Question 1

What is a standard descent gradient?

Accepted Answer

The 3:1 rule equates to a 3-degree descent angle and roughly 300 ft/min per 100 kts of groundspeed. Airlines typically plan 1,000-1,500 fpm for en-route descent, steepening to 700-800 fpm on the approach.

Question 2

How early should I start descending?

Accepted Answer

Multiply the altitude to lose (in thousands of feet) by 3 to get the TOD distance in NM, then add 10% as a buffer for ATC routing or crossing restrictions. This calculator does that automatically in Normal mode.

Question 3

How do you calculate top of climb (TOC)?

Accepted Answer

Divide the altitude to gain by your average climb rate to get the climb time in minutes, then multiply by ground speed to get the distance: TOC distance = (altitude to gain / climb rate) x (ground speed / 60). Climbing from 5,000 ft to FL350 at 1,800 fpm and 300 kts ground speed puts TOC about 83 NM out.

Question 4

How do you calculate the required rate of descent?

Accepted Answer

Divide the altitude to lose by the time available: time (min) = distance (NM) / ground speed (kts) x 60, then required V/S = altitude to lose / time. The Descent Rate mode does this for a fixed distance to go - useful when ATC clears you to cross a fix at a given altitude.