Handy Calculations While Flying

If we wish to navigate through space with a level of precision, it is necessary to determine numbers such as: Top of Descent, Required Rate of Descent, and other information we usually have to look up or plug into our EFB or E6B.

For some situations a “That-Looks-About-Right” calculation will help to determine if our decision process is on the right track or needs further scrutiny for safety or regulatory compliance.

For example, if you’re at the airport and would like to get an idea how strong the wind is, a wind of 8-10 knots will register with the windsock pointing at 45 degrees from horizontal. A fully extended windsock will register at least 15 knots or more. If it’s 90 degrees to the runway a quick go-no/go might be forthcoming.

Scenario: You’re returning to your home airport traffic pattern at 2,000 MSL from a cross country altitude of 6,500 feet. That means you need to lose 4,500 feet sometime between where you are and the traffic pattern.

How far out should you start your descent without popping your passenger’s ears? You’d like to reach for your iPad, but the turbulence is too much to keep steady fingers on the menu selections.

Not to worry; the wonders of That-Looks-About-Right is at your service. All one needs to do is multiply the altitude to lose by three and drop the zeros to get a distance to Bottom of Descent (2,000 feet) (4,500 X 3 = 13,500/100 = (13.5 nm) Your Top of Descent should be not from the airport, but the traffic pattern. Therefore, add four to five miles to your number to be at TPA sufficiently before you enter the traffic pattern.

This is also important for making sure one does not encroach upon Class Bravo when flying in the MSP area. The aforementioned formula is based upon a 3 degree glide slope.

That only provides one piece of the puzzle. The next number to be discovered is the rate of descent required to achieve the desired arrival. Let’s say you started your descent with a ground speed of 110 knots. Divide 110 knots by two, add a zero and, viola! You’re on a three degree glide path with your power and trim set to a Required Rate of Descent of 550 feet per minute, which will make you look like a pro. Are these numbers exact? No, they are the TLAR formulas that will get us in the ballpark when the iPad overheats, or our Foreflight subscription expires mid-flight.