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I have noticed on commercial flights (Ryanair, Boeing 737 specifically), that during the last 20 - 30 minutes of the flight, the aircraft alternates between descending and ascending pitch.

Why is this? Why not just gradually descend?

Further info: The route flown was London, UK to Budapest, Hungary.

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  • $\begingroup$ We can only guess without more specifics as to the route flown, and the airport being flown into. You could look at the Approach Charts for the airport being landed at, see what is published for flight levels to be flown. Farther out, ATC has control as to the levels flown. $\endgroup$ – CrossRoads May 3 '18 at 13:08
  • $\begingroup$ @CrossRoads I have added the detail to the question $\endgroup$ – Cloud May 3 '18 at 13:17
  • $\begingroup$ You'll need to wait for an answer from your side of the pond I'm afraid. I'm only familiar with US airspace. $\endgroup$ – CrossRoads May 3 '18 at 13:23
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    $\begingroup$ @MichaelKjörling OK, well basically the nose was pointing up and down (alternating) for 30 mins or so and I could visibly see use ascending and descending from the window. Why? $\endgroup$ – Cloud May 3 '18 at 13:48
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    $\begingroup$ This answer adds relevant information IMO. $\endgroup$ – ymb1 May 3 '18 at 15:30
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The plane wouldn't be ascending during the descend sequence. It instead follows a stair-step sequence. The times where the plane remained level may have felt like ascending even though it wasn't. It is very hard to judge that in the passenger cabin.

It is much easier for the pilot to have a sequence of descends to a specific altitude alternating with keeping altitude until the next marker. It also provides a timeline during which he can handle his checklist.

Doing a full continuous descend brings the danger of ending up too high and needing to re-intercept the glideslope from above. This needs more vertical speed and if something goes wrong can end up with the plane undershooting the runway in the worst case. Intercepting from below each time is much safer and makes that the airplane will never descend steeper than the glideslope and the horizontal sections allow for a margin to correct any minor mistakes.

It is also safer to change the configuration of the airplane (flaps/gear) while in level flight rather than during descend. Staying level for a bit also allows the plane to bleed off the airspeed gained during the descent.

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The Step-down process is because of the shape of Class B airspace, which has a decreasing series of steps. The airplane follows it by descending to a specific altitude, holding level for a time, then descending to the next lower altitude in a series of "stair-steps". This is very easy to follow, but requires the engines to draw back to idle, spin up to cruising speed, and drop down to idle again for each descent/cruise/descent step.

With the FAA's NextGen program, there are new approaches that can let the plane glide down on a continuous path. This is described here, saves fuel, has less noise, and is more comfortable for passengers.

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    $\begingroup$ I doubt London, UK to Bukarest, Hungary involves much FAA-controlled airspace, although it might possibly include class B airspace. (I'm mostly familiar with Sweden, which pretty much only uses class C and class G.) $\endgroup$ – a CVn May 3 '18 at 14:45

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