What causes the slight "pause" at around 10 degrees during rotation in the 767 and 73

[Tee Emm]

Appreciate any factual technical information on why does the 767 and 737 have a "pause" around 8-10 degrees nose up as the aircraft is rotated for take off? Do any of the FCOM or FCTM mention the reason for the pause?

One explanation is it happens as the tail goes down into ground effect and causes a stick force change. That theory is shot when one considers these aircraft have hydraulic controls with artificial feel. Another theory is the relative positions of the C of G and Centre of Pressure change during rotation, causing a nose down change of trim.

The third answer is that the "pause" is designed into these aircraft to minimise tail strike if the rotation is too fast?

[john_tullamarine]

I suspect ground effect considerations. A change in pitching moment is still going to require a change in pilot input regardless of what is between pilot and surface.

[Wizofoz]

I think it is because that is when the aircraft unsticks- the center of rotaion then shifts from the bogies to the C of P.

[wheelie my boeing]

Presume it's the same thing as what happens in the A319/320/321. More backstick is required around 10 degrees to continue the rotation

[Stuck_in_an_ATR]

Maybe it is due to the stabilizer getting into the wing wake/downwash, which would reduce the stab AoA and the downward lift? Just a thought...

[Fullblast]

it is due to the stabilizer getting into the wing wake/downwash

That's correct. It is called "dead band".

[Lord Spandex Masher]

It may well be called that but it woud increase the stab alpha and increase it's lift.

[crwkunt roll]

Only really noticeable in the sim.

[Centaurus]

Only really noticeable in the sim.

If that is correct, it suggests the fidelity of the simulators needs to be re-examined. The simulator is supposed to accurately replicate operation of the actual aircraft which is why the zero flight time concept was introduced all those years ago. Hands up all those who have never experienced this "pause" while rotation on the real 737,767 et al

[Fullblast]

To me, al least on the 73, is more noticeable in the plane than in the sim, I will keep an eye on it in the next opc anyway.

[737aviator]

The tail going into ground effect is the reason given by our training department on the 737 which has always made sense to me as the tail is getting close to the ground (especially on the 800/900) and we need to keep the tail going 'down' whereas the result of ground effect is to stop that happening.
I don't understand why 'hydraulic controls' discredit this theory as all that needs to be done as one approaches the dead band is increase the elevator deflection.

[Dash 42]

It may well be called that but it woud increase the stab alpha and increase it's lift.


Yes, and hence the slight pause in rotation....

[Lord Spandex Masher]

I assume NT means Northern Territories and thus you're in Oz where everything is upside down.

But just think what happens when you increase horizontal stabiliser lift in the northern hemisphere where the the HS is the right way up ;)

[sevenstrokeroll]

Its because it is cool as hell to do it that way.

[mono]

I assume NT means Northern Territories and thus you're in Oz where everything is upside down.

But just think what happens when you increase horizontal stabiliser lift in the northern hemisphere where the the HS is the right way up

Not sure what you're getting at here?

If you increase stab alpa, the stab will want to go up. However at rotation what one wants it to do, is go down! Hence the pause at rotation!!!

[Dash 42]

Quote:
I assume NT means Northern Territories and thus you're in Oz where everything is upside down.

But just think what happens when you increase horizontal stabiliser lift in the northern hemisphere where the the HS is the right way up


Not sure what you're getting at here?

If you increase stab alpa, the stab will want to go up. However at rotation what one wants it to do, is go down! Hence the pause at rotation!!!

Thanks Mono!::8

[aerobat77]

Appreciate any factual technical information on why does the 767 and 737 have a "pause" around 8-10 degrees nose up as the aircraft is rotated for take off?


there are some ugly scratches on the tail when you pitch significantly higher with the mains still on the ground. :ooh:

[misd-agin]

Memory is that the 727 had the most noticeable lag. You'd rotate and it would sit there 'yeah, what do you want me to do now?' and then it would say 'ok, I'll fly'.

737 has it also. Guys ask what a 727 flew like - a 737NG with derated 22K. :ooh:

Don't recall it on the 767.

Like sevenstroke said - why's it do that? Because it's cool. ;)

[Lord Spandex Masher]

If you increase stab alpa, the stab will want to go up. However at rotation what one wants it to do, is go down! Hence the pause at rotation!!!

What happens to the elevator when you pull back on the stick and what does that do to the stab alpha??

Even my girlfriend can understand it!

[parabellum]

Only really noticeable in the sim.


Disagree there, flew the B767-300ER for three years, thought the 'lag' was very noticeable. Can't say I ever noticed it on the B737-200, but that was a long time ago!

[Vidab]

Not the case in the 747-400/-8. There is a distinct change in the force required around the 9-10 degree range. The tail moving into ground effect is the cause.

Proper technique is to rotate at 2.5deg/sec with no pause to target 12.5 deg. initial (which is tail strike territory). However, executed properly this ensure about 18 inches of tail clearance at max weights prior to liftoff. A two stage rotation is not required to protect the tail. Mostly, on the 747 at least, if you see it it's just poor technique.

Of course, none of this precludes the PF from adjusting rotation rates and pitch attitudes for the given conditions (W/S, gusty conditions, eng failure, etc).

[sudden Winds]

too lazy to read the whole thread to check if this has been already explained.

That apparent "pause" is basically a change in the pitching characteristic as you transition from rotating the airplane about its main gear, to pitching it up about its longitudinal axis, that is forward. That´s all. If you´ve rotated the plane a few times already, you know that by the time you are passing by those 10 degrees, you apply about 2 inches more of aft elevator displacement, so that the 3 degrees per second are not interrupted.

[blind pew]

Sudden I think that your theory is incorrect as the change of the pivot point moves further away from the force; ie stab. Therefore there is a greater moment on transition from gear to CofG.
The ground effect explanation gets my money.
If you want an even better mystery then why do some gliders hunt in pitch during a winch launch...stick on the aft stop?

[Conductor]

The lag is certainly noticeable on the 767 and I was under the impression it was due to the tail plane flying into ground effect.

[de facto]

On the 737, it is the result of the mach trim system action after take off when the aircraft is in the FCNSE region.(within 10 secs after lift off).
The elevators will move to a neutral shift position and will allow the aircraft to have a larger nose up attitude for a specific reason.
This neutral shift is felt by you as a 'pause'...
You can pitch up through this 'pause' for the loss of a few knots...
The third answer is that the "pause" is designed into these aircraft to minimise tail strike if the rotation is too fast?

No,proper training and reading/applying FCTM do.

The elevator neutral shift is to allow a higher attitude in case of engine failure.

[Lord Spandex Masher]

Mach trim is only active above .615M. Are you talking about the speed trim system?

[de facto]

Excuse me my Lord:E but the Mach trim works in high speed flight regime (as you said above Mach 0.615 but also within 10 secs after lift off as described above.
The speed trim by the way moves the stabilizer not the elevator , directly at least.

[Lord Spandex Masher]

That's why I ask as my tech manual doesn't mention the 10 seconds airborne in the Mach trim section only in the speed trim bit.

Classic not NG by the way, are they different?

[de facto]

I fly the NG and only have a few hours on the CL so not sure,but i dont see why not,both have mach trim and speed trim system.
The speed trim after take off is for speed stability when in a low speed-high thrust scenario,the FCC will command the stabilizer CPU to mive it downwards to induce an increase in speed.
The mach trim in our case moves the elevator to a neutral position so the elevator can be moved more upwards in case anenginefailsduring take off,a different function of the mach trim other than speed stability at mach .615 and above.

[JammedStab]

Not the case in the 747-400/-8. There is a distinct change in the force required around the 9-10 degree range. The tail moving into ground effect is the cause.

Proper technique is to rotate at 2.5deg/sec with no pause to target 12.5 deg. initial (which is tail strike territory). However, executed properly this ensure about 18 inches of tail clearance at max weights prior to liftoff. A two stage rotation is not required to protect the tail. Mostly, on the 747 at least, if you see it it's just poor technique.


40 (plus or minus one) inches tail clearance depending on the flap setting is the appropriate number for the -400, slightly different for the -8(at least according to the FTCM). As for the change in force, it seemed more like at 12.5 degrees to me but I will check it out again in a couple of days. As well, 15° is the initial target attitude according to the FCTM.

I had been wondering what the reason was for the force change. Quite a convenient aerodynamic feature to have if it is just the ground effect as the cause.