How accurate is your ASI??

[Guest OzChris]

Recently I have been thinking about the ASI in the aircraft that I fly...in particularly, how accurate is it?

 

So I set out to find out more about ASI's and how to calibrate them - properly.

 

I have read some interesting articles and general information on ASI's and their accuracy...and the need to calibrate them accurately - thought Id share what I have found here.

 

Please feel free to add comments on your methods or to correct me if something I have said doesn't sound right.

 

John Gilpin suggests that there are several ways that we can calibrate an ASI, however many consistent and known conditions need to be present to perform accurately. He suggests that the GPS is the answer.

 

That doesn't mean, of course, just flying along and compare the Ground Speed with the ASI and assume no wind component. Or even fly in a circle at a constant IAS and pick the highest and lowest GPS readings and average the results - this is just not good enough, not by far!

 

He suggests using the method published by Randy Schlitter of Rans Aircraft - its easy to do and should produce a good degree of accuracy, even with some wind drift.

 

Here is the method using a GPS to calibrate your ASI:

 

Stabilize your aircraft at the chosen indicated airspeed and altitude, on a heading of 120 degrees by the track bearing on the GPS

 

 

 

Hold that same track for 10 minutes, recording the ground speed on the GPS every minute.

 

 

 

Turn to 240 degrees as indicated by the track bearing on the GPS.

 

 

 

When the track and airspeed (same IAS as the 1st leg) are stabilized, fly 10 minutes in that direction, recording the ground speed on the GPS every minute.

 

 

 

Turn to 360 degrees as indicated by the track bearing on the GPS.

 

 

 

When the track and airspeed (same IAS as the 1st and 2nd legs) are again stabilized, fly 10 minutes in that direction, recording the ground speed on the GPS every minute.

 

 

 

You should now have 30 readings of Ground Speed. To calculate the average, add all the figures and divide by 30. This is what the ASI should have indicated (within a knot or 2) if not that is the amount it is out FOR THAT DENSITY AND IAS

 

This method should give you a pretty accurate calibration, regardless of wind drift, but it does require that the wind drift be constant on all legs...so its important to maintain constant altitude.

 

The preferable time to do the test would be early morning on a still day to avoid thermal effects.

 

The ground speed readings along any leg should be very consistent...if they're varying more than a couple of knots on the same leg, then either they're wind changes or you're not holding the airspeed steady enough.

 

This would also be very good practice at Straight and Level...

 

 

[Guest rrudd]

"This method..(the change of heading bizzo) should give you a pretty accurate calib. regardless of drift.'???...and then goes on to say 'drift should be constant on all legs' Which is it??? Unless its a perfectly still day this wont be the case.. with a steady wind flow .. the drift will change with every heading change. Thats why in the good old days they flew 'wind triangles' to get an accurate wind speed and direction, by observing all the drifts.

 

A simple out and return will do it... or build yrself a simple manometer to check it on the ground.

 

Spent many years getting drift angles for camera offsets for line aerial mapping. Vale the Mk1 eyeball and the 'bomb'sight! GPS rules.!

 

 

[Guest OzChris]

Yes thats right - the emphasis was on "constant" drift (wind), not variable.