B 767 pitchup on touchdown

[RetiredBA/BY]

Came up in conversation recently.
Just after touchdown when the spoilers lifted the B 767 would pitch up slightly.
The Boeing FTM warned against it (tail strike) but did not explain the cause.

I always assumed it was because the wing Center of pressure moved forward as the spoilers lifted but does anyone know of any other cause?

[pattern_is_full]

If the vertical Cg is below the spoilers (and whether that is true sometimes, always or never in the 76, I have no idea) then the drag from the deployed spoilers would induce a pitchup moment. Just as go-around thrust with engines below the vertical cg will induce a pitchup.

[RAT 5]

B737/757/767 all had as tendency to pitch up as spoilers were deployed IF you kept too much back pressure on elevator. Guys who tried to cruise down the runway with the nose wheel off the ground were surprised and nearly caught out.
There will be aerodynamic reasons better understood by others than I, but from a practical point of view this is what I found to be an operational consequence.
Land the nose wheel and no problem.

[Capt Fathom]

Never stop flying the aeroplane!

[stilton]

In addition to the spoiler contribution if your pitch angle after
touchdown is high (I forget the exact number) autobrakes will
not activate until that angle is decreased



The initiation of autobraking counters the pitch
up tendency

[parabellum]

Boeing definitely teach NOT holding the nose wheel off, wing needs to be much nearer to a full delta before there is any aerodynamic braking advantage, apparently. Boeing teach flying the NW on and not letting it drop.

[RetiredBA/BY]

So no one really knows the fundamental cause of the pitch up!
Being dual rated on the 75 and 76 I never allowed pitchup to develop (tail strike risk, never aerodynamic braking) but on a landing by an FO when I was new on type he did allow a marked pitchup to develop which I stopped. He, being much more experienced on type thought I was being overcautious, but since pitchup is specifically warned against in the Boeing FTM I sometimes wondered about the fundamental aerodynamic cause!

I always assumed it was aerodynamic but never heard a full explanation.

[Stampe]

I converted onto the 767 in 1988 there was great emphasis placed on training to land the mains and then land the nose wheel.Boeing produced a very good video reinforcing this.The video has been lost over the years and is I believe no longer available.This is highlighted in an aaib incident report which is very helpful on the subject.
https://assets.publishing.service.gov.uk/media/5422eb45ed915d137100005d/Boeing_767-324_G-OOBK_05-12.pdf
Sometimes as a type matures in service hard learnt lessons are forgotten.Modern airline training and trainers are rich in keyboard skills but sadly lacking in stick and rudder ability.Weak or non existent regulatory oversight exacerbates the problem.
Regards Stampe

[gtseraf]

I found the pitch up tended to occur with reverser deployment, not always but my guess is that it would be a combination of factors, included T/R deployment. I suppose the fact that it may happen and be prepared for it is more important than the why it happens.

[FCeng84]

I learned early on in my time as a flight controls engineer that spoilers do just that, spoil lift. They are not very effective as direct drag inducing devices. The way that they work when employed for speedbraking is different in-air vs. on ground (both resulting from their lift dump characteristic):

In-air, deploying spoilers dumps lift requiring increase pitch attitude (AoA) to continue to fly at the same flight path angle. The increased AoA is what really brings on the increased drag and slows the airplane. If spoilers are deployed without increasing pitch attitude, the lift loss will result in lowering the flight path angle. On a related noted, care must be taken as to how quickly spoilers are deployed when in air as the lift loss can be a real surprise to the passengers if done too fast! A feature on some newer models that include fly-by-wire control augmentation is to slow in-air spoiler deployment so that a quick extension of the level on the flight deck does not result in the bottom dropping out for everyone in back.

On-ground, deploying spoilers dumps lift allowing more effective wheel braking. The spoilers by themselves (without wheel brakes) don't contribute much to deceleration during rollout.

An interesting thing about spoilers is that at certain angles of attack extending them slightly will actually increase lift. I suspect that was what was happening during the 747 flight testing mentioned above. This is because they act like vortex generators energizing the flow over the wing and helping keep it attached. Once spoilers are extended beyond a couple of degrees their net effect is a loss of lift.

On the subject of pitching moment from spoilers, one key aspect for a swept wing airplane is the coordination of inboard and outboard spoiler extension. Get the mix right and the net pitching moment is fairly neutral. Too much inboard spoiler compared to outboard and the airplane with pitch nose down. Too much outboard compared to inboard and the reverse will happen - i.e., a nose up tendency. Airplanes with fly-by-wire augmentation have the added degree of freedom of combining elevator motion along with spoiler deployment to tailor the pitch response.

[Broomstick Flier]

Currently on the 67 and from personal observation, I noticed that the pitch-up tendency is not so strong when employing auto-spoiler upon touchdown.

Conversely, all the time spoilers were manually extended (without previous info) by the captain, I noticed the plane had a strong tendency of raising the nose.

Maybe auto-spoiler actuation has some dampening and when manually extended (and without finesse) this is not the case.

[RetiredBA/BY]

Gentlemen thank you for all the most interesting replies, more involved than I thought!

[piratepete]

I seem to recall that , like other types, on the 767 the autobrakes will not activate until the pitch attitude is at or less than 1 degree nose up, i.e. you must derotate upon mainwheel touch down.One effect of this is that other inputs, pilot not relaxing the back pressure, spoiler activation, reverser deployment etc etc may cause the pitch attitude to INCREASE or cause a pitch up if the lowering of the nose by the PF is not applied in a timely fashion.Not all FCOMs contain the reference to the 1 degree requirement.Im unsure why that is.

[Volume]

Looking at some wind tunnel data (https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760019130.pdf) it seems that spoilers have no major effect on the airfoil pitching moment, so the pitch-up must be related to the spanwise lift distribution, the main wing wake effect on the horizontal tailplane or some other more complex relation. In ground effect things are different anyway, so this may as well play an important role. Boeing flaps extend pretty far and create a hell of a ground effect on the inner wing.

[SR-22]

I seem to recall that , like other types, on the 767 the autobrakes will not activate until the pitch attitude is at or less than 1 degree nose up, i.e. you must derotate upon mainwheel touch down.One effect of this is that other inputs, pilot not relaxing the back pressure, spoiler activation, reverser deployment etc etc may cause the pitch attitude to INCREASE or cause a pitch up if the lowering of the nose by the PF is not applied in a timely fashion.Not all FCOMs contain the reference to the 1 degree requirement.Im unsure why that is.

In my outfit's FCOM it is stated that on the 757 autobrake deceleration is limited until the pitch angle is less than one degree. But in the 767 autobrake deceleration is limited until main landing gear truck untilt has occured.

Interesting discussion, I for myself have a few times experienced this pitch up tendency on the 767 but never really got any explanation as to why. I do remember experiencing this on a 737 although rarely but never on a 757. Perhaps something to do with the fact that the 757 column forces required are usually higher?

And on this note interestingly a 757-300 (which apparently has the same speedbrake/spoiler system as 767s) during landing, the nose will drop fast if the speedbrakes are raised too sharply manually. The YSM (yaw damper/stabiliser trim module) which only 757-300s and B767s have apparently is connected to the speedbrake system among other things. And I wonder if that might have anything to do with this, although I believe the 767 does not include the EFL (elevator feel limit) and EFSM (elevator feel shift module) functions of the YSM like the 757-300 does, perhaps that possibly explains the different behaviour.

[oceancrosser]

In my outfit's FCOM it is stated that on the 757 autobrake deceleration is limited until the pitch angle is less than one degree. But in the 767 autobrake deceleration is limited until main landing gear truck untilt has occured.

Interesting discussion, I for myself have a few times experienced this pitch up tendency on the 767 but never really got any explanation as to why. I do remember experiencing this on a 737 although rarely but never on a 757. Perhaps something to do with the fact that the 757 column forces required are usually higher?

And on this note interestingly a 757-300 (which apparently has the same speedbrake/spoiler system as 767s) during landing, the nose will drop fast if the speedbrakes are raised too sharply manually. The YSM (yaw damper/stabiliser trim module) which only 757-300s and B767s have apparently is connected to the speedbrake system among other things. And I wonder if that might have anything to do with this, although I believe the 767 does not include the EFL (elevator feel limit) and EFSM (elevator feel shift module) functions of the YSM like the 757-300 does, perhaps that possibly explains the different behaviour.

I have seen pitch up numerous times on the B757 associated with delayed selection of Reverse Thrust by new F/Os, too preoccupied with landing the airplane to remember to promptly select reverse thrust on main gear touchdown. Same situation is possible on the 767.

[RAT 5]

I do remember experiencing this on a 737

Certainly a near gotcha on an auto land, with the nose up trim, if you don't land the nose wheel correctly. The slightest back pressure as the spoilers come up before you pull TR's and the nose can rise like a boat rising up the swell.