Reducing thrust in cruise for overspeed
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Reducing thrust in cruise for overspeed
Have never had to do it but it has been recommended to only reduce thrust levers to the mid-position. A friend recently told me that he saw someone bring a pair of CFM's back to idle once in cruise and the time to spool back up was in the order of 30 seconds or longer.
Has anyone seen similar(or better or worse) performance on other engine types?
Has anyone seen similar(or better or worse) performance on other engine types?
Has anyone seen similar(or better or worse) performance on other engine types?
One test involved pulling the power back to idle at something like 40 thousand feet. We recorded the stable idle rpm and then set full military thrust; i.e. full power without afterburner. We timed how long it took to reach 100% and if I remember correctly the time was usually around 35 seconds or so. And the J-57 had a reputation for pretty good throttle response.
Based on that, I would guess the time for a high bypass fan engine to go from idle to high power at high altitude would be much longer than many pilots would imagine. I flew the 767 and I was always aware of the relatively long spool up time of those engines; however, I don't recall ever letting them stabilize at flight idle at high altitude and subsequently call for high power settings. But even so, thirty seconds or more doesn't sound unusual to me.
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At high level it has been recommended to only reduce thrust levers to the mid-position.
Good idea, and use speedbrakes.
A friend recently told me that he saw someone bring a pair of CFM's back to idle once in cruise
Bad idea, unless you intend to descend.
Good idea, and use speedbrakes.
A friend recently told me that he saw someone bring a pair of CFM's back to idle once in cruise
Bad idea, unless you intend to descend.
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Do any of you 737 guys have the Engine Response Time Bulletin? There is a lot more to it, but it includes this about the cruise:
3. If the airplane experiences a sudden increase in airspeed that causes the autothrottle to reduce thrust, manually guard the thrust levers to maintain a minimum of 60% N1, if possible. If thrust is reduced below 60% N1, a significantly longer time will be required for the engines to spool up if the time at idle thrust is less than 60 seconds.
4. If the airplane experiences a sudden increase in airspeed, consider using smooth extension of the speed brakes to increase drag and to avoid large thrust reductions.
4. If the airplane experiences a sudden increase in airspeed, consider using smooth extension of the speed brakes to increase drag and to avoid large thrust reductions.
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AIRBUS has given following procedures for overspeed.
OVERSPEED PREVENTION PROCEDURE
1.Keep AP and A/THR
2.Select a lower speed
3.Monitor speed trend
4.Speedbrakes (as required)
OVERSPEED RECOVERY PROCEDURE
1.Keep AP and A/THR
2.Full Speed brakes
3.Monitor IDLE or set thrust levers on IDLE
OVERSPEED PREVENTION PROCEDURE
1.Keep AP and A/THR
2.Select a lower speed
3.Monitor speed trend
4.Speedbrakes (as required)
OVERSPEED RECOVERY PROCEDURE
1.Keep AP and A/THR
2.Full Speed brakes
3.Monitor IDLE or set thrust levers on IDLE
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If the concern is Vmo, why are we so terribly concerned about reducing speed so quickly ?
Vmo is a limit but not of a "fall-out-of-the-sky" concern if one has a nominal exceedance ie bringing the indication back should be a more disciplined and steady operation.
Vmo is a limit but not of a "fall-out-of-the-sky" concern if one has a nominal exceedance ie bringing the indication back should be a more disciplined and steady operation.
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If the concern is Vmo, why are we so terribly concerned about reducing speed so quickly ?
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I empathise .. in my own fleet I have the same sort of problem with pilot concerns about exceedances and big brother ... we know about it from the black boxes after the flight ... even if I don't care much about this particular one, most of the pilots do, unfortunately.
However,
(a) folk should be aware than Vmo is not a full-on-must-fix-the-exceedance-before-the-plane-falls-out-of-the-sky sort of thing
(b) presuming the heavy iron folk still descend on the barber pole, modest/nominal Vmo exceedances are a routine fact of life.
However,
(a) folk should be aware than Vmo is not a full-on-must-fix-the-exceedance-before-the-plane-falls-out-of-the-sky sort of thing
(b) presuming the heavy iron folk still descend on the barber pole, modest/nominal Vmo exceedances are a routine fact of life.
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presuming the heavy iron folk still descend on the barber pole, modest/nominal Vmo exceedances are a routine fact of life.
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I empathise .. in my own fleet I have the same sort of problem with pilot concerns about exceedances and big brother ... we know about it from the black boxes after the flight ... even if I don't care much about this particular one, most of the pilots do, unfortunately.
However,
(a) folk should be aware than Vmo is not a full-on-must-fix-the-exceedance-before-the-plane-falls-out-of-the-sky sort of thing
(b) presuming the heavy iron folk still descend on the barber pole, modest/nominal Vmo exceedances are a routine fact of life.
However,
(a) folk should be aware than Vmo is not a full-on-must-fix-the-exceedance-before-the-plane-falls-out-of-the-sky sort of thing
(b) presuming the heavy iron folk still descend on the barber pole, modest/nominal Vmo exceedances are a routine fact of life.
Any one else have any experience with spool up times at altitude, perhaps on engines other than CFM?
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I flew with a CA recently who balked at my use of speed brake in mountain wave to avoid an overspeed. I told him about the "Bodie Maneuver" issue with N1 spool times and what the FCTM directs us to do.
His response, "I'd rather write up an exceedance than use speedbrakes in cruise."
His response, "I'd rather write up an exceedance than use speedbrakes in cruise."
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I have done it once, and yes it takes forever for the engines to spool back up (in the order of 30+ secs is probably about right). However, I did have "forever" available, as with idle thrust and speed brakes out the speed was still increasing reaching a point well into the clacker (reaching almost .85 if I remember correctly).
Still in the clacker, I increased thrust as soon as the trend vector reversed and it took a long time to spool up, but an equally long time for the speed to decrease. But if you wait with increasing thrust, until the speed is back in the normal range you might set yourself up for a problem.
I agree 100% with john_tullamarine. It was a non issue, and no need to get stressed about. Just reduce speed calmly and no reason to panic. I could have started climbing, but it went so fast that the speed was already approaching .85 and the increasing trend vector starting to become less and expected to start reversing, that I didn't find it made any sense. My thinking was it was better to let it fly straight ahead, rather than to introduce G into the equation. The whole experience was unnoticeable, no turbulence, no previous warnings, no big changes in the wind was forecasted, no nothing. MMO is just a line drawn on a piece of paper in some manual that someone had to give an approval stamp. I can attest to that a 737 flies just fine at .85, and doesn't fall out of the sky or get's bend out of shape (in order not to be sued, I will however add the disclaimer "please respect the limits set out by the Boeing, and I assume no responsibility if you exceed them either inadvertently or on purpose").
Had it been the other way around (decreasing headwind), I think it would have been a non issue, as the sheer was really smooth and gradual. I think the increase in thrust would have been much faster to hold the speed up. What I am getting at is, that the engines really spool down quite slowly too, which was contributing to the speed getting that high.
I would never let the thrust reduce to idle, for a small speed excursion, but in this case there was nothing else to do, and obviously with .85 there is a lot of time to before the speed decays to a critical level, to increase thrust again. For normal small increases in speed (like flying at .79ish having .815 and increasing trend vector), I recommend to start pulling the speed brake, before even touching the thrust levers - it usually works by itself. Due to the slow spool down time, speed brakes are much more effective (instant drag as opposed to slow decrease in thrust).
As for paperwork, it's one line in the Techlog, a 5 min inspection of the flaps and another line by the technician to sign it out again. And filling out a report, which takes 5 mins. No big deal really.
Still in the clacker, I increased thrust as soon as the trend vector reversed and it took a long time to spool up, but an equally long time for the speed to decrease. But if you wait with increasing thrust, until the speed is back in the normal range you might set yourself up for a problem.
I agree 100% with john_tullamarine. It was a non issue, and no need to get stressed about. Just reduce speed calmly and no reason to panic. I could have started climbing, but it went so fast that the speed was already approaching .85 and the increasing trend vector starting to become less and expected to start reversing, that I didn't find it made any sense. My thinking was it was better to let it fly straight ahead, rather than to introduce G into the equation. The whole experience was unnoticeable, no turbulence, no previous warnings, no big changes in the wind was forecasted, no nothing. MMO is just a line drawn on a piece of paper in some manual that someone had to give an approval stamp. I can attest to that a 737 flies just fine at .85, and doesn't fall out of the sky or get's bend out of shape (in order not to be sued, I will however add the disclaimer "please respect the limits set out by the Boeing, and I assume no responsibility if you exceed them either inadvertently or on purpose").
Had it been the other way around (decreasing headwind), I think it would have been a non issue, as the sheer was really smooth and gradual. I think the increase in thrust would have been much faster to hold the speed up. What I am getting at is, that the engines really spool down quite slowly too, which was contributing to the speed getting that high.
I would never let the thrust reduce to idle, for a small speed excursion, but in this case there was nothing else to do, and obviously with .85 there is a lot of time to before the speed decays to a critical level, to increase thrust again. For normal small increases in speed (like flying at .79ish having .815 and increasing trend vector), I recommend to start pulling the speed brake, before even touching the thrust levers - it usually works by itself. Due to the slow spool down time, speed brakes are much more effective (instant drag as opposed to slow decrease in thrust).
As for paperwork, it's one line in the Techlog, a 5 min inspection of the flaps and another line by the technician to sign it out again. And filling out a report, which takes 5 mins. No big deal really.
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P.s.
I never heard a word from our safety department after sending that report. What kind of airline would punish their pilots for a weather phenomena out of their control?? Excuse me, but if I worked for such an airline, I would start looking for another job. That's a very poor safety culture, to have the employees living in fear of making a mistake and most like makes for an automation culture with inevitably decrease in skills.
I never heard a word from our safety department after sending that report. What kind of airline would punish their pilots for a weather phenomena out of their control?? Excuse me, but if I worked for such an airline, I would start looking for another job. That's a very poor safety culture, to have the employees living in fear of making a mistake and most like makes for an automation culture with inevitably decrease in skills.
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His response, "I'd rather write up an exceedance than use speedbrakes in cruise."
This has got to be the first pilot I've ever heard about who would rather do paperwork after a flight than avoid it and be first in the pub. Amazing!
This has got to be the first pilot I've ever heard about who would rather do paperwork after a flight than avoid it and be first in the pub. Amazing!
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Can testify to very long spool up times. I did it once when I was a new(er) first officer. It wasn't even idle, reduced to maybe 50 % N1 and it was painful to watch the slow response back up. A humbling experience and remember since the even humbler captain's suggestion to limit reduction to 70 %