Pitot Tube Drain Hole

How to keep the Cessna 170 flying and airworthy.

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wa4jr
Posts: 437
Joined: Mon Apr 29, 2002 2:44 am

Pitot Tube Drain Hole

Post by wa4jr »

I am having a devil of a time keeping moisture out of my pitot line. I've had it blown out once becuase of airspeed indication errors. Mechanic said quite a lot of water came out, and airspeed indications were then normal. I went to the practice of using a tight fitting plastic sleeve over the pitot tube now to seal out water instead of relying on the silly little flipper. My last flight in temps below freezing had me flying with no airspeed indication....well the needle started at the 12 Oclock position and as my airspeed increased the needle traveled counter clockwise to about the 10 Oclock position saying I was cruising along at 180 MPH! Moisture must have infiltrated the pitot line again and froze. Altimeter normal so static line is not a concern here. My question is can I drill a small hole in the bottom of the unheated pitot tube a few inches aft of the opening and prior to the rise up to the wing leading edge? With the airplane sitting on the ground, there is a perfect place for moisture to gather and either blow back into the pitot line or simply freeze. I know the heated tubes have a drain hole and they work just fine. Until I get a heated pitot tube installed, and a sump bottle as well, I need to do something to prevent the moisture from entering and/or drain it from the system. Ideas and comments guys? (gals?)
John, 2734C in Summit Point, WV
russfarris
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Joined: Wed May 15, 2002 2:25 am

Post by russfarris »

Hi John, I did exactly that a couple of years ago. I drilled a 1/16 hole in the bottom, at the low spot just prior to the bend. It didn't work 8O

Maybe the hole wasn't big enough, but I was reluctant to make it bigger because of the possible ram air loss to the airspeed indicator. Perhaps a little trial and error is called for here.

My water in the static system problem didn't go away until I installed the heated pitot and plastic sump bottle. The bottle went in first, and it was an improvement. It cost around 8.00 two years ago, from Van Bortel I believe. Try that first, it's cheap and easy.

Flying in heavy rain, the static instruments still occasionally jump. I'm not sure what's left to be done, except installing an alternate source... Russ Farris
All glory is fleeting...
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n3575d
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Pitot tube drain hole

Post by n3575d »

I had constant problems with water in the pitot-static system. The airplane is tied down outside so I decided to plug up all the openings. I put tape over the static port and on the front of the pitot tube and plastic plugs on the front of both venturis. That solved the problem. Pete
Pete Tufts
N1277D
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Pitot Tube

Post by N1277D »

One should not drill a small hole in the pitot tube. Drilling a small hole will lead to incorrect airspeed indications. The small plastic condensate bottle added to both the static and pitot tube should be the best fix for water/condenstate in the lines. The condensate bottles can be purchased for about $10.00 from Chief Aircraft.
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wa4jr
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Post by wa4jr »

So now the logical question is how can the heated pitot tube heads have a drain hole and still provide for accurate airspeed indications? Is the airspeed indicator itself calibrated in such a way as to compensate for the drain hole? I'm going into a hangar at long last in a couple of days so maybe this thing will be a moot point...but I will look into the installation of sump bottles...$9 from Spruce when the WX warms to above 15 degrees or so! Has anybody installed the heated pitot head kit from Spruce PN AN5812-12? I don't think the spearhead type tube contained in the Association drawing has a drain hole...but then maybe I didn't look close enough.
John, 2734C in Summit Point, WV
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Roesbery
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Post by Roesbery »

When we installed the wings on my current magic carpet the pitot was a bit bent; knowing we get around 150 inches of rain annually in this area, I bent the tube in a curve downward and then forward so that parked it has a slight downward tilt at the end. So far no water problems in the pitot and it's been 4 or 5 years. Indicated airspeed has not been affected.
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bgiesbrecht
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Re: Pitot Tube Drain Hole

Post by bgiesbrecht »

I'm resurrecting this thread after almost two decades :D See? Some of us do use the search function...

Anyway, I am having the same issue in my 170. I park outside and after every rain (thankfully not TOO often in Socal but often enough in winter...), I can pretty much guarantee water in the lines. If I tap the static port with my finger, I notice water is extracted out with my finger. Sounds like I need to cover the pitot tube (i.e. the little flap doesn't do anything to stop water...?) and the static port. Does water get in via the venturi tubes as well?

It's always fun taking off and ASI jumps from ~40mph on climbout to 80 when the system clears itself.
Former owner of:
1953 170B
N1977C
s/n 26122
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GAHorn
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Re: Pitot Tube Drain Hole

Post by GAHorn »

The static port is not connected to the pitot tube in any direct way…so no transfer of water is possible between them. Chances are any water you observe at the static port is only at the surface due to capillary action and is unimportant.

The Original pitot tube (unheated) was a 1/4” aluminum tube with a specified shape (illustrated in the Service Manual) that meets the relative-wind straight-on in level, cruise flight. The accuracy of the indication is not particularly sensitive but the tube should be formed per the Service Manual illustration.

When the airplane is tied-down outdoors and a heavy rain occurs…the nose-up/3-point-stance of the airplane may allow water to enter into the lower curve of the tube. Then when the airplane accelerates for takeoff the water will “bubble” or “percolate” in that tube until relative wind eventually dries it out. The water does not actually travel all the way into the wing plumbing, or down the leading edge or get to the airspeed indicator…. the erratic needle movement of the indicator is only due to varying air-pressure playing with that water inside the tube in front of the leading edge. Eventually it will dissipate as the water dries out.

The small aluminum “flapper” pitot-cover is only intended to discourage insects.

If you store your airplane outdoors and this occurs frequently enough to be bothersome for you….. take a piece of aluminum foil and loosely wrap it around the end of the tube…. not air-tight…only loosely to create a “tent” to prevent water ingress.
Don’t forget to remove it next preflight inspection. :wink:
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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bgiesbrecht
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Re: Pitot Tube Drain Hole

Post by bgiesbrecht »

GAHorn wrote:The static port is not connected to the pitot tube in any direct way…so no transfer of water is possible between them. Chances are any water you observe at the static port is only at the surface due to capillary action and is unimportant.

The Original pitot tube (unheated) was a 1/4” aluminum tube with a specified shape (illustrated in the Service Manual) that meets the relative-wind straight-on in level, cruise flight. The accuracy of the indication is not particularly sensitive but the tube should be formed per the Service Manual illustration.

When the airplane is tied-down outdoors and a heavy rain occurs…the nose-up/3-point-stance of the airplane may allow water to enter into the lower curve of the tube. Then when the airplane accelerates for takeoff the water will “bubble” or “percolate” in that tube until relative wind eventually dries it out. The water does not actually travel all the way into the wing plumbing, or down the leading edge or get to the airspeed indicator…. the erratic needle movement of the indicator is only due to varying air-pressure playing with that water inside the tube in front of the leading edge. Eventually it will dissipate as the water dries out.

The small aluminum “flapper” pitot-cover is only intended to discourage insects.

If you store your airplane outdoors and this occurs frequently enough to be bothersome for you….. take a piece of aluminum foil and loosely wrap it around the end of the tube…. not air-tight…only loosely to create a “tent” to prevent water ingress.
Don’t forget to remove it next preflight inspection. :wink:
Thanks, this is great information. I do have the original shaped aluminum tube (and I have shaped per the available template here as well). Maybe I'll just fabricate some type of cover that won't damage the flapper.

EDIT: looks like something like this could work. It's a plug instead of a cover - designed not to pressurize the system when inserting the plug either. Review mentions a C140 user that likes it. Might give it a whirl, I can always return it.

https://www.aircraftspruce.com/pages/ps ... ights3.php
Former owner of:
1953 170B
N1977C
s/n 26122
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GAHorn
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Re: Pitot Tube Drain Hole

Post by GAHorn »

Yes…keep in mind that the little “flapper” type of pitot cover is “aftermarket” …and that is what is contributing to the problem. A cap or plug would not introduce water. (As late as the B-model…no pitot “flapper” was depicted as OEM.
3AFD60EE-8D70-4448-A153-F1663A994D77.png
79F8AC92-79E0-4859-B87C-524D7978CF38.png
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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bgiesbrecht
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Joined: Wed Feb 13, 2019 10:51 pm

Re: Pitot Tube Drain Hole

Post by bgiesbrecht »

Referencing my earlier post, I bought this:

https://www.aircraftspruce.com/pages/ps ... ights3.php

and happy to report it works perfectly!! There are 3 pitot cover sizes included and the smallest one fits. There are holes in the cover such that you won't pressurize the system by putting the cover on. Materials/construction appears high quality and durable.

Looking forward to not having my ASI/VSI jump around on takeoff/climbout...
Former owner of:
1953 170B
N1977C
s/n 26122
hilltop170
Posts: 3481
Joined: Sat May 06, 2006 6:05 pm

Re: Pitot Tube Drain Hole

Post by hilltop170 »

I had the same thing happen to my pitot, water collecting in it after every rain. What I did about 45 years ago was to gently bend the pitot tube down along its full length, so as not to kink it, until the end was pointing ever so slightly down where it would drain in the 3-point attitude. I did not notice any change in indicated airspeed and it never gurgled again.
Richard Pulley
2014-2016 TIC170A Past President
1951 170A, N1715D, s/n 20158, O-300D
Owned from 1973 to 1984.
Bought again in 2006 after 22 years.
It's not for sale!
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