Flight Planning: Time to Waypoint

Why groundspeed

Flight planning is about planning a trip from point A to point B, across a distance over the ground. How long that trip will take boils down to how quickly your plane covers the ground distance, which is why we need to know our groundspeed.

In the air, we think in terms of airspeed, but that doesn't directly tell us how long the trip will take. Only groundspeed does.

Where the wind comes in

Groundspeed is true airspeed (TAS) adjusted for wind. If you hold a constant TAS into a headwind, you'll fly a slower groundspeed, because the wind is resisting your forward movement. Whereas with a tailwind, at the same true airspeed you'll move faster over the ground -- the wind is helping you to cover the distance faster.

Say your POH gives you 100 knots true at cruise and the leg is 50 nm. With no wind, that's 30 minutes. Add a 20-knot headwind and your groundspeed drops to 80 knots, so the same leg takes about 38 minutes. Swap it for a 20-knot tailwind and you're there in 25.

Working the estimate

On the ground, before you fly, here's how you get there:

• True airspeed comes from the POH cruise chart. TAS is calibrated airspeed corrected for altitude and temperature.
• Groundspeed is calculated using the wind-triangle solution described in PHAK, Chapter 16. Most E6Bs print the steps on the top of the wind-calculator side. You'll use the winds aloft forecast for the time of your flight.
• Time is distance divided by groundspeed, times 60 to get the time in minutes.

It's an estimate because the wind is a forecast, which is why the FAA nav log labels the column "GS Est." Once you're airborne, your checkpoints tell you how good the estimate was. Reach one late and the winds aren't what the forecast said, so you'll need to adjust on the fly.

What about the GPS and the climb?

Once you're in the air, the GPS measures your actual groundspeed and works out time to waypoint on the fly. That's handy, but this quiz is about the planning you do beforehand, before any of that exists.

And yes, the climb is its own animal. The POH time-to-climb chart covers that segment. For the cruise legs, it's distance over groundspeed.