DESPITE two catastrophic air crashes (the disappearance of MH370 over the Indian ocean, and the shooting down of MH17 over Ukraine), 2014 was the safest year on record for civil aviation—with only one accident for every 4.4m flights. All told, 73 airline accidents occurred, including 12 involving fatalities. Some 641 passengers and crew died as a result. Even if the 298 lives lost over Ukraine are included (aircraft destroyed through acts of war are not classified as accidents), 2014 would still have been the safest ever.



Yet, for all the flying aids and safety measures fitted to modern aircraft, mistakes made in the cockpit continue to claim more lives than other causes, be they mechanical, electronic or weather related. Even among commercial airlines, carelessness, fatigue and lack of experience by flight crew account for around 60% of fatal air crashes. For private planes, pilot error is responsible for a greater proportion still. As the leading cause of fatal air crashes, pilot error has remained stubbornly high since the 1950s, and shows no sign of decreasing.



The most dangerous phases of flight are take-off and initial climb (which account for 20% of fatalities, despite amounting to just 2% of the duration of a typical flight), and final approach and landing (36% of fatalities during 4% of flight duration). In both of these phases, the aircraft is at its most vulnerable, aerodynamically speaking, because it is close to its stall speed. And, because the flight crew has a lot to do during these brief periods, they can easily make mistakes.



More than 40% of pilot fatalities are the result of a stall that turns into a spin. If this happens at low altitude, there is little room for recovery. And low altitude is just where it does tend to happen. Some 80% of stall-spins occur within 1,000 feet (300 metres) of the ground.



An aircraft stalls when its wings present too great an angle to the oncoming airflow. In level flight, this “angle of attack” is 3° or 4°. Increasing it increases the lift generated, but only up to a maximum of between 15° and 20°. Above that, eddies start to form in the airsteam flowing over the wing, causing that flow to separate from the wing's surface. When this happens, the amount of lift starts to fall off dramatically. If the angle of attack increases further, the separation point works its way forward from the wing's trailing edge until it reaches the fattest part, near the front. That creates a large turbulent wake with lots of drag and little lift. As a result, the plane starts to fall out of the sky.



A stall is nothing to fear, provided the plane is high enough for the pilot to recover by pushing the nose down. The danger comes when trying to change direction while flying slowly at low altitude. In this case, the air flowing over the wing on the inside of the turn moves more slowly than that over the wing on the outside of the turn. That can cause the inner wing to drop while the outer one flies normally. The difference in lift between the two wings then makes the plane roll, while the difference in drag makes it yaw. A stalling aircraft that is both rolling and yawing is spinning towards the ground, corkscrew fashion.



Clearly, if planes could be designed so they would not stall, they would be unable to spin in this way. Surprisingly, this has proved hard. The only aircraft in series production to be spin-resistant is the Icon A5 (see photo above), a recently launched amphibious light-sport plane built in California. Several other spin-resistant prototypes have been built over the years, but have failed to make it into commercial production.

One reason is cost. Fabricating a spin-resistant airframe with leading-edge “cuffs” and other lift-generating devices is not cheap. It is less expensive to endow a plane (at least small ones) with a parachute all of its own, to lower it down gently in case of an emergency. Another reason is weight. The spin-resistant features required Icon to seek dispensation from the FAA for breaching the amphibian light-sport aircraft's 1,430 pound (650kg) class limit. The plane has now been approved for a maximum take-off weight of 1,680 pounds



Why some aircraft are prone to spinning and others not was a mystery for much of aviation’s history. It was not until the 1970s, when NASA’s Langley Research Centre, in Virginia, started a systematic study, that a proper understanding of spin-resistance came about. The outcome was the FAA’s Part 23 spin-resistant standard, introduced in 1991. Since then, aircraft designers have known broadly how to make an aircraft handle more benignly when close to a stall and thus avoid the risk of spiralling into the ground.



The most common approach is to build “washout” (twist) into the wings, so the part towards the tips has a lower angle of attack than the part nearer the fuselage. With its greater angle to the airflow, the inner section stalls first, leaving the outer part to continue generating lift—and thus prevent the wing from dropping abruptly. This arrangement also allows the ailerons (located on the outboard trailing edges) to remain effective, countering any tendency of a stalling plane to roll.



Another modification is to add a slightly drooped extension, or cuff, to the outboard part of the wings’ leading edge, as Icon has done. Like washout, this drooped extension reduces the angle of attack of the wings’ outer portions, preventing a stall from spreading towards the tips. Also, because a cuff forms a sharp discontinuity about half way along the leading edge, it acts as a strong vortex generator that sends swirling air over the wings and ailerons. This prevents stalled air from migrating outwards from the wing root towards the tip, while at the same time energising the air flowing over the ailerons. Again, that helps the pilot retain control during a stall.



Though originally devised for other purposes, “winglets”—common on passenger jets and increasingly on light aircraft—help curb stalls, too. The winglets’ main job is to block air circulating around the tips of the wings from the high-pressure underside to the low-pressure region above. Normally, this circulation causes powerful vortices to swirl off the wing tips, inducing extra drag. Winglets spread these tip vortices over a larger area, reducing the drag and increasing the lift. That helps lower the stall speed, and makes it easier for the plane to recover from a stall.



Other tricks include adding stall strips along a wing’s leading edge; metal "fences" running from front to back across the top of the wings; vortex generators in the form of vertical plates located near the leading edge; and slots through the leading edge that feed high-pressure air from the wing’s underside to its upperside, energising the boundary layer and keeping it attached to the surface for as long as possible. All such design features can help prevent a stall turning into a spin.

Besides adding leading-edge cuffs to the outer halves of the wings, Icon's designers also modified the A5's aerofoil sections, so that the airflow would separate more gradually as the wing approached its critical angle of attack. However, doing so meant adding flaps to the trailing edge to improve the plane's take-off performance on water.

Turning a spin-recoverable aircraft into a spin-resistant one incurred cost- as well as weight-penalties for Icon. But making a plane that is as easy to fly as driving a car has opened a whole new market for the A5. Even before the first customer took delivery in July, Icon had racked up over 1,200 orders for the stall- and spin-resistant aircraft.

Now the Icon A5 has proved that it is possible to build planes that cannot stall or spin, other makers may well follow suit. All it takes, like breaking the four-minute mile, is for someone to prove it can be done.