A good pilot customarily calculates a ground speed, duration and estimated fuel burn before each flight — all from forecast winds — and that’s not so good…
Problem: A forecast is not the same as actual.
The Winds and Temperatures Aloft Forecast (FD) is a great tool, but if you plan a long cross-country flight using forecast winds, you’re asking for trouble. You must always be ready to rework the plan on the fly — literally — or the day will come when you find yourself flying against a headwind that you are not aware of. Translation: You will run out of fuel.
SOLUTION: Calculate the actual (not forecast) winds while en-route to your destination. Here’s how…
- Check your planned ground speed estimate by timing how long it takes to fly between your first and second checkpoints. You know the distance between checkpoints because you measured that on the chart and recorded it on your navigation log — right? Now, all you need to know is how long it took to travel that distance. The math is simple: Distance divided by time equals actual (not forecast) ground speed. If your actual ground speed comes out close to your original estimate, then your forecast and actual winds can’t be too far off and you have a dependable plan. Important: If the estimate and actual is off, then you should do the ‘missing wind equation!’
- The missing wind equation is just a way to work ground speed calculations “backward.” You will need to know the airplane’s True Course (TC), True Airspeed (TAS), the actual ground speed (GS), and the airplane’s Wind Correction Angle (WCA). Don’t worry, it’s easier than it sounds. The TC and TAS would have been calculated prior to takeoff and we solved for actual ground speed in step 1. The only item needed is the WCA and this is acquired visually, so it helps if you have good checkpoints. Example: Let’s say that you are using two towers as a checkpoint. If you have encountered an unexpected crosswind you will notice the wind drift as you approach the towers. Correct for the drift, by using your eye to set up a crab angle. Hold the crab angle for a few minutes to check it, then compare your new heading to the True Course. The difference is your WCA.
- You now have TC, TAS, GS, and WCA. The missing wind can be calculated using the wind-face side of a standard E6-B flight computer. First: Place the TC under the true index of the flight computer. Second: Slide the movable card so that the GS is under the computer’s center hole (grommet). Third: Mark a ‘wind dot’ at the intersection of WCA and the TAS. Fourth: Turn the computer’s wheel so that the ‘wind dot’ stops under the true index. Last: Read the actual wind direction under the true index — the actual wind velocity as the space between the ‘wind dot’ and the center hole.
- Now you have actual (not forecast) wind information and you can make accurate flight planning adjustments. Maybe your missing wind problem will reveal that you have more tailwind or less headwind. But you could also discover a greater headwind or less tailwind than planned on. Make fuel adjustments accordingly – even if that means making an extra fuel stop.
- Submit a PIREP to report actual winds aloft at your location.
If you’re not familiar with TC, TAS, GS, and WCA, plan to brush up by talking with a flight instructor! Note: Many GPS units can calculate winds aloft and if that applies to you, you’d be wise to learn how to use that feature with your eyes closed. But remember: The only thing you can always count on having in the cockpit is you. Know the missing wind equation. It will make you a better pilot and it could save your life.