By definition, the top of descent (or TD) is the point at which an aircraft initiates a descent to a lower level for arrival at its destination airport. Every pilot should know when s/he should start to descend and how to adjust the rate accordingly. Your ability to quickly calculate your top of descent is useful both in real-world aviation and in Infinite Flight.
Here is my preferred method of how to quickly calculate your top of descent (TD):
- Your current ALT (a)
- Subtract destination airport’s elevation (b)
- Multiply that sum by 3.
- The given product is (c)
- Divide (c) by 1000.
- And voila, your (d)!
TD = a – b (3)
TD = c /1000
TD ~ (d)
Example: 30000 – 100 = 29900 => 29900 x 3 = 89700 => 89700/1000 = 89.7
Thus, your TD ~ 90NM (Your destination ICAO is ± 9NM, just enough time for your final checks)