Inappropriate rotation technique

[spud3]

I've heard that basically this can "degrade the stalling margin by up to 3 degrees in some cases"

I'm looking for an explanation.

Specifically, if pilot A rotated at an excessive rate, to the computed take off pitch attitude, (and not beyond), and let's say the pilot even commenced rotation before reaching Vr, will this cause any problems at all if the pilot does not exceed the computed take off attitude?

Please note, I am not refering to an increase in ground roll distance, as I see this may be the result, and so please do not reply to this thread with that response.

Of course, icing conditions may be present, so if you have some thoughts on how this may factor in, please respond.

Or does the correctly computed rotation pitch angle have nothing to do with an aircraft stall, even with contaminated wing?

thoughts?

[john_tullamarine]

Interesting question.

Can't see any problem with stall onset.

However, such a technique

(a) with a concurrent engine failure sets the pilot up for not hitting V2 as expected. Generally, rotation rate is geared to suit the programmed VR-V2 split. OEI, rotate slow results in V2 exceeded, rotate fast results in V2 not achieved.

(b) depending on Type airframe geometry, may set the pilot up for a tail strike.

(c) sets the pilot up for an overrotation in the event of any distraction - now that might, in turn, set up a stall scenario

(d) sounds pretty silly to me.

[ImbracableCrunk]

Could it be as simple as the private pilot ground effect-type question? You rotate early, get airborne early (Vmu or "slow") and are at a pitch/airspeed mis-match (ie AOA).

I'll pitch for 15 degrees on takeoff, but not while I'm on the runway. I think on the 737, you should reach 15 degrees about the same time you get to V2+15.

[5LY]

A proper rotation ensures that your performance targets are made. Too fast a rotation, too slow a rotation, under rotation, or over rotation all will reduce your screen height. If you are performance limited in any way, you've lined up a hole in the cheese.

[galaxy flyer]

One issue I know of (part of the findings in the G650 test crash) is that a rapid rotation can cause the plane to be airborne in ground effect and the rapid rotation can slow acceleration. Where a slower rotation rate results in the plane passing thru ground effect and accelerating. A wing stalls at a lower AoA in ground effect than in a free stream. so, the wing can rapidly reach its stall AoA in ground effect and roll off.

GF

[de facto]

I think on the 737, you should reach 15 degrees about the same time you get to V2+15.
Correct.higher speed you under rotated,under speed You over rotated.
The 15 deg is a general nr obviously, one should be in fd pitch when rotation maneuver is finished at v2+15.

[paulsalem]

Most people rotate way too fast. I believe most AFMs state 3deg / second. So it should take you 5 seconds to go from the ground to 15deg up.

Rotating too fast is uncomfortable for pax and crew and dangerous as the g loads increase stall speed. The last thing you want low and slow is to raise your stall speed.

[sheppey]

In the simulator it is quite common to see pilots rotating harshly so that within 1.5 seconds they are beyond ten degrees, then slow the rate of rotation so as to be around 15-17 degrees by the time five seconds has passed since the first rotation movement. It is hard to convince them since they invariably claim they reach 15 degrees by 5 seconds as per recommended procedure.

It was the extremely rapid rate of rotation when using Flap one for take off on the 737-200, that caused marked wingdrop and pitch up associated aymmetric wing icing. The report into the Potomac River 737 accident discussed this.

Way back in 1989 a crusty old and not too bloody competent ex Royal Navy chief pilot of long since defunct UK 737-200 operator criticised my rotation rate as being too slow. For the next several flights I timed his own rotation to 15 degrees. In practically every case he reached 15 degrees with a savage jerk of two seconds and he would not believe the figures when I showed him. He didn't change his technique even after the evidence was presented to him. There were quite a few of his type around in those days. His first officers were terrified of him.

[JammedStab]

737-200 and old DC-9's used to rotate at 6°/second interestingly.

[Capt Claret]

The Douglas/Boeing 717, a DC9 on steroids, recommends a rotation rate of 2.5º per second. It surprises me that the DC9 would have been more than double that. (Not disagreeing or arguing JammedStab, as I've not flown the DC9, just surprised at the difference).

[flyboy410]

I know of a chap who does his rotations in a strange way. He makes a sharp pull on the yoke and abruptly settles it around 8 or 9 degrees ANU narrowly avoiding a tail strike. Once he sees RA and VS increasing, he'll yank back again and take it fast up to 15 degrees or as commanded by the FD bars and abruptly settles it. Have not heard from him in sometime now...;)

[FullWings]

I'm sure we all aim to rotate at the correct rate, as per the AFM. Personally, given the natural spread of results with manual manoeuvres, I think it's better to err on the slower side than the faster one. Reasons in no particular order:

a) Less chance of a tailstrike.
b) It's better to be above V2 than below it, in case of power loss.
c) You can always convert excess speed to height, the other way round doesn't work so well near the ground, especially when you're slow.
d) A rapid rotation at a slower speed generates more drag than normal so leaves you worse off in total energy terms.
e) If the aircraft is certified to clear all obstacles OEI from V1, with everything running you will be well clear - no need to snatch it into the air.
f) If you rapidly rotate to a high pitch attitude then lose an engine, it can require some quick reactions and deft handling to recover the situation.
g) It gives the person sitting next to you the willies...

[Microburst2002]

Galaxy flyer seems to have answered the question.

If you rotate very rapidly you achieve a high aoa while your stall angle is reduced, so margin is reduced.

Also it seems to me that a fast pitch rate will create higher an aoa than a slow one, since the flight path "lags" behind pitch. If you rotate slowly, flight path "catches up" with pitch.

[ViciousSquirrel]

Galaxy and Micro have it as far as I'm concerned.

I see this kind of agressive rotation technique on the EMB 145 a lot, at varying degrees of severity. We have a Pitch Limit Indicator on our PFD which is basically our angle of attack gauge. It doesn't even display on the PFD until you're approaching the critical AOA. For some pilots I've flown with the PLI makes regular appearances when rotating the aircraft. I would take that as an indication that your technique might need a little modification.

Someone mentioned I think the private CRJ that crashed in Birmingham a few years back. This also demonstrates the point nicely. The final report mentioned frost on the wings and an aggressive rotation as the cause. They also mentioned that either one of these by themselves wasn't enough to cause the loss of control.

Bad technique like this is inconsequential 99.9% of the time and might just make the chap next to you a little uncomfortable. It's that 0.1% where the perfect combination comes together that it's going to bite you and bite you hard.

I guess my question then is this: If it says 3 degrees a second in the manual then why not just rotate at 3 degrees a second?

[Microburst2002]

In the bus, more than in other models, lo houred pilots tend to rotate very rapidly, chasing the pitch FD bar. I did it myself a couple of times. Its easy with the sidestick, lack of experience and confusion about how FD works. This was detected by foqa in my airline and corrected since then by explaining the guys that the FD bar doesn't give pitch rate command.

If there is no engine failure, rotating slowly, even less than 3 per sec, is ok and it is smooth and nice. If then you have tahe failure your ass is covered.

If there is engine failure it is probably better to rotate slowly than rapidly, since flight path will lag more behind pitch.

So in conclusion I always favour a slow rotation (but without delay, at VR)

[JammedStab]

The Douglas/Boeing 717, a DC9 on steroids, recommends a rotation rate of 2.5º per second. It surprises me that the DC9 would have been more than double that. (Not disagreeing or arguing JammedStab, as I've not flown the DC9, just surprised at the difference).

Not done anymore due to contamination related incidents.

[sheppey]

737-200 and old DC-9's used to rotate at 6°/second interestingly.

My copy of the Boeing 737-200 FCTM 1975 states three degrees per second up to fifteen degrees. The Boeing Airliner magazine same era says same thing. Certainly Boeing never recommended more than three degrees per second when the 737 first came out.

I bet it was your chief pilot's personal technique rather than a manufacturerer. Sounds lethal to me.

[Nubboy]

Hi guys. Good thread and interesting points.
However the other side of the coin to slow rotation rates could place you close to, or even above, tyre speed limits. 195 kt tyres at places like Addis or Tehran (about 7500' amsl) in a heavy 321 on a hot day spring to mind. Doubtless there are lots of others.

[hawk37]

Well I apologize to the group for posting and then not being around for follow-up. The last 4 days have been unexepectedly busy for me.

The first post, JT says "Can't see any problem with stall onset" and when I read it I thought I agreed, remembering I had said "to the computed take off pitch attitude, (and not beyond)" clearly in my post. Subsequent posts seem to challenge this.

I Clunk says:
"You rotate early, get airborne early (Vmu or "slow") and are at a pitch/airspeed mis-match (ie AOA)".
Would you really get airborne if you don't exceed your computed attitude?

G Flyer says:

"a rapid rotation can cause the plane to be airborne in ground effect.... Where a slower rotation rate results in the plane passing thru ground effect and accelerating. A wing stalls at a lower AoA in ground effect than in a free stream. so, the wing can rapidly reach its stall AoA in ground effect and roll off"
Airborne before the computed take off pitch attitude, is this really possible to have these effects?

Sheppey says:
"It was the extremely rapid rate of rotation when using Flap one for take off on the 737-200, that caused marked wingdrop and pitch up associated aymmetric wing icing." Again, was this before reaching take off pitch attitude?

Maybe it's just pilot lore, but I thought the phrase "rotation rate affects the stalling angle of an airfoil" has been around for quite awhile.

Explanations?

[de facto]

Maybe it's just pilot lore, but I thought the phrase "rotation rate affects the stalling angle of an airfoil" has been around for quite awhile

Not sure about the phrase itself :E but yes a higher rate of rotation will increase your angle of attack ,increasing gload would be one factor that comes to mind.
I think you could see it increasing by comparing the flight path vector motion to your pitch motion ..

[BOAC]

A rapid rotation can take a wing past its normal stall angle and still have 'attached' flow ie remain unstalled for few seconds. This has been shown in tests, and I am sure helped the Lightning 'rotation' take-offs where flight path rapidly caught up pitch angle.

The big no-no with 'slow' rotations on take-offs in normal a/c is......... - remember the s/e screen height on a wet runway and then rotate slowly? No thanks.

[ImbracableCrunk]

hawk37 wrote:

I Clunk says:
"You rotate early, get airborne early (Vmu or "slow") and are at a pitch/airspeed mis-match (ie AOA)".
Would you really get airborne if you don't exceed your computed attitude?
Sure, ground effect increases your lift, so you can get airborne early. However, the final pitch attitude shouldn't be attained until you're already airborne. The wheels will come off around 6-8 degrees on a normal rotation and you'll continue to rotate. Rotate to 15 degrees on the runway in a -900, you'll have some paperwork.