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Reason For De-rated Climb?  
User currently offlineshufflemoomin From Denmark, joined Jun 2010, 470 posts, RR: 1
Posted (3 years 2 months 5 days 18 hours ago) and read 5484 times:

Hi everyone,

Long time reader and finally signed up to pick the brains of the far more experienced people here than I am. My question: On many flights I've taken, within about 60 seconds of take-off, there's a clear throttling back of the engines and you can feel a definite decrease in speed. Other than maybe noise restrictions, what are the reasons that an aircraft might want to climb slower than usual?

31 replies: All unread, showing first 25:
 
User currently offlineMir From United States of America, joined Jan 2004, 21425 posts, RR: 56
Reply 1, posted (3 years 2 months 5 days 18 hours ago) and read 5461 times:

Quoting shufflemoomin (Thread starter):
On many flights I've taken, within about 60 seconds of take-off, there's a clear throttling back of the engines and you can feel a definite decrease in speed. Other than maybe noise restrictions, what are the reasons that an aircraft might want to climb slower than usual?

Takeoff power cannot be maintained indefinitely, so the engines will be throttled back shortly after takeoff.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineshufflemoomin From Denmark, joined Jun 2010, 470 posts, RR: 1
Reply 2, posted (3 years 2 months 5 days 17 hours ago) and read 5452 times:

Quoting Mir (Reply 1):

So it's perfectly normal procedure to throttle back around a minute or so after take off? What sort of power decrease are we talking about? In some of my flights, the tone of the engines and the sensation of speed can decrease very noticeably? In some cases, it's been a cause for concern for a second or two. I was curious why it would be more pronounced in some flights more than others, other than for noise restriction.


User currently offlineKELPkid From United States of America, joined Nov 2005, 6340 posts, RR: 3
Reply 3, posted (3 years 2 months 5 days 17 hours ago) and read 5440 times:

Quoting shufflemoomin (Thread starter):
Other than maybe noise restrictions, what are the reasons that an aircraft might want to climb slower than usual?

Air Traffic Control restrictions come to mind...also, if the aircraft is following a SID (standard instrument departure), it may be at the upper limits of the segment of the procedure it is on.



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 4, posted (3 years 2 months 5 days 17 hours ago) and read 5440 times:

Quoting shufflemoomin (Reply 2):

So it's perfectly normal procedure to throttle back around a minute or so after take off?

Yes. Saves fuel and wear and tear on the engines.

In rarer occasions, the de-rated take-off thrust may in fact be less than the climb thrust. So after take-off instead of a power decrease you'll have a slight throttle up.

Quoting shufflemoomin (Reply 2):
What sort of power decrease are we talking about?

Depends on a myriad of factors, but a very rough guesstimate off the top of my head, anywhere between 5-15%

Quoting shufflemoomin (Thread starter):
what are the reasons that an aircraft might want to climb slower than usual?

Nobody wants to climb slower than usual. It's just the price you pay for being "nice" to the engines and fuel budget.

[Edited 2011-05-10 14:44:58]

User currently offlineflipdewaf From United Kingdom, joined Jul 2006, 1561 posts, RR: 1
Reply 5, posted (3 years 2 months 5 days 16 hours ago) and read 5412 times:
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Quoting Fly2HMO (Reply 4):
Yes. Saves fuel and wear and tear on the engines.

If I remember correctly it only saves the wear and tear on the engines as it is actually more fuel efficient to get up at full bore but this may just be takeoff.

Fred


User currently offlineshufflemoomin From Denmark, joined Jun 2010, 470 posts, RR: 1
Reply 6, posted (3 years 2 months 5 days 16 hours ago) and read 5403 times:

Perfect. Thanks to you all for the information. I'm just a very curious individual and next time the engines are throttled back, I'd like to have more of an idea about why it's happening.

User currently offlinevikkyvik From United States of America, joined Jul 2003, 9710 posts, RR: 27
Reply 7, posted (3 years 2 months 5 days 16 hours ago) and read 5388 times:
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I'm not a pilot, but my guess is that the speed doesn't actually decrease typically. Your forward acceleration may slow down (or stop), as you don't want to bust through 250 kts below 10,000 feet (at least in the US).

Also, flaps/slats are usually retracted around that time as well, which will decrease your drag. As a result, you may not need as much thrust to maintain the selected climb rate and forward speed.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineMir From United States of America, joined Jan 2004, 21425 posts, RR: 56
Reply 8, posted (3 years 2 months 5 days 15 hours ago) and read 5369 times:

Quoting flipdewaf (Reply 5):
If I remember correctly it only saves the wear and tear on the engines as it is actually more fuel efficient to get up at full bore but this may just be takeoff.

Jet engines are more efficient at higher power settings (all else being equal), and that's true across the board, but you run into problems with engine wear, which is the reason for the takeoff time limit. That's also the reason for derated takeoffs.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineshufflemoomin From Denmark, joined Jun 2010, 470 posts, RR: 1
Reply 9, posted (3 years 2 months 5 days 15 hours ago) and read 5363 times:

Quoting Mir (Reply 8):

So you're saying there may be an entry in the maintenance log for the aircraft that would lead the crew to climb at a lower thrust rating?


User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 10, posted (3 years 2 months 5 days 14 hours ago) and read 5323 times:

Quoting shufflemoomin (Reply 9):

So you're saying there may be an entry in the maintenance log for the aircraft that would lead the crew to climb at a lower thrust rating?

No. What he means if that over the life time of the engine, not doing de-rated take offs will take its toll eventually. Most take offs are done with about 85% power. The only time you'll see 100% is in an emergency really.

Take your car for example. Most likely you can drive 60mph down the freeway on 3rd gear, but do you really want your engine screaming at 5000RPM, if not more, the whole time? That's not healthy for your engine.

So on the same token, there's no point using 100% thrust if the plane is lightly loaded, there's no obstacles to worry about, and you have a nice long runway. You only need enough thrust to safely (and legally) get you in the air.

Then again, I've known more than a few pilots that would launch at 100% power when doing an empty ferry/repositioning flight. Tis' fun  


User currently offlineimiakhtar From , joined Dec 1969, posts, RR:
Reply 11, posted (3 years 2 months 5 days 11 hours ago) and read 5257 times:

Quoting flipdewaf (Reply 5):
it is actually more fuel efficient to get up at full bore but this may just be takeoff.

You are correct. A max thrust climb will result in lower trip fuel burn.

Quoting Fly2HMO (Reply 4):
Saves fuel and wear and tear on the engines.

See above, a de-rate does not save fuel. An A345/6, depending on the de-rate level will burn anywhere from 50-200kg extra compared to a max climb thrust setting.

Quoting Fly2HMO (Reply 10):
The only time you'll see 100% is in an emergency really.

Generalisation on your behalf? At some airlines it's standard procedure to do a max thrust/TOGA takeoff on contaminated runways. Depending on where you are, TOGA takeoffs could be very common.

Ultimately, it boils down to increasing engine life as the engines have greater operating margins with de-rate. When a GE-90 costs north of $30 million, it's not surprising to see airlines trying to extend engine life, even with fuel at $3.50/gallon

There are two excellent publications on this topic, one by RR/B777 and another by Airbus, see below:

Derated Climb Performance In Boeing 777

Flex/Derate, Engine Bump and Derated climb Related issues

[Edited 2011-05-10 20:31:33 for poor grammar]

[Edited 2011-05-10 20:42:54]

User currently offlineMax Q From United States of America, joined May 2001, 4325 posts, RR: 19
Reply 12, posted (3 years 2 months 5 days 10 hours ago) and read 5199 times:

On the longer, westbound Transatlantic flights returning from Europe to the US in the 757 we are often very tight on fuel.


There are several techniques we use to squeeze every last mile out of every pound of fuel.


One of mine was to do a full power take off, followed by a climb at Maximum Continuous thrust all the way to cruise altitude.


This ensures the quickest possible climb to altitude where you burn the least fuel when established in cruise.


It seems counterintuitive to use more power (and fuel, initially) to save fuel but overall trip fuel is less, and this technique works well.



The best contribution to safety is a competent Pilot.
User currently offlineimiakhtar From , joined Dec 1969, posts, RR:
Reply 13, posted (3 years 2 months 5 days 8 hours ago) and read 5175 times:

Quoting Max Q (Reply 12):
On the longer, westbound Transatlantic flights returning from Europe to the US in the 757 we are often very tight on fuel.

There are several techniques we use to squeeze every last mile out of every pound of fuel.

One of mine was to do a full power take off, followed by a climb at Maximum Continuous thrust all the way to cruise altitude.

This ensures the quickest possible climb to altitude where you burn the least fuel when established in cruise.

It seems counterintuitive to use more power (and fuel, initially) to save fuel but overall trip fuel is less, and this technique works well.

Thanks for your post. Is climb thrust derate available on the 757? I know it's available on the 737NG and 777, but can't recall if it's an option on the 757.

With the over powered nature of the beast, I would suspect it is?


User currently offlineMax Q From United States of America, joined May 2001, 4325 posts, RR: 19
Reply 14, posted (3 years 2 months 3 days 11 hours ago) and read 4839 times:

Quoting imiakhtar (Reply 13):

Thanks for your post. Is climb thrust derate available on the 757? I know it's available on the 737NG and 777, but can't recall if it's an option on the 757.

With the over powered nature of the beast, I would suspect it is?

By Derate I think you mean, is a reduced power climb setting available. Yes it is. But we don't use it.We switch to climb power at 1000 or 1500' and maintain this to cruise altitude. Maximum Continuous thrust is available at any time and will give you more power than climb power up until the high 20,000's where it becomes the same.


Derate actually means something quite different. It refers to the 'artificial' lowering of the available thrust of the engine.


This cannot be changed by the Pilots at any time.



The best contribution to safety is a competent Pilot.
User currently offlinejackmidd From United Kingdom, joined Jan 2010, 12 posts, RR: 0
Reply 15, posted (3 years 2 months 3 days 5 hours ago) and read 4757 times:

Whilst fuel savings are likely a big part of it the car example was good. Imagine driving your car along a straight, increasing speed to go up a steep hill with a speed limit at the top of it, you wouldnt continue to accelerate otherwise you'd end up speeding. LHR has good examples of thrust settings on the climb out, the changes in thrust are frequent, sometime more so than others, suppose it depends on assigned altitutes as well. As someone else pointed, they are generally following a flight plan, they should be at waypoints at a certain time, perhaps power settings are also time related.

User currently offlineAA737-823 From United States of America, joined Mar 2000, 5722 posts, RR: 11
Reply 16, posted (3 years 2 months 3 days 1 hour ago) and read 4723 times:

We also have to keep in mind here that the speed limit below 10000 feet is 250 kts. An airplane will often reach 250 kts WAY before 1000 feet, due to standardized departures, etc.

Further, takeoff thrust is only available for a short period of time; that is, on 737-200 aircraft, you're not supposed to maintain takeoff EPR for more than one minute on the ground, and I assume there's a similar restriction once you're in the air as well.


User currently offlineInnocuousFox From United States of America, joined Dec 2003, 2805 posts, RR: 15
Reply 17, posted (3 years 2 months 2 days 21 hours ago) and read 4668 times:

Isn't it a noise issue as well? They need to get up above certain levels as soon as possible and then they are able to proceed with a normal climb?


Dave Mark - Intrinsic Algorithm - Reducing the world to mathematical equations!
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 18, posted (3 years 2 months 2 days 20 hours ago) and read 4648 times:

Quoting imiakhtar (Reply 11):

Generalisation on your behalf?

Yes, and I forgot how you should always put a generalization disclaimer on this board   


User currently offlinevikkyvik From United States of America, joined Jul 2003, 9710 posts, RR: 27
Reply 19, posted (3 years 2 months 2 days 18 hours ago) and read 4601 times:
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Quoting InnocuousFox (Reply 17):
Isn't it a noise issue as well? They need to get up above certain levels as soon as possible and then they are able to proceed with a normal climb?

It can be, but that's dependent on the airport and the noise restrictions on the surrounding area.

SNA had quite a severe climb restriction previously (I think they've relaxed it somewhat). It's sort of nonsensical though, because allowing aircraft to use a higher climb thrust will allow them to climb faster, therefore likely bothering fewer people on the ground.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineMax Q From United States of America, joined May 2001, 4325 posts, RR: 19
Reply 20, posted (3 years 2 months 2 days 17 hours ago) and read 4585 times:

Quoting AA737-823 (Reply 16):


We also have to keep in mind here that the speed limit below 10000 feet is 250 kts. An airplane will often reach 250 kts WAY before 1000 feet, due to standardized departures, etc.

Not sure what you mean there. Regardless of the type of departure you are flying (ICAO A or B) you will never reach 250 knots before 1000 feet.


The normal profile has you operating at V2+10-15 up to 1000-1500 AGL, on the heaviest of jets that could be as high as 180-190 knots.


Certainly not 250.

Quoting vikkyvik (Reply 19):


SNA had quite a severe climb restriction previously (I think they've relaxed it somewhat). It's sort of nonsensical though, because allowing aircraft to use a higher climb thrust will allow them to climb faster, therefore likely bothering fewer people on the ground.

You have that a little confused. Departing SNA, you will actually use full power initially until you reach the cutback altitude, this does, as you say allow you to climb faster. But, at the cutback altitude you will reduce power much more than the usual climb power setting, usually only enough to allow a 500 feet per minute rate of climb.


You maintain this until clear of the 'sensitive' area, at which point you will set climb power , clean up and climb away.



The best contribution to safety is a competent Pilot.
User currently offlinevikkyvik From United States of America, joined Jul 2003, 9710 posts, RR: 27
Reply 21, posted (3 years 2 months 2 days 17 hours ago) and read 4571 times:
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Quoting Max Q (Reply 20):

You have that a little confused. Departing SNA, you will actually use full power initially until you reach the cutback altitude, this does, as you say allow you to climb faster. But, at the cutback altitude you will reduce power much more than the usual climb power setting, usually only enough to allow a 500 feet per minute rate of climb.

Ah OK, thanks for the clarification. Though I was talking about climb thrust, not takeoff thrust.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlinemusang From United Kingdom, joined Apr 2001, 861 posts, RR: 7
Reply 22, posted (3 years 1 month 3 weeks 5 days 4 hours ago) and read 4210 times:

Quoting Max Q (Reply 14):
Derate actually means something quite different. It refers to the 'artificial' lowering of the available thrust of the engine.

This cannot be changed by the Pilots at any time.

On the 737 Classics it can via the PERF INIT > N1 LIMIT page in the pre-flight set-up. Our proceduress don't allow for these to be modified so we just check its at "TO" i.e. not derated. If we should line-select "TO-1" or "TO-2" it then shows the new engine thrust limit, down from 23,500 lbs to (I think) 21,500 or 18,500. A semi permanent derate can be set by the engineers, on for example the lighter -500 or -300 aircraft.

The "Assumed Temperature" method achieves the reduced thrust calculation for take-off. We enter a fictional (but not random! Its worked out!) temp to fool the engine into thinking its warmer outside than it is, and it comes up with a reduced thrust setting. If we don't, the system just takes actual temp. and works the thrust out based on that. We would allow this on a short/limiting runway where we have to go at "max chat".The thrust reduction after take-off in such a case would be very noticeable, e.g. take-off at 94% thrust, reducing to about 86%. That would get your attention down the back. The "deceleration" you "feel" is actually the shallowing of the climb gradient.

Many people refer to the assumed temp process as "inputting the derate" or similar, but as MaxQ points out, technically its not.

Quoting AA737-823 (Reply 16):
We also have to keep in mind here that the speed limit below 10000 feet is 250 kts. An airplane will often reach 250 kts WAY before 1000 feet, due to standardized departures, etc.

Unlikely. Typical airline practise is to climb to 1000 feet or perhaps 1500 feet at T/O thrust, then begin accelerating combined with a thrust reduction.

Quoting vikkyvik (Reply 7):
but my guess is that the speed doesn't actually decrease typically.

Correct. See above comment. Standard routine - on reaching 1000' above airport or whatever the acceleration height is, set climb power (as mentioned, usually a reduction but occasionally an increase) and set the climb speed, i.e. typically increasing from say, 155 to 210 knots. As it accelerates, retract the flaps in stages as the flap minimum speeds are reached. Example - through 170 knots, flaps from 5 to 1, and through 190 knots, flaps to UP. Flap retraction is conditional upon accelerating, not the other way round!

For noise abatement the procedure sometimes varies e.g. at Naples thrust is reduced at 1000 feet above airport as usual, maintaining the initial climb speed, but acceleration is delayed until 3000 feet. I seem to recall Madrid is the same....

Regards - musang


User currently online747classic From Netherlands, joined Aug 2009, 2079 posts, RR: 14
Reply 23, posted (3 years 1 month 3 weeks 5 days 2 hours ago) and read 4174 times:

Thirty five years ago I already used derated climb thrust on early PW JT9D series 3 and 7 powered 747's to increase engine life. (hot section of the engine was extremely critical regarding high temperatures during a prolonged time )

The procedure was :
After the climb-out speed was reached and V/S was higher than 600ft/min, CLB thrust setting was manual reduced by 0.04 EPR. The PF and FE coordinated that procedure (no A/T available during CLB).
When the V/S dropped below 600 ft/min power had to be increased again to (full) CLB thrust.
This procedure however increased the fuel used until TOC considerably , but back then the fuel was cheap , certainly compared to the extra maintenance of the hot section of the PW engines.



Operating a twin over the ocean, you're always one engine failure from a total emergency.
User currently offlinetom355uk From United Kingdom, joined Nov 2007, 336 posts, RR: 3
Reply 24, posted (3 years 1 month 3 weeks 5 days 1 hour ago) and read 4157 times:

Quoting shufflemoomin (Thread starter):
there's a clear throttling back of the engines and you can feel a definite decrease in speed.

The classic somatographic illusion - cause of many, many accidents in the past. That change in rate of acceleration makes you feel like you are slowing down and falling at the same time, because you don't have any outside visual cues to 'calibrate' your inner ear with. See Flash 604, Kenya 507 and Air India 855.



on Twitter @tombeckett2285
25 shufflemoomin : Interesting. I was just getting curious when some of the people here said that when the engines are throttled back during climb that the airspeed con
26 AA737-823 : Thanks for catching that- I left of a zero, and intended to say 10,000 feet!!!
27 tdscanuck : You're feeling a *change* in acceleration. The change is negative (you're not accelerating as quickly) but not big enough that you're actually decele
28 vikkyvik : Think of it this way: If you're moving at a constant velocity, you don't feel any extra forces that you wouldn't feel being stationary. When you get
29 Post contains images shufflemoomin : Thanks for the post vikkyvik, what you say makes perfect sense. In some flights, there's been a sprightly take-off and after a minute or so, it feels
30 Mir : What you're experiencing is a reduction in climb rate. This is what allows the speed to build up (in essence trading what you could gain in altitude
31 Post contains images vikkyvik : Ugh, I just noticed that this was inaccurate: Should have been "changes in acceleration, which is called the jerk". You're feeling the jerk, not the c
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