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Bob Axsom

Well Known Member
When I did my annual in January I replaced the tubes with Michelin Airstop units. Everything was fine flew to Key West, flew to Taylor Texas and a lot of local testing. In preparation for a trip to California I converted from the racing configuration to the cruise configuration and pumped the mains up to 45 psi. A couple of days before we were to leave (a couple of days after pumping up the tires) I went to the hangar to continue work on the plane and the left main was flat to the point that the wheel fairing was resting on the floor. I reinstalled the old tube but I also inflated the Michelin. The tube has remained inflated. The only thing I can think is the valve was leaking due to contamination, a freak close condition that resulted in a one time only imperfect seal or a valve defect that could recur at any time.

This is a request for reports of similar experience.

Bob Axsom
 
Tube

Bob:

Had the same thing happen to me with one of my Michelin Air Stop tubes. I couldn't duplicate it nor could I find a leak. I replaced the tube anyway.
 
HAve had the same thing happen

I decided that with the installation of tire and new tube that the tire ,wheel and all, captured the air between the new tube and tire. The tire being tight to the rim, captured the air like a tubeless tire would . Over couple days the tire did leak down and allow the new tube to inflate properly. With that the pressure that I thought was adequate went away! Bottom line was the tube was not inflated properly inside the tire. At least that was my thoughts when i found no leaks in the tube. I think the small volume tire and wheel may have something to do with that :confused:/ DRR
 
Rationale doesn't quite match my timeline but ...

DennisRhodes said:
I decided that with the installation of tire and new tube that the tire ,wheel and all, captured the air between the new tube and tire. The tire being tight to the rim, captured the air like a tubeless tire would . Over couple days the tire did leak down and allow the new tube to inflate properly. With that the pressure that I thought was adequate went away! Bottom line was the tube was not inflated properly inside the tire. At least that was my thoughts when i found no leaks in the tube. I think the small volume tire and wheel may have something to do with that :confused:/ DRR
Click to expand...

Rationale doesn't quite match my timeline but the fact that your tube deflated is significant. I had been flying with mine for months and only added air before the trip last weekend. My experience was not just a reduction in pressure like you would experience with an air bubble escaping from between the tube and the tire - the plane was sitting on the wheel fairing (without my racing subfairings thank goodness). Mannanj's experience makes three cases if yours was a Michelin AirStop tube.

Bob Axsom
 
Michelin

Bob Axsom said:
Rationale doesn't quite match my timeline but the fact that your tube deflated is significant. I had been flying with mine for months and only added air before the trip last weekend. My experience was not just a reduction in pressure like you would experience with an air bubble escaping from between the tube and the tire - the plane was sitting on the wheel fairing (without my racing subfairings thank goodness). Mannanj's experience makes three cases if yours was a Michelin AirStop tube.

Bob Axsom
Click to expand...

Bob:

Mine had been flying for months too, before I discovered it totally deflated and sitting on the wheel pant. I could not find anything abnormal. I removed the tube and checked for leaks by by inflating it to over twice it's normal size and immersing it in water. I could find absolutely no leaks.

My conclusion was that it must have been the inflation valve.
 
Same here

I did the emersion test and I covered it with soapy water but could detect no leaks. I decided it had to be the valve which of course was reseated when the tube was re-inflated. Thanks for providing your experience.

Bob Axsom
 
This is one reason that I always use a metal valve cap with a seal on my tire valves. It provides a redundant seal to the valve itself. We used to use tire valves in various inflater devices when I was in the diving business (decades ago), and it was not uncommon to see little leak bubbles, so I don't like to depend on a Schrader valve as a single-point seal.

Bottom line - I agree that a valve leak is likely Bob - put a cap on it and see if it happens again. (BTW - all flight-critical QD's have redundant pressure-sealing caps for flight on the Shuttle.)
 
A bit Of Spit

A bit of spit, thats all it takes to check the valve if it is leaking. Learnt that as an after school hanger brat tending DHC2's. I always check valves after putting air in and it is quite common to get a leak through the valve.

If a leak is found just give a lift under the valve stem with the tool that removes tha valve and it will usually seal OK

As Paul says a metal cap with an airtight seal also doubly secures it.

Rob
Very Very Slow build RV8
 
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Yeah I do that too but ...

I had a brass cap on the stem at the time the failure occurred. It was only finger tight though and obviously didn't seal. I think I am guilty of a missunderstanding of the cap hardware. There is an extra nut that I have been using as a jam nut to lock the cap in place but I suspect the nut has another purpose (like at the base on some applications) and should never be used as I have been using it - In many instances it will back off the cap as you try to lock it in place. Oh well, another well intended misapplication. This does not eliminate the basic valve failure but the change would allow the cap to seal more reliably. I don't know about the valve consistency, the maintenance of standards was given a real down turn in the Gore led reinventing of government in the US at least.

I looked in the Aircraft Spruce 2008-2009 catalog and found three valves offered for sale on page 245

High Pressure Valve Cores
#2300HV (Long Type, Length 1-3/16") Operating pressure range 70-2000 PSI from -10 F to 165F. Conforms to AN809-1. P/N 06-00397 ..... $3.85
#9914A (Short Type, Length 7/8") Same operating range as #2300HV, but doesn't conform to AN809-1. P/N 2525 .............. $1.39

Aircraft Tire Valve Core
#6035 (38-830) (Short Type) Operating pressure range 0-400 PSI from -65F to 350F. P/N 6035 .............$0.40

I removed the valve (core) from the Michelin AirStop tube and inspected it. I found part of the black plastic band was torn away at the rear edge (referencing the end inserted into the tube stem as the forward end) and distributed across the gap between the forward body and the rear threaded portion of the valve and continuing up across the threads in a non linear manner, like a scallop pulled off of the plastic band with one point of highest pressure - not spirialing around in conformance with the thread pattern. The tube contains the following marking:

AIRSTOP MICHELIN
5CG67 (Type III) Valve TR67
5.00-5 PN 092-308-0
3 15 08 MADE IN ITALY

I measured the length of the valve (the overall length does not change) with a micrometer and found it to be 0.774 inch or 6.192/8" which is less than the high pressure short type's 7/8" listed in the catalog. I am going to order a few of the P/N 6035 Aircraft Tire Valve Cores which have no length specified but look like the the high pressure short type cores.

I conclude that the problem is a manufacturing defect in the assembly and the failure will occur intermittently when the valve is actuated as when air is added to the tube or the pressure is checked.

Bob Axsom

P.S. I will add the spit test to my process.
 
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I had some bad experiences with Michelin Airstops recently.

I bought 4 tubes and all of them had the valve cores inset too deep into the stem. This prevented me from filling them with air or letting air out.

I removed the cores on two of them and ground the stem down by about 1/16", which fixed the problem. One other tube had a bad core when it was removed (left half of it stuck in the stem). I returned the two unused tubes to my supplier.

It looks like some bad quality control at Michelin.

Vern
 
Follow up - The ACS valves didn't work

The valves were received and I installed one. The tire held some pressure for maybe an hour. I cleaned up the original by removing the cut off seal material, reinstalled it and it held pressure just fine. Tried another ACS valve - it didn't pass the spit test so I reinstalled the original Michelin valve again and pumped it up. I don't know if I would trust it enought to reinstall it on the plane but it appears OK. ACS valves center pass function seams to work OK but the outer shell seal and the "mating" surface in the stem appear to be the source of the leak.

Bob Axsom
 
Bad valve core

I had the same problem recently with a new set of Michelin Airstop tubes. The original set never had a problem, but one of the new tubes went flat overnight. I re-inflated, and no leaks the second time. I decided to leave well enough alone until after OSH.

Today I decided to pull the valve core from the tube that had leaked, and it sheared in half while I was unscrewing it from the stem. 30 minutes and lots of bad language later I got the second half out. The design is a bit different (cheaper?) than the cores on my previous set of Michelin tubes; perhaps someone saw an opportunity to save a $0.05 in production cost.

I put one of the old cores in and its holding fine so far.
 
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