Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Under-rotation and FDM (FOQA) events

Tech Log The very best in practical technical discussion on the web

Under-rotation and FDM (FOQA) events

Old 13th Mar 2018, 09:29
  #1 (permalink)  
Thread Starter
 
Join Date: Jan 2003
Location: UK
Posts: 304
Likes: 0
Received 0 Likes on 0 Posts
Under-rotation and FDM (FOQA) events

We all know about over-rotation and the risks associated. Well, what about under-rotation? I fly for a 737 operator that has become increasingly interested in rotation rates as part of its FDM analysis. Recently, they have been pushing low rotation rates as a risk factor. The argument is that low rotation rates mean that the aircraft is not able to meet calculated performance figures and that is a risk. The airline safety magazine tells us that a good rotation rate is 3 degrees per second and that if this is achieved we will be V2+20 by climb out attitude.

I do have my doubts about some of what they are saying and the extent to which this is considered a threat. While the FCTM states under-rotation will cause problems, it states 2-3 degrees as the rotation rate. 2-2.5 degrees is quite slow and (I suspect) would trigger an FDM event. But would, it truly prevent the aircraft from meeting the performance calculations? As for meeting V2+20, by climbout attitude; well a lot seems to depend on aircraft weight. We often fly very light, and rotating at a normal pitch rate results in speeds in excess of V2+20. Yet the aircraft climb rate is rocket like.

So, when do you consider under-rotation to be a problem and when will it truly cause problems in meeting performance calculations?

Last edited by Fair_Weather_Flyer; 13th Mar 2018 at 11:54.
Fair_Weather_Flyer is offline  
Old 13th Mar 2018, 11:06
  #2 (permalink)  
 
Join Date: Nov 2004
Location: bkk
Posts: 285
Likes: 0
Received 0 Likes on 0 Posts
FWF.As always, the FCTM is your guide on these and many other issues with respect to HOW to fly your aircraft.In an ideal world we would all rotate at the correct rate, but many factors can interfere with this.The key element is to know the tailstrike pitch attitude and be wary of this during all takeoffs, but the situation you are describing , under rotation, has two immediate effects, namely a reduced climb gradient, at least initially, and secondly a higher speed than ideal during the early departure.A good way to counter act this is to remind yourself, on each and every departure, ideally just before you line up, what your TARGET speed is.For most takeoffs V2 plus 15-20 for all engines operating and V2 plus 5 with an engine out, will give you very very close to the optimum climb performance.I suggest you start doing this now and dont stop doing it until you retire from flying.Pete.
piratepete is offline  
Old 13th Mar 2018, 11:21
  #3 (permalink)  
 
Join Date: Mar 2002
Location: Seat 1A
Posts: 8,546
Received 73 Likes on 42 Posts
Watching 737-800s takeoff a lot, and having to rotate my non-737 at 2.5°/sec, I'll wager that 2-3°/sec is not used during most rotations. It is obvious that the rate drops off a lot as unstick occurs and is nowhere near 2-3°.

My bet is that your FDM slow-rotation witchhunt will result in tailstrikes.

The rotation rate is academic on 2 engines.
Capn Bloggs is offline  
Old 13th Mar 2018, 14:15
  #4 (permalink)  
 
Join Date: Dec 2003
Location: Tring, UK
Posts: 1,833
Received 2 Likes on 2 Posts
But would, it truly prevent the aircraft from meeting the performance calculations?
Highly unlikely. To go below the OEI floor on a twin with AEO would take some doing.
The rotation rate is academic on 2 engines.
I agree with the sentiment. Obviously the target is the FCTM rate but I am happier when any deviation is to the slow side as opposed to the fast.

If you’re too quick you’re into tail strike territory and vulnerable at a high pitch attitude and low airspeed, should you have a loss of thrust at that point. If you’re a little on the slow side, you use a bit more runway (of which you have plenty AEO) and have a higher climb out speed (which often actually increases performance). Balancing one against the other, I know where I’d rather be...
FullWings is offline  
Old 13th Mar 2018, 14:40
  #5 (permalink)  
Thread Starter
 
Join Date: Jan 2003
Location: UK
Posts: 304
Likes: 0
Received 0 Likes on 0 Posts
Well, this is as I would see it. I would have thought that for two engine performance considerations, a slower (within Boeing limits) rotation is no big deal. Most of the FDM events generated within the airline appear to be either fast or slow rotations. I feel that the fast ones are a real risk but equal emphasis seems to be given to the slow ones. The airline seems to believe that the ideal rate of rotation is a hard 3 degrees. To me, that is the upper limit of Boeing’s recommendation. As for tolerances, I don’t know what they are using. However, there is no witch hunt as such (yet) and there is supposed to be a tool due to be rolled out allowing crew to self assess rotation rates. Of course, this will be tracked religiously by some, but ignored by the worst offenders.
Fair_Weather_Flyer is offline  
Old 13th Mar 2018, 15:13
  #6 (permalink)  
 
Join Date: Jun 2000
Location: last time I looked I was still here.
Posts: 4,507
Likes: 0
Received 0 Likes on 0 Posts
I wonder which airline it is, and what motivates this. When I flew B738 it was normally nose heavy on load sheet trim setting. That, plus the dead-band just before unstick, contribute to 'hesitant' rotation rates. I was always taught, due to tail strike paranoia, 2.5-3.0 on all engines & 2.0-2.5 on engine out. The former was fine in reality, and the latter seemed to work in the sim.
Back to realty, and the distinct lack of balanced fields we operated from, I would prefer a smooth controlled continuous rotation in all circumstances, rather than the more rapid pull/push/pull one sometimes sees, especially on 50% grunt.
RAT 5 is offline  
Old 14th Mar 2018, 11:39
  #7 (permalink)  
 
Join Date: Apr 2010
Location: IRS NAV ONLY
Posts: 1,229
Likes: 0
Received 0 Likes on 0 Posts
Under-rotation might lead to tyre speed limit exceedances, among other things well discussed above. This is especially the case when departing from a hot and high airfield at performance-limited weights. When you start the rotation, the aircraft still accelerates and if commenced late or at a slower rate than prescribed (which is what AFM performance is based on), you could quite easily go past the tyre speed limit with MLG still on the ground.
FlyingStone is offline  
Old 14th Mar 2018, 12:33
  #8 (permalink)  
 
Join Date: Jun 2000
Location: Australia
Posts: 4,188
Likes: 0
Received 13 Likes on 5 Posts
The situation changes somewhat when there is a significant spread (e.g.10-15 knots) between V1 and VR with engine failure while the aircraft is still on the runway. Of course we can only use the simulator as an example, as information on real engine failures at V1 is practically non-existent.

In the 737 at least, there is a huge difference between the stick force to rotate the aircraft all engines and that of rotating following an engine failure on the runway

Since the vertical component of thrust is halved with engine failure, it takes a much harder and prolonged pull at VR to get airborne since the stab trim setting is set for a normal take off; not a single engine take off. In the simulator we see pilots often rotate much more slowly with engine failure on the runway. This is not done on purpose but because the stab trim setting is now invalid and a much heavier initial pull is needed to get the nose up off the ground.

It is common, in fact, to see pilots inadvertently allowing the aircraft to skip back on the ground during rotation because the slightest relaxation of stick pull caused by momentary surprise at the amount of back pressure needed, slows rotation rate; especially when passing through 10 degrees nose up when the aircraft always seems to hesitate momentarily. It used to be called the "two step rotation" and happened to the 727 as well.

Because of the risk of tail-strike, it is prudent to avoid using any stab trim to help with dragging the aircraft clear of the ground. One therefore has to manually "muscle" the aircraft to the desired 12-14 degrees climb attitude before relieving the heavy stick force with stab trim at about 300 feet where there is no chance of tail strike.

Also with engine failure at V1 and continue to VR, as far as I recall and providing rotation rate is correct, V2 is reached by around 35 feet and the climb continued at V2. Different on two engines of course where V2 plus 15 to 25 knots is reached by the time the aircraft reaches 15 degrees body angle.
Centaurus is offline  
Old 14th Mar 2018, 15:37
  #9 (permalink)  
 
Join Date: Jun 2000
Location: last time I looked I was still here.
Posts: 4,507
Likes: 0
Received 0 Likes on 0 Posts
Agreed. And what was alway emphasised is that, single engine, the pull/push/pull MUST be avoided at all costs. It was prudent, as you explain, to brief that the trim would be nose heavy and delicate muscling was required. This was more so because the dead-band at 9-10 degrees was very close to the 12-13degree target than the all engine 15 degree target. A large agricultural heave at 10 degrees, with the FD acting like a magnet at too high an attitude, would see the speed dive below V2 and the FD, once airborne would attempt to accelerate you back to V2. With gear down this would be level flight, and I've seen guys, in a panic, call for gear up at RA 5' and VSI zero, and it be selected. Thank god for ground effect, but when frozen in that position it was a very scary salient self-teaching 'never do that again, sir' moment.
RAT 5 is offline  
Old 15th Mar 2018, 01:20
  #10 (permalink)  
 
Join Date: Nov 2004
Location: bkk
Posts: 285
Likes: 0
Received 0 Likes on 0 Posts
For all types with engines under the wing (on airbus there may be auto trim), the take off calculated STAB TRIM is only valid for all engines operating.In the case Centaurus is discussing, engine out while still on the ground, I teach the following.

Apply enough rudder to keep on centerline, and plant your heel on the floor, so it doesnt move.
At Vr rotate slightly slower than for normal take off to the target pitch attitude (10-12?)
Apply 2 units of NOSE UP STAB TRIM.
Then apply rudder trim to unload the pedal pressure.Now there is very little control force needed to fly the machine.

It is worth noting that the requirement to apply a CORRECTION ie 1-2 more units of nose up trim than the uncorrected loadsheet figure which is typically produced based upon full thrust, when using a large thrust derate is often hidden in the weight and balance manual somewhere, but it makes sense doesnt it?
piratepete is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.