IAS accuracy

[Veetail88]

Seeking knowledge and wisdom regarding the importance of indicated airspeed accuracy.

I recently installed a heated pitot on my 8 having flown it for the past year and a half with the standard Vans bent aluminum tube. During phase I testing, I verified it?s accuracy with the usual GPS triangulation method and established stall speeds and whatnot.

I fly airspeed in the pattern and my normal speeds have been 90 downwind, 80 crosswind and 70 on final and this has been working out fine, although probably a bit faster than required, but I?m good with that. I also have AOA, and in the pattern I check it here and there if I feel I?m getting a bit slow or if I?m cranking a turn to final to avoid overshooting, a scenario I keep a close eye on. The AOA (AFS flavor) is calibrated a bit conservative, so I don?t get the aural warning ?angle angle push push? in normal patterns until I?m actually right about in the flare. Of course the chevrons give me visual feedback along the entire range but the aural part comes in when I still have maybe 4 red chevrons left.

Due to the length of the pitot mast I have, the distance from the bottom of the wing to the center of the pitot inlet is shorter, like around 4 inches, and if I recall the standard one is 6 or more. I figured I?d install it with what I had, fly it to see how things were and adjust as required. Feeling safe with this approach since the AOA is present.
I?ve only had time to make a couple of flights with it so far, but those had purpose and I didn?t have time to run it through a stall sequence. It appears the pitot reads accurately at higher speeds, but when the angle of attack increases I?m pretty certain it?s reading higher than the old one, probably quite a bit. On downwind at 90 knots, I?m getting the AOA aural warning, so of course I carried quite a bit extra indicated air speed to keep her quiet and me safe. I?ve been using the AOA as a backup check so I never actually tracked its indications during normal pattern work otherwise.

I?ll do more testing when I can get to it, but my real question here is, does it really matter if it?s actually right? If it?s consistent and I fly the new numbers instead I don?t really see a problem with it, other than when I let another RV guy fly the airplane and he might expect the more normal numbers.
With multiple GPS units on board, I really don?t see any use for IAS en route. I get ground speed and time to waypoint and that?s what really matters for trip and fuel planning. Am I missing some point as to the need for accuracy here other than just to be correct? Like I said, I?ll likely order a new mast and redo the installation, I?m just pondering?..

[Raymo]

I recently installed the Dynon heated pitot tube (not flying yet) but it said that the mast should be at least 6 inches long for accurate readings.

As for the importance of inaccuracy, it may have a negative affect on other calculations that depend on it, such as wind speed. I believe ADS-B uses GPS data for speed, not IAS.

[krw5927]

One pretty important reason to verify TAS (and thus IAS) accuracy during a test program is to verify that the static system is designed, installed, and operating properly. Usually there will be more potential for error in the static system than the pitot. (Remember that your airspeed indicator requires both dynamic (pitot) and static pressure.)

If the static system has gross errors, then not only will your airspeed be off, but more importantly your altitude will as well. That could mean a world of difference when shooting that VOR approach to minimums.

[RV10inOz]

Double check you have the pitot and AoA lines connected to the correct orrifice.

Do not assume anything........I have seen this before when test flying another persons RV7. We wondered why things were weird in IAS and AoA.

No amount of me describing the evidence would convince the builder, until we started testing which hole was pressurising the ASI.

[ChiefPilot]

Originally Posted by krw5927

If the static system has gross errors, then not only will your airspeed be off, but more importantly your altitude will as well. That could mean a world of difference when shooting that VOR approach to minimums.

Such an error would be caught by the IFR Pitot/Static/Transponder check so unless you're doing the approach without the required certifications/tests I think this is a remote possibility.

[Veetail88]

Thanks for the responses folks, but to be clear, I only changed the pitot. Didn't mess with any other tubes or fittings and there's no reason to think there's anything wrong with any other component. My static line and AOA tubing are entirely separate.

[Walt]

Originally Posted by krw5927 View Post

If the static system has gross errors, then not only will your airspeed be off, but more importantly your altitude will as well. That could mean a world of difference when shooting that VOR approach to minimums.

Originally Posted by ChiefPilot

Such an error would be caught by the IFR Pitot/Static/Transponder check so unless you're doing the approach without the required certifications/tests I think this is a remote possibility.

Not really true, the static system check on an "IFR" cert is merely a system leak check, this has nothing to do with the dynamics of the static system during flight. Airspeed/altitude errors from misplaced or non-standard static ports cannot be checked during ground testing.

[ChiefPilot]

Originally Posted by Walt

Not really true, the static system check on an "IFR" cert is merely a system leak check, this has nothing to do with the dynamics of the static system during flight. Airspeed/altitude errors from misplaced or non-standard static ports cannot be checked during ground testing.

The key point is gross errors. An error which indicates 50' high or low - nope, might not be caught. A static error that results in a 1000' deviation for test altitudes of (for example) 5000' MSL would be problematic and I'd submit that if the pitot/static check didn't catch this then there is no point in doing one.

[Walt]

Originally Posted by ChiefPilot

The key point is gross errors. An error which indicates 50' high or low - nope, might not be caught. A static error that results in a 1000' deviation for test altitudes of (for example) 5000' MSL would be problematic and I'd submit that if the pitot/static check didn't catch this then there is no point in doing one.

Maybe we are talking about 2 different things, an IFR cert indeed checks the accuracy of the altimeter plus static system integrity. The FAR's "assume" that the static system design criteria have already been met by the manufacturer.

The OP was asking about indicated airspeed errors, airspeed errors can be introduced thru the static system but those type errors (generally static port installation) cannot be checked on the ground.

[GalinHdz]

IMHO I think people are saying the same thing but with different baselines. There is only one way to insure everything is correct in an EAB aircraft.

1 - Have a complete pitot/static ground check to include system leaks.
2 - Do an in flight verification of IAS using the GPS method.
3 - Adjust pitot/static locations if needed until all indications are correct.