shufflemoomin
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Reason For De-rated Climb?

Tue May 10, 2011 8:47 pm

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?
 
Mir
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RE: Reason For De-rated Climb?

Tue May 10, 2011 9:16 pm

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
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shufflemoomin
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RE: Reason For De-rated Climb?

Tue May 10, 2011 9:32 pm

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.
 
KELPkid
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RE: Reason For De-rated Climb?

Tue May 10, 2011 9:43 pm

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.
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Fly2HMO
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RE: Reason For De-rated Climb?

Tue May 10, 2011 9:43 pm

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]
 
flipdewaf
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RE: Reason For De-rated Climb?

Tue May 10, 2011 10:17 pm

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.

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shufflemoomin
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RE: Reason For De-rated Climb?

Tue May 10, 2011 10:22 pm

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.
 
vikkyvik
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RE: Reason For De-rated Climb?

Tue May 10, 2011 10:53 pm

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.
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Mir
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RE: Reason For De-rated Climb?

Tue May 10, 2011 11:23 pm

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
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shufflemoomin
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RE: Reason For De-rated Climb?

Tue May 10, 2011 11:30 pm

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?
 
Fly2HMO
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RE: Reason For De-rated Climb?

Wed May 11, 2011 12:30 am

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  
 
imiakhtar
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RE: Reason For De-rated Climb?

Wed May 11, 2011 3:27 am

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]
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Max Q
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RE: Reason For De-rated Climb?

Wed May 11, 2011 5:07 am

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.
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imiakhtar
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RE: Reason For De-rated Climb?

Wed May 11, 2011 6:21 am

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?
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Max Q
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RE: Reason For De-rated Climb?

Fri May 13, 2011 3:35 am

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.
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jackmidd
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RE: Reason For De-rated Climb?

Fri May 13, 2011 9:47 am

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.
 
AA737-823
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RE: Reason For De-rated Climb?

Fri May 13, 2011 1:19 pm

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.
 
InnocuousFox
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RE: Reason For De-rated Climb?

Fri May 13, 2011 5:38 pm

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?
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Fly2HMO
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RE: Reason For De-rated Climb?

Fri May 13, 2011 6:20 pm

Quoting imiakhtar (Reply 11):

Generalisation on your behalf?

Yes, and I forgot how you should always put a generalization disclaimer on this board   
 
vikkyvik
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RE: Reason For De-rated Climb?

Fri May 13, 2011 8:49 pm

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.
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Max Q
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RE: Reason For De-rated Climb?

Fri May 13, 2011 9:49 pm

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.
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vikkyvik
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RE: Reason For De-rated Climb?

Fri May 13, 2011 10:13 pm

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.
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musang
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RE: Reason For De-rated Climb?

Fri May 20, 2011 10:34 am

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
 
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747classic
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RE: Reason For De-rated Climb?

Fri May 20, 2011 12:46 pm

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.
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tom355uk
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RE: Reason For De-rated Climb?

Fri May 20, 2011 2:15 pm

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.
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shufflemoomin
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RE: Reason For De-rated Climb?

Wed May 25, 2011 7:32 am

Quoting tom355uk (Reply 24):
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.

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 continues to increase. To me,at least, it feels like there's a definite deceleration. So you're saying that's just an illusion? I'm unfamiliar with the term "somatographic" so I guess some googling is in order here. I could understand how that sensation would cause someone to think they're slowing down (as it does with me) but surely a pilot has more than enough instrumentation to tell him that's not the case and they would be aware of the illusion this sensation causes?
 
AA737-823
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RE: Reason For De-rated Climb?

Wed May 25, 2011 8:01 am

Quoting Max Q (Reply 20):
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.

Thanks for catching that- I left of a zero, and intended to say 10,000 feet!!!
 
tdscanuck
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RE: Reason For De-rated Climb?

Wed May 25, 2011 2:13 pm

Quoting shufflemoomin (Reply 25):
I was just getting curious when some of the people here said that when the engines are throttled back during climb that the airspeed continues to increase. To me,at least, it feels like there's a definite deceleration.

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 decelerating.

Tom.
 
vikkyvik
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RE: Reason For De-rated Climb?

Wed May 25, 2011 5:49 pm

Quoting shufflemoomin (Reply 25):

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 continues to increase. To me,at least, it feels like there's a definite deceleration. So you're saying that's just an illusion? I'm unfamiliar with the term "somatographic" so I guess some googling is in order here. I could understand how that sensation would cause someone to think they're slowing down (as it does with me) but surely a pilot has more than enough instrumentation to tell him that's not the case and they would be aware of the illusion this sensation causes?

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 up from your seat on an airplane during cruise, you don't suddenly go rocketing back toward the rear of the airplane due to nothing holding you.

So any force you feel is due to acceleration. During a climb and a forward acceleration, you feel like you're being pushed back into your seatback and down into your seat. Since it's very difficult to look out the window and discern whether you're climbing, descending, accelerating or decelerating forward, your eyes are pretty useless, and these feelings are basically all you have to go on. Plus, after a minute or two of climb, you probably get used to the forces you're feeling (I know I do!) and don't really notice them.

So when the airplane goes from, say, 10 feet/second forward acceleration to 5 feet/second forward acceleration, the feeling of being pushed back into your seatback just got cut in half.

And when the airplane goes from, say, 1000 feet/minute climb to 500 feet/minute climb, the feeling of being pushed down into your seat just got cut in half.

Add all that up, and it feels like you're suddenly being pushed forward and up, when in actuality you're just being pushed down and back less than you were before.

As for pilots, I'm sure they are trained to deal with it, and yes instrumentation will tell you as well, assuming it's functioning correctly. But in panic situations, the ability to rationally deal with information hitting your brain is reduced. So you feel like you're falling, and you automatically react to it, without being able to process and realize that you're not actually falling.

Sort of related - I remember learning in one of my classes that a similar situation can happen/has happened in a deep stall. The airplane can be perfectly level, giving the appearance of level flight. But in actuality, you're descending at a huge rate; the AOA is great enough that you're stalled.

Random trivia: what you're feeling is changes in jerk. The jerk is the name for change in acceleration (at least the one I learned). So it would have units of m/s^3 or ft/s^3.
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shufflemoomin
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RE: Reason For De-rated Climb?

Wed May 25, 2011 11:37 pm

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 like the aircraft has essentially hit a wall. Similar to the feeling you get from suddenly trying to swim in water. Obviously I've deduced that I'm experiencing a reduction in speed/climb rate and not a complete stop in both, otherwise I wouldn't be here.   It can just be quite pronounced at times and in a handful of occasions, has given me a momentary cause for concern. As I think I mentioned before, one of those was a night time departure in an A330 from YYZ when after a minute or so of climb, due to the sudden sensation of lose of climb rate/speed and the engines throttling WAY back it caused a feeling of panic for a moment. I was looking for explanations for what I experienced and your post, plus a few others, have given me the answers I was looking for. I'm not a particularly nervous flyer, but I would say a curious one and knowledge isn't always power, regardless of what some people say.  
 
Mir
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RE: Reason For De-rated Climb?

Thu May 26, 2011 12:56 am

Quoting shufflemoomin (Reply 29):
Obviously I've deduced that I'm experiencing a reduction in speed/climb rate and not a complete stop in both, otherwise I wouldn't be here.

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 for some additional airspeed). Eventually the climb rate will come back as the speed builds toward the wing's optimal climb speed, but you don't notice this as much.

-Mir
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vikkyvik
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RE: Reason For De-rated Climb?

Thu May 26, 2011 1:11 am

Ugh, I just noticed that this was inaccurate:

Quoting vikkyvik (Reply 28):

Random trivia: what you're feeling is changes in jerk.

Should have been "changes in acceleration, which is called the jerk".

You're feeling the jerk, not the changes in the jerk.

Quoting shufflemoomin (Reply 29):
I'm not a particularly nervous flyer, but I would say a curious one and knowledge isn't always power, regardless of what some people say.

Certainly can be true. If you have an irrational fear of flying (or anything else), all the knowledge in the world probably won't help. Otherwise it wouldn't be "irrational".  
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