I got a call from a King Air B200 owner/operator. He had been a regular customer of mine when I had my maintenance shop, but we hadn’t spoke for several years. He always made time to chat with me whenever dropping off or picking up his King Air, and I was glad to hear from him. Besides catching up, he wanted my opinion on a persistent squawk: His nose tire kept going flat.
He had been fighting this problem for many months. Everyone involved felt they had attacked the problem from every possible angle and were at their wits end. (In case you’re wondering, the tire was being serviced with nitrogen, not air.)
Troubleshooting a Flat Tire
If a tire is low, the first thing I check is the valve core. They are the most susceptible to failure and an easy fix. If that doesn’t take care of the problem, I would check the tire for leaks. This is properly done by immersion in a water bath. In a pinch, I’ve used a spray bottle to drench the tire/wheel assembly, looking mainly for a leak around the bead. I’ve found many leaks this way, but some leaks are not so forthcoming. There is no substitute for removing the tire/wheel and submerging it in water.
New Tires and O-rings
Whenever any tire – nose or main wheels – is changed on your King Air, it gets a new O-ring as well (at least it should). This large O-ring, sandwiched between the wheel halves, can be tricky to install. If not seated perfectly, they can get pinched between the wheel halves and cause the new tire to lose pressure. Rarely do you get a brand-new tire that is defective. If it’s a new tire going flat, then you’ve got a pinched O-ring. The maintenance manual even calls for an “air retention check” on a new tire by letting it sit for 24 hours after mounting.
Running Out of Options
With the old customer’s B200, they replaced the valve core, but it didn’t help. So, they put it in a water bath and still found nothing. Next, they installed a brand-new nose tire and waited the obligatory 24 hours and the tire held pressure. But after a few flights, it went flat! I think he tried a couple of brand-new nose tires, all to no avail.
I should mention at this point that this nose tire problem always cropped up at the home hangar. There was no loss of tire pressure due to a change in altitude. By that I mean if you operate most of the time near sea level, then you take a monthlong vacation in Leadville, Colorado, at 9,934 feet, your tire pressures might need a little attention. Finally, out of desperation, the B200 owner/operator bought a brand-new nose wheel assembly and that fixed it.
The Nose Wheel Valve Stem
As I listened to his tale of nose tire woes, I thought about the valve stem. They can leak. On all King Air nose wheels, the valve stem is mounted from the inside of the wheel wall, going to the outside. The inside end of the valve stem is flanged. On the outside, there’s a jam nut that screws down and pulls the flanged end snug against the wheel. Sandwiched between the jam nut and the outside wheel wall is a rubber gasket or grommet; there’s another one between the flange and the inside wheel wall. When the jam nut is screwed down, the gaskets compress and form a seal. In the valve stem photo (above) instead of two gaskets (one inside and one outside), there is a one-piece, sheath-like grommet that protrudes slightly at each end. When the jam nut is tightened, the grommet is compressed in the same way at both ends.
My theory is that these valve stem gaskets or grommets shrink in the cold temperatures at altitude and this allows the tire to lose pressure. Back on the ground, the grommets have returned to normal, but the nose tire is low. “Nose tire – low pressure” goes on the squawk list. The tire/wheel gets the water bath treatment, but no leaks are found. Now you are going down the same path as my B200 friend. I wish he had called me before he ordered that new nose wheel; so does he.
King Air Tires with Tubes
If putting in tubes sounds wrong to you, then you probably don’t have a model 90. All the wheels on the 90 models have the type of valve stem discussed above, not just the nose wheel. I learned early on to keep those valve stems in stock, and that practice stood well with me.
Some 90 owner/operators didn’t want to wait for a valve stem to fail and bring down the pressure in one or more tires. Their proactive solution was to put tubes in their tires, including the nose tire. This way they could maintain consistent tire pressure. Any King Air with double truck main gear, including the F90, has a different valve stem on all main wheels. It’s shorter and it screws directly into the wheel. The O-ring at the end is much smaller than its nose wheel counterpart and is tucked away from exposure. So that’s four out of five wheels you don’t have to worry about.
Generally speaking, the King Air nose wheel assembly is not a problem-prone area. It does what you want, when you want it to. You can’t ask for much more than that.