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I rode a smaller twin-tail-engined jetliner for a few short hops last month, and I noticed that their airport approaches are much rougher than the widebody airliners I am used to. I believe that it was an MD-80 with United Airlines.

As soon as the pilot began the descent, it seems like the he/she was going wild with the throttle; the engines would appear to idle for 10 seconds, then suddenly and audibly power to full for another 10 seconds, and the cycle would repeat until we were right above the runway. I know that this was not crew specific because I took the commercial flight on 6 different occasions on three different routes.

Question: Why did the pilots increase and decrease the throttle so rapidly and so many times on descent to the runway?

edit: Terrain: flat, Atlanta area; Meteorological conditions: heavy cumulus, twice during the night and the other four during dusk, light to moderate rain under and in the clouds. The clouds had no breaks and often had a strange formation where in some areas they would rise much higher than others. Imagine a series of mountains and valleys with several? miles of distance between cloud peaks and cloud dips.

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    $\begingroup$ without knowing the precise meteorological conditions I am afraid it will be hard to answer unambiguosly. $\endgroup$ – Federico Aug 27 '15 at 13:46
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    $\begingroup$ It would help if you know the aircraft type. Which airline / routes? I suspect a not so well tuned autopilot / autothrottle. Smaller jets often use generic autoflight systems which are tuned to the aircraft model. But it can also be meteorological effects above hilly terrain for example $\endgroup$ – DeltaLima Aug 27 '15 at 13:53
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    $\begingroup$ I haven't much experience with those models. I have noticed a similar oscillation in Fokker 70 / 100 aircraft. $\endgroup$ – DeltaLima Aug 27 '15 at 13:57
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    $\begingroup$ Are you sure that it was the engines and not the air conditioning packs? $\endgroup$ – Simon Aug 27 '15 at 14:03
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    $\begingroup$ Possibly just a stepped descent: en.wikipedia.org/wiki/Continuous_descent_approach In the process of being phased out due to the increased noise and fuel consumption of repeatedly throttling up and down vs. cruising in on a direct line. $\endgroup$ – Andrew Medico Aug 27 '15 at 20:47
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I think your note about this taking place in the Atlanta area with heavy Cumulus clouds in the area is the best clue as to why such large throttle inputs were necessary. It sounds like there were thunderstorms in the area in the cumulus and mature stages. The pilot was most likely encountering heavy updrafts and downdrafts passing through the air and had to compensate accordingly with the power levers to maintain approach airspeed and glidepath during the approach.

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