Pass/Fail criteria for pitot leak?

Kevin Horton · #1 05-18-2008, 12:08 PM

I'm looking for pass/fail criteria for a pitot leak. I've spent four hours today chasing a small pitot leak, and am starting to wonder whether maybe it is good enough as-is. It leaks a bit less than a knot a minute at 220 kt. I will eventually want to track this leak down and fix it, but maybe I can defer it until after I get flying.

I've searched the web, and can't find any authoritative info on acceptable pitot system leaks. It looks like it is up to each aircraft manufacturer to define pass/fail criteria. The one reference I found was in a service bulletin for a Robinson R-22 helicopter - they allow a leak of 10 kt a minute at 70 kt. Mine is much, much smaller than that.

Does anyone know what Cessna, Piper, Beech etc allow as an acceptable pitot leak?

Mel · #2 05-18-2008, 01:52 PM

My transponder guy is coming out tomorrow afternoon to do a couple of certifications. If you haven't heard anything by then, I'll ask him. He does a lot of spam cans and experimentals, both VFR & IFR.
Of course US & Canadian limits may differ.

Kevin Horton · #3 05-18-2008, 02:34 PM

Quote:

Originally Posted by Mel

Can you wait 'til tomorrow?

Yeah, I can wait several days, easily.

The good news is that I have lots of other stuff to do, so after I reached the point of total frustration with the pitot leak I moved onto tidying up the FWF wiring, etc. I've just arrived home from the hangar, and I'll be on the road all week, so my next work session won't be until next weekend.

The leak rate didn't change one iota, no matter which connections I tightened or resealed, so I haven't come close to finding it yet. When I do next attack this leak, I'll be armed with some soapy water. There is a chance the problem is actually on the water manometer end of things, or at the connection to the pitot tube, and maybe not in the aircraft at all. That would be very nice.

RV6_flyer · #4 05-18-2008, 02:36 PM

Kevin:

Not sure about airspeed leak but I do know altimeter.

FAR 43 Appendix E Altimeter System Test and Inspection.

FAR 23.1325 Static pressure system

This is the only thing that I could find about airspeed.
FAR 23.1323 Airspeed indicating system.

As a DAR, I have never checked to see if the airspeed indicator has been leak checked. Other than my initial pitot static check, I do not remember a leak check on the airspeed indicator at any time in the past 10+ years flying my RV.

Kevin Horton · #5 05-18-2008, 03:15 PM

I suspect that the lack of specific regulatory guidance is a good sign that it would take a very large pitot system leak to cause a significant airspeed system error. If you are at a constant speed, there is no air moving in a leak-free pitot system. The whole system is at the same pressure, which is equal to the total pressure sensed by the pitot tube. If there is an air leak, the air that is lost will be replaced by air flowing from the pitot tube towards the leak. Fluid flowing in a tube always incurs a pressure loss, so there will be a loss in pressure that is proportional to the size of the leak and the distance from the pitot tube to where the leak is. But, the rate of air flow due to a small leak like I have must be very small, thus the loss in pressure is probably insignificant.

If all else fails, I'll try to estimate the airspeed error as a function of leak rate, but for the moment I'll hope someone has some leak pass/fail criteria from a manufacturer that I can use.

Mel · #6 05-18-2008, 03:24 PM

There is one particular brand of ASI that has a tendency to leak around the glass. I believe it is "Aeromarine". The leak is so common that most instrument shops won't even certify them any more. However, having said that, I believe the common leak is on the static side.

David-aviator · #7 05-18-2008, 03:40 PM

Quote:

Originally Posted by Kevin Horton

I suspect that the lack of specific regulatory guidance is a good sign that it would take a very large pitot system leak to cause a significant airspeed system error. If you are at a constant speed, there is no air moving in a leak-free pitot system. The whole system is at the same pressure, which is equal to the total pressure sensed by the pitot tube. If there is an air leak, the air that is lost will be replaced by air flowing from the pitot tube towards the leak. Fluid flowing in a tube always incurs a pressure loss, so there will be a loss in pressure that is proportional to the size of the leak and the distance from the pitot tube to where the leak is. But, the rate of air flow due to a small leak like I have must be very small, thus the loss in pressure is probably insignificant.

If all else fails, I'll try to estimate the airspeed error as a function of leak rate, but for the moment I'll hope someone has some leak pass/fail criteria from a manufacturer that I can use.

There is an ASI error check, I had one done once. But it does not check for a leak, it simply compares the indicated air speed to that on the test equipment. In an unpressurized airplane, I don't think it is a big deal.

Kevin Horton · #8 05-18-2008, 05:37 PM

Quote:

Originally Posted by Mel

What "brand" is your ASI?

There is one particular brand of ASI that has a tendency to leak around the glass. I believe it is "Aeromarine". The leak is so common that most instrument shops won't even certify them any more. However, having said that, I believe the common leak is on the static side.

It is a United. It was completely leak-free when I checked my ASI and Dynon EFIS for ASI instrument error, back in 2005 (details). When I did the check in 2005 I had a line going direct from the water manometer to a T, and then to the ASI and EFIS. Now I have the whole pitot system installed, which adds a few more places for leaks.

Quote:

Originally Posted by David-aviator

There is an ASI error check, I had one done once. But it does not check for a leak, it simply compares the indicated air speed to that on the test equipment. In an unpressurized airplane, I don't think it is a big deal.

I was actually trying to recheck my EFIS ASI calibration today. I had checked it back in 2005, but the latest firmware has a "calibrate zero pressure" button, and I foolishly pressed it. Now I don't know if that changed the calibration curve I developed in 2005, so I wanted to check it again. But I noted the pitot leak, which would have made it impossible to get an accurate calibration using a water manometer.

az_gila · #9 05-18-2008, 07:08 PM

Quote:

Originally Posted by Kevin Horton

....
But I noted the pitot leak, which would have made it impossible to get an accurate calibration using a water manometer.

Kevin.... I know it's not totally desirable, but if you calibrate your pitot system with a leak in it, wouldn't your calibration correct for the leak?

I realize that the leak could change slightly due to outside factors, but if it really is a small as you think, the calibration procedure should correct for it if you put the water manometer right at the pitot hole...

gil A

John C · #10 05-18-2008, 07:41 PM

I have the AOA pitot probe that was added with my D-10. There is a drain in the pitot that you have to plug and there is some external leakage that we could see using soapy water. We dropped the probe from the mount and found a substantial leak around the potting. It was also leaking around the mounting screw holes. We used black RTV over the potting and in the screw holes. The guy doing the calibration did not mind minor leaks but most were fixed anyway.

My first calibrations at home did not reveal any leaks but the leaks at the pitot showed two years later.