Tundra Tire Nation

Until recently the popularity of oversize tires was largely limited to operators of single-engine aircraft used for essential transportation in rural Alaska, Canada and other places with little or no road infrastructure. But these days, many pilots who fly strictly for fun and seldom land on any surface but smooth pavement or manicured grass are putting big rubber on their airplanes as well. First a ubiquitous work tool and now a fashion statement, the tundra tire is fast becoming an icon of popular culture.

"It's the baby boomers," says Bill Duncan, president of Alaskan Bushwheels, a Joseph, Oregon-based manufacturer that's riding the crest of the tundra tire wave. "Here you have a group of people that lost their shirts in the stock market after 9/11 but still have disposable income and are tired of sitting in the office watching the clock. They've figured out that a light utility aircraft-a Super Cub, a Husky, a Maule or a Scout, to name the top four, will keep its value over time and may even appreciate in value. And they want to get outside and play."

Duncan has seen his business quadruple in size since 2000, the year he bought the company from an Anchorage, Alaska-area partnership building one tire per day as the Alaska Tire & Rubber Company. At that time, virtually all Bushwheel production was destined for the Alaskan and Canadian markets, but these customers now only account for around half of the annual production total. Foreign customers (among them operators in several African nations, Venezuela and New Zealand) account for 10 or 15 percent, but by far the fastest growing market segment has been the continental United States. In 2004, operators in the Lower 48-many of them recent converts to aviation-bought 4 of every 10 Bushwheels sold.

Though Bushwheels come in various sizes, the most popular model by far is the 31-incher, a treadless rubber donut best suited for operations on rough, unpaved runways, river gravel bars, beaches, woodland clearings and tidal mud flats. Now FAA-approved for use on more than 150 aircraft types and sub-variants, these low-pressure tires roll quite easily over rocks the size of shoeboxes. And, while Bushwheels certainly allow pilots to do things with their airplanes that they wouldn't ordinarily be able to do, their greatest attribute could be the confidence they inspire, even in high-time pilots.

"There's the coolness factor, plus it's nice to have a little more capability than you may need," says Charles McDowell, a longtime Aviat Husky pilot and owner based in Cedar Rapids, Iowa. "It's true that you can land on almost any airstrip in the Lower 48 with [standard aircraft tires], but if you have 26-inch or 30-inch tires you can do it comfortably all the time instead of rattling your fillings out and shaking the gauges out of the instrument panel.

"Without question there are psychological factors at play. Almost none of us have a legitimate 'need' for a Husky-type airplane at all. We want a rugged plane because it makes us feel a little more independent from the infrastructure. And remember, most of us are doing this as a form of recreation. If I have more fun flying around on sunny weekend afternoons with giant tundra tires, who cares? Heck, even most of the guys in Alaska don't really 'need' to be landing on gravel bars either-they simply enjoy it, or are ferrying hunters who could, instead of hunting, go to the grocery store and buy beef at one-tenth the price."

Other owner-pilots, such as Husky flier Bob Carlson in Red Wing, Minnesota, regard their tundra tires as nothing less than a key safety enhancement, a hedge against the unlikely event of an engine failure while flying over rough terrain. "I have the 31-inch Bushwheels and I think the biggest reason I have them is for forced landing insurance," he says. "I land on sand bars, snow-covered lakes and some not-so-smooth grass. The most likely option for a forced landing around here is a plowed field. With the 31s it would be just another landing."

"Balloon" Tires and Drag SlicksNot unlike the wheel itself, the tundra tire emerged at various times and places around the globe. In North America, Canadian World War II veteran Welland Wilford "Weldy" Phipps is generally credited with being the first. Using a Piper Super Cub equipped with oversized balloon tires he crafted himself, Phipps opened much of Canada's high arctic to air transportation. Later, operating a fleet of 19-seat de Havilland Twin Otters also furnished with balloon tires, Phipps brought airline service to Resolute Bay, Grise Fiord and numerous other remote Inuit communities.

Alaskans knew a good thing when they saw it and were quick to embrace the tundra tire concept. In the more or less liberal atmosphere of the day, pilots felt free to experiment and a great variety of tire/aircraft couplings were sampled. To try out a new configuration, all a pilot had to do was contact his local Civil Aviation Authority (CAA, precursor to the FAA) representative, who would come out and inspect the workmanship for obvious flaws and double-check the aircraft's weight and balance data. He then would grant a "field approval" for the modification and the pilot was free to take his creation (and often his unwitting passengers) into the air. Less successful efforts included tractor and even automotive racing tires, though some of the latter can still be seen at some Alaskan airports.

"Those Mickey Thompson [Schneider] drag slicks were absolutely incredible," says Duncan, the tire maker. "They weighed a ton and were about as aerodynamic as a barn door. They weren't too bad in the rough stuff, but if you tried to land on a paved runway with them, they'd stick hard immediately and flip the airplane over on its back."

Back when 17,000-hour bush pilot F.E. Potts began his Alaska flying career in the mid-1960s, the most popular tundra tire among professional Super Cub pilots was the well-rounded 24-inch balloon tire similar to Phipps' original design. These early tundra tires, he says, struck an adequate balance between shock absorption, weight and drag, and the light footprint needed for off-airport operations. Modifications and improvements to the original 1950's Super Cub design also contributed to the popularity of oversize tires among hunting guides, charter operators and other everyday fliers in the far North.

"It pretty much became the standard to equip the Super Cub with the oversize Borer prop, which greatly improves takeoff and climb performance," says Potts, whose 1993 book, F.E. Potts' Guide to Bush Flying, is viewed by many as one of the best references yet written on the subject. "The STC [supplemental type certificate] to do this requires increased ground clearance, which is most easily achieved by installing large tires."

Such modifications notwithstanding, accidents involving tundra tire-equipped Super Cubs still happened with alarming frequency but at the time, most were attributed to factors unrelated to tire selection. Pilot error was cited as the most common cause of bush crashes, specifically, poor judgment with regard to the harsh Alaskan outback environment. Also contributory was the so-called "bush pilot attitude," a tendency on the part of some operators to take unnecessary risks, such as flying into deteriorating, non-visual weather conditions, attempting ill-advised landings on rough, inhospitable terrain, flying overloaded or carrying insufficient fuel. The general inexperience of some pilots new to the Alaskan scene was another probable cause of many light aircraft accidents, a trend that persists to this day.

"I encountered a lot of 'wannabe' bush pilots when I was living in Wasilla during this period, and they did show their ignorance and ego in a lot of ways," says Potts. "But the issue of tire size never came up, because all Super Cubs in Alaska used the large tires as a matter of course."

Moose and the GovernmentTire size eventually did become an issue, however, after a string of low altitude stall/spin accidents involving Super Cubs occurred during Alaska's 1994 moose-hunting season. Pilots and passengers would be flying at minimum airspeed just above the trees in a steep turn, focused outside the airplane looking and pointing at a moose, and they'd get too slow, stall, spin and crash.

Called in to root out the cause of the carnage, investigators from the National Transportation Safety Board (NTSB) discovered what Alaskans had been up to for the past three decades. Not only was nearly every accident aircraft equipped with tundra tires-established practice dictated that the field approvals for the tires were issued without requiring the owner-operators to perform flight-testing or supply other data regarding the potentially adverse effects of the modification.

In response to the NTSB's Safety Recommendation dated February 7, 1995, the FAA in April of that year conducted a series of tests designed to determine the effects of tundra tires on aircraft performance, stall behavior and overall handling qualities. Mounted on a generic PA-18-150 Super Cub, five tire sets were evaluated at various weights and center of gravity positions-the factory standard 8.00 X 6 tires, McCreary 8.50 X 10 tundra tires, McCreary 29.00 X 11 tundra tires, Schneider Racing Slicks (14.00 X 32) and Goodyear Airwheels (35.00 X 15).

As the flight tests proceeded on the Lake Hood Strip, a 2,200-foot gravel airstrip three miles southwest of Anchorage, they revealed that forward field of view during taxiing is inversely related to tire size. As the tire size increases, the ability to see over the nose decreases, requiring pilots to make "S" turns in order to see ahead. Brake effectiveness was also reduced, specifically the ability to hold against takeoff power. Still, ground handling during takeoff from a gravel runway was found to be satisfactory for all of the configurations tested.

During landings the fly-off revealed a noticeable nose-down pitching moment when the tires contact the ground, particularly when a wheel landing (main tires contact first, then the tailwheel touches down) is performed. Though no tests were undertaken on paved runways, the FAA concluded that the ground handling characteristics of tundra tire-equipped aircraft are substantially poorer on pavement than on gravel, grass and other surfaces that allow the tires to skid easily.

In the air, tundra tires clearly had an adverse effect, reducing the aircraft's top speed, rate of climb, angle of climb, range, useful load and stall warning (buffet) margin. Data from the tests did not, however, validate the theory that oversize tires raised the PA-18's stall speed. In fact, stall characteristics were found to be normal in level flight and in a turning stall, as the airplane generally rolled to a wings-level attitude. But in maneuvering flight, the nose tends to drop as bank angle is increased. If the pilot uses "top rudder" to counter this effect and stalls the aircraft while cross-controlled in this manner, the airplane tends to roll rapidly over the top and enter an incipient spin.

Should this sequence occur at low altitude, such as while circling to spot a moose, the aircraft typically crashes before the pilot can recover. To make matters worse, some Super Cub operators had modified their wings for higher lift by removing the standard 2.5 degrees of washout at the wing tip. Aircraft designers usually incorporate a measure of washout to ensure that the inboard section of the wing stalls first, a practice that promotes benign stall characteristics. Stalling a cross-controlled aircraft in a turn with the washout removed accentuates its already pronounced tendency to depart controlled flight and spin.

Resistance to the FAA's findings was vocal, to say the least. Super Cub operators maintained that the tests only proved that the airplane's tendency to go "over the top" in an uncoordinated stall occurred even when the standard factory tires, the 8.00 X 6s, were used. This problem, they say, is common to all light aircraft flown low and slow, not just the Super Cub. "In other words, it's a little airplane thing, not a tire thing," asserted one high-time Alaska-based pilot who observed the proceedings firsthand and requested that his name be withheld. "Virtually any airplane, when stalled in an uncoordinated condition, will have some fairly aggressive stall tendencies."

Tundra Tires Come of Age Ultimately, the FAA ruled that while tundra tires did degrade Super Cub performance to some degree, the effect was "not significant with regard to safety." It has been said that the agency knuckled under to pressure from the Alaskan aviation community; others believe the FAA embarked on its study with the preconceived notion that tundra tires are unsafe but backed off when its data failed to support that conclusion. Still others are convinced that the government correctly weighed the potential risks of tundra tires against the safety enhancement they provide in off-airport operations.

One action the FAA did take in the wake of the 1995 study was to suspend its long-standing field approval process pending an overhaul of the system. It now requires its Air Safety Inspectors to receive specific training prior to signing off any tundra tire installation. All field-approved tundra tires must have either Technical Standard Order (TSO) or Parts Manufacturer Approval (PMA) approval-the same requirements imposed on all parts destined for certified, as opposed to homebuilt, aircraft. All new installations must be flight-tested and operators must ensure that all wing, tail and control surfaces have not been improperly altered. Their aircraft must also be weighed, and worst-case forward and aft center of gravity limits for its new weight determined. Passengers may not be carried on the test flights and tire diameter is limited to 35 inches.

This tightening of the rules has proven a boon for Alaskan Bushwheels, whose tires-like those of a small number of competing manufacturers-have the government's blessing and then some. In March 2002, the company received TSO approval for an improved Bushwheel, a 31-inch radial with a stiffer, Kevlar-reinforced sidewall. This tire is lighter and has a narrower profile than the original bias-ply model, so it generates less parasite drag and thus imposes less of a performance penalty.

Duncan and his team are also offering a new "tundra tailwheel," built to be used in concert with their oversize main wheel tires. The newest supersized Bushwheel, unrepentantly massive at 35 inches in diameter, is earmarked for a pair of revered bush aircraft-the Canadian de Havilland Beaver and the Swiss-made Pilatus Porter. This tire, in theory, can safely negotiate boulders as large as living room televisions. Though most recreational pilots seldom encounter obstacles this big during the course of their weekend hamburger run, at least they'll be ready if they do.

"This really isn't about tundra tires at all," says Potts, who survived 22 years of Alaskan bush flying without ever damaging an aircraft. "The issue is that we need frontiers, both physical and mental, to lead a full life, and frontiers are something we no longer have. There is no question that the type of life one could lead in the Alaska of the 1960s, in its freedom, self-reliance and overall quality, is a compelling image for many people today. Huge tires, along with skis and floats, are unquestionably part of that image."

Paul Richfield is an aviation writer living in Roswell, New Mexico.