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GPS and Airspeed Calibration

Verifying your velocity with satellite technology

By Ed Kolano (originally published in EAA Sport Aviation, September 2001)

The August “Test Pilot” completed our discussion of flight path stability, which showed how to take the data gathered using the flight-test techniques described in the July “Test Pilot” and transform the raw test-day numbers into a meaningful plot. This flight path stability plot showed you how the example airplane’s final approach path angle varied when the airspeed changed.

This is catch-up month. In February and March we explained how to calibrate your airplane’s airspeed indicator using the traditional, but somewhat cumbersome, ground course method. We also asked for your experiences using a GPS (global positioning system) to calibrate your airplane’s airspeed system, and this month we’ll present the ideas several readers have suggested.

There can be several advantages to using GPS over the ground course method. Safety is the most important advantage because you can fly GPS methods at safer altitudes. To accurately time your checkpoint passage, the ground course method requires you to fly at low altitudes, where engine problems, bird strikes, and near-stall speeds can be disastrous.

Changes in wind speed or direction during your test run can contaminate your data. With the ground course method your test run must be long enough to minimize the effect of tiny timing errors but short enough to remain in an area where the wind is constant. The longer your course, the longer you’re exposed to wind shifts and airspeed variations. Whether using a GPS solves this problem depends on which GPS method you choose.

The ground course method requires a ground course. Obvious, yes, but needing one means you’ll have to find a suitable course, coordinate with the owner of the property you’ll fly over, and be subject to the weather conditions at that site. A GPS doesn’t care about what’s beneath you or how far beneath you it is. So, you can conduct your test at a safe altitude wherever you find suitable weather.

Find the Wind

Wind is a problem during airspeed calibrations. The constant heading method of the ground course takes care of the wind by flying reciprocal headings and eliminating the wind during data reduction. Still, good results require a steady, near-calm wind. Calm, constant temperature air is ideal. If calm air isn’t available, flying directly into and away from the wind during reciprocal heading runs is the next best thing. Determining the wind direction is not always easy, however.

Several readers suggested a GPS method for determining wind direction. In smooth air fly a standard-rate (3 degrees per second) 360-degree level turn. Keep an eye on the GPS ground speed readout and note your heading when the ground speed reaches its maximum and its minimum values. If you’re flying in a steady air mass, these two extremes should occur 180 degrees apart during your turn.

For this technique to work your airplane must be stabilized before you begin. This means your engine controls are set, your bank angle is established, and your observed airspeed (from your airspeed indicator) is steady. When everything is stabilized, note your heading and start comparing ground speeds and headings for the next 360 degrees of turn.

After you’ve determined the wind direction, there are a couple of ways to proceed according to our respondents.

Quick Look-Let’s say your airspeed indicator showed 100 knots all the way around the circle. Your GPS showed a 115-knot ground speed when heading 360° and a 95-knot ground speed when heading 180°. You determine the wind is from the south at 10 knots by subtracting the maximum from the minimum ground speeds and dividing by 2.

Applying this 10-knot wind correction to the GPS ground speeds, you determine your true airspeed was 105 knots (115 minus 10 and 95 plus 10). You can repeat this circling test at a variety of airspeeds, recording observed airspeed, maximum and minimum ground speeds, pressure altitude, and outside air temperature (OAT).

You’ll still have to convert the test pressure altitude and OAT to density altitude. Then you’ll convert the true airspeed determined from the wind-corrected GPS ground speeds to calibrated airspeed using a flight computer. You can then compare the calculated calibrated airspeed with your observed airspeed to determine your airplane’s airspeed calibration. If your test airspeeds exceed 200 knots, you should also make an equivalent airspeed correction.

Longer Look-Once you know the wind speed and direction, fly reciprocal-heading test runs directly into and away from the wind. Assuming the wind doesn’t change from your circle determination, flying the reciprocal-heading method is similar to the ground course method with the simplification of merely adding the head wind and subtracting the tail wind to find each true airspeed. Disagreement between calculated true airspeeds for the reciprocal test run pairs could indicate a change in the wind.

One advantage to this longer look method is that it allows you to stabilize your airplane for straight and level flight while taking data. Another benefit is the two-way true airspeed comparison as a quality check.

Perform the density altitude calculation from pressure altitude and OAT measured during your two-way test runs, convert true airspeed to calibrated airspeed, and compare the results with your observed airspeed.

Ground Course in the Sky

Pegasus Technologies uses this technique in its flight performance software. It requires reciprocal, constant-heading test runs, but you make them while heading true north and south. You fly the stabilized test run just as you do for the ground course method, but instead of using landmarks as checkpoints, you use lines of latitude.

By stipulating true north and reciprocal headings, the GPS can show when you cross your latitude checkpoints. The GPS knows the distance between lines of latitude, which is 59.95 nautical miles per degree of latitude or approximately 1 nautical mile per second of latitude. Of course, this is the book answer for the Earth’s surface at its mean radius, but considering the planet is 7,900 nautical miles in diameter, the error introduced by you flying a couple of thousand feet above the surface is negligible.

Using the GPS latitude readout, you know the north-south distance traveled during your test runs. Divide this distance between lines of latitude by each test run time to calculate your ground speed for each run. Then average the reciprocal ground speeds to determine your true airspeed. Perform the true-to-calibrated conversion using pressure altitude, OAT, and true airspeed, and you’re done.

The Pegasus software does all this for you, but there’s no reason you couldn’t perform these calculations yourself using your own GPS for latitude markers. Because this method is nothing more than a ground course in the sky, the wind is accounted for by averaging the reciprocal calculated ground speeds. Notice we said the calculated ground speeds. These north-south speeds are different from the ground speed readout on your GPS, which indicates your ground speed along your flight path.

Wind & Other Bad Things

The lower the wind speed, the better your results will be. Although the calculations account for wind, significant errors have been reported when drift angles are too large. How large is too large? Don’t know, but the smaller the better.

Using GPS might entice you to fly longer test runs than you would on a ground course. This isn’t always the best idea. The longer your test, the more likely there’ll be a change in wind speed or direction. You’ll also be exposed longer for a rogue gust to ruin an otherwise Yeager-ish test run.

There are other GPS airspeed calibration techniques. One has you fly toward and away from a waypoint that’s thousands of miles away. Another involves three legs that are 120 degrees apart, and another uses three legs 90 degrees apart. There are probably several techniques that can provide sufficient accuracy for the recreational flying most of us do. The FAA does not universally accept any of these methods as legitimate airspeed calibration techniques for testing certificated airplanes, but it does allow the use of differential GPS and certain post-test software. The methods we’ve presented are still taboo for several aircraft certification offices.

GPS receivers are magic boxes. Color displays, moving maps, airspace warnings, and more flight data than most of us really care about can make them seem like the digital authority. Be careful. Although the elimination of the selective availability allows greater GPS accuracy, all GPS units are not created equal. Proprietary processing algorithms and display update rates can have an effect on your airspeed calibration data. Unless you know how old the numbers on your GPS display are, it’s a good idea to take your time gathering the data to ensure your airplane is really stabilized on the test condition.

Finally, and most importantly, think safety. Just because you fly these GPS airspeed calibrations a few thousand feet above the ground doesn’t mean it’s risk-free. There are still airways, airspaces, birds, and other airplanes to avoid. Don’t bury your head in the cockpit-and don’t forget to fly your airplane. Engine temperatures, navigation, fuel management, etc. don’t go away just because you’re on a test flight.

This month we passed along a few airspeed calibration techniques provided by EAA Sport Aviation readers. If you try them, let us know how it goes.

Thanks to everyone who took the time to send along their GPS airspeed calibration techniques, especially Paul Lipps, Don Saint, Bernie Wilder, and Bill Dalton.

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