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Calibrating the Airspeed Indicator (ASI)
A sometimes overlooked part of the flight test program is calibration of the airspeed indicator (ASI). The airspeed indicator is (arguably) the single most important instrument we have on board our airplane, and it's important that owners/pilots make sure that the ASI has been calibrated (that it has been corrected for errors).
Indicated Airspeed (IAS) is the speed of the aircraft as shown on the airspeed indicator.
Calibrated Airspeed (CAS) is the indicated speed of the aircraft after it is corrected for position and instrument error. CAS is equal to true airspeed in standard atmospheric conditions at sea level. It is important that builders/owners calibrate their airspeed, both with flaps up and down, and mark their ASI accordingly.
An excellent source for information and details on how to do this is the National Test Pilot School website (www.ntps.edu), and specifically, their download page on Using GPS to Determine Pitot-Static Errors.
According to the worksheet, we're going to note GPS speeds as we maintain specified IAS while maintaining specific headings at a given altitude. We'll do this for different given headings to account for prevailing winds.
Additional equipment required: A handheld GPS and a portable outside air temperature (OAT) gauge (shown above).
Roger is ready to fly with clipboard and GPS in hand:
Here's a video clip as we fly:
Filling in the worksheet as we fly:
This is the spreadsheet that we used:
- AirSpeedCal-1.xls (Microsoft Excel Spreadsheet with two worksheets)
After flying, Roger inputs the data in the worksheet:
...and then transfers the info to the ASI:
It's important that the builder/owner properly document the CAS in the pilot operating handbook (POH).
For a good example of this, see page 5-4 in the AMD Zodiac series POH (11/09).
Comment
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Comment by Jim and Amelia Isaacs on January 15, 2017 at 8:19am
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Great post, thanks. Link for the documents has moved to here: http://www.ntps.edu/information/downloads.html
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Comment by Ricardo Rodriguez on February 20, 2010 at 6:43am
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Great post! Very useful.
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Comment by Tommy Walker on February 14, 2010 at 5:03pm
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Thanks for the useful information. This is valuable.
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Comment by Tim Garrett on February 12, 2010 at 5:52pm
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What were you two test pilots laughing at on final?
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Comment by Jonathan Porter on February 12, 2010 at 1:50pm
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A very useful post. Thanks Sebastien. For interest, when I take up an unknown aircraft I always get to a safe altitude using a 'higher than normal' indicated cruise speed. Once there, I 'hunt' for the incipient INDICATED stall speed on the ASI - ie just before the stall. I then make sure that I always keep my speed at least 1.3x that indicated speed. (1.3xVs is the 'recommended' approach for many aircraft' but I often use Vs x 1.5 and allow the speed to bleed close to the ground - but that is because of the wind changes, etc here).
More posts like this would be great (shame we cant see the video here).
Take care.
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