Aircraft Tire Pressure
Every aviator has their soapbox issues, and when it comes to maintenance, my top two are constant-speed propellers and aircraft tires. I may touch on the former in a future article, but for now let’s focus on the latter.
Tires are one of the most vitally important — yet frequently ignored — parts of an aircraft. It’s easy to see why: they’re relatively simple elements which work day in and day out without problem, and as such are taken for granted. In addition, some of the typical pilot’s attitude toward tires is transferred from the way they treat their automotive counterparts. Be honest, how often do you inspect your car’s tires? When was the last time you checked the pressure on all four wheels? I’m about as anal as a person gets when it comes to car maintenance and upkeep, and I might check the tires once every couple of months at best.
In a light GA aircraft, tire failure on the takeoff or landing roll can lead to loss of directional control, runway excursion, and/or a ground loop. These things are unlikely to be fatal but are frequently embarrassing and inconvenient as they’ll shut down the runway for a while. I’ve had several of those in my career. Ironically, it always seems to happen during a student’s softest, smoothest 3-point landing in the Decathlon. Well, almost always.
I once lost a tire — literally — while taxiing a Pitts S-2C. I had stopped for cheap fuel in Limon, Colorado, a paved but little-used strip on the edge of a small town near the Nebraska border. After a great landing in a 25 knot crosswind, I refueled and started to back-taxi on the runway when the left main tire slowly deflated. I shut down and opened the canopy, but as soon as my feet came off the brakes, the airplane weathervaned into the wind, taking the tire right off the rim as it pivoted. Now I couldn’t even move the plane.
Oh, and did I mention the sun was setting soon and the runway had lights? Suddenly the flat tire was less important than ensuring some wayward pilot didn’t attempt a night landing with my disabled, unlighted biplane sitting on the runway.
Light aircraft flats are more often caused by the failure of the tube than the tire itself. The Pitts incident taught me the value not only of proper tire inflation, but also of alighting at airports with maintenance services when flying cross-country. Making that trip today, I’d at least carry a spare tube.
Of course, that won’t always save the day. I once had to rescue a friend from Death Valley when his underinflated aircraft tires melted into the tarmac on a 120+ degree day. Tire condition can be difficult to judge on fixed-gear aircraft on account of the fairings which often hide 90% of the rubber from view.
In turbine aircraft, improperly inflated tires are more likely to lead to expensive damage, if not outright catastrophic consequences, due to the higher speeds and heavier weights of those aircraft. There’s simply a lot more kinetic energy for the tires to absorb. This is why turbine aircraft tires are stronger and more advanced than those found in their lighter brethren. It also explains why those tires are filled with nitrogen instead of air. Nitrogen doesn’t expand at altitude the way air does. It has a low moisture content so it doesn’t freeze, and it will not support combustion.
The FAA recently issued Safety Alert for Operators (SAFO) bulletin 11001, “The Importance of Properly Inflated Aircraft Tires”, which notes:
Research has shown that transport-category airplanes can lose as much as five percent of tire pressure per day under typical operations. At a pressure rate loss of five percent per day, it would only take a few days before they require servicing.
Tires not serviced within an acceptable range may require tire replacement due to under inflation limitations specified in the maintenance manual. Additionally, servicing of underinflated tires without proper protection such as a tire screen or other protective devices may cause damage to the aircraft or injury to the individual servicing an underinflated tire.
The FAA’s not alone in their crusade to get us to pay more attention to our tires. Last week, Gulfstream reviewed tire safety “best practices” in their weekly Breakfast Minutes publication. It referenced the Goodyear Aircraft Tire Care and Maintenance publication. It’s an excellent read.
A bit of research revealed that tire failure has caused a variety of high-profile jet accidents, including Air France flight 4590, Nigeria Airways flight 2120, Mexicana flight 940, and most recently, the 2008 crash of a Lear 60 in South Carolina.
That last accident was cited in their SAFO bulletin. In fact, the FAA recently issued an Airworthiness Directive for the Lear 60 which requires a tire pressure check no less than 96 hours before any flight.
Obviously it’s not possible to prevent every instance of tire failure, but we can skew the odds in our favor by paying more attention to them. That means checking the tire pressure at appropriate intervals. Mounted tube-less aircraft tires lose significant pressure every single day, and underinflated tires cannot necessarily be detected by simply looking at them. If nothing else, proper inflation leads to longer tire life and better ability to survive FOD damage should it be encountered. Remember too that the landing and takeoff distances listed in the Aircraft Flight Manual are predicated on proper tire inflation!
To show the importance of proper tire pressure, consider that as a tire leaves the deflected area (aka the ground) as it turns, it attempts to return to its normal shape. Due to centrifugal force and inertia, the tread surface doesn’t stop at its normal periphery but overshoots, thus distorting the tire from its natural shape. This is called a traction wave. Assuming the tire is turning at 250 mph:
At this speed, it takes only 1/800 of a second to travel 1/2 the length of the footprint (CX). In that same time, the tread surface must move radially outward 1.9 inches. This means an average radial acceleration of 200,000 ft./sec./sec. That’s over 6,000 G’s! This means the tread is going through 12,000 to 16,000 oscillations per minute.
Tires are designed to withstand traction waves… but only while inflated to the appropriate pressure. Under or overinflation will magnify the effect of traction waves. Suddenly the AFM recommendation on tire pressure seems pretty important, doesn’t it?
Speaking of the AFM, the landing gear may have other limitations which must be observed. In 2007, I witnessed first-hand what happens when those limitations are exceeded.
Whatever you call them — tyres, hides, skins, rolling stock, stickers — aircraft tires are certainly one of the most highly-stressed yet least respected parts of an aircraft. Next time you fly, think closely about the punishment they take and whether you’re sure those babies are truly airworthy.