# Modern method to verify speed of a homebuilt aircraft

Summary: In 2001, airspeed of homebuilt aircraft was calculated by flying in a ⊔-shape and using trigonometry to subtract wind effects. Now that we have portable computers, is there a more modern method that is more accurate?

A paper published in 2001 by David Rogers outlines his "Horseshoe Heading Technique" which involves flying the aircraft in a ⊔-shape and recording GPS-derived groundspeeds so that you can then adjust for implied winds aloft to calculate the average TAS of the aircraft over the maneuver.

Now that we have significantly more advanced computer equipment in regular use, including AI, Extended Kalman Filters (the math behind AHRS), etc, are there more advanced ways of doing this that would use probabilistic methods to account for things such as pilot inaccuracy, wind variation during the flight, etc? Or even ways that would allow for TAS to be calculated while performing arbitrary maneuvers?

The motivation for this question is that I'm considering making an app that calculates airspeed while taking into account lack of pilot precision. I'm just not sure if anything like this exists already.

• Hi Zaz. Please make it clear you are asking about GA or kit planes (we have tags for both), i.e. those with simple equipment. I only realized that's what the Horseshoe Heading Technique is for after checking an AOPA article that mentioned it and the linked paper itself. Flight testing of airliners, where the air data computers are calibrated, which is what came to mind first, don't need those methods at all, so please make the XY problem clear, to help get the right answer and also so no one spends time on an answer that is not wanted.
– user14897
Commented May 17, 2022 at 0:00
• I'm totally confused as to what the distinction is: Aren't the calibration of airliners' air data computers tested using the Horseshoe Heading Technique? If not, what are they tested with? And if a certain method is used to verify the calibration in airliners, can that same method not be replicated in smaller aircraft?
– Zaz
Commented May 17, 2022 at 0:04
• Summary: I'm asking about test flying any size of aircraft and certified or non-certified. I just want to know what the options are for flight testing to verify airspeed calibration in general.
– Zaz
Commented May 17, 2022 at 0:05
• Replicated on small planes? No. Because they have much simpler equipment. And here lies the XY problem, which is good! Now you know how to reword the question. PS that method you mentioned is navigational more than calibration or flight testing.
– user14897
Commented May 17, 2022 at 0:06
• Well if there are differences in technique, I would love to hear about them. The most current I am aware of for both commercial and GA is the Horseshoe Heading Technique, so any information on any state-of-the-art improvements since then would be welcome.
– Zaz
Commented May 17, 2022 at 0:10

The "Brauniger IQ Comp" variometer which is at least two or three decades old now is a rate-of-climb instrument for hang gliders. It is capable of producing a wind direction and wind speed estimate any time the glider is engaged in circling flight, simply based on the ground track generated by a handheld-style GPS that interfaces with it. (Obviously, there is an assumption that at least over the short run, the pilot is not making erratic changes in bank angle, so at least over the short run the glider is assumed to be describing round circles with respect to the airmass.) If you know wind speed and direction, and you know ground speed and direction, then it is trivial to calculate TAS (especially when the glider happens to be going directly upwind or downwind.) The much higher-tech computerized variometers of more recent design for sailplane usage undoubtedly have similar capabilities and more. Therefore in any aircraft that spends a lot of time circling, or in any aircraft where the pilot is willing to fly a few sample circles without erratic changes in bank angle, it would seem trivial to use such instruments to come up with a wind calculation that is independent of any data from any airspeed probe. This in turn, when compared with the ground track data, leads directly to a True Airspeed calculation that is independent of any data from any airspeed probe. And when air temperature and pressure are taken into account, this leads directly to a calculation of what the Indicated Airspeed ought to be.

Just as any modern soaring flight analysis software used to display and analyze actual flight tracks based on GPS position data alone can come up with an estimate of wind direction and speed and present a table of how this is varying both over altitude and over time. The same software can easily present a graph of groundspeed and direction of ground track. If the actual data from the airspeed indicator is also saved, a post-flight analysis of possible airspeed calibration errors should be entirely possible.

So no, this sort of thing is not something that can only exist in the world of Boeing airliners, corporate jets, etc.

As for a list of precisely which instruments or programs are best suited for, and often used for, actual real-time (or post-flight) calculation of errors in Indicated Airspeed -- that will be left to another answeree to complete the story.

(I guess I'm interpreting the question as one about what is the most state-of-the-art equipment that would be practically available to pilots of home-built aircraft, sailplanes, etc, and would avoid the need to fly horshoe patterns, etc. A constant-banked circle, sustained over many revolutions, seems simpler and more redundant than a horseshoe pattern. If it is truly desired for the system to work as the pilot is maneuvering arbitrarily, then it would appear that incorporating accurate aircraft heading data (not just direction-of-ground-track data) would be critical. But that too is easily available with today's technology--technology that is realistically available far beyond the realm of airliners, corporate jets, etc.)

On the newest experimental development programs they use what are called "Smart Booms", or Smart Air Data Test Probes, for the test a/c, which is a flight data probe or boom that doesn't have the traditional angle vanes etc, but just has a series of holes that allow dynamic pressure to be measured around the nose of the boom.

All of the parameters measured by the boom, dynamic/static pressures, angles of slip and attack etc, are derived by software. Below is the Smart Boom (the orange bit) used on the developmental C Series (Airbus A220) that replaced the traditional long boom with vanes etc.

This is a Smart Probe developed for UAVs that produces very precise software-derived air data and may be what you have in mind.