Keeping tire pressures correct

[Kent Shook]

We've got a DA40 in a 30-member, 3-airplane equity club. We love it, it's been a wonderful addition to the club...

BUT...

Man, do we go through tires like mad! Just had another flat away from home, and the A&P said it looks like the tire spun on the wheel just a hair - enough to damage the valve stem and allow the tire to go flat. He said that the opposite tire was about 8 pounds low, and that was probably enough to have caused the flat we got.

Checking tire pressure on the DA40 is a real pain, so I think that some (many) of the people who fly it aren't doing anything more than taking a look at the shape of the tire on preflight.

Is there anything that can be done to make checking tire pressure easier short of removing the wheel pants for good (which makes the plane MUCH less attractive)?

And has anyone certified wheel pants that aren't below the bottom of the wheel itself so that they don't get all scraped up in the event of a flat tire?

Thanks for any ideas!

Kent

[rwtucker]

Kent,

Beyond rotating the tire with the pant door open until you can see the valve stem, the only tricks I use are: (a) use an angled tire gauge and filler stem to facilitate a well seated connection and (b) use a loop of twine or a smooth steel hook (no sharp edges) to lift the valve stem gently into optimal filling position.

I'm not sure which pants you have but the XLS low profile tires and pants are a component of the DA40's speed. Change them and you will probably decrease your cruising speed.

Your A&P is spot on with respect to pressure. Tire pressure should be checked frequently and a pound or two high is better than a pound or two low. (As I recall, Goodyear recommends plus 4% for fully loaded aircraft tires.) Goodyear has a number of general recommendations. Some of these apply more to larger aircraft but the advice is useful. http://www.goodyearaviation.com/resourc ... manual.pdf

As you have seen, a flat damages the fairing significantly on our Diamonds and they are costly to repair unless you do your own fiberglass work. I don't know your club's rules but, in my opinion, a flat is likely the fault of the PIC that day if pressures are low. I have never had a flat from runway hazards, although some of a 2007 run of Goodyear tubes were reported to have had some weak spots.

Bottom line, your tires are losing pressure daily. Using dry nitrogen only slows the loss a little. (The main advantages of nitrogen for light aircraft are that it does not oxidize the tube and does not expand and contract quite as much.) In the best of conditions your tires will be below optimal in less than a month, sooner during seasonal changes and/or or hard landings.

So far, you have been lucky enough to have only gradual flats. A touchdown blowout can be a memorable experience with the Diamond's castor steering. In principle, a sudden loss of tire pressure in one MLG could be fatal. In my opinion, your should check tire pressure with a gauge at least once a week.

[Steve]

I check mine every 4-6 weeks. Air loss has been negligible since I installed the Leak-gard tubes. Before that I would typically lose 1 PSI per week.

Steve

[rwtucker]

How long have you had the Leak-Gard Steve? Did they require any special consideration in installation?

[Steve]

On the mains for 3 years, nose gear for one. No special considerations at all, I simply used them when I installed new tires. Aircraft Spruce has them. For my (large) tires:

https://www.aircraftspruce.com/catalog/ ... kkey=27761
https://www.aircraftspruce.com/catalog/ ... kkey=16064

I carry a spare one of each size in the cargo tube in case of a flat tire away from home.

[Kent Shook]

Has anyone used the "easy-flate" version of the Leak-Guard tubes? Does the valve fit inside the pant correctly?

http://www.desser.com/store/products/15 ... VE%29.html

[mdieter]

This is my helpful hint. Get a red or blue grease pencil / china marker. Position the plane so you can see one of the tire stems in the pant door. On the inboard side of that tire, draw a vertical line on the tire. The line should be perpendicular to the ground when the tire valve is properly positioned in the pant door. Do that for all three tires.

When it is time to check the tires, pull the plane until you see the china marker line on one of the tires. Check that tire and continue for all three tires. It's not magic, but it is significantly easier than pull a bit, walk around all three to see if any valves are in the pant door, repeat infinitely!!!

Touch up the china marker lines every now and then.

http://www.uline.com/Product/Detail/S-1 ... lsrc=aw.ds

[rwtucker]

I just had an interesting conversation with a guy at Desser. He says that Goodyear and Michelin tubes are made of the same low leak butyl compound as used by Leak-Guard. There is no need to buy special tubes except that the Leak-Guard brand is $20 less expensive. I asked about defect rates. He said the number of defects that they get back is about the same for all brands.

He also claimed that Goodyear III tires (Custom) get more landings than the II but that retreads are probably the best overall for a couple of reasons. One AOPA article seems to agree. http://www.aopa.org/News-and-Video/All- ... raft-tires

Just when I thought I had it figured out.

[jd4727]

I would echo Steve's advice to keep spare tubes in the plane. I fly in the Western US and land at some small places. The DA-40 tube must not be a common size and is therefore never in stock when I have had a flat. In my first 100 or so hours with the plane I must have had four flats including a blowout of sorts on landing in LeGrande Oregon...which in this case was pretty much a non-event luckily, aside for some minor wheel pant damage. It took me a while to learn how sensitive the wheels are to under-inflation and ham footed braking technique on landing. I was tearing the valve stems. On a slow leak event the A&P showed me how under inflation was causing the tube to chafe against the inside of the tire eventually causing small pinhole leaks and a flat tire overnight. Now I use a grease pencil to mark where the stems are and check pressure frequently. Since then and correcting my braking technique no flat tires.

[Steve]

A couple of folks have mentioned using a grease pencil to mark the valve stem position on the tire. I go one step further. When you replace a tire, there is a red mark denoting the light point on the tire. The tire should be installed with this mark aligned with the valve stem (the heavy point on the tube). After installation I spray paint a red line on the stem side of the tire, which is visible below the wheel pant.

The painted line is more permanent than a grease pencil mark (plus, this is the way the airplane came from the factory).

Steve