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Peterk

Well Known Member
About a week ago I notice that my pitot tube would move horizontally about a qtr inch every time I put the cover on it. I would pull it back and it would stay there. I assumed the set screw needed tightening. Today I took on that small task. I was wrong to assume of course.

What had happened was the entire white plastic pitot block had moved away from the gearbox backing. The pitot tube and set screw were still in tact. The safety wire between the two 6mm cap screws was still in tact. What no longer existed were the threads on the aluminum gearbox that the cap screws turned into. The cap screws were not out of the holes, they were just floating there. Nothing left the aircraft of course because the pitot tube/block set-up hadn't changed and the retainer cap screws were wired together.

Now I have to decide how to re-attach the cap screws. The original plans tell you to mount the pitot block and thread the cap screws with appropriate washers in place...turn until they touch the surface of the washer...then turn another qtr turn...safety wire. I gathered at the time they were concerned about too much torque against the plastic. Apparently they just vibrated their little ***** off until they ate up the aluminum. So I see the following options:

1. Replace cap screws with a M6x35.
2. Helicoil (something from my younger days says don't mix steel helicoils with aluminum?)
3. Bondo the plastic pitot block to the gearbox and say yuck...lol

The airframe has 375 hours. Looking to thread repair right now but that doesn't really prevent it from returning. Not sure what caused it in the first place. Will report to Van's via the appropriate document but curious if anyone sees other options for repair. Thanks

Pete
 
I don't remember the geometry, but if possible drill and tap to the next size. As a last resort I'd fill the hole with liquid steel epoxy, insert the cap scres and clamp tight until it cures.
 
Pitot Tube installation

Here is what it looks like:
i-XfqSWpH-M.jpg

There are two more tapped holes above the two securing the FF-1201 Pitot Block. If the bottom two tapped holes are deep enough then you could use the M6X35 you suggest. But, even better would be to make a larger FF-1201 Pitot Block and use all four of the holes to bolt it to the gear case. I also think that low strength Loctite #220 would be wise also to keep the bolts from vibrating in the threads.

Just my initial thoughts, sounds like a problem easy to fix. There may be even better solutions offered.

Tony
 
Thanks Tony, I think you're onto something. Since the cap screws were never tightened within torque values to begin with, aren't they basically "loose" from the start? And since they are wired together and "stuck" in plastic it is difficult for them to come out. Rather they are likely to wallow in place until they wear away the threads. A drop of blue Loctite at the beginning would no doubt provide the required tension to keep them in place. Might be something for new builders to consider.
 
I have a feeling there will be many cowlings pulled over the next few days. Thanks for reporting this.
 
Is it possible that the wrong screws were supplied or used I.e. M5 rather than M6? Since they were screwing into the plastic there would have been resistance felt during insertion. Seems pretty strange that an M6 screw into M6 threads can rattle around like that. Please verify.
 
Bill_H said:
Is it possible that the wrong screws were supplied or used I.e. M5 rather than M6? Since they were screwing into the plastic there would have been resistance felt during insertion. Seems pretty strange that an M6 screw into M6 threads can rattle around like that. Please verify.
Click to expand...

Yes they were (and still are M6). They did not wallow around when I put them in. They went thru a washer, then the plastic block, another washer and into the threads where they began to turn. When the cap touched the first washer it received another qtr turn per directions. All was tight, but not torqued. Since it was not within torque values it was in fact not tight and began to move. It could not move far because it was safety wired to cap screw #2. There was nothing left for them to do but bounce around in the aluminum channel and nibble away metal.
 
What kind of pitot cover are you using? Possibly the cover may be tight. If it doesn't slip on easy maybe the pitot gets a push-pull cycle every time it's installed.
 
RFSchaller said:
What kind of pitot cover are you using? Possibly the cover may be tight. If it doesn't slip on easy maybe the pitot gets a push-pull cycle every time it's installed.
Click to expand...

that is most certainly a possibility. mine is tight and that may well have been the cause. think I'll take your recommendation and just leave it off after I get it fixed. thanks!
 
If you use a pitot cover like page 19 of the modifications sticky thread, you can put it on and off without any movement of the pitot at all. Hold the pitot with 2 fingers of one hand while applying with the other.
 
I checked the screws on New Blue last night. At 440 hours, they are still tight with no sign of movement whatsoever.
 
Pitot Tube Check!

Just checked my today. Both bolts still snug.
2012-09-18%2010.04.20_zps969ee617.jpg

However the bolts are only 1 inch long and the block is 1/2 inch thick.
2012-09-18%2010.11.15_zps3c2e5bf7.jpg

2012-09-18%2010.11.42_zpsa7844a1d.jpg

Not very much thread in that hole so I went ahead and took the initiative, removed both bolts, put blue loctite on them then reinstalled them. When I removed them they were still snug but not tight. Now I don't have to worry about if I miss checking them next time I have the top cowl off. I believe that's called peace of mind. ;)
 
Geico266 said:
Jet guy! Get a real shop ruler! A yard stick? Really? :rolleyes:
Click to expand...

Give him a break Larry - that's the same ruler he uses to calibrate his metric crescent wrench!;)

Actually John, good idea. I thnik I will do the Loctite trick next time I have the top cowl off. "An ounce of prevention......" etc. etc.

John
 
It has meaning!

Hey I built my whole plane with that ruler, that was a gift
From my 11 year old daughter. :rolleyes:
 
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