Flat tires

[smoss]

Seems to me the biggest downside to running the tires with higher pressure is that braking would be diminished. More pressure = smaller contact patch = less braking power and more likely to skid. If you don't fly to short strips ever, I suppose that is not a huge issue, but I've had a landing even at a 2800 ft strip at sea level (unanticipated tailwind after an IFR approach to a towered airport) where if not for max braking I likely would have hit the fence (not a pleasant feeling), and I don't think I would want to give up that safety factory by jacking up my pressure at all. Go experiment on your bicycle with increasing air pressure--it becomes very very easy to skid.

[Tommy]

I've never had a problem going into short fields with tires 10% over inflated.
If you go into short fields a lot you should have no problem getting in under 2,000'
over a 50 obstacle. If you're not comfortable doing it, don't do it.

[rwtucker]

Overall, I think Steve has it right. The best pressure in most situations is probably the pressure the engineers decided was best for the tire and aircraft. I say "probably" because the tire pressures are generally set by gross weight. To be more precise, they should be adjusted for type of landing gear (springs, shocks, elastomeric) and the hardness of the field (concrete, asphalt, dirt, grass).

If you are the kind of person who is willing to check pressures every week or two, I would go with specs. I'm not that person so I inflate to plus 10% and try to re-inflate when the pressure drops down to specs.

The problems with under- and over-inflation are not symmetrical. Under-inflation is worse for reasons already discussed. At worst, 10% over-inflation will reduce braking power slightly but, in all the years I have flown, I have never been in a situation where I had to stand on the brakes throughout the roll-down. If you fly in those situations, the science says you would be better off under-inflated and just live with those consequences (and buy brake pads by the gross ). If you land on smooth wet runways frequently, over-inflation will be better for you.

Some of this discussion is more theoretical than practical. I can't tell the difference between spec and +10%. Also, significant changes in altitude and OAT between inflation and landing conditions can push these pressures well outside the ranges we are discussing. Eventually, I'm going to nitrogen, It provides a more consistent landing configuration over varying landing conditions.

[Rich]

Eventually, I'm going to nitrogen, It provides a more consistent landing configuration over varying landing conditions.

Sounds like an "old pilot's tail" (like getting "on the step" in cruise), plain old air is 78% nitrogen to begin with, so its an incremental change, if any.

[rwtucker]

Rich,

This is a bit much for this forum but a few points to consider:

- While air is 78% N2, as you say, the 21% that is O2 burns (i.e., oxidizes) organic matter, and is a much smaller molecule. N2 is inert. Try storing you leftover wine in O2 vs N2 and let's see if you can drink the O2 .

- Compressed air typically contains significant amounts of water. You can remove the water from compressed air but doing so puts you in the same cost effort zone as Nitrogen (depending on your methods and sources).

- While differential psi losses are not all that great at automotive pressures of 30 psi (a Consumer Report study showed a 2.2 psi loss for N2 vs. a 3.5 psi loss for compressed air), this 37% delta for 30 psi increases with pressure. I don't know what it is with aircraft tires running 40-60 psi but it is reasonable to expect a delta of maybe 50% or more. Worth it? Depends.

A lot of the "anti-N2" rhetoric we all read about these days has to do with the fact that a few tire shops are charging as much as $25/tire to evacuate the air and fill with N2. Crazy! On the other hand, if you already have a tank and regulator, you can't beat N2. If you don't have access to tank-priced N2, I agree with you, except for the pilot's tale part. A lot of high performance, high pressure aviation tires do not run on compressed air.

Bob

[Rich]

With the Michelin tubes, I lose about 1 psi of air per month. Yes, O2 is, by definition an oxidizer, but it surrounds your tire and it takes years to do any damage. My mains are 5 years old. My nosewheel tire is 11 years old.

Moisture can be a factor, but this is a hydrous vs. anhydrous issue. But I've also used plain old air in climates where the temperature gat way, way, way, below freezing and have not had any tire/tube issues. In fact, now that I think of it I've never encountered a flat tire on any airplane I've owned or rented. And they were all filed with air.

In any case, it's impractical/wildly expensive in my situation (tied down at a airport 20 miles and a ferry ride away) to try to manage to keep my tires inflated with anhydrous nitrogen.

[RC7]

David, thanks for sharing your experience! I'm curious, when you say "lift on a dolly", do mean you physically lifted up the strut yourselves without using a jack? If so, where were you all holding on to? I've always wondered what would happen in such a scenario with no readily available low wing jack.

I can't speak for him, but we have some experience with getting these things off of the runway without a jack/dolly. For whatever reason, the FBO on field here has adopted a policy of not helping us out with equipment to get disabled aircraft off of the runway. With the DA40, it's quite easy to lift up one of the wheels by getting 2 or 3 guys to stand under the wing and lift with your back. We've had to do this twice to get the tire up onto a dolly (which we had to purchase I might add!).

It's not so easy with the 42. We used 4 guys and managed to lift and walk it off of a runway one night. Basically we had the 4 of us lifting the wing so the weight was off the tire while a truck pulled a winch attached to the nose gear. We were only able to make it about 50 feet at a time, but eventually we got it off the runway. It caused quite a bit of drama at our airport because we had no help and it took us 3 hours to figure out how to get it off without damaging the aircraft. Delta wasn't too happy with diverting for us

Made for a cool picture though

[CFIDave]

Wow -- Landmark the FBO at Roanoke is being awfully irresponsible if they're unwilling to help clear the runways of an aircraft with a flat tire. Usually when a company like Landmark has a contract to become the FBO at an airport, they are also responsible for emergency assistance. They just spent big bucks remodeling their building at Roanoke; you think they could afford to help clear the runway of disabled aircraft. Our FBO does this at KJYO; even when we had Landmark as our FBO before being replaced.

If I were you I'd complain to the local airport commission or politicians who oversee Roanoke airport operations. It's especially ridiculous that a commercial Delta flight had to divert because of this. You might consider joining forces with the airline local ops people on this issue.

[rwtucker]

I guess I'm wandering off-topic but I agree with Dave. At KMYL, any pilot who needs assistance is usually met by the FBO and a few fellow pilots who happened to be working in their hangars. Everyone gets the aircraft to the public parking area to give the pilot time to decide on the next step. Friendly, helpful people. The self-serve 100LL was down one day so they pulled up the full-serve truck and fueled the handful of us who were stuck in the self-serve line. I didn't have an account but was told to stop by to pay later, whenever I had time. They don't have loaner cars anymore but for $10, a pilot can check out a brand new rental car to to grab a meal and drive through town.

[RC7]

I should clarify somewhat - the guys from Landmark did come out to help at first, but they wouldn't let us use their equipment. One of the mechanics jacked the airplane up and tried to put air in it, but the tube wouldn't hold it. After that, they said that they hadn't had anything to eat for dinner so it was our problem (real nice, right?). We were also told that Piedmont didn't have any equipment that could do it. The airport director was made aware of the situation the next day and was not at all pleased with Landmark.

When we got a flat last week, we were able to get it off the runway within 20 minutes. There was a friendlier bunch working the morning shift and Landmark let us use their tug and we used our own dolly. It's not my intent to bash Landmark or hijack the thread, just wanted to share our flat tire experiences.