Another "Out of Annual" out-of-body, experience

[Gone Flyin]

So, I had planned on taking my C150 up early yesterday morning. First flight since getting it out of annual. My "annual from hell", but you all know that.

So, there I am getting ready to do a short field take off from this 2200' runway. Off I go barreling down the runway when I notice my airspeed indicator is straight up. Abort the take off and slow to a stop.

Then I think, well it has been over a month since I've flown so maybe I'm being too cautious, here. Did I really get up enough speed for the AI to indicate? In the back of my mind I am hearing that little voice say "out of maintenance is when you have the most issues, so be careful".

I turn around at the far end of the runway and try again. Nope. This time I'm sure I had the speed. Back to the hangar I go.... ****ed off.

I peer into the pitot tube and see nothing. I check the static port and it looks fine. I had no issues prior to annual so what can this be. Send off a few emails for ideas. Come back this morning, fresh.

I take a piece of safety wire. Make it straight and send it down the pitot tube. It goes all the way to the back no problem. So, I remove the pitot tube assembly.

Take off the wires for the heater and unscrew the top-most nylon nut and the tube assembly comes off in my hand.

I blow into the end and nothing. It is completely blocked up! Off to the bench we go.

Using a long wooden Q tip I run that down the tube. This time I feel it go "crunch" and out pours some white powerdury substance. Corrosion? Fungus? Who knows.

Use some electronics cleaner in both ends and my air gun and I get it fully clean.

Reinstall and test and all is well.

Rain the next two days

BTW: Mine used four T15 Torx bolts to hold the tube to the mounting flange. Hard to see with the paint and rust. I ended up slotting them with my Dremel and removed with a screw driver. Will replace with some flat head 6-40 x 1/4" Allen key bolts.

 

[RyanShort1]

Ah, you don't need a Pitot tube to know how fast you're going... as long as you have a GPS, right???

 

[nrpetersen]

Those heated pitot tube mounting threads being 6-40? I have never heard of any application using 6-40 threads. Check again, and see if maybe they are 6-32 screws.

 

[Doc Holliday]

So, I had planned on taking my C150 up early yesterday morning. First flight since getting it out of annual. My "annual from hell", but you all know that.

So, there I am getting ready to do a short field take off from this 2200' runway. Off I go barreling down the runway when I notice my airspeed indicator is straight up. Abort the take off and slow to a stop.

Then I think, well it has been over a month since I've flown so maybe I'm being too cautious, here. Did I really get up enough speed for the AI to indicate? In the back of my mind I am hearing that little voice say "out of maintenance is when you have the most issues, so be careful".

I turn around at the far end of the runway and try again. Nope. This time I'm sure I had the speed. Back to the hangar I go.... ****ed off.

I peer into the pitot tube and see nothing. I check the static port and it looks fine. I had no issues prior to annual so what can this be. Send off a few emails for ideas. Come back this morning, fresh.

I take a piece of safety wire. Make it straight and send it down the pitot tube. It goes all the way to the back no problem. So, I remove the pitot tube assembly.

Take off the wires for the heater and unscrew the top-most nylon nut and the tube assembly comes off in my hand.

I blow into the end and nothing. It is completely blocked up! Off to the bench we go.

Using a long wooden Q tip I run that down the tube. This time I feel it go "crunch" and out pours some white powerdury substance. Corrosion? Fungus? Who knows.

Use some electronics cleaner in both ends and my air gun and I get it fully clean.

Reinstall and test and all is well.

Rain the next two days

BTW: Mine used four T15 Torx bolts to hold the tube to the mounting flange. Hard to see with the paint and rust. I ended up slotting them with my Dremel and removed with a screw driver. Will replace with some flat head 6-40 x 1/4" Allen key bolts.

So do you keep a pitot cover on the tube when not in use?

 

[aftCG]

Those heated pitot tube mounting threads being 6-40? I have never heard of any application using 6-40 threads. Check again, and see if maybe they are 6-32 screws.

Yeah but with 6-40 it's kind of like a self locking feature.

 

[BryanAV8R]

Those heated pitot tube mounting threads being 6-40? I have never heard of any application using 6-40 threads. Check again, and see if maybe they are 6-32 screws.

Just had to fix mine yesterday; loose poly hose in the fitting...they're 6-32 machine screws, countersunk, 3/16" long.

 

[geezer]

Since the OP described the fasteners as Torx, they were probably not factory installed, and might actually be 6 40. Measuring carefully is in order, and returning to the original if possible.

Insects work fast, we left our pitot cover off for one day, and at the end of the day had a blockage with insect eggs. That was on a day trip, landed mid morning, blocked late afternoon when we were going to return. Fortunately, the shop was still open, and took care of us in just a few minutes.

 

[Dan Thomas]

They're #6-40 and hard to find sometimes.





If they're something else, someone has tapped a different thread in there or just forced them in. Gets interesting later on, usually for someone else, who tries to remove such screws and they break off in there. You either find someone real sharp with a TIG welder, to weld a small bit of steel to the broken stub and try to work it out, or you buy a new pitot head.

I've had to deal with too many such "owner-maintenance" messes, fixing their "fixes." New owners are dismayed when they encounter that.

 

[RyanShort1]

I've had to deal with too many such "owner-maintenance" messes, fixing their "fixes." New owners are dismayed when they encounter that.

Yep. I found a spark plug that should have had some anti-seize applied last week.

 

[nrpetersen]

They're #6-40 and hard to find sometimes.

Maybe a left hand thread too?

 

[Dan Thomas]

Maybe a left hand thread too?

Haha. Nope. 6-32 is common enough. 6-40 is not so common. Not many places you'd find them in any airplane, especially the flat head.

 

[Gone Flyin]

Picked up a 12" long 1/8" drill bit for keep my pitot tube clean going forward. Exact size of the tube.

Run it gently in and out with a few twists and it should take care of any debris that may find its way in there from flying or sitting.

 

[Gone Flyin]

Haha. Nope. 6-32 is common enough. 6-40 is not so common. Not many places you'd find them in any airplane, especially the flat head.

They were, in fact, 6-40 x 1/4" long flathead. I chose to use a hex head this time

The head was a bit wider than the countersunk hole but worked fine.

Easy to find on Ebay.

 

[Doc Holliday]

Picked up a 12" long 1/8" drill bit for keep my pitot tube clean going forward. Exact size of the tube.

Run it gently in and out with a few twists and it should take care of any debris that may find its way in there from flying or sitting.

A pitot cover protects the pitot just as well, without the risk of damaging the inside diameter of the tube.

 

[Dan Thomas]

A pitot cover protects the pitot just as well, without the risk of damaging the inside diameter of the tube.

Yup. One can loosen small debris that then ends up plugging the tiny drain hole in the pitot under its elbow.

 

[NordicDave]

Picked up a 12" long 1/8" drill bit for keep my pitot tube clean going forward. Exact size of the tube.
Run it gently in and out with a few twists and it should take care of any debris that may find its way in there from flying or sitting.

A pitot cover protects the pitot just as well, without the risk of damaging the inside diameter of the tube.

I agree with Doc's point about covering rather than reaming the hole clean. Knocking lose even the smallest particle will flow straight to the delicate Airspeed indicator and likely damage it. A good A&P will disconnect the Pitot line and blow it clean with pressurized air as required.

Edit: replaced Altimeter with Airspeed Indicator

 

[Dan Thomas]

I agree with Doc's point about covering rather than reaming the hole clean. Knocking lose even the smallest particle will flow straight to the delicate altimeter and likely damage it. A good A&P will disconnect the Pitot line and blow it clean with pressurized air as required.

Except that the pitot doesn't feed the altimeter. It only goes to the airspeed indicator.

The worst blockage I found in a pitot was a mud-wasp nest. They are tiny bugs that look for tiny places like that pitot to build their nest, which is made of a thick, waxy goop that is hard to remove. One almost has to take things apart to get at it.

 

[texasclouds]

Pitot to the Altimeter? Thanks for the laugh guys, I needed it!

 

[Gone Flyin]

A pitot cover protects the pitot just as well, without the risk of damaging the inside diameter of the tube.

I suppose if one could leave the tube covered all the time, this might work well.

For me, I'd prefer to be able to fly and in order to do that I need the airspeed indicator to be working to make it legal.

The 1/8" drill bit will not damage the tube if inserted gently, as I suggested, and rotated a few times then withdrawn.

The flutes in the bit will likely carry out any debris.

 

[NordicDave]

Except that the pitot doesn't feed the altimeter. It only goes to the airspeed indicator.

The worst blockage I found in a pitot was a mud-wasp nest. They are tiny bugs that look for tiny places like that pitot to build their nest, which is made of a thick, waxy goop that is hard to remove. One almost has to take things apart to get at it.

HA! Right! I meant airspeed indicator. Shouldn't post and multitask at work. :7) I had to coincidently replace both Airspeed and Altimeter at the same time.