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During takeoff, the Boeing 737 encounters a distinctive "dead band" phenomenon after the initial pitch reaches approximately 10 degrees. Essentially, a slight increase in back pressure is necessary around 4-5 seconds into the takeoff procedure or when reaching the 10 degrees pitch to sustain the rotation rate.

While this heightened back pressure demand within the “dead band” is quite noticeable during initial training on the 737, it progressively becomes ingrained and intuitive through experience, etching itself into muscle memory.

Rotate at a rate​ of 2.0 to​ 2.5 degrees per second. Beware of a “‭dead‬​ ‭band‬” ‭​‬ around 10 degrees, and maintain a co‭nstant rate of rotation towards 15 ​‬ degrees nose up to a​ p‭itch limit of 20 degrees.‬

Boeing Flight Crew Operations Manual (FCOM) - Takeoff Procedure

enter image description here

Although the Flight Crew Operations Manual (FCOM) addresses the "dead band" effect during the takeoff procedure, as far as I am aware, it is not mentioned or discussed in the Flight Crew Training Manual (FCTM) or the Airplane Flight Manual (AFM).

Based on my understanding, this "dead band" effect during takeoff is attributed to the combination of downwash off the wings and ground effect phenomena, resulting in diminished elevator effectiveness. Is this explanation accurate?

Furthermore, does this dead band phenomenon occur to the same extent in other Boeing aircraft or similar narrow or wide-body commercial aircraft?

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    $\begingroup$ WIth T tails on some airplanes the problem is the opposite; you have to relax a little bit to keep the same pitch rate. In that case it's mostly the T tail moving deeper into the downwash field and the fact that the gear with tail mounted engines is farther aft to keep the thing from tipping back when empty. $\endgroup$
    – John K
    Commented Jun 13, 2023 at 18:58
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    $\begingroup$ To your second question, it affects the 767 as well. To your point, primarily ground effect, it does impact other airframes but I think the 737 got so much press around it due to flight simulators emphasizing the effect relative to what some pilots felt the effect was on real aircraft. Most flight instruction curriculum lists it as tail going into ground effect, but the downwash also effects it. Pilots will tell it's noticeable on the 767 as well. $\endgroup$
    – Benloper
    Commented Jun 14, 2023 at 11:52
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    $\begingroup$ It is noticeable on the real aircraft too. 👍 $\endgroup$
    – tedioustortoise
    Commented Jun 14, 2023 at 12:23
  • $\begingroup$ It seems you already asked this question two years ago. Related discussion at PPRuNe. $\endgroup$
    – mins
    Commented Jun 23, 2023 at 11:06
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    $\begingroup$ Even from your drawing you can see that the wings are aligned at the same height of the elevators. You have then turbulent flow reaching them, reducing their effectiveness (negative lift in this case). Ground effect will also augment the pressure of the lower part of elevators, which is not what you want in this case (lift should be downwards) $\endgroup$
    – maximusboscus
    Commented Sep 15, 2023 at 12:13

1 Answer 1

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A question about dead-band has been posted previously by the OP. That one was about a deadband in response of the flight controls with hydraulic systems OFF, this present question is about a deadband during take-off with flight control hydraulics fully functional.

Centre deadband

In systems control and response, a deadband is a region where a change in input does not cause a change in output, like the centre deadband in the figure above. In the takeoff rotation deadband that the FCOM mentions, input is the pitch stick position, and output is the aeroplane Nose Up angle. So in the deadband region, further stick deflection does not result in further Nose Up angle.

And we can see in the pictures in OP why this happens: with the aeroplane on ground, nose up means tail plane very much closer to the ground, causing more lift at the tail plane due to ground effect, like depicted in this answer. So more and more stick input is required to further up the nose, described as a deadband.

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