ASI calibration Q

[IO540]

I have just had my ASI checked and at 140kt indicated (TB20) it was reported as reading 5kt high.

This doesn't seem right, relative to fuel flow and previous performance over the 5 years I've had the plane.

Today I flew the GPS cal method, where you fly at 120 deg apart headings, 3 legs, and plug the ground speeds into a formula, and the error is about 1kt.

I suppose my Q is how does the standard ASI calibrator work? It is just a pipe which fits over the pitot tube, and uses static pressure only. How can it be accurate? The GPS method is definitely accurate, yet its result is 4kt (~ 3%) different.

['India-Mike]

IO540 - how high did you fly for the calibration check? Averaged GPS will give you a TAS. Need to apply the squart root of the relative densities....
I would have thought that if your averaged GPS reading was equal to your ASI at altitude, then your ASI over-reads....
(I know it's not the answer to your question, viz how does a calibrator work, but might be useful to think about)

Thinks - does your formula include altitude?

[IO540]

The formula I used was

TAS and windspeed from three (GPS) groundspeeds.
Determine your groundspeed on three headings that differ by 120 degrees (eg 40, 160 and 280 degrees), call these v1, v2 and v3
Let
vms = (v1^2 + v2^2 + v3^2)/3
a1= v1^2/vms -1
a2= v2^2/vms -1
a3= v3^2/vms -1
mu= (a1^2 + a2^2 + a3^2)/6

Let bp and bm be the roots of the quadratic b^2 -b + mu =0
ie:
bp= 1/2 +sqrt(1/4-mu)
bm= mu/bp

The TAS and windspeed are then given by sqrt(vms*bp) and sqrt(vms*bm) provided that the TAS exceeds the windspeed. If this is not the case, the roots are exchanged.

I flew the three headings at a constant 140kt IAS, at 4200ft, 1024QNH, +10C OAT. The 3 ground speeds were 153, 140, 147kt.

The calculation yielded 146kt and that is TAS. Correcting this back to IAS for the pressure altitude and OAT (using the Jepp CR-5 circular calculator) gives about 142kt IAS.

Furthermore, it is obvious that for low wind speeds (i.e. not significant relative to TAS) the above formula reduces to

TAS = (V1+V2+V3)/3

i.e. the average of the three speeds, and this was indeed the case yesterday, with winds aloft being somewhere around 10kt. Calculating that gives 146.6kt TAS which is v. close.

I am not doubting the formula; it is obviously right and I have two GPSs telling me the GS: a KLN94 and a G496.

What I do query is the conventional method of ASI calibration which uses the "box" with a static pressure generator inside it and which obviously assumes that all pitot tubes have the same properties.

['India-Mike]

Yep, main thing's the density correction, and it's obviously included. One might conclude your ASI's ok!

Never used a calibration test kit, so I'm afraid I'm of no help there.

[Two_Squirrels]

All the calibration kit does is measure the INSTRUMENT error. The GPS method is actually attempting to measure the PRESSURE error, or the POSITION error which is caused by the relative pressure around the pitot and the static ports. I obviously don't know what your plane is, but assuming that it is a fixed wing, then the chances are is that your pitot (or total) pressure error is zero, and that all the pressure error is coming from the static error. It is possible therefore that the pressure error is the same magnitude, but in the opposite sense to your intruemnt error and is thereofre cancelling it out. I suspect the real reason though, is that you cannot really just use one tets point to make a conclusion. The pressure error will probably vary with airspeed, and you really need to cover a wide range of airspeeds to measure the errors properly. In addition, only using one test point, especially with the GPS method, may not be entirely accurate, as it assumes that the weather conditions do not change throughout the runs, and that you have been able to maintain your test airspeed and heading sufficiently correctly throughout.

[IO540]

The GPS method is obviously measuring the total system error. GPS ground speed measurement is highly accurate; much more accurate than position measurement.

It's easy enough, on a nice day like yesterday, especially doing it over the ocean and on autopilot, to fly at a constant and accurate IAS, and fly constant headings.

The effect of flying a heading that is very flightly off (say a few degrees) has a negligible effect when using the three-headings method.

The thing which puzzles me, not understanding pitot tube physics much, is how one can just stick a plastic hose to the front of a pitot tube (and another one to the static vents) and declare the ASI has such and such error.

The aircraft was a TB20.

[Genghis the Engineer]

When a new aeroplane design is certified (I'm including pretty much everything except for homebuilts and microlights here) you usually need to determined PPEC (Pitot PEC) and SPEC (Static PEC) so as to determine airspeed measurement error (which is a function of TPEC - Total PEC = PPEC+SPEC) and altitude measurement error (which is a function of SPEC).

Errors then fall into two parts - the overall system error (which is generally a function of the shape of the aeroplane) and the individual instrument errors.

Looking at the airspeed measurement error - this is down to TPEC (which lends itself well to being measured by a GPS method, of which the triangle method is one of 4 that I'm aware of). But, at the same time, instrument errors apply. The ground test set only measures instrument error.

If you look in the POH, you'll find a graph of airspeed measurement error - which will have had to comply with JAR-23 requirements that over the range 1.3Vs to Vne the maximum error may not exceed the greater of 5knots or 5% (so it could be 10 knots out at 200kn and be quite legal).

So, all being well, what you see in the cockpit is the sum of those errors (which are specific to type) and the errors in the instrument (which are individual to that gauge and were measured by that test set).

All being well, then IO540 what you are seeing in your GPS based calibration is the sum of that 5kn instrument error and presumably a 4-6 knot underread due to PEC, which you should see in the POH.

G

[IO540]

The TB20 POH, under Airspeed Calibration, gives

120kt: IAS=120 CAS=120.5
150kt: IAS=150 CAS=151

This doesn't appear to support a -5kt correction factor.

The avionics shop used a big yellow DRUCK box for the test, which appears to be fairly standard.

[Genghis the Engineer]

In which case one or more of the following is likely to contain an error:

(1) The test set
(2) The static
(3) The pitot
(4) The relationship between actual OAT and your estimate / measurement of OAT in determining density altitude.
(5) Your maths
(6) The accuracy of your airspeed maintenance / measurement.


To be frank, given that lot, I'd consider myself pretty fortunate to be within 5 knots of where I should be on a fairly quick and dirty single point flight test.

I probably sounds like I'm shrugging my shoulders. Plead guilty as charged m'lord.

G

[IO540]

1) very likely IMHO
2) and 3) these are just pipes, and the test set connects to them, and injects some static pressure in them, so a leak there could explain it during the test
4) and 5) can be checked from what I posted
6) it was spot on, carefully avoiding parallax on the indicator :)

It does sound like something to do with the ASI test setup.

[Two_Squirrels]

I would be very careful about saying the test set was broken on your evidence. I have done a quick calculation, and taking into account a 2kt error in maintaining airspeed, a 2 kt variation in GPS ground speed, a 100 ft error in measuring/maintaining altitude, 2° error in temperature gives a measurment error of nearly 3 knots. These figures are based on real data, (although I have added a bit on to the altitude and airspeed taking into account you are not a test pilot. Sorry :) ) I have not attempted to look at the effect of heading, or the variation in wind conditions. The maths will take too long. As I said before, you cannot make a judgement on one test point. You need to at least cover the entire speed range to see what the trend is. Ideally, you need to repeat the tests several times.

[IO540]

Fair enough :) but I can deal with some of this right away: the autopilot holds altitude to 10ft, the OAT gauge has been checked with a PT100 (platinum sensor) reference thermometer accurate (and certified to) +/-0.1C but it does display only whole digits so could vary by as much as 1C without it showing, the GPS GS is accurate to better than 1kt, the IAS instrument can be read to about 0.5kt resolution. Heading accuracy is within a few degrees (on autopilot).

I will more tests and report.

[Two_Squirrels]

Fair enough :) However, (there is always a 'however'!) you are talking about reading accuracy and instrument error. In addiiton there is also, for want of a better word, experimental error, which is effectively how accurate the point can be flown in the given conditions. Again, based on experiecne with test pilots, the GPS variation during a given run was between 1-2 knots, and the airspeed variation was ±1 knot. All this was captured on flight test instrumentation, which gives a better picture tha just a simple snapshot which you have probably done. Please note I am decrying your attempts, at both flying skilss and anlaytical skills, I am just trying to pass on some some real life experience.

[Pilot DAR]

Hello IO540,

To be properly calibrated, the airspeed indicator would have to be removed from the aircraft. The means of calibrating the airspeed indicator would probably involve direct connection to a meriam fluid (an oil with a density more precise than, but otherwise the same as water) manometer. This means is able to provide calibration accuracy well within one knot in the speed range you mention.

Due to the design of the mechanism inside the instrument, it is possible for there to be either one or two points along scale which are right on, and all others a little out. Depending how far out (knot or two), a correction card might have been provided. More error than that should be adjusted out.

If the process was accomplished with the instrument in the panel, there would have been several extra opportunities to introduce errors (though I would have expected them to be the other way).

It is also necessary to vibrate the instrument continuously during this process. The normal forces within the mechanism are so low that it is easy to have a many knot error due to internal friction (even in a perfect condition instrument). The most effective way to vibrate the instrument is to gently hold the blade of a screwdriver in your fingers, rolling it back and forth, while letting the plastic handle bounce off the top of the instrument case.

I spent 5 hours of test flying a deHavilland DHC-2 Beaver, following the relocation of the engine, to verify the static error had not changed (which would affect the accuracy of the airspeed). I tried the GPS method, and out of frustration gave up, and did 10 zero wind timed miles instead - that worked better. There was no change anyway.

I'm sure the GPS method works with precise flying and patience (as it was recommended to me by Transport Canada Flight Test department), but with the client watching the mounting bill for flying hours, the simpler method answered the question faster!

Cheers, Pilot DAR

[Genghis the Engineer]

I'm sure the GPS method works with precise flying and patience (as it was recommended to me by Transport Canada Flight Test department), but with the client watching the mounting bill for flying hours, the simpler method answered the question faster!


It's worth mentioning that there is not "a GPS method", there are several, and all require good understanding of the maths to make them work well.

Either the May or June issue of Aeronautical Journal will have a paper that goes into this in considerable depth and may be worth interested flight testers reading. :8

G

[IO540]

A very good point about the vibration... it is necessary to overcome stiction within the instrument. Same with a vertical card compass I have.

The instrument was certainly not vibrated during that test. But then neither were the altimeters, which are described as "sensitive altimeters"; the operating ceiling is 20,000ft.

[Pilot DAR]

Hi IO540,

The term "sensitive altimeter" describes an altimeter whose most precise increment of indiction would be less than 100 feet (usually 20 feet). It would be unlikely that pilots of "modern" certified aircraft would have ever flown with anything else. I did fly an Ercoupe many years ago which had an "altimeter". It's most precise pointer would indicate an altitude change of two thousand feet per one revolution of the pointer - was that ever confusing! I believe now minimum equipment regulations for aircraft, require the installation of a sensitive altimeter, which excludes the use of any other type for primary altitude information.

A sensitive altimeter is even more in need of vibration during calibration, as it has a mechanism somewhat similar to ht of an airspeed indicator, but also a reduction gearbox which drives the thousands and tens of thousands pointers from the primary pointer. It's a lot of mechanical drag to overcome with only atmospheric pressure driving everything.

Cheers, Pilot DAR