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Took off from LHR in a B747-400 on Sunday and I experienced something new that is puzzling me.

After take off as the aircraft was climbing it seemed to continually speed up and slow down at the rate of once every one or two seconds. Kind of like an oscillation in the direction of travel.

After a few minutes of this it stopped.

What caused this to happen?

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    $\begingroup$ What was it that caused you believe it was speeding up and then slowing: a feeling of acceleration then deceleration physically, change of pitch of the airplane, change in engine noise, or something else? $\endgroup$
    – Terry
    Mar 29, 2016 at 7:16
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    $\begingroup$ Could have been gusting head/tailwinds - would cause both feeling of acceleration and pitch to change. $\endgroup$
    – IanF1
    Mar 29, 2016 at 8:18
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    $\begingroup$ One "normal" reason for (a wrong) sense of airspeed oscillation could be the air conditioning pressure variation caused by a malfunctioning engine bleed or pack valve. $\endgroup$
    – Sami
    Mar 29, 2016 at 9:44

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There are a whole bunch of possible reasons.

The first thing to note is that, from inside the cabin it can be very difficult to tell the difference between speeding up/slowing down, pitching up/down, and the rise and fall from turbulence (particularly when in a climb). And one I wasn't aware of but as pointed out by Sami, the air conditioning pressure varying because of a malfunction in the engine bleed or pack valve.

So why could it be happening?

  1. Turbulence causing the aircraft to gentle rise/fall, which, in a climb, is fairly akin to small acceleration/decelerations - both to our inner ear, and in terms of what's physically happening to the aircraft
  2. The air conditioning pressure rising and falling more than usual rather than maintaining a constant pressure
  3. Slight pitch up/down manoeuvres, possibly from a relatively inexperienced pilot flying the aircraft
  4. The auto-throttle throwing a hissy fit or, if climbing at 250 knots under 10,000 ft, oscillating slightly in order to stay at/below the 250 knot speed limit
  5. The pilots adjusting thrust manually, possibly for the same reason as above
  6. A gusting head/tailwind. Particularly noticeable near the ground and when in a climb, and can cause a feeling of acceleration/deceleration.
  7. A malfunction in one of the engines causing the power to vary slightly. It's possible this was only noticeable at takeoff/climb thrust levels, and disappeared when throttling back

Typically you'd expect it to be due to either turbulence or a gusting wind, but there are several systems which could develop a small, fairly benign fault and have this symptom.

Edit: As noted by user Jamiec, Storm Katie (a reasonably strong storm) was over the UK at the time, which would certainly back up turbulence/gusting winds being the most likely culprit

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  • $\begingroup$ A more general #4 would be the auto-throttle or auto-pilot in speed mode (at any speed) often does this, which is why I usually avoid the speed mode and use something more stable. $\endgroup$
    – Lnafziger
    Mar 30, 2016 at 16:56

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