View Full Version : Calculating a distance in TOD

18th Nov 2006, 17:48

Can anyone please help me with the following?

I need to calculate the distance where I have to comence my descent from any given altitude, with a constant speed. I fly a MD 80 and Im currently using the x3 factor. Altitude/1000 X 3. But this calcutation is based on a speed schedule of M0.76 and 290 KIAS.

what I want to know is where to star my descent is Im going to do it at 250 Kias for example.

Thanks a lot


LRdriver II
18th Nov 2006, 19:05
Cant you just modify it with GS? ie 300kts = 1500ft/min (divide by two and add a zero for a 3deg decsnt path)

18th Nov 2006, 19:53
(divide by two and add a zero for a 3deg decsnt path)

Or GS X 5.... :)

Desert Budgie
18th Nov 2006, 21:40
I use GS x 6

At 250 kts, to descend 10000 ft requires a descent rate of about 1400ft/min over 30 nm. So using GS x 6 gives me a descent rate of 1500ft/min. I find it gives me a bit of altitude to play with when I have to be below a certain altitude by any given waypoint. Then again, I handfly a B1900, cant keep it quite as accurite as an autopilot when Im bouncing through the soup :}

DB :ok:

Old Smokey
19th Nov 2006, 04:39
The 3 times formula works just fine for the major part of the descent at high speed, but it does not allow for the 2 major decelerations needed during descent, i.e. (1) the deceleration to 250 KIAS at 10000 ft / Terminal Area entry, and (2) later deceleration to approach speed.

Try using 3 X Ht +10 for TOD. The +10 makes good allowance for these 2 decelerations which "stretch" your glide. After the decelation has been made, use a simple "3 times".

This works pretty well with aircraft such as the MD-80 which are pretty good at slowing down. Others like the B777 are more reluctant to slow down, and a +20 additive is more appropriate. I'm sure that by experimentation you'll find the appropriate additive for your operation.


Old Smokey