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On chart plates it is written the minimum climb gradient with which an aircraft should climb for obstacle clearance or after ATC request (for traffic flow management etc.).

However, since this is the minimum required climb gradient, I am curious about what practically happens during real flights. For example, if a chart plate instructs an initial climb with a minimum climb gradient of i.e. 320 feet per nautical mile, will the aircraft try to maintain this minimum climb gradient until the termination altitude? Will it climb with the maximum climb gradient that its performance characteristics allow?

In other words, given an assigned climb gradient from a chart plate (i.e. from DEEZ5: "Standard with minimum climb of 250' per NM to 300), what would the average climb gradient of an aircraft during the initial climb phase be, provided that this value is taken into consideration not only for a lower but also for the calculation of an (hypothetical) upper limit?

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    $\begingroup$ The average gradient has very little practical meaning. It varies with aircraft type, engine type, take-off weight, airport altitude, air temperature, engine setting (e.g. derate / assumed temperature), airline procedures etc. etc. The average on one airport can be totally different from the average on another airport, from a summer day to a winter day, from one airline to another. What are you trying achieve? $\endgroup$
    – DeltaLima ♦
    Sep 23, 2019 at 8:11
  • $\begingroup$ @DeltaLima I am trying to understand which criteria an aircraft takes into consideration in order to decide its climb rate / climb gradient during the initial climb phase of a departure procedure. In other words, if it always aims to achieve the maximum climb gradient that its performance characteristics allow or if it takes other factors into consideration and, mainly, the minimum climb gradient that is written on a SID plate. $\endgroup$
    – Vector Zita
    Sep 23, 2019 at 18:29

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The upper limit is a function of the airplane's performance at rated climb power at the normal speed used, modified somewhat by the wind (the gradient is based on ground speed). What actually happens on a departure is simply whatever numbers that airplane's performance produces using the standard speed and power setting for that airplane.

In a jet you are generally going somewhere around 200 kt once the flaps are up, then accelerate to 230-250 kt once past 3000 ft up to 10000 ft.

From the table below, assuming no wind, an airliner climbing at 3000 FPM at 200 kt will have a gradient of roughly 1000 ft/nm between the initial acceleration altitude (say 1000 ft) and 3000 ft:

Climb Table

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  • $\begingroup$ Thank you John for your input. In your example, you mentioned a climb rate of 3000 FPM. I guess that what I am actually asking is if the climb rate is decided based on the mandatory climb gradient which is written on a SID plate e.g. for obstacle clearance purposes. If not, does an aircraft generally try to achieve the maximum climb rate, based on its performance, during the initial climb phase, without "caring" to be close to the SID's minimum climb gradient? $\endgroup$
    – Vector Zita
    Sep 23, 2019 at 18:22
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    $\begingroup$ What it means is you just do whatever you normally do without concern for the published gradient, which is a minimum requirement for obstacle clearance, IF you know you can exceed the requirement. Those limits really only apply for marginal cases. If you're in a piston twin flying a departure in a mountainous area that requires a 350 ft/nm gradient, well, no problem cuz my baby can climb well over 526 fpm at 90kt with both engines operating, but what if an engine quits and it can only climb 300 fpm? If I do the departure and one goes down, I'm in trouble. Now I have to think hard about it. $\endgroup$
    – John K
    Sep 23, 2019 at 20:08

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