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Boeing..Reason of F/D being 8º pitch up on preflight

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Boeing..Reason of F/D being 8º pitch up on preflight

Old 12th Mar 2018, 07:45
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Boeing..Reason of F/D being 8º pitch up on preflight

I remember a colleague long time ago giving a plausible explanation on Boeings logic to have the Flight director pitch mode at aprox 8º pitch up while on ground.
I have tried to find an explanation on the manuals but have been unsuccessful.
Possible explanations found and ruled out have been:

Initial pitch after T/O: Initial pitch should be 15º, Flight director after airborne would target a pitch of V2+15..... (so based on speed) Also applicable on Engine failure.

Tail strike avoidance: Doesn't make sense as 777 varies from 12.1º to 10º and 787-9 around 9.7º so an 8º would still cause it.

Any other ideas?
bluefalcon is offline  
Old 12th Mar 2018, 07:57
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On the B737, the FDs command 15 degrees pitch up during takeoff roll above 64/84 knots and then revert to the normal speed function after liftoff.

Two points:

1. FDs are NOT to be used for any kind of rotation guidance. Disregard them until airborne.

2. You can't always see the pitch the FD is commanding. It only shows you the direction to which you need to pitch to satisfy the command and how close you are to the target pitch attitude. I.e. if you pitch to -5 deg on takeoff, you will still see the pitch FD bar, despite the target will probably be 20+ deg, which you won't be able to see on the attitude indicator.
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Old 12th Mar 2018, 08:08
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Thank you Flying stone,
I agree with all you say, however I just remember a logic on the flight director being at 8º on the preflight on ground for 787/777 with an interesting explanation, nothing to do operationally wise for flying guidance, just as a curiosity logic, and trying to find out which one. Any other thoughts welcomed.
cheers
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Old 12th Mar 2018, 09:17
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Originally Posted by Old Smokey
It's a constant pitch value to be flown for the Airborne portion of the Takeoff from Vloff through to V2. It does cater for the worst case condition.

Certification testing is based upon flight at the nominated fixed pitch attitude from Vloff through to V2. This is not unique to Boeing, at one of the manufacturer's acquired by Boeing the figure was / is 9°, and applies at all weights and environmental conditions.

Think about it, in the 1st segment we are told to fly V2 (and the FD is programmed for this), in the 2nd segment we are told to fly V2 or V2 plus a particular additive if a higher speed has been achieved (and the FD is programmed for this), in the 3rd segment we are told to fly a constant altitude (and the FD is programmed for this), and so it goes. What are they going to tell you to fly between Vloff and V2? An attitude which will ensure correct acceleration between the two is what is provided, 8° for the case in point. It COULD be varied at the manufacturer's discretion to squeeze the last drop of performance from the aircrat, but at this point in time, it's constant.
Hope that helps, search around on the forums, there’s a few discussions covering your question in more detail
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Old 12th Mar 2018, 14:49
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Dear Fuzuma,
Thanks for your reply.
Sounds like a good reason. Dont remember well the one I heard years back but this explanation seems good enough.
Cheers
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Old 12th Mar 2018, 18:25
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If your FD on the 737 NG is showing 8 degrees this is not normal;

The AFDS commands pitch attitude in the following order:
• 10 degrees nose down until 60 knots IAS
• 15 degrees nose up after 60 knots IAS
• 15 degrees nose up after lift–off until a sufficient climb rate is acquired. Then, pitch is
commanded to maintain MCP speed plus 20 knots.( in this case, as observed it varies as it does with the windshear program )
No idea where 8 degrees comes from Type?
Avenger is offline  

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