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Pitot Static Position Error

What is it ?

Position error normally manifests as the ASI (airspeed indicator) reading a very low speed at high angles of attack. It is at it's greatest at approach speeds, and particularly when close to the stall speed (high angle of attack).

It normally occurs due to airflow effects around the static ports, and airflow meeting the pitot tube at high relative angles. Essentially it means that the pitot tube is no longer sampling the full velocity of incoming air, and/or the Static port is not sampling the correct static pressure. This can affect all aircraft types.

Why does it matter ?

This is not normally an issue if you do stall testing to establish approach speeds and fly them on your aircraft. You will know what IAS your Bearhawk stalls at and you will plan your approach speeds accordingly.

Let's say your Bearhawk stalls at 42 KIAS and that you've verified this by stalling tests.

Hypothetically you might fly an approach beside another Bearhawk to a parellal runway, both flying at exactly the same speed over the ground, side by side. Both aircraft are flying at exactly the same TAS, and the same groundspeed.

Your airspeed indicator might indicate your preferred approach speed (say 50 KIAS). The other pilot might be flying the exact same speed over the ground, side by side, yet their airspeed indicator might be showing an airspeed of 40 KIAS. Their aircraft might actually stall at 35 Indicated Airspeed (for their aircraft). That's position error. So long as both pilots fly their own speeds as derived during flight testing, no problem. Where it becomes an issue is if the other pilot hears that you're flying at 50 KIAS and suggests that it's much too fast. He flies his approaches at 40 KIAS. If you were to attempt to fly an approach at 40 KIAS on YOUR aircraft, clearly it wouldn't end well.

In some cases there can be 10-15 KIAS difference between aircraft of the same design. It's all about comparing apples with apples. Well, in this case get the idea.

The problem with comparing IAS with TAS on an EFIS

For STOL aircraft when flying at high AOA, it's particularly common to see very low IAS for a given TAS. However, many of the modern EFIS screens will display a TAS that is derived from the IAS, so there can be a corresponding error in the TAS at high AOA such that the low TAS appears to confirm a low IAS and making it appear as though there is no error.

One common method of determining position error is to fly on a day with very light winds and record the groundspeed over 3 legs, each 120º apart. It should be noted that this method is only accurate in absolute nil winds and that any small amount of wind will render the results inaccurate. The amount of error increases at low airspeeds - the ones we're really interested in testing.

This Kitplane Article explains everything in greater detail and has a link to a spreadsheet used by the National Test Pilots School. The advantage of this spreadsheet is that it utilizes GPS, is extremely accurate, and is relatively simple to perform.

NTPS Spreadsheet

Here is a spreadsheet (Excel and Numbers format) for calculating position error (in the air). It requires a GPS of some type such as a Garmin, Dynon, or iPad / iPhone with a Nav app.

You simply fly a constant HEADING, at a constant IAS for say 30 seconds. Then enter into the spreadsheet what your resulting GROUNDSPEED (Vg) is, and the resulting TRACK. Nothing will happen until you have all 4 entries. It's best (but not essential) to do the 4 headings 90° apart - 360, 090, 180, 270. Then read the average TAS at the bottom right yellow square (V True). The spreadsheet calculates Wind direction, Wind speed, and TAS. TAS is the one we're interested in, to compare to our IAS. It is normal to test for example an IAS of say 35 KIAS (just above the stall) and to find that the TAS is (say) 40 KTAS. This is position error. Once completed, you'll know what your TAS is at high AOA when the IAS reads very low.

You can repeat this for any IAS you wish to test. I did mine at 120kts, 100, 80, 60, 50, 40. But the ones we're usually interested in are the ones closer to the stall speed. I tested mine with Flaps 3 and Flaps 4 at 50KIAS and 40KIAS in the approach configuration.

Note: The STD DEV is used to validate the data. It was recommended that if the STD DEV is above 0.5, that test (for that IAS) be repeated. It usually indicates an error.

Position Error 4
Download XLSX • 161KB

Position Error 4
Download NUMBERS • 792KB


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