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convair880mfan
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Joined: Fri Jun 25, 2021 12:33 am

Two noticeable pitch angles after takeoff in the MD-90?

Wed Aug 18, 2021 8:22 pm

On my flights on the MD-90 I noticed that the aircraft rotated to a rather shallow seeming angle before taking off and then moments later rotated into a fairly steep angle. I am wondering about this. Was the initial shallow angle made so that the rear fuselage wouldn't scrape the runway?

I noticed the same thing while watching passenger window videos of a DC-8 stretch aircraft taking off. First a shallow angle for several moments and then a much steeper angle.

I realize that perhaps airliners go through many pitch changes during these times, but this was quite noticeable even to me a passenger. Can anyone explain this or am I just reading the situation incorrectly?
 
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Starlionblue
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Re: Two noticeable pitch angles after takeoff in the MD-90?

Thu Aug 19, 2021 12:04 am

I can only speak for the airliners I fly, but we rotate to an initial pitch angle target first (15 degrees on the A330). Once the flight director pitch bar stabilises, we pitch further to that commanded pitch rate. You shouldn't chase the flight directors during the rotation manoeuvre.

So it could indeed seem like a two-step manoeuvre.
 
convair880mfan
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Re: Two noticeable pitch angles after takeoff in the MD-90?

Thu Aug 19, 2021 12:24 am

Thank you for your response Starlionblue. Do do you do this on the Airbus 330 to avoid a tail strike or because of the flight director? I wouldn't think one would have to worry about a tail strike on the A330, but the MD90 and those long stretched DC-8's might be a different matter. Of course I have no idea and am just guessing.

On the Boeing 727, I noticed that frequently the plane pitched up and up during the takeoff roll but seemed to hang on to the ground for quite a bit n before the mains broke free with that thud of extended struts. I've never been on the A330 but I would imagine it just flies off the runway without hanging there a bit like a half truck half plane waiting for overwhelming aerodynamic forces to take over.
 
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Starlionblue
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Re: Two noticeable pitch angles after takeoff in the MD-90?

Thu Aug 19, 2021 4:21 am

It isn't because of tail strike. It is because the flight director should not be used for dynamic maneuvres. If you do that you end up chasing it.

Where tail strike comes in is the gradual pitch up, I suppose. You shouldn't just yank it to 15 degrees. Three degrees a second or thereabouts. So it is a gradual thing, giving time for the mains to lift and the tail to climb.
 
BoeingGuy
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Re: Two noticeable pitch angles after takeoff in the MD-90?

Thu Aug 19, 2021 5:44 am

Starlionblue wrote:
It isn't because of tail strike. It is because the flight director should not be used for dynamic maneuvres. If you do that you end up chasing it.

Where tail strike comes in is the gradual pitch up, I suppose. You shouldn't just yank it to 15 degrees. Three degrees a second or thereabouts. So it is a gradual thing, giving time for the mains to lift and the tail to climb.


On Boeing airplanes, the Flight Director pitch bar is parked at an initial rotation angle of 8 degrees while on the ground (I think it’s 10 degrees on the 747). At lift off it goes to 15 degrees, then transitions to a speed through elevators control law.

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