Runway length requirement is a function of the square of the minimum flying speed (stall speed), and most design effort is spent on reducing this number. For takeoff, large power/weight ratios and low drag help the plane to accelerate for flight. The landing run is minimized by strong brakes, low landing speed, thrust reversers or spoilers (less common). Overall STOL performance is set by the length of runway needed to land or take off, whichever is longer.
Of equal importance to short ground run is the ability to clear obstacles, such as trees, on both take off and landing. For takeoff, large power/weight ratios and low drag result in a high rate of climb required to clear obstacles. For landing, high drag allows the aeroplane to descend steeply to the runway without building excess speed resulting in a longer ground run. Drag is increased by use of flaps (devices on the wings) and by a forward slip (causing the aeroplane to fly somewhat sideways though the air to increase drag).
Normally, a STOL aircraft will have a large wing for its weight. These wings often use aerodynamic devices like flaps, slots, slats, and vortex generators. Typically, designing an aircraft for excellent STOL performance reduces maximum speed, but does not reduce payload lifting ability. The payload is critical, because many small, isolated communities rely on STOL aircraft as their only transportation link to the outside world for passengers or cargo; examples include many communities in the Canadian north and Alaska.
Most STOL aircraft can land either on- or off-airport. Typical off-airport landing areas include snow or ice (using skis), fields or gravel riverbanks (often using special fat, low-pressure tundra tires), and water (using floats): these areas are often extremely short and obstructed by tall trees or hills. Wheel skis and amphibious floats combine wheels with skis or floats, allowing the choice of landing on snow/water or a prepared runway.
Micro Dynamics vortex generators mounted on the wing of a Cessna 182K
A number of aircraft modification companies offer STOL kits that can be installed on aircraft to improve their short field performance.
Horton, Inc of Wellington, Kansas offers STOL kits under the brand name Horton STOL-Craft, emphasizing that the modifications increase safety by allowing forced landings to occur at lower speeds and thus improve survivability. The Horton modifications include a drooped leading edge cuff, conical cambered wingtips, control surface gap seals and wing fences. The company says: "On an average you can expect to get a 4-7 knot reduction in stall speeds. Flying at these lower stall speeds you can reduce the take-off and landing distances by 10%". Horton STOL kits are available for several Cessna and Piper PA-28 models.
Micro AeroDynamics markets vortex generator modification kits for "STOL benefits". The Micro kits are small vortex generators that are glued to the wing leading edge, as well as the underside of the elevator and on the fin. Kits are available for a large number of light aircraft types.
Sierra Industries sells Robertson STOL kits, marketed under the name R/STOL, incorporate a drooped leading edge cuff, wing fences, drooping ailerons and an automatic trim system. The company says that installation "allows 15 to 25 MPH slower approaches and requires up to 40% less runway distance". R/STOL kits are available for various Cessna models.
Stolairus Aviation of Kelowna, BC has developed STOL Kits for the de Havilland Canada DHC-2 Beaver and de Havilland Canada DHC-3 Otter to increase lift and reduce stall speeds. The DHC-2 Beaver STOL Kit includes a contoured leading edge, flap-gap seals, wing fences and drooped wingtips. The DHC-3 Otter STOL Kit includes a contoured leading edge and drooped wingtips.
A STOLport is an airport designed with STOL operations in mind, normally having a short single runway.
CESTOL or Cruise Efficient Short Takeoff and Landing, is sometimes used to describe aircraft with both very short runway requirements and high cruise speeds (greater than mach 0.8).
Many different definitions of STOL have been used by different authorities and nations at various times and for a myriad of regulatory and military purposes. Some accepted definitions of STOL include:
short takeoff and landing: (DOD/NATO) The ability of an aircraft to clear a 50-foot (15 meters) obstacle within 1,500 feet (450 meters) of commencing takeoff or in landing, to stop within 1,500 feet (450 meters) after passing over a 50-foot (15 meters) obstacle. Also called STOL.
Department of Defense Dictionary of Military and Associated Terms (JP 1-02)
"STOL (Short Take Off and Landing). STOL performance of an aircraft is the ability of aircraft to take off and clear a 50-foot obstruction in a distance of 1,500 feet from beginning the takeoff run. It must also be able to stop within 1,500 feet after crossing a 50-foot obstacle on landing."
"An aircraft that, at some weight within its approved operating weight, is capable of operating from a STOL runway in compliance with the applicable STOL characteristics and airworthiness, operations, noise, and pollution standards" and ""aircraft" means any machine capable of deriving support in the atmosphere"
"A STOL aircraft is an aircraft with a certified performance capability to execute approaches along a glideslope of 6 degrees or steeper and to execute missed approaches at a climb gradient sufficient to clear a 15:1 missed approach surface at sea level... A STOL runway is one which is specifically designated and marked for STOL aircraft operations, and designed and maintained to specified standards."
"Heavier-than-air craft that cannot take off and land vertically, but can operate within areas substantially more confined than those normally required by aircraft of the same size. Derived from short takeoff and landing aircraft."
McGraw-Hill Dictionary of Scientific & Technical Terms
"short takeoff and landing aircraft (STOL), heavier-than-air craft, capable of rising from and descending to the ground with only a short length of runway, but incapable of doing so vertically. The precise definition of an STOL aircraft has not been universally agreed upon. However, it has been tentatively defined as an aircraft that upon taking off needs only 1,000 ft (305 m) of runway to clear a 50-ft (15-m) obstacle at the end of that distance and upon landing can clear the same obstacle and then land within 1,000 ft."
"The STOL mode of flight is one during which an airplane taking off or landing is operated at climb-out and approach speeds lower than the conventionally accepted margins of airspeed above the power-off stalling speed of the airplane."
Lieutenant Colonel Walter P. Maiersperger, USAF (Ret)
Additionally some aircraft manufacturers market their products as STOL, without providing evidence that the aircraft meets any accepted definition.