Calibrating your Air Speed Indicator
By: Jeff Shultz


Stall speeds are always a favorite topic of discussion with pilots. Have you heard someone confidently declare "My aircraft stalls at 25 mph!", but your nearly identical airplane stalls at 35 mph? You may ask yourself, What is going on here? The explanation may have nothing to do with those wingtips, chisel nose, or flaprons. It might simply be instrument error in your/his air speed indicator (ASI). ASI errors can creep in due to a variety of reasons, including obstructions or leaks in the pitot/static plumbing, improper placement of the pitot tube or static source, or simply a sloppy ASI gage. Troubleshooting these errors is notoriously difficult. But calibrating your ASI is quick and easy using a handheld GPS. The following method and spreadsheet will allow you to determine a "correction factor" for your aircraft that converts the indicated airspeed on the gage to actual airspeed (also known as calibrated airspeed).

The GPS calibration method involves flying at a constant indicated airspeed (say 60 mph indicated) at three different headings. One of the difficulties in using GPS speeds to calculate airspeed is the effect of wind. By flying three different headings and mathematically crunching the geometry, we can precisely determine both the speed and direction of the wind, and then subtract those wind effects to give us the actual airspeed of the airplane.

Here is how the flight test procedure goes:

  1. Gather the necessary supplies. You will need a handheld GPS, set to display heading in degrees and speed in mph, plus a note card to record your test data.

    Sample Data Note Card

  2. Select a time when the air is free of turbulence. Steady wind is OK, but gusty conditions and lots of bumps will give you poor data.

  3. Pick a starting altitude and airspeed. It is important to maintain these as precisely as possible.

  4. Fly the first test heading and allow the aircraft to become stabilized before recording the indicated airspeed (V indicated), GPS speed (Vg 1) and GPS track/direction (Track 1).

  5. Repeat step 4 for two additional directions (any two will serve just fine) and record GPS results. Be sure to maintain a constant altitude and indicated airspeed throughout the test.

  6. Enter all the test data into the spreadsheet. It is recommended to repeat the test for one or two more indicated airspeeds.

All of the number crunching is done for you in the form of an excel spreadsheet. To use the spreadsheet, you simply fly the test procedure and fill in the data in the blue shaded cells, and the program does the rest. It will give you the calibration results in the form of an equation:

Calibrated Air Speed (CAS) = X (%) * Indicated Air Speed (IAS)

X in the equation is expressed as a percentage. A typical result might be:

CAS = 95% * IAS

This means that in reality, your actual airspeed (CAS) is only 95% of your indicated airspeed (IAS). Your gage says 100 mph, but you're only going 95 mph!

Likewise, if:

CAS = 120% * IAS

Then in reality, your actual airspeed is 20% greater than indicated on the gage. ASI says 25 mph, but you're really going 30 mph! This is good for creating an easy to remember rule of thumb for your plane. It might be something like "My ASI reads 10% low." or "My ASI is 5 mph low at stall, and 10 mph low at cruise". Just fill in the blanks to match your aircraft.

Additionally, the spreadsheet will graph the results if you repeat the test at several different airspeeds. The graph will serve as a handy reference tool where you can use any indicated airspeed for your plane and find the resulting calibrated airspeed off the graph.

Let's say you performed three test runs and different airspeeds (i.e. close to your stall speed, cruise speed, and somewhere in between), plugged all that into the spreadsheet, and now had the following graph.

It is a simple matter to find CAS. Say your ASI is reading 65 mph, you find 65 on the bottom axis (IAS), follow it up to the plotted line, and over to CAS on the left axis. Doing this, you determine that the actual calibrated airspeed is 68 mph.

So stop wondering how accurate your ASI is. Grab that handheld GPS, take a few measurements, and go find out!

 

Download the ASI Calibration Spreadsheet (MS Excel) file.

Jeff Shultz