Re: Descent Calculator

Date:         22 Nov 97 20:41:27 
From: (Edward Hahn)
Organization: The MITRE Corporation
References:   1
Followups:    1
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In article <airliners.1997.2688@ohare.Chicago.COM>, (Ted Landy) wrote:
>I am trying work out how to create a small program to calculate
>descent  profiles for aircraft to load into a handheld computer.
> I want to be able to enter height, speed, wind component and weight
>to come up with a distance.
>Originally I thought I could just enter a L/D ratio and presto come
>with an answer. Unfortunately for me it is not so easy. For a given
>IAS on descent the L/D seems to constantly change (increase).
>Does anyone know of any useful formulas, available programs or have
>suggestions that may be of use to me?

This is not a simple topic, and the answer will greatly depend on exactly
what you are trying to accomplish with your program.

The problem is that an aircraft's descent profile in the real world is
only somewhat dependent on the aerodynamic performance of the aircraft.
Typically, ATC instructions or other procedures (such as a STAR or the
10,000/250knot rule in the US), weather, or airspace restrictions
(restricted areas, etc.) will throw off any purely aerodynamic calculation.

Also missing is what the pilot and/or operator of the aircraft is trying
to accomplish - is the plane late for a bank of arrivals/departures (i.e.
flying fast)?, is the mission to save as much fuel as possible (fly optimum
L/D speed)?, some combination of the two?, etc.

Even throwing away these non-aerodynamic factors, the problem will require
knowledge of the density profile with altitude (related to the temperature
lapse rate) of the atmosphere on the particular day and time you are

BTW,  in the real world, airline pilots use a rule of thumb to determine
the distance required for descent:  3 nautical miles for every 1000 feet
of altitude - so a descent from 30,000 feet to the surface will take
approximately 90 nm.  This actually works out reasonably well.


>>>>  Ed Hahn    |    |    (703) 883-5988  <<<<
The above statement is the opinion of the author.  No endorsement
or warranty by the MITRE Corporation is expressed or implied.
Really, I wouldn't kid you about a thing like this.