[Jamie]

Way back when I was building my wings I decided to look for a good used pitot from an aircraft salvage yard rather than pay > $1000 for what amounts to nothing more than a tube with a heater element in it.

I called up a couple of salvage yards and found a good pitot off of a 1976 Skyhawk. I had read that the Gretz mount would work fine with this pitot so I bought it ($80.00!). A little filing and it fit perfectly.

Unfortunately, from my first flight I have been having IAS problems -- my IAS was typically 8 knots below where it should have been. I have two airspeed indicators (mechanical unit from Van's and my Dynon) and they read exactly the same airspeed. I did all of the usual debugging on this issue, including checking for static errors and pressure testing both pitot and static systems. In the course of my testing I found a small pitot leak on the back of my Dynon but that was easily remedied with the application of teflon tape to the threads. Alas, this still didn't fix my problem.

Since I had no pitot system leaks and no static position error or leaks it only left one thing -- an error with the pitot itself.

I borrowed a Gretz Pitot from a builder friend of mine (Steve Ashby, RV-8A builder, VAF forum member and one of the nicest guys you will ever meet) which fit right into my mast and flew it. Airspeed indication was definately up so I knew I was on the right track.

Back on the ground I re-installed my pitot. On the Cessna pitot there is a little drain hole on the bottom/back of the unit that allows the line to drain if the pitot freezes over (meaning that it will drop to 0 on the ASI). For my pressure testing I taped over this hole (obviously) but would remove the tape to go and fly.

Last night I decided to tape over that hole and fly. Voila...everything was perfect. I did 4-way GPS runs and entered the numbers into the GPS PEC spreadsheet (everyone should use this document for airspeed testing). My TAS on the Dynon was within 1 knot of the value calculated by the GPS PEC spreadsheet.

I will mix up some epoxy and plug that hole. Of all of the other heated pitots I have seen, none have a drain hole like the one on the Cessna pitot, so plugging that hole will give me the same safety level as those other pitots.

I had suspected the drain hole from the start, but everyone was telling me that couldn't be the problem. I should have gone with my instincts from the start.

My assumption is that Cessna airspeed indicators are calibrated to account for this hole. It seems to calibrate the ASI on the ground one would need to be able to provide a constant pressure to the pitot which would obviously leak air out of the drain hole.

I know that by searching the archives on VAF there are a lot of people who are using Cessna Pitots. I just can't imagine that no one else has seen this problem. After searching the archives to see if the drain hole was an issue, I don't think anyone else has ever posted about this issue. This is the primary reason for my post -- to get in the archives to hopefully save someone else the trouble of all that debugging.

So, has anyone else seen this problem?

 

[DGlaeser]

I've got one too...

I got my Cessna pitot on Ebay a few years ago for $125 + shipping (should've gone to a junk yard ). I've only got 4 flights so far, but my ASI and GPS match very well.
How big is your drain hole? Mine is very very small. I haven't measured it. I think it is for when you fly in rain more than anything else. The hole is so small I don't thing water would drain unless it were under pressure (from forward flight). I will test with it taped over and see if I find a difference.

 

[Jamie]

My drain hole is tiny as well, probably 1/32 in diameter max, but enough that if you blow through the pitot (with it disconnected from the pitot line of course!) you can certainly feel air coming out of the bottom.

 

[Kevin Horton]

I had suspected the drain hole from the start, but everyone was telling me that couldn't be the problem. I should have gone with my instincts from the start.

My assumption is that Cessna airspeed indicators are calibrated to account for this hole. It seems to calibrate the ASI on the ground one would need to be able to provide a constant pressure to the pitot which would obviously leak air out of the drain hole.

Look at a catalog from ACS, Wicks, etc. You don't see ASIs that are sold as only for installation on Cessnas, because serviceable Cessna pitot tubes put out the same pressure as any other serviceable pitot tube. There is a TSO for ASIs, and it lays out the required ASI reading vs pressure. An ASI is an ASI.

In principle, the drain hole is supposed to be very small, and designed so it gets pressure from quite close to the pitot tube opening. In this case, the tiny amount of air that escapes through the hole is quickly replaced by air coming in the front of the pitot tube, and the hole has no practical effect on the pressure in the pitot line. Perhaps your pitot tube had some sort of issue, such that the drain hole was larger than normal. Maybe someone ran a drill through it for some stupid reason.

I will mix up some epoxy and plug that hole. Of all of the other heated pitots I have seen, none have a drain hole like the one on the Cessna pitot, so plugging that hole will give me the same safety level as those other pitots.

I would be a bit cautious. Those drain holes are there for a reason. If you fly in rain, it is possible to get water in the pitot system. If that water cannot escape through the drain hole in the pitot tube, it is possible that it will later freeze in the pitot system.

I know that by searching the archives on VAF there are a lot of people who are using Cessna Pitots. I just can't imagine that no one else has seen this problem. After searching the archives to see if the drain hole was an issue, I don't think anyone else has ever posted about this issue.

Please consider the possiblity that no one else has had this problem because the problem should not exist if the Cessna pitot tube is serviceable. Maybe the junk yard sold you a piece of junk.

 

[Jamie]

Look at a catalog from ACS, Wicks, etc. You don't see ASIs that are sold as only for installation on Cessnas, because serviceable Cessna pitot tubes put out the same pressure as any other serviceable pitot tube. There is a TSO for ASIs, and it lays out the required ASI reading vs pressure. An ASI is an ASI.

Yes, I would generally agree with this statement and it is consistent with everything I have read about this issue. BUT.....there is a reason ASIs have little calibration screws, right? My airspeed indicators would work fine with the pitot WITH the hole if I simply calibrated out the error.

In principle, the drain hole is supposed to be very small, and designed so it gets pressure from quite close to the pitot tube opening. In this case, the tiny amount of air that escapes through the hole is quickly replaced by air coming in the front of the pitot tube, and the hole has no practical effect on the pressure in the pitot line. Perhaps your pitot tube had some sort of issue, such that the drain hole was larger than normal. Maybe someone ran a drill through it for some stupid reason.

Is this from experience? The hole in my pitot is tiny. This is what others have told me and it was the assumption I was working with the entire time. Obviously plugging the hole increased the pressure in the line, so I don't know what to make of this.

Maybe this isn't an issue with the pitot at Cessna speeds? Perhaps I should do a speed run at 110kts IAS and see how off it is.

I would be a bit cautious. Those drain holes are there for a reason. If you fly in rain, it is possible to get water in the pitot system. If that water cannot escape through the drain hole in the pitot tube, it is possible that it will later freeze in the pitot system.

Hmm...sort of like the stock setup with the bent tube? Or the Gretz Pitot? Or the ANXXXX pitots? None of them have drain holes (that I have seen). I don't see how I could be worse off than them.

Please consider the possiblity that no one else has had this problem because the problem should not exist if the Cessna pitot tube is serviceable. Maybe the junk yard sold you a piece of junk.

Maybe others are just not as anal as me about correcting airspeed errors?

FWIF, the pitot was yellow tagged (I know, I know...doesn't mean a whole lot). Again, it's just a tube with a heater element in it...and comparing the hole in my pitot vs. the pitot drain hole in a Skyhawk sitting on the ramp there is no visible difference. I just don't see how the pitot could be 'defective'.

The only other culprit I can think of is on the Cessna the pitot is mounted very close to the wing, whereas my mast places my pitot down quite a bit. I'm not an aeronautical engineer, but I do know that the pressure under the wing varies with respect to distance from the bottom of the wing.

 

[PaulR]

Pitot Hole

For what it's worth, the Dynon pitots have holes for drainage in them. I've never heard of it being a problem with them.

 

[Jamie]

After doing considerable reading on this topic last night I decided the drain hole is very important for IFR work. There are actually several pretty scary incidents related to pitot drains being clogged, including a couple of very dangerous near catastrophes in airliners and business jets. These incidents have led to several ADs requiring periodic checks to ensure the drains are cleared. I also found at least one accident report where the pitot drain being clogged by a bug was cited as a contributing factor to an accident.

I'll definately look for a better option when I do my planned IFR panel upgrade. For VFR work I should be fine for now.

Thanks for the comments, guys.

 

[Kevin Horton]

The only other culprit I can think of is on the Cessna the pitot is mounted very close to the wing, whereas my mast places my pitot down quite a bit. I'm not an aeronautical engineer, but I do know that the pressure under the wing varies with respect to distance from the bottom of the wing.

Yes, the static pressure will vary with location. But the total pressure will not vary, as long as you are far enough from the wing to be out of the boundary layer. The pitot tube measures total pressure, so unlike a static port, the accuracy of a pitot tube does not vary with location (assuming you don't put it in the wake of something, in the prop wash, in the boundary layer, or have it misaligned with the local air flow by more than 15 degrees or so).

 

[SkywayCaptain]

I have a Cessna type pitot on my RV. It is mounted much farther away from the wing than on a Cessna and it is very accurate. I wonder if you can take your pitot back to the junkyard and exchange it.