The International Cessna^® 170 Association

I just got flew plane back from the shop at a different airport than my home base. My Airspeed indicator wasnt working as I took off at 45 MPH and 3pointed my landing at 110.

I suspect its got water in the system and Froze(It was 6degF today). Any suggestions on how to remedy? Should I just take it to the Instument shop and let them deal with it?

Thanks

Is it going to get over freezing or can you get the airplane in a hanger?

It is unlikley the water/ice is in that far in the pitot. If it's going to get warm enought that it will melt chances are it will then dry out on its own. I've had this happen several times to me here in PA. I just park the plane and come back in a few days and it's OK. Even in the middle of the winter when it may not get over freezing.

BTW it's probably no more than a drop.

It doesnt look like it will get above freezing soon, but I could tent it pretty easily or rent a hanger for the day.

Should I put the Tail Up on a drum to aid in draining or bend the pitot down a little(Returning to original position before flying?).

When the plane was getting fixed it was in a hanger so Im surprised it didnt drain then. I think the water got in their when my mechanic was nice enough to wash my plane while it was inside(Mixed blessing I guess).

Like I said there isn't likely much more than a drop of water. If you can heat up the pitot that may be enough.
Depending on the plumbing of the pitot maybe you can discontect it real close to the pitot and after heating blow out the water. I'd also make sure your static port is not blocked with water while your at it.

With the static port after the water is melted some time just pushing in gently on the side of the aircraft around the port like an oil can will force the ater out.

Craig,

Renting a hangar for the day to fix this is overkill in my opinion. This is my procedure when my airspeed doesn't work. If you suspect ice in the standard 170 curved pitot (aluminum tube sticking out about 6-8") take a short piece of safety wire and probe for it. Then if you feel it is blocked, take your red dragon preheater to it for a few minutes to melt the ice, while bending it just enough so the water drains out. A propane torch is way too much but if you use it very lightly, it would work too. You may have to tweak it a little when done so that it points to the same angle as before.

Your pitot has one of the little flapper deals on the end that opens when moving faster than about 20mph right? If it doesn't, it should.

By the way, you meant -6F this morning, right...I assume your based at Birchwood?

Bruce C.

The problem with the red dragon trick is that you don't know where the blockage is, and you won't know till you fly it again.

If it were me, I'd get it in a heated hangar (or tent if you want) for a few hours. Disconnect BOTH the pitot and static lines at the back of the gauges.

Gently blow them out with shop air at very low pressure, from the cockpit end out. DO NOT use high pressure air, or blow them out with your breath, which contains a lot of moisture, which got you here in the first place.

Bending the pitot tube is a bad idea, in my mind, though most of them have been tweaked enough times it might not hurt, but why work harden something that costs that much, rather than doing it right?

Warm the whole system blow it out backwards, and you'll not have a recurrance. Heat it up outside, and don't get it all out, and it'll just be driven further into the system.

Might as well fix it the first time.

Mike Vivion

There is low point in the pitot line behind the upholstery on the left side of the fuselage just ahead of the forward door post. That is most likely the furthest point water will be able to penetrate the pitot line toward the air speed instrument. I've had this happen often enough that I finally installed a pitot head under the wing, but without pitot heat.
The first time the problem arose, I opened up the upholstery, cut the 1/4" aluminum pitot line, installed a piece of 1/4" ID hose and back blew the water out with lung power. This was back in the days my airplane was tied down outside in the rain. After the initial "blow job", the pitot line was spliced back together with a piece of 1/4" ID hose. After about the fourth time, I put the pitot head under the wing. Haven't had it happen since.
The moisture in the short puff of breath needed to purge tie tube is not going to put enough condensate in the to be of concern.

I'm with Bruce, walk out to your plane and stick a piece of safety wire in the Pitot to find the ice plug 6" back. When we flew down to America, our Pitot froze several times, we used our single burner camping stove and a FBO borrowed propane torch once. Tilt the tube down so the water can drip out. It only takes a few minutes to be done with this problem. No need to rent a $40 per night (AK) hanger but if you do, go ahead and swab the tube out when your done. Before the last time it froze, we put the plane in a heated hanger overnight just to thaw everthing out. The pitot was froze again on take off the next morning, so I circled around, landed and asked for a propane torch, heated, and swab the tube dry with some kind of absorbing material and was done with it.

Get yourself a pipecleaner and swab the Pitot out whenever being exposed to moisture in the cold country. At one time I used the pipecleaner as part of my preflight, but lost it somewhere. Next time I come across one, I think I'll stash it. Although I doubt I'll ever need it for a frozen Pitot again! Thanks for the weather update, guess I'll take a heavier coat up tomorrow!

mvivion wrote:The problem with the red dragon trick is that you don't know where the blockage is, and you won't know till you fly it again.

Bending the pitot tube is a bad idea, in my mind, though most of them have been tweaked enough times it might not hurt, but why work harden something that costs that much, rather than doing it right?

Might as well fix it the first time.

Mike Vivion

The whole point of testing the pitot with safety wire before doing anything as mentioned in my post is to insure you know the blockage is in the low point of the pitot sticking out from the wing. If you can't feel the blockage with 5 or 6" of safety wire stuck down the end, things are more serious and the heated hangar option is next. Also, If you are on straight skis, hangars can be a big pain with the usual almost clean of snow asphalt leading up to the door.

The amount of bending of the pitot to get it back to correct shape should be minimal, about 10 degrees of tweak is all I have had to do. This is not a procedure I advocate doing weekly either, only about once or twice a winter to get going quickly and avoid the expense and hassle of a hangar. Dave mentioned used a pipe cleaner or swab to soak up the water instead of bending the tube. This is an excellent plan if you have the time and materials to make a little swab. Occasional light bending is more for getting going right NOW. I have done both with good results and in fact is fixing right the first time.

Bruce Christie

In retrospect, I guess I really had a different problem. My pitot line never did freeze up. It just, over time, accumulated enough rain water run back to settle in the low point in the line and finally choke it off. The pitot pressure was never sufficient to force the water plug uphill from the low point to the instrument, but it stopped the air speed indication.

Just bend it down enough to drain when parked and leave it there. End of problem. All it needs is for the opening to be facing the realitive wind in flight. It could be as radical as the 172 fuel vent under the wing and still work. If you bend it in a gentle S or backward S depending on which way you look at it, it will not get water beyond the bottom of the S even if you spray a hose at it. As long as the end slopes down it will run back out unless it's cold enough to freeze in place. Your static vent will freeze sometimes, but if you put a alternate static source inside the cabin it will solve that. Just open it and you have airspeed indicated, it'll be off about 10 mph but you can live with it. The altimiter will work fine.

mvivion wrote: Disconnect BOTH the pitot and static lines at the back of the gauges.

Gently blow them out with shop air at very low pressure, from the cockpit end out. DO NOT use high pressure air, or blow them out with your breath, which contains a lot of moisture, which got you here in the first place.


Mike Vivion

WHOA! STOP! DON'T!

Disconnecting the Static line from the back of the Airspeed Indicator and blowing through it is a BAD IDEA! That line is also connected to your Altimeter and your Vertical Speed Indicator and you could damage them.

(Not to mention the fact that your complaint does not indicate any problem with the static line anyway!)

Blowing the Pitot line out (in reverse) is not disqualifying, however, and since nothing else is connected to that line, it won't hurt to use a bit more pressure in the process. BUT....keep in mind that most shop air compressors also have moisture in them (if you're worried about that small amount.) Dry Nitrogen or compressed air from a dry source is certainly better.

In any case, ...Be aware you may not have properly diagnosed your problem. Just because the temperature is below freezing may be an excellent clue....it's not definitively your problem. Other reasons for blockage may exist (such as insects.) Be certain your mechanical pitot-tube cover is working properly, of course.
Disconnecting the pitot line from the back of the instrument and confiming it's blockage is the first step. Blowing it out in reverse with about 25-50 psi should clear most insect nests. Using a hair dryer can be helpful in melting water (....if that's the problem.)
But the first order of business is confirming the problem IS IN FACT a blocked pitot line. (And leave that static line alone if your altimeter and VSI are working normally.)

My VSI & Altimeter appear to be working properly, so that lead me to the Airspeed Indicator. Also, when I taxied it out from a warm hanger and took off it appeared to accelerate normally. It then seemed to stick on take off. When I put the nose down it jumped up to 110. It then stuck at 110 until I hit the runway for landing and it dropped down to 40 and stayed.

I keep my plane at Merrill Field in Anchorage, Ive considered moving it to Birchwood especially now as I heard the municipality was increasing fees. Ive heard up to 2X. Hangers are currently running $50-$65 for a day, so if I can get it out without putting it in a hanger that will give me more gas money.

I went by today and put my gust locks on and meant to stick the pipe cleaner in the pitot to see if it was blocked. After putting on my gust locks, my hands were freezing and I forgot to stick the pitot.

Anyway I try it tommorow. I will get it out by this weekend as I have the itch to go flying. Thanks for all the info.

N2830C, did you fix this problem? On mine I would take off the front door post / wing root vent trim, disconnect the pitot connection there and blow the water out through the pitot. Easier than getting under the panel, and you can also watch through the windshield to see if any water comes out. My advice: don't bend your pitot tube. There is a diagram of the proper pitot tube shape in the Cessna 100 series maintenance manual, in case you want to check to see if yours is correct.
Max