Pitot Tube System help

[juredd1]

Looking for some advice on my pitot tube system and air speed indicator.

I have dreaded a failure in the system ever since I started flying.

This past Monday morning with just over 25 hours in 28C it happened. Thus far I can only blame myself for the issue as I would not really consider it a failure since I failed to put the cover on the pitot tube. I check the hole before flying and could not see any obstructions this time, the only other time that I left the cover off ( On the 152) it was obvious I had a problem, something had filled it in with dirt.

So seeing nothing in the hole I went flying. It was registering around 45 or so as I was rotating for takeoff so there were no big alarms going off in my head and then it didn’t move. I felt like I was picking up speed but the instrument was telling me different. Finally realized it had a problem and thought oh boy what now.

My uncle, who just happened to be with me, has always told me you need to learn how to fly the wing but I wasn’t ready to be put in a position to fly the wing just yet. So we flew for a bit and then it was time to come down. I just did the best I could, lined it up and drove her home. I ended up coming in a bit fast but all went well.

My apologizes for the long stories as you can tell I like to provide some detail to some of my questions. So on to the question.

Is there a safe way to test the pitot tube system before flying? I asked my Uncle and he said that some old timers had told him when he was a young pilot that if you blow in it that it will tear things up but neither could figure out why that would be an issue.

I did stick some safety wire in the whole to attempt to clean it out if there was something beyond sight. Maybe that was a bad thing to do but it’s done so you can let me know if I should not do that in the future. Anyway I did get a very small amount of debris out of the tube. Just not sure that was my problem or if I could have pushed my problem all the way to the airspeed indicator.

Thanks for any advice you can offer.

Justin

[voorheesh]

From your description, it is hard to diagnose what went wrong with your pitot system. It sounds like you are saying the airspeed indicator hung up at 45 mph. Is that what happened? Is that the highest it would indicate? If so, you may have contamination or a leak somewhere in the tube and this is blocking air from getting to your airspeed indicator. Your mechanic should be able to troubleshoot this problem and you may need to find a radio shop that works on pitot/static systems if a repair involving disassembly of the system is necessary. The next time you fly with an instructor or experienced pilot friend, cover up your instruments and practice flying by visual reference, sound, and feel. This is good practice in any aircraft and will get you more comfortable if this ever happens again. Your Uncle is right when he says "fly the wing". Good job getting it back down in one piece!

[GAHorn]

Yep. Level flight attitude....with 2500 RPM should be about 100-110 mph.
Level attitude with 1500 RPM should be 100 in a descent.
Level attitude with 1500 and 30 or 40 degrees flaps...should be about 60 mph.
Full power, no flaps, with nose just below horizon should be 70-80 mph in a climb.

These numbers should keep you safe in a 170.

An insect only needs a few minutes to plug up an uncovered pitot tube.
Get somone qualified to fix this for you, as it appears you may not be equipped to do this yourself. (Proper repair should probably involve disconnection of the system at the instrument and blow compressed air backward out the tube, and a check of the instrument also, as well as a check of the static-side of the system. Did you know that a plugged STATIC side of the instrument can indicate just like you described? No blockage of the pitot tube may be the case at all...it could be an instrument failure or a static system/side failure.)

[Bruce Fenstermacher]

What ever you do, you do not want to blow into the pitot. You can break your asi that way. No way to really test it before flight. You need to get real comfortable flying and not looking at your asi.

How sensitive is a asi? If you placed your thumb over the inlet of the gauge, then pushed in. The deflection of you skinn should move the gauge. Don't know if that will work from the pitot.

[cessna170bdriver]

You can safely blow TOWARD the pitot tube as long as you don't wrap your lips around it and try to blow it up like a balloon. You can generate about 1psi static pressure with your mouth, equivalent to about 240 mph when applied to a pitot tube. NOT good for our 160 mph ASI's.

[bagarre]

Yep. Level flight attitude....with 2500 RPM should be about 100-110 mph.
Level attitude with 1500 RPM should be 100 in a descent.
Level attitude with 1500 and 30 or 40 degrees flaps...should be about 60 mph.
Full power, no flaps, with nose just below horizon should be 70-80 mph in a climb.

Good data to write down on a card and stick in your glove box.

I lost my static system shortly after buying 81D. Got some water in the line (long story)
I was very grateful that my tailwheel instructor had me do quite a bit of pattern work with everything covered up but oil pressure. It makes you very comfortable flying the airplane.

[GAHorn]

A method which may be used safetly to test the pitot system for leaks is to slip a clear 1/4" vinyl tube of about 18" in length over the pitot tube entrance. Pinch the end of the tube, and slowly roll it up ...upon itself...thereby developing pitot-pressure, while someone inside the cockpit observes the ASI response. Do not exceed 2/3-scale (about 120 mph) or you risk imparting a "hysteresis" or stretch to the capsul.

If you don't have a helper....then use a 12-foot long tube and observe the ASI yourself.

[juredd1]

Thanks to everyone for the responses. I apologize for the lack of response but the last two days have been hectic in my home. My wife left town today for several days and I had my hands full with to kids under 4. So they are in the bed so I had a few minutes before I hit for a 4am early rise to work.

After a bit of flying that morning the ASI did end up getting to around 60mph I think. Not much over that but for the initial speed was round 40-45mph. I don't recall exactly how long before it bumped up to 60 but it was definitely after we got up to a good level cruise.

I need and want to feel like this plane is an extension of my body but I'm just not there yet.

Thanks to everyone for the advice and thanks to George for the numbers that I can attempt to go by if this happens again before I become comfortable without those instruments.

Justin

[GAHorn]

Thanks to everyone for the responses. I apologize for the lack of response but the last two days have been hectic in my home. My wife left town today for several days and I had my hands full with to kids under 4. So they are in the bed so I had a few minutes before I hit for a 4am early rise to work.

After a bit of flying that morning the ASI did end up getting to around 60mph I think. Not much over that but for the initial speed was round 40-45mph. I don't recall exactly how long before it bumped up to 60 but it was definitely after we got up to a good level cruise.

I need and want to feel like this plane is an extension of my body but I'm just not there yet.

Thanks to everyone for the advice and thanks to George for the numbers that I can attempt to go by if this happens again before I become comfortable without those instruments.

Justin

I urge you to have the airspeed system inspected/repaired before further flight. My suggested numbers were not intended to encourage flight with a known airspeed indication discrepancy....but instead to provide some guidance in the event of an in-flight failure of the system.

Hope the wife also comes back.

[juredd1]

Thanks George.....I intend to verify things are working before I give it a go. I get the feeling you wouldn't misguide anyone on purpose but none the less I did not plan to put that info on a sticky and tape it to the dash as my new ASI. I also understand you don't know me from Adam at this point so you needed to put out the warning.

[juredd1]

Well I didn't see any sense in starting another thread on this subject since it's me with the problem again.

So on take off I realized I was having another ASI issue. The Pitot tube cover was in place this time so I didn't think it was a bug issue but could have gotten on in there on take off I guess.. So after safely making it back to the ground I did what I have always done (152 and 170) I attempted to unscrew the cone tip off the pitot tube so I can better see what might be up in the hole and I usually run a small piece of safety wire up in it to see if I get any type of debris on it. Well that didn't go to well as the tip snapped right off the threaded tube. In hind site I would have just took the 4 screws off and removed the whole part that sets outside the plane and unscrewed the rtubing and blew through it the other way to see if it was clean. Of course that doesn't change the fact I have a broken pitot tube now.

This happens to be a heated pitot tube. I guess I am wondering is everything that seats outside the plane one piece of can parts be replaced such as the threaded center tube that twisted off? Sorry guys in the preview it shows all the pictures turned sideways. That is not how they were taken but not sure how to rotate the in this thread.

If everything that seats outside has to be replaced where do I start looking?

Thanks for any advice you can provide.
Justin

[DaveF]

The L-shaped pitot tube assembly fits into the sleeve bracket that extends down from the wing. You can get a replacement from any salvage place. That's not the original pitot tube, so you may need to check the 337 for the Cessna part number they used in the conversion. Look at a 172/182/? parts manual for more info.

To replace it, remove the pitot pressure line and wires inside the wing, then remove the four flat-head attachment screws (the rusty one in your picture). Remove and replace the pitot tube assembly. Of course, start a few days early with penetrating oil. And be careful with the four screws -- they're some kind of unusual size and thread, 5-40 or something.

[juredd1]

Thank you for the reply Dave, I have already had the 4 screws out they came out without any issues.

Justin.

[Karl Towle]

Those screws are #6x40, which is absolutely NOT a common size/pitch combination. I had one missing for some time. Finally was able to order some stainless, countersunk, socket head replacements at a local nut & bolt wholesale place (Denton Bolt Company (940) 383-1483). The other possible source is a gun-smith. But they are unlikely to have stainless - which I definitely preferred. I did have to do a bit of field modifying on the overall length, and head thickness though.

[blueldr]

An uncovered stock pitot head can and will get water in it that will settle in the lowest point in the pitot line to the instrument and block the line. Pitot pressure will not clear the ;ine since there is normally no flow of air it. The answer is to disconnect the pitot line at the air speed instrument and back blow the line to clear it. A pitot head cover is strongly suggested to be installed when the airplane is not flying.
Removal of a pitot head cover is strongly suggested prior to flight. However, if your left door window is capable of opening completely in flight and you happen to have EXTREMELY long arms, in flight removal of a pitot cover may be possible.
In flight removal of a pitot cover is considerably easier if you happen to fly with a pet orangutang.