The Danger with Rapid Rotations

[JammedStab]

Originally Posted by Capn Bloggs

6° per second? You have got to be joking. The 717 (shortish DC-9) FCOM recommends 2.5° per second and that feels too fast.

The 6° per second rate of rotation can result in higher than desirable angle of attack as the airplane becomes airborne.

Understatement of the century.

This accident involved an ABEX DC-9 in PHL many yaers ago in freezing drizzle conditions without being de-iced. Aside from rapid roll reversals, their power dropped off significantly due to turbulent airflow into the nearby engine inlets(during certification MacDac had seen this stall/power loss combination when doing high altitude stalls which happen at a lower AoA). Similar happened to TWA and Ozark DC-9's in earlier incidents. Apparently longer body DC-9's are not vulnerable to this as the turbulent airflow goes above the tail.

It is interesting to note that for baby DC-9's, the slower rotation rate recommended by the NTSB and the procedural changes concerning rotation rate made by ABEX only applied to icing conditions.

[dhardesthard]

Slow Vs fast rotation rates.

I agree with the 3 degree rotation rate. If you are not going to do 3 degrees per second then slower is better than faster. I have often shown my Co-pilots how close some A/C tails come to touching the runway on take-off by making them observe actual take-offs at various runways while on duty. A slow rotation rate also gives the A/C time to get to correct flying speed if the take-off speeds have not been correctly worked out (within reason).

[Checkboard]

The chief pilot criticised my rotation rate as being far too slow (in his opinion) and warned this could compromise obstacle clearance with engine failure.

Bad science! It's a cost/benefit analysis between lower obstacle clearance (1% of take-offs are first or second segment obstacle limited) VS running the speed on one engine too low (100% of take-offs with an engine cut at V1 are at risk of this problem)

If your rotation is too low - energy is still being added from the engines, so if you see a tree in the window, you have that energy still available to pullup.

If your rotation is too fast - well, the closest a normal flight EVER gets to the stall is at rotation, and stall, flip, crash, burn, die is the classic.

If you are TWO engines, then you have so much extra power over the ONE engine case the performance is planned on, it's not a problem.

If you lose an engine, you will have less pitch-up inducing thrust, which will lower your rotation rate in any case.

[thermostat]

Another very important point. Never "double rotate" i.e. rotate to say 8 degrees and stop, then rotate to 15 degrees. When at 8 deg, the aircraft may leave the ground but will not climb more than a few feet. So there you are flying just a few feet off the runway then you rotate a second time, driving the tail into the ground. With a constant 3 degrees per second, as the angle increases the rate of climb also increases. This allows the tail to descend towards the runway at about the same rate as the aircraft is ascending. A win-win situation.
Know your airplane.

[JammedStab]

Originally Posted by thermostat

Another very important point. Never "double rotate" i.e. rotate to say 8 degrees and stop, then rotate to 15 degrees. When at 8 deg, the aircraft may leave the ground but will not climb more than a few feet. So there you are flying just a few feet off the runway then you rotate a second time, driving the tail into the ground. With a constant 3 degrees per second, as the angle increases the rate of climb also increases. This allows the tail to descend towards the runway at about the same rate as the aircraft is ascending. A win-win situation.
Know your airplane.

I have to admit that I did both in the 727. A continuous 3° per second rotation to 15° and sometimes a hesitation at about 10° pitch later in my timeperiod flying them, but the mains were still on the ground for the momentary hesitation. I don't believe that Boeing says anything about a slight hesitation being the way to do it.

Constructive criticisms are welcome.

[cwatters]

If so critical.. do any jets have an auto rotate feature or rotation limiting features?

[cosmo kramer]

I have in other recent threads been advocated strongly that people look out the window when completing the final part of the approach. By that I mean stop looking at the instruments (except for airspeed) and maintain lateral and vertical path by looking at the touchdown point at below 300 feet latest (weather dependent of course).

Apparently, the need to look out the window needs to be emphasized when it comes to takeoff as well.

When PM calls V1, I have a confirming look myself and thereafter only look out the window until established in the climb, completing the takeoff by going back to the instruments and adjusting pitch to maintain V2+20.

By looking out you have a complete feel for how the aircraft is reacting to the rotation and adjustment is then much easier. Plus it makes spotting drift and maintaining centerline much easier as well (from lining up behind departing aircrafts apparently a discipline that few care about in these days). The feel for the right rate is much easier to spot in a real 3 dimensional world, than on a 2D instrument.

To look out the window you have to be seated properly at the designed eye reference position. All too many are seated completely wrong, too low and too leaned back (but I guess they think this increases the coolness factor ).

[de facto]

Exactly!
Looking out during last few hundred feet and during rotation is essential.
Rotation should be a continuous increase of pitch(just as the flare)to be inside the flight director/15deg at V2+15.If you end up with an airspeed below you over rotated, if above you under rotated.
3deg /sec too complex for me.

[5LY]

Years ago I saw a film of a live demo by Boeing on the 727 in which they set up a 35' stick, and a camera at the DER and did a series of t/o's. Using all engines they did under rotation, over rotation, too slow a rotation, and too fast a rotation. On each of these 4 t/o's with all engines running they were under the 35' mark as they launched. On the last t/o, they pulled the center engine to idle at V1 and continued the t/o. With a normal rotation, they exceeded the 35' mark as they launched. This was a perfect illustration to a simple guy like me of the importance of proper rotation technique.

[yippy ki yay]

I agree in theory with if you are at or around V2 + 20 having a done a smooth continous rotation then you are doing it at the correct rate....although when you're very light I find this not to be the case.

Just a quick one - I believe the above poster flies 737ng where the FDs command V2+20 after rotation not 15..

[Tee Emm]

On an allied subject and that is FD reading after VR. Assuming engine failure just after VI where the aircraft has not yet reached VR on one engine. Providing the rate of rotation is correct it is my understanding that the FD will give the correct angle to maintain V2. If the rotation rate on one engine is slow then it is probable the FD will not give V2 but a different angle in order to maintain the airspeed at some figure well beyond V2.
Question therefore: If the engine failed while on the runway at V1 and the take off is continued, and the pilot is slow to commence rotation with commensurate increase in airspeed above the scheduled V2, does the FD command V2 or some other speed and what speed is that. It is known that the FD will adjust its angle to maintain the speed at which the engine failure occured after airborne - but how does the FD change with slow rotation rate if at all?

[Dani]

Depending on type of aircraft it is not advisable that you fully look out of the window until end of rotation. On widebodies, your margin between not climbing and striking your tail is so narrow that you have no chance to guess your pitch by looking out of the window - you basically only see your dashboard as a line witout horizontal reference of the wings or other parts of the aircraft.

I recommend a "fading in", i.e. let your scan change from fully looking out to fully looking in in a gradual sense, like you do it during landing when approaching to minimum.

About the rotation rate:
It is widely accepted that you fly 3°/sec. On bigger aircraft and during OEI, it is slightly lower. I also heard from old school DC-9/MD-80 pilots that you should rotate fast, because if you don't get your climb attitude fast, you will not climb.

[Meikleour]

Dani: Amen!

With the "look out the window during the whole rotation" brigade I have always wondered just exactly they thought that they were looking at during a low visibility take-off!

[Capn Bloggs]

It is widely accepted that you fly 3°/sec. On bigger aircraft and during OEI, it is slightly lower.

99% of the jets of 737-800 size and larger I watch never go close during liftoff. 5 seconds from 0° to 15° ish? It just doesn't happen. In my little MD machine, ours is recommended at 2.5° per second, and if done at that rate, the rotation feels positively horrible. The only thing that saves us from scraping the rear end is...we don't have one.

I also heard from old school DC-9/MD-80 pilots that you should rotate fast, because if you don't get your climb attitude fast, you will not climb.

I suspect only because of the very inefficient wing. 20° nose up with a flight path of 12°. Looks impressive though.

[Denti]

Boeing suggests a rotation rate of 2° to 3° per second on the 737 depending on variant. Longer airplane variants should be close to 2°/second, smaller close to 3°/second. The slower rate is probably a good guideline for the 900ER and 800 variant, 700 should be somewhere in the middle and 600s at the faster end of the range. Target attitude is usually 12° but depends on conditions. Usually take offs are done using both derated thrust and assumed temperature which leads to low power take offs with a low pitch attitude. Unreduced take offs however, especially on light weight, are closer to 20° pitch.

There is not one rate that suits all airplanes, simply check your FCOM use the correct technique for the type in question. A blanket 3°/sec won't work.

[cosmo kramer]

Depending on type of aircraft it is not advisable that you fully look out of the window until end of rotation. On widebodies, your margin between not climbing and striking your tail is so narrow that you have no chance to guess your pitch by looking out of the window - you basically only see your dashboard as a line witout horizontal reference of the wings or other parts of the aircraft.

That's because you are most likely seated too low.

With the recommended seating position to obtain optimum eye reference position you should have "sight along the upper surface of the glare shield with a small amount of the airplane nose structure visible" (according to the NG FCOM, most likely your aircraft will have a similar recommendation).

So you should have the nose visible and in that case it is very easy to judge the angle (and rate of change) compared either to horizon or other ground reference - far easier than on a 2D instrument, depth perception does wonders in judging speeds, distances and accelerations.

When we learned to fly a SEP straight and level, we looked at nose versus the horizon - basic flying. Obviously there are situations where we have to use instruments as primary reference when flying airliners, such as when the nose is pointed 20 degs upwards and it's no longer possible to see the horizon, but that doesn't mean we have to discard basic flying techniques all together, in particular not in situations where they are superior.

With the "look out the window during the whole rotation" brigade I have always wondered just exactly they thought that they were looking at during a low visibility take-off!

Low vis takeoffs are another story and just because we use a special technique, doesn't mean we have to apply that to the 95% other takeoffs we make - we don't do auto lands all the time either, do we?

Besides, described technique can be used when the weather misty too, you don't need 10+ km visibility to have a reference point on ground when at such a low altitude. If the cloud cover is low, switch to instruments earlier - common sense?

[Dani]

That's because you are most likely seated too low.

Nope, as all modern jets we do have those nice little ball thingies giving you the exactly correct eye height (in my case: Airbus).

Look (litteraly), a human brain is simpy not able to judge attitude to the last degree. In a long wide body jet aircraft, the difference between 10 and 11° is in the air or tail in the tarmac. Look at the skidmarks in Melbourne...

btw you don't see the nose if you are seated correctly in an Airbus, and I think to remember the same is valid for most other jets - except the Concorde with nose not lowered.

May I ask what type you're flying?

Even during landing we have to check our primary flight display. You simply cannot make a landing purely by feel and luck.

Dani

[cosmo kramer]

Dani,
I am flying 737. Above I wrote an exact quote from the 737NG FCOM, to repeat:
"sight along the upper surface of the glare shield with a small amount of the airplane nose structure visible".

If you don't see the actual nose of the Airbus, it won't make a difference. My point is that if you sit high enough you should still be able to see the horizon and not have it "hidden" below the glare shield. The upper surface of the glare shield should be just as good a reference as the nose structure.
On a day with low weight and a long runway (hence heavy de-rate and improved climb), the V2+20 pitch is often only 10-12 degs, from this pitch it is still possible to see the horizon when sitting in the correct position.

I seriously doubt that there is a 1 deg margin on any longer jets (wide body or not), unless someone screwed the rotation up, or like Melbourne used wrong data (which makes it a pretty bad example).

Hence, it shouldn't be necessary to judge to the last degree, if that's necessary something was done wrong (like the "too fast initial rotation, pause" technique). You should be judging a rate instead, that you don't need to make a pause since the aircraft is safely in the air long before passing 10-11 degs. That is my point in case - you will be much better at judging a rate looking outside than on a 2D instrument.

I am flabbergasted and speechless by your comment about landing. Just to make sure, you are saying that a safe landing accomplished by looking out the window is due to "luck"??

[Dani]

Not by pure luck, but by feel and luck, you twisted my words.

Let me assure you: On a wide body it is one degree that makes a difference. An A340-600 has something like 9.5° tail strike clearance but needs 12.5° initial lift off pitch attitude - make the maths. A widebody you cannot fly just by looking out of the window. And responsible TRTOs on medium range aircraft teach their pilots to fly them the same way like their bigger brothers - to get a common mindset.

Dani
(A340-600 rated)

[cosmo kramer]

Again, I can't see what relevance the width of the body has to do with tail clearance. What is it that you would like to say with that? What is it that in your opinion makes a (oh lord) wide body different from other aircraft?

And responsible TRTOs on medium range aircraft teach their pilots to fly them the same way like their bigger brothers - to get a common mindset

Actually this is what Boeing has to say about it:

737 300-900 FCTM:
When a smooth continuous rotation is initiated at VR, tail clearance margin is assured because computed takeoff speeds depicted in the PI chapter of the FCOM, airport analysis, or FMC, are developed to provide adequate tail clearance.
....
For optimum takeoff and initial climb performance, initiate a smooth continuous rotation at VR toward 15° of pitch attitude. The use of stabilizer trim during rotation is not recommended. After liftoff, use the attitude indicator as the primary pitch reference.

I don't have a wide body Boeing manual at hand. But normally, as you say, they maintain a uniform philosophy across their range of aircrafts. Exactly my point again. Focus on getting the rate right, and the performance calculations takes care of the rest of your worries. The topic is "the danger with rapid rotations", with the right rate, the rotation won't be rapid. I am giving my point of view on how to achieve the right rate.

An A340-600 has something like 9.5° tail strike clearance but needs 12.5° initial lift off pitch attitude - make the maths.

You are missing that the wheels unstick at a considerable lower pitch than 12.5 degs (hopefully, otherwise you are missing 3 degs on every takeoff ).

To take care of the landing as well, here is what Boeing has to say about that:

When the threshold passes under the airplane nose and out of sight, shift the visual sighting point to the far end of the runway.

They don't recommend looking at the PFD during the landing. Again, I am sure e.g. a 777 manual will have the same wording.

The techniques above are applicable to all 737 models, including the 900ER which have a Tail Strike Pitch Attitude of 10 degs during takeoff and 8 degs for landing.

If you can provide a reference from the your Airbus manual with a recommendation to look at the PFD during rotation and flare, all I can say is strange aircrafts, strange ideas! However, I doubt you can...?