Easiest way to calculate top of descent point
Thread Starter
Joined: Jun 2006
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From: somewhere civilised
Easiest way to calculate top of descent point
Hi I was wondering if any of you have an easy method that you use in your mind to roughly calculate the point where you want to initiate your top of descent?
Joined: Dec 2005
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From: UK
Thats pretty much it. 3 times height to lose plus 5 miles to slow and configure.
ie. 20000ft to 5000ft = 15 x 3 = 45 miles + 5 = 50 miles.
Rate of descent required to achieve this is 5 x groundspeed
So 250 kts GS - 5 x 250 = 1250ft per minute.
ie. 20000ft to 5000ft = 15 x 3 = 45 miles + 5 = 50 miles.
Rate of descent required to achieve this is 5 x groundspeed
So 250 kts GS - 5 x 250 = 1250ft per minute.
Joined: Apr 2008
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From: Somewhere
Rate of descent required to achieve this is 5 x groundspeed
For example, 250kts divide by 2 = 125 , 1250ft/min.
Joined: Apr 2006
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From: Land of the Raj
This works fine for me,
Take your altitude and multiply it by 3. That equals your distance in miles to begin your descent. 2. Now take half your ground speed. This is your rate of descent in hundreds of feet. Ex. If you are flying at 12000ft at a ground speed of 150kts and you need to descend to 2000ft, the difference is 10000ft. Multiply 10*3=30 miles out you must begin your descent. Half your ground speed is 75, add a zero, and 750 ft per minute is your rate of descent.
KW
Take your altitude and multiply it by 3. That equals your distance in miles to begin your descent. 2. Now take half your ground speed. This is your rate of descent in hundreds of feet. Ex. If you are flying at 12000ft at a ground speed of 150kts and you need to descend to 2000ft, the difference is 10000ft. Multiply 10*3=30 miles out you must begin your descent. Half your ground speed is 75, add a zero, and 750 ft per minute is your rate of descent.
KW
Joined: Dec 1998
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From: Escapee from Ultima Thule
All well and good in a pressurised aircraft but not in non-pressurised types or those with engines that don't appreciate high speed/low power descents (think cylinders...).
In non-pressurised types I target 500'/min to keep my pax. ears comfortable. If piston as well then it also lets me avoid low power at high speed. Easiest way I know to calculate it is to use 2 x height-to-descend in thousands of feet = minutes required to descend. eg 12,000 height to lose = 24 mins prior to ETA is when to start the descent. In these types the change in TAS at lower altitudes is usually sufficient to allow for any speed reduction that may be necessary to configure for the approach. Further, starting a power reduction to approach or holding power from cruise power using 1" MAP/min at the appropriate time prior to ETA will have most types I know at an easily achieved approach speed for configuration changes.
In most cabin class piston twins this equates to ~6 or 7nm / 1000' if you prefer that method.
If you need a reduced rate of descent eg a pax. with ear trouble then target 300'/min and use 3 x height-to-lose instead.
In pressurised turboprops then I still tend to use minutes-to-ETA at whatever descent rate I care to use. The equivalent nm per 1000' height-to-lose depend on type eg a Kingair 90 I used to fly worked best at ~5nm / 1000' while a Kingair 200 worked for me using ~4nm / 1000'.
This was all based on arriving at the IAF or FAF configured for the approach.
In non-pressurised types I target 500'/min to keep my pax. ears comfortable. If piston as well then it also lets me avoid low power at high speed. Easiest way I know to calculate it is to use 2 x height-to-descend in thousands of feet = minutes required to descend. eg 12,000 height to lose = 24 mins prior to ETA is when to start the descent. In these types the change in TAS at lower altitudes is usually sufficient to allow for any speed reduction that may be necessary to configure for the approach. Further, starting a power reduction to approach or holding power from cruise power using 1" MAP/min at the appropriate time prior to ETA will have most types I know at an easily achieved approach speed for configuration changes.
In most cabin class piston twins this equates to ~6 or 7nm / 1000' if you prefer that method.
If you need a reduced rate of descent eg a pax. with ear trouble then target 300'/min and use 3 x height-to-lose instead.
In pressurised turboprops then I still tend to use minutes-to-ETA at whatever descent rate I care to use. The equivalent nm per 1000' height-to-lose depend on type eg a Kingair 90 I used to fly worked best at ~5nm / 1000' while a Kingair 200 worked for me using ~4nm / 1000'.
This was all based on arriving at the IAF or FAF configured for the approach.
