Rotation technique
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Rotation technique
Sometimes i hear controversial opinions about rotation technique when reaching Vr speed.. .My personal way is to rotate to the target attitude (18~20 degrees) in a continuos movement (it takes about 5 seconds to reach the attitude).. .I've seen other guys, during rotation, pausing at ~10 degrees attitude, for about 1 to 2 seconds, just to build some speed, then going to the target attitude.. .I'd like reading your opinions about your way to rotate, also taking into consideration a possible windshear encounter or engine failure during the rotation.. .. .Thanks. .B_D
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On big planes, tailstrikes can ruin your day. As long as I have excess runway and obstecles aren't a big factor. I'm more inclined to rotate the aircraft to 9-10 degrees and let it fly away from the ground in this attitude before I rotate it to the full 18-20 degrees.
Mistrust in Management
I was always taught 2.5 to 3 degs/sec resisting the tendency to reduce that rate (due to horizontal stabiliser in ground effect) which occurs at approximately 10 degs on the 737. In other words, pull through that ground effect.. .. .Always worked for me. So far!!
Dutch Roller
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The 737-800 is very critical with rotation, you have about 22 inches between tail and tarmac during liftoff. . .I use the technique airbrakes describes, let it fly..... .I was also told that the resistance you feel at about 10-12 degrees pitch is aerodynamically built in (more than just the ground effect) so as to reduce the risk of tailstrikes. . .-------------------------. .Track
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Just a point....According to the questioners profile he is flying a MD not a B737. Possibly he is seeking advice for his current machine, which is fine if they are all similar. Are they? . .We must be careful not give him any duff gen which could get him in trouble on his machine.
Mistrust in Management
Quite right Scan+3,. .. .That is why I specified the 737-2/3/4/5. (Don't know about 7 & 8) The scarebus I now drive is different again. I would presume that any MD type would not suffer from the 'ground effect' I previously mentioned. . .. .By the way I'm not claiming any expert knowledge here, I've just been applying what I've been taught many moons ago.. .. .One other point, I would think that performance is worked out on the assumption that the A/C is rotated smoothly at the correct rate through to the correct pitch attitude. . .. .Regards. .Exeng
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Can't be bothered looking up the exact wordage but certainly on the 757 Boeing prescribe a "continuous smooth rotation at 2.5 degrees per second to the target pitch attitude"(or words to that effect)They most certainly do not mention anything about "pausing" at 10 degrees-which is roughly the point of MINIMUM tail clearance.I find many people do that unintentionally due to the afformentioned characteristics.As a wise old American said on pprune some time ago "Son,if Boeing don't recomend it-don't go stickin' your dick in there"
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Airplanes are certified with a 2.5-3deg/sec rotation technique at Vr.. .. .The effect at 10 degrees happens in most airliners, even in my FBW-scarebus. It is purely aerodynamical, and not built in to prevent a tailstrike.. . . .If your take off calculations are right, and you rotate at Vr with the given 3 degrees per second, your tail won't strike the ground. . .. .Even worse, if you apply your house-built rotation-technique, your performance calculations wouldn't be right, because the V2, the 35ft-screenheight point and your 2nd segment climb would be somewhere different than calculated.
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Pegasus,HSL,exeng. .. .that's what i wanted to get at: performance issues. Never heard stabilizer ground effect issues on the MD. .I know that with runway ahead and no obstacles, pausing a moment around 10 degrees of pitch attitude isn't going to ruin the take-off; but what about if at 10 degrees your engine decide to quit? or you encounter a windshear? I think the aircraft will sink much more than if it was at a steeper attitude.. .What I'm saying is that if you reach your target attitude ,and "trouble" starts, you are going to experiencing just a flattening of your take-off gradient, but if you are "pausing" at ~10 degrees pitch attitude, and "trouble" starts, you are in for a bad sinking, even if (pausing at ~10 degrees) your are building speed to trade to regain the positive gradient.. .So,what do you think?. .. .Thanks. .B_D
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Black Dawn. .. .I think it is rather a personal call. I must however say, on all large wide-body jets the normal AOM will state a continuous 3 degrees per second to the required T/O pitch angle for all engines operating. My personal preference is to rotate to one engine out pitch attitude, say 13 to 15 degrees, take a quick glance at the speed to see it is above V2 and accelerating at a normal rate, then continue onto normal all engine pitch attitude. At least I am then sure if I do have an engine failure all I have to do is go back to the one engine-out attitude and I will be well within the required performance criteria!. .. .Regards. .Jumbo Mouse
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I don't pilot airliners for a living, but I'd like to get a word in here anyway.. .. .As you'll have more drag at 10 degrees pitch than before rotation, why would you want to accelerate at 10 degrees pitch? If the airspeed is what is worrying you, you could accelerate faster if you just delayed your rotation.. .. .You all know the reasons for not delaying rotation I guess and I think the same applies here.. .. .Cheers,. . /ft
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"but what about if at 10 degrees your engine decide to quit? or you encounter a windshear? I think the aircraft will sink much more than if it was at a steeper attitude.". .. .As for windshear, many manuals tell you to delay rotation and build up "smash". Some companies do this procedurally to improve revenue loads. My company calls this delayed rotation, "Improved Performance Takeoff. As for an engine quitting, you'll have so many more visual refrences available to you at 10 degrees. Look out the window and "step on" the runway centerline. I've flown military heavies where, accelerating in ground effect with an engine loss was standard procedure. In the equation of lift, V squared is the dominant figure, not AOA. At 20 degrees, you'll have a more dynamic situation and you're closer to the ground. . .. .Anyways my 10 degree technique presumes excess runway and no obstecles. I've always felt the plane will fly when it's damn well good and ready to. I'll only force it to fly if I'm running out of runway or have to clear an obstecle.
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Black_Dawn,. .your handling technique seems perfect to me. Rotation of 2.5-3.0°/sec was SOP for 747 classic when I received training in Roswell 25 years ago. At 10° we used to stop just a moment(1-2 sec)as you describe and than continued rotation straight to required T/O attitude. The related point as tailstrike(there are others)has been covered by some posters. Kept this habit on heavy AB too.. .. .To stop the rotation at 10° and remain there til lift-off as proposed here(misunderstanding on my side?)is a procedure I wouldn't dare to mention. Performance requirements, engine failure case, windshear, obstacle clearance a.s.o. have to be kept in mind. <img border="0" title="" alt="[Wink]" src="wink.gif" /> . .Generally, stick to aunt Boing manuals to avoid being trapped in a place you can't enjoy as "dick"! <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />
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If I may offer some thoughts ... .. .(a) the manufacturer prescribes recommended rotation practice and one probably ought not to vary from this without a compelling reason .. much harder to explain the accident away. In general, the intention is to schedule Vr such that, . .. .(i) with a V1 failure and . .(ii) at the nominated pitch rate and. .(iii) using the nominated technique. .. .the aircraft will achieve somewhere in the vicinity of V2 at screen.. .. .(b) if one is concerned about windshear, surely it is better to delay rather than to interrupt rotation for reasons of predictable rotation distance and tail clearance. Others have highlighted this concern above.. .. .(c) many aircraft with a low tail will experience a very noticeable ground effect reduction in tail lift during the rotation. In such cases the rotation becomes a manipulative artform if it is to be performed smoothly. . .. .(d) the 737-800 has, I believe, an intentional stab trim mistrim schedule to increase the apparent control loads during rotation to reduce the risk of excessive pitch rate and tailstrike. Certainly I understand that one operator discovered that, by trimming a little more noseup, the rotation was much easier. When queried, Boeing suggested that this was not a very good idea.. .. .(e) the rotation is a skill like any other .. it is learnt and an appropriate level of skill achieved. .. .(f) if one is scheduling an overspeed or improved performance takeoff, then the story is the same as for a minV2 schedule - there is no "delay" in rotation, the rotation speed schedule is quite intentionally increased to achieve an improved OEI second segment climb gradient.. .. .My main concern in reading this post is in regard to SOPs .. don't we adopt SOPs precisely to strive for a repeatable, reliable outcome ? Do it yourself techniques and procedures may have a place in some areas of aviation .. but, in scheduled operation ?
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As usual, JT is right on target. . .Have found that at MAX weights, a "slightly slower" rate of rotation is helpful (TriStar)...but have found that my F/O's don't always agree, but they still do it my way nonetheless...never insist, just suggest.
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To all:. .. .The makers of the aircraft you fly know best. Do what they suggest, stick to the SOPs and use the armrests because the armrests are the secret to good rotation.(Boeing and Lockheed suggest they should always be used). Why think too much, keep some space in your brain for something real.. .. .Prince of Dzun
I would have thought that an absolute text book rotation rate would only be critical when at length or obsticle limit weight and you have an engine failure right at V1. After all, that is what all the performance figures are based on.. .. .Once you reach V1, every moment after that spent operating without an engine failure is bringing you further and further away from the edge of the required performance envelope.. .. .So if you've made it all the way to Vr and you still have all engines operating, bonus! I think in that case you can afford to REDUCE your rate of rotation from say 3 deg/sec to maybe 1 degree per second passing through 9-12 degrees nose up.. .. .So it takes an extra 2 seconds to reach the target attitude (3 seconds to pass through 9-12 degrees nose up instead of 1 second)(6.8 seconds not 5.6 seconds to reach 17 degrees nose up). So what. The recommended rate of rotation is 2.5 to 3 degrees per second for the B733 so there is a difference of 1.2 seconds for a rotation to 17 degrees nose up right there.. .. .If a reduction in rate of rotation when the tail is just 2 feet off the runway greatly reduces the chance of a tail scrape, and it's not absolutely critical for performance reqirements for the reasons stated earlier, I think it's a good compromise.. .. .Anyway I doubt whether people are actually stopping the rotation at 10 degrees nose up. I think it's just a matter of greatly reducing it for 3 seconds or so.. .. .If however you are taking off at limited weights and the brown stuff hits the spinning bits, then yes, you might have a point. But even then I come back to the point that it's only a matter of an extra 2 seconds to reach the target attitude (in this case 13 to 15 degrees or so).. .. .Maybe that sounds unreasonable. Tell me if it does and why. I always like to share ideas and learn.. . <img border="0" title="" alt="[Smile]" src="smile.gif" />
Union Goon
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If you don't always rotate at 3 degrees per second, how are you gonna do it when you absolutely need it?. .. .With the advent of reduced power takeoff every takeoff is the equivelent of a max weight performance limited takeoff so should you chuck an engine you will NEED that 35 feet.. .. .Delaying rotation is an enourmous hit.. .. .Just do it right, not half assed. .. .Cheers. .Wino
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As far as technique is concerned, how do you experienced guys actually rotate to achieve the 2.5/3 deg/sec? Is it the old "one thousand, two thousand" etc?