Challenger Ultralight Aircraft Maintenance

Tip #5 – Flat Tire Mystery

It began when I noticed that the right main tire was flat on my Challenger. I assumed that I must have suffered a punctured tube. I re-inflated the tire and then I rolled the plane back and forth to see if the tire had suffered any obvious damage.I could not see any but the view was restricted by the wheel pant. The tire seemed to be holding pressure but the next day it was flat again. I jacked up the aircraft and removed the right wheel assembly.

I examined the tire but did not find any obvious pouncture. Then I disassembled the wheel hub so as to remove the inner tube. When I examoned the tube, I again could find no obvious puncture that would cause an air leak. When I pressurized the tube to about 20 psi, I listened and felt for a leak, but none was found. Finally I took the pressurized tube and put it in a tub of water. This would show the leak by an obvious stream of air bubbles. Again the tube was leak free but I noticed a bubble form at the end of the tire valve stem. Then another and another. It was a very slow leak coming out the tire valve. Not a lot but enough to flatten a tire overnight.

A quick trip to the hardware store procured a tire valve removal tool. I unscrewed the little valve from the stem. It has a flat rubber face that sits down flat on the brass valve seat. Embedded in the rubber face was a very tiny metal filing. Just enough to unseat the valve and let the compressed air in the tire escape. It must have entered the valve when I last checked the tire pressure.

I wiped off the metal filing and reinstalled the tire valve into the stem. Then I reassembled the tire and wheel hub assembly and put it back on the plane. I pressurized the tire to exactly 25 psi. The next morning the tire was still inflated and a quick pressure check confirmed that it was holding pressure at 25psi.