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?

Here's one data point (sort of). It's been a LONG time, so don't hold me to the exact numbers. I was qualified to do squadron level test flights on the U.S. Navy F-8 Crusader powered by a single P&W J-57, a conventional (non fan) turbojet engine with afterburner (reheat). These tests were done after major, squadron-level maintenance, like an engine change.

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.

I have seen the CFM-56's pulled back to approx 45% in cruise. It does take longer than 30 seconds ( maybe 40) to get back to 85-90%.

Use speedbrakes.

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.

It seems unanimous then:)

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:

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

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.

Maybe because everyone hates paperwork

Agreed, but one should still one's best to avoid it without creating additional problems!

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.

I've never had the clacker in my thus far relatively short professional career. Any activation of the clacker is mandatory techlog entry, a call to maintenance and hopefully a deferred defect and authorization to continue.

The problem is....some airlines treat an exceedence whether it is over landing weight by 1 pound or over VMO by 1 knot as almost the end of the world. As a pilot who can expect to be demoted or severely admonished, it becomes a big deal.

Any one else have any experience with spool up times at altitude, perhaps on engines other than CFM?

AIRBUS FOLKS
Over speed does not necessarily require inspection but pilots must report it. VMO>20KT or VMO and >2g requires inspection.

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."

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.

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.

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!

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 %

I've never had the clacker in my thus far relatively short professional career.

One presumes your operation descends with a margin to the barber pole. No problem there. I'm a bit dated but we routinely had minor clacker exceedances (I'm talking 5-10kt rather than a significant exceedance).

Any activation of the clacker is mandatory techlog entry, a call to maintenance and hopefully a deferred defect and authorization to continue.

With the greater emphasis on DFDR/QAR exceedance monitoring, that makes good sense as it indicates attention on the flight deck and adherence to SOP. I would be extremely surprised if the base response was other than to continue.

some airlines treat an exceedence whether it is over landing weight by 1 pound or over VMO by 1 knot as almost the end of the world.

That, of course, is both true and most unfortunate. Unless those monitoring exceedances have a suitable engineering/certification background, there needs to be plenty of guidance from the totem pole folk as to what is, and what is not, terribly important in the overall scheme of things.

As a pilot who can expect to be demoted or severely admonished, it becomes a big deal.

Absolutely correct .. we all have our crosses to bear unfortunately


The main value of this thread is to get the acceleration deterioration at height message across to the general heavy aircraft pilot population ...

Boeing have a current bulletin out for the NG, recommending that thrust not be brought back below 60% N1 for short term speed excursions & to use speedbrake if required.

The engineering manual at our joint requires a rather involved inspection for overspeeds greater than M0.02, which isn't much of an overspeed. It's 15kts for a VMO overspeed, if I remember correctly.

I just recently had an over speed in the cruise CAS went from 247kt to 272kt (.85 Mach)in 3 seconds due to a 6deg temp drop in that time. Pulled the speed brake but speed was still increasing so also pulled thrust back towards idle. Yes it does seem to take forever for the engines to spool up again, but speed never fell below what was bugged.

Yes, you just got to anticipate when to add thrust again. Then it's no problem, i.e. NOT wait with adding thrust again, until speed is back at bugged speed, but rather as soon as the speed starts decreasing again.

And of course not overreact for small speed excursions as Boeing writes in their bulletin. Keep calm. Rather a little too fast, than a little too slow. ;)

I am with you in saying that it is not a fall-off-the-sky deal, but I wouldn't downplay it to that extent.

The margin set between MMO and MD (cleared for flutter and loads) is not based on the crew assumption that MMO is a rubber-stamped line.

I am not saying to disregard MMO, obviously. I am just saying that unintentionally exceeding it is no cause for panic (unless you work for an airline run by people that have no understanding of aviation and will use any excuse to punish you).

MMO is just a line drawn on a piece of paper in some manual that someone had to give an approval stamp

Vmo/Mmo is a limit the same as any other and is required to be observed. However, the limit has a lot more fat than the older style certifications using Vne.

Indeed, most (not all) AFMs have the rider that the limit may be exceeded for test or training.

I was just reviewing a 747-8 AFM revision (something I need to do with some regularity), this is what it says:
"The maximum operating limit speed shall not be deliberately exceeded in any regime of flight."
I think the key word here is "deliberately".
BTW, 747 MMO is 0.90 Mach. During the 747-8 certification, I looked at a whole lot of data that was 0.97 Mach and higher (flutter testing and the such), and the airplane didn't break :ok:

I believe a key differenciator for test pilots is that they act in a particular way when they know the first or even the second hole in the cheese is lined up already.
This is why you will not find MD published in the AFM.

Test pilot during acceptance of new aircraft demonstrated MMO exceedance. No big deal at MMO + 0.2, apart from noise of the damn clacker!

Yeah probably a good thing what with 1% of pilots being clowns and all.
Do Boeing and AB use the same process for determining MMO relative to MD?

What is the term "MD" being referred to here?


Oops. Disregard. Just googled it. in case anybody else was as ignorant as me: Vd/Md are the absolute maximum speeds. I think referred to as dive speeds. 0.96 apparently for an A320, probably something similar for a 737.

Maximum Mach in a Dive...

Kfuddle
A320 VMO350KTS, MMO M.82, VD381KTS and MD.89.
Only A380 has MD M.96
According Airbus
1. critical loss of lift due high Mach is well beyond the normal envelope and MD.
2. In level flight at high altitude, drag increase prevents MD from being reached. In flight tests MD is reached through specific manoeuvres (dives with full thrust are necessary)

Thanks Vilas. So the difference between 0.82 and 0.89 isn't really that great considering the shear that can be experienced crossing a jetstream!

I think you overestimate the shear of even a very strong jet stream. Going from .79 to .85 (like I described previously) was a huge speed increase. And it took time. Had the speed continued to increase, it would have been fairly easy to pull back on the yoke an climb a bit.

Anyway, an Easyjet 737 was recorded to go 450 knots, in a botched manual reversion test. No damage to the aircraft afterwards! Though the Mach number was probably relatively low, as it happened around 10-15k feet, it is still a testimony, that these machines are stronger than what you might think.

And for those that think I am making that up/are not familiar with the incident:
http://www.aaib.gov.uk/cms_resources...JK%2009-10.pdf

It's an interesting read...

Deefer dog, I sure hope you meant +0.02 Mach, not +0.20 Mach :eek:


Most airliners don't like going supersonic :}

Thanks Cosmo,

You're probably correct about my over estimation, it would be something like 40kt increase from 0.82. However, we know how astonishingly quickly things can unravel at 40,000 once problems occur.

Thanks also for the report, the FDM recorded max was an astonishing 429kts! This was quoted as Mach 0.719. I guessing the FDM sampling rate is the reason why the pilot noted a higher speed. Looking at this "Diving" Into A320 Diving Speeds the A320 Vd is 381kts, which must be similar to the B737!

Check you this video of the A380 flutter tests (also at the end of the linked blog)

Speed brakes are not necessarily the best solution to an overspeed for all aircraft types. At highish levels, like 410, aggresive airbrake usage will easily take an aircraft like an E190 to the other end of the speed tape and beyond, without a significant change in airspeed. But reducing thrust to idle will provide relief. Fortunately, E190s have such small engines that they very quickly spool up to cruise thrust from idle.

PM