How do you calculate TOD?

That is the age old question. It’s never a fun experience for the controller or pilot to miss when a pilot misses their TOD (Top of Decent).

The goal is to contact approach when you are at or below 18,000ft AGL, 50nm away from the airport, at or below 260knts IAS. Here’s an example I’ll use from a flight I did a week ago, from NZWN to NZAA, cruise at FL240.

  1. First step is to open up a precision descent profile calculator. Google “descent calculator” and you’ll get a bunch of results.
  2. I’d like to do a gentle descent of 1500ft VS (f/m) to 18,000ft AGL from my cruising altitude of 24,000ft MSL, at 280knts IAS.
  3. So open up the descent calculator. To achieve that 1500ft VS descent, I’d need to start my descent 76nm out (50nm + 26nm = 76nm). So that’s my TOD, 76nm.
  4. As I get closer to the 50nm mark, I’ll continue to decrease my IAS to 260knts or below, change my VS if I’m too high or too low, then contact Approach when I’m below 18,000ft AGL, 50nm out.
July 17, 2019 by Kyle Boas