc. height? Maybe some genius figured out that at that height people, that reside in noise sensitive areas, don't get bothered with the sound that aircrafts generate and it is thus safe to derate the climb rate and accelerate. These thoughts would lead me to belive that most European countries will use 1500 ft. as THR red. height and 3000 ft. flap height setting to allow greater climb out, this according to SAS VA. Of course, saftey comes always first... and if some aircraft can't preform the published noise abetment procedure... then so be it.But this is just my theories, if somone can come with some real experience or have other thougts.. that would be much apprecaited.

[FLEX1978 373]

Just want to point out guy's that using a reduced climb thrust is good for engine life but 'increases' fuel consumption. CheersRob

[Guest chrut]

Just want to point out guy's that using a reduced climb thrust is good for engine life but 'increases' fuel consumption. CheersRob

Yes, very good and you're actually right. Use of full rate takeoffs and bringing up those flaps as soon as possible will save fuel. Look here, page 76. But then again, long-term thinking: with the use of derated takeoffs TOTAL costs will be reduced.