Short Matters

Tools and tactics for safe STOL operations

August 1, 2008

There’s an island in the French West Indies where the jetsetters play, but you can’t land a jet there. It’s called Saint Barthélemy —St. Barts for short—and it’s got a 2,100-foot airstrip with a mountain on one end and the ocean on the other. Approach from the beach side if you dare, but there’s no go-around, so the recommended procedure is to skim over the top of the mountain, then nose down, and hug the terrain until reaching the touchdown zone. Local pilots refer to this as “flying the solid glideslope.”

Setup is the key—the trick is to be stabilized in landing configuration at least a couple of miles out, at an airspeed slow enough to prevent floating, but fast enough to compensate for jittery crosswinds and turbulence. Once over pavement, you’ve got about two seconds to get the airplane planted, raise the flaps, and brake hard without flat-spotting the tires. If you float, or hold it off in hope of a smooth touchdown, a water evacuation is probably in your immediate future.

St. Barts is challenging, but other strips in the developing world are in another league. Papua New Guinea, for example, is known for hundreds of challenging airstrips hacked into mountainsides. Kanainj, located in the eastern highlands, is notorious. To get there, airplanes thread their way between peaks reaching 16,000 feet or higher, and a 9,500-foot-cruise altitude only keeps you a spear’s throw from the dense equatorial jungle below. Virtually all flying is done early, before the daily cloud buildups close all the passes.

Since the airport is lodged between two steep ridges, you’re committed to landing before having the severely up-sloping runway in sight. Plan to slam it down hard on the near end, and power taxi uphill to the parking area, where you’ll see a native village along with an impressive mound of rusted aircraft wreckage that for many years represented a significant percentage of the Britten-Norman Islander production run.

Area tribesmen consider airplane crashes entertainment at its finest, and the distant thrum of engines quickly attracts a crowd. The takeoff from Kanainj—downhill regardless of the wind conditions—is not unlike an Olympic ski jump, minus the snow.

Airplanes with attitude

Even in New Guinea, most remote area airstrips are built with commerce in mind, and are built to be accessible to light aircraft and the occasional heavy one as well. Still, twin turboprops in the de Havilland Twin Otter class are about as large as most remote area operators elect to go. There are myriad exceptions, of course. A former employer of mine once landed a lightly loaded Douglas DC-3 at St. Barts with room to spare, a spectacular feat of airmanship that came perilously close to impressing the French.

But the need to operate from truly short fields and rough clearings, while providing a valid alternative to short-ranged helicopters, has led to the development of dedicated short takeoff and landing (STOL) aircraft. Notable models include the Fieseler Storch, the Westland Lysander, the Scottish Aviation Twin Pioneer, the Dornier Do 27/28, the Socata Rallye, the PZL Wilga, the Maule M-series, the de Havilland Beaver, the Helio Courier, the Pilatus Porter, the Islander, and its larger offspring, the Trislander trimotor.

Other, relatively mundane general aviation types have also proven capable STOL performers, including the Piper Super Cub and its many variants and imitators, the larger Cessna piston singles and light twins, the Piper Aztec, and the Partenavia (now Vulcanair).

And then there’s the Caravan. While Cessna’s 208 single-engine turboprop has grabbed much of the remote area transport/piston twin replacement market, it should never be considered a modern-day Storch. This is because of the decision to go with the thin, relatively fast wing from the 300/400-series twins. To obtain a 180-knot cruise speed, extreme short field performance was sacrificed.

The 208’s newest rivals in the utility/heavy hauler market include the Quest Aviation Kodiak, the Gippsland Airvan from Australia, and Canada’s Found, which recently launched the up-engined and tricycle-geared Expedition version of the Bush Hawk. For those needing a twin, there’s the pusher-configured Angel 44. Cessna’s 182 and 206 are back in production but the latest iterations of these airplanes, with their glass cockpits and plush interiors, seem more about neckties and briefcases than grass skirts and shrunken heads.

High-wing, tailwheel-equipped aircraft dominate the low end of the FAA-certified STOL market, with Aviat Aircraft’s Husky series leading a small but aggressive pack. (Full disclosure: I’ve got one of these). Cubcrafters offers a line of Super Cub derivatives while American Champion has the Scout and the High Country; two evolutions of the Champ/Citabria line. Equipped with optional skis, floats, or high-flotation “tundra” tires, this class of airplane has enjoyed a renaissance in recent years.

For the majority of general aviation pilots, however, remote area STOL operations are an additional capability, something they’d like to do should the need or opportunity arise, without sacrificing too much in the way of passenger comfort or cruise speed. Lucky for them, various modifications are available, proven, and if properly applied, can turn even the most prosaic of pattern plodders into formidable short-field assault craft.

STOL modifications

For many general aviation airplanes, small aluminum tabs known as vortex generators (VGs) offer an inexpensive way to improve low-speed handling and pad the safety margin above the stall. They work by stimulating the airflow over the upper surface of the wing and the lower surface of the horizontal stabilizer, creating vortices that “energize” the boundary layer and delay the onset of a stall. As long as this boundary layer remains intact, the pilot can maintain control authority despite low airspeeds and high angles of attack.

At cruising altitudes above 10,000 feet, VGs provide the added benefit of alleviating the control mushiness common when airplanes near their performance limit in thin air. This attribute can significantly reduce pilot workload, making long flights without an autopilot a lot less tiring.

VG installation on most light aircraft is easy; two people can do the job in a few hours if sufficiently caffeinated. One of them should probably be an A&P/IA who can sign off the work. The kit typically comes with transparent Lexan templates with cutouts for the individual VGs, which are glued in pairs on the wing in a spanwise line a few inches aft of the leading edge. Don’t worry about the glue holding—it’s a two-part Loctite epoxy of ridiculous strength.

VGs have their critics. Some allege that the devices induce drag that results in minor speed losses; others believe that tampering with an airplane’s stall characteristics opens the door to unpredictable behavior—such as severe wing drop—when the stall finally occurs at a very high angle of attack.

I can confirm that taxiing a VG-equipped taildragger rapidly in high winds requires full attention, because the wing never really stops flying and an errant gust can launch you back into the air. With such concerns in mind, it makes sense to review anecdotal information regarding VG performance for your particular aircraft type prior to buying the hardware and the STC.

Beyond VGs

For operators looking to take the next step toward STOL supremacy, a number of vendors produce various wing and control surface modifications. These products have been available for decades now, and enjoy a loyal following and steady repeat business. Horton, for example, offers a STOL conversion kit encompassing a wing upgrade, conical cambered wing tips, control surface (gap) seals, and stall fences.

The wing modification consists of inboard and outboard leading edge cuffs that extend the chord line forward and down, while enclosing a new landing light and increasing wingspan by 10 inches. The conical wing tips help by increasing wing area and improving lift, decreasing stall speed and improving aileron effectiveness at low speeds. They also help to reduce drag, by reducing the strength of wing tip vortices.

Gap seals—typically aluminum strips—work by keeping high-pressure air beneath the wing from leaking through the space between the aileron and the wing. If allowed to pass freely, this air can cause early burbling of airflow over the aileron at high angles of attack, potentially inducing a stall. Wing fences, the last element of the Horton STOL conversion, control the spanwise progression of a stall, preserving aileron effectiveness as the angle of attack is increased.

Robertson’s R/STOL Hi-Lift System overlaps the Horton package and then takes the idea a step farther. It includes leading edge cuffs and stall fences, but with the addition of drooped ailerons to increase outboard wing lift, and an automatic trim system linking the stabilizer to the flaps. The latter feature relieves the pilot from the chore of frequent re-trimming during flap application on an approach, and while reconfiguring the airplane after takeoff or during a missed approach.

Unlike most other STOL kits offering amended speeds “equal or lower” than book figures, the Robertson system comes with a new airspeed indicator faceplate, adding the weight of authority to the STC’s performance claims. My own limited experience with the R/STOL mod was entirely positive—a Cessna 402B I flew for six months down in the Caribbean had the full package (plus extended “zap” flaps), allowing me to approach the hill at St. Barts with complete confidence at a very un-twin-like 75 knots with nine people and their luggage aboard. (If only it stopped the passengers’ screaming!)

No substitute for training

While these modifications and others—such as automatic leading edge slats, spoilers, and bolt-on canards—may improve an airplane’s low-speed performance, they’re no substitute for maturity, good habits, and experience.

STOL work isn’t just about flying the airplane more slowly—short-field operations in remote areas imply a sophisticated decision-making process that pushes limits closer to the physical and regulatory edge than many GA pilots might be accustomed.

Loading is critical; often the runway length at an interim destination dictates the amount of fuel and payload that can be safely carried from the start. Beware of common STOL kit pitfalls, such as the tendency to climb out at inordinately high pitch angles, just because you can. This is a good way to overheat a closely cowled engine. Also, resist the urge to approach at ultra-slow, backside-of-the-power-curve airspeeds, where you need gobs of power to compensate for the lack of airflow over the wings and tail. These types of landings, in an area of the flight envelope that engineers (and know-it-alls) call “the region of reverse command,” are for experts only.

Don’t underestimate the value of training, and grab it when and where you can. Professional pilots, and those flying for the various missionary and relief organizations, typically undergo a long apprenticeship and often must complete a series of supervised flights and a flight evaluation with a government examiner or designee before being let loose on their own. Then they’re checked at periodic intervals, and checked again.

Unfortunately, there are few training resources available for private fliers seeking entrèe into the STOL world, although a number of schools offer courses in related topics, such as mountain, ski, or float-flying. To begin, it’s a good idea to read everything available on the subject, and cruise the hangar rows until you find a pilot with thousands of hours logged in actual, remote-area flying.

With luck, this person will be amenable to passing on some of his or her hard-won knowledge. Your local, airline-hungry flight instructors might talk a great game, but their expertise in perfunctory “short- and soft-field techniques” has little or no application here. Are STOL pilots ever self-taught? Of course—the tribesmen over at Kanainj are some of their biggest fans.