# When does a passenger plane leave the cruising phase?

I have Flightradar24 data here. Which are in the following structure.

latitude    longitude   altitude    lastUpdate      velocity
....
49.34240    8.77556 19650   2018-11-01 09:38:09     415
49.35040    8.77490 19550   2018-11-01 09:38:15     414
49.35617    8.77442 19475   2018-11-01 09:38:20     414
49.37045    8.77308 19275   2018-11-01 09:38:24     412
49.38132    8.77208 19125   2018-11-01 09:38:30     411
49.38964    8.77132 19025   2018-11-01 09:38:34     410
49.39796    8.77054 18900   2018-11-01 09:38:39     409
....


This is data for a specific flight. My goal is to split the flight in two parts.

• Pt 1: Airborne -> Exit cruise time / start Approach
• Pt 2: Approach Start --> Touchdown

Well, I am not to deep into aviation. My naive approach would be:

If the flying altitude has dropped continuously over the last k points, then the first timestamp is the approach start.

Now I ask myself, in what period of time does a passenger plane leave the cruising phase?

This is different from the pilot calculating the top of descent explained here: What is a rule of thumb for when to begin a descent?

How much ft altitude does an airplane usually lose when it comes out of the cruising phase? And how long does he need for that?

The background is that I want to understand how long a passenger plane usually takes to land at a specific airport from the approach phase. Optimally, I then see that on days when the airspace over the airport is fuller on other days, times, the approach phase takes much longer. Or do you think there are other factors playing an important role? Or is the approach not so correct. Maybe it would be better if I do not look from Approachphase, but from the time where a certain radius was reached. 80km before arrival?

• What you're trying to identify is the "top of descent" point. Clue: Planes try to descend in glide mode as much as possible to save fuel, so the location of the TOD and the rate of descent are linked to the glide ratio of the plane. See What is a rule of thumb for when to begin a descent? and all the linked questions (column at the right of the question) for an overview.
– mins
Feb 7 '19 at 10:35
• Note that aircraft sometimes climb/descend during the en route phase, typically due to weather or traffic. It may be difficult (if not impossible) to reliably distinguish between this and the start of a typical stairstep descent on approach. Feb 7 '19 at 16:13
• In a similar project I worked on years ago, we defined to use as top of descent the last point of the horizontal segment preceding the first descent segment that undercut 20000ft. Intermediate level-offs up to I believe four minutes duration were still considered part of descent segment. For flights never exceeding 20000ft we used the last horizontal point at highest altitude where at least five minutes level flight were attained (matching that airline’s short flight cruise planning policy), and if that condition wasn’t matched either, we simply used last point at highest altitude obtained. Feb 7 '19 at 17:05
• What is the purpose of this exercise? Feb 7 '19 at 17:11
• @ymb1 I don't see how this is a duplicate. That question is, basically, "When should I, the pilot, choose to start my descent?" This question is, "How can I, an observer detect that the descent phase has begun." The pilot of the flight may or may not be following the guidance given in answers to the other question. Feb 7 '19 at 17:58