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There’s only one foolproof way to ensure you don’t end up as an accident statistic in yet another stall/spin fatal crash: Avoid stalling in the first place. Yet if the goal is to avoid stalling, why do we spend so much time practicing stalls? After all, 97 percent of stall/spin accidents that start at or below pattern altitude are unrecoverable. In the typical light single-engine airplane, if you enter a stall and it progresses into a spin from this height, your chances of surviving to tell the tale are close to zero.
We practice stalls so we can recognize their symptoms and etch into our muscle memory how to recover with a minimum of altitude loss. But such practice is not of much value if our primary goal is to teach recovery from the types of stalls that are most likely to get us killed — for example, accelerated stalls on takeoff or while maneuvering at low altitude.
Think about the last time you practiced stalls with an instructor. Chances are one of the setups involved adding full flaps, pulling the power and easing back on the yoke until the nose dropped and you initiated a recovery. It’s valuable for enhancing our understanding of how the airplane behaves at low airspeeds, but as a practical maneuver to save our bacon after we’ve made a critical error in airmanship while low and slow, we might as well be learning how to program the music stations on the XM radio receiver.
The stall/spin accident I always think back to is the crash of a Comanche 250 at the Sun ’n Fun Fly-In in 1996. That was the first year I flew into Lakeland, Florida, for the airshow. At the time, I could not fathom how the pilot had managed to make a smoking hole a quarter mile short of the runway on a gorgeous day that was just about perfect for flying. The fact that he crashed only minutes after I’d arrived on the same day and headed for the same runway left an impression.
It was the classic stall/spin accident scenario on the base-to-final turn: The pilot entered a tight downwind for Runway 9L and then, on the left base, he flew through the extended-centerline course to final. Sensing that he was overshooting, he tightened his turn by increasing his bank angle to about 45 degrees. At this slow airspeed and high load factor, the Comanche predictably stalled. As drag on the left wing increased, it dropped suddenly and the airplane spiraled into the ground, killing all three on board.
I think you’d agree that this pilot didn’t have a thorough understanding of stalls. Sadly, that’s probably true of a lot of pilots, who fear stalls simply because they seem so mysterious. If we could see the air, a stall would be no more enigmatic than the flow of water over and around rocks in a river. As we pull back on the yoke to practice a stall, the air flowing smoothly over the wing begins to separate from its surface like water splashing over a boulder. The control forces become light as the air burbles and eddies and backfills until lift is destroyed. We feel a rumble and hang against our seat belt as the nose falls and maybe a wing drops. We’re no longer flying; we are falling.
Safe Flight supplies angle-of-attack systems in a variety of airplanes, such as this Challenger 604. Improved flight-path stability is just one of their many benefits.|
You may have noticed that up to this point I’ve avoided mentioning the most important factor of all: angle of attack (AOA). That was intentional, so that I could make the point that lift and not airspeed determines when a wing will stall. How strange, then, that we talk in terms of “stall speed” and not critical angle of attack when discussing stalls and spins. For people in the business of designing angle-of-attack systems, “airspeed” is a dirty word when talking about stalls. The worn maxim “Speed equals life” simply isn’t true. “Lift equals life” is more accurate. And the only way to accurately provide a presentation of lift to the pilot is with an angle of attack indicator, not an airspeed indicator.
There are really only four reasons why the airplanes we fly have airspeed indicators anyway, and none of them have to do with predicting or avoiding stalls. The first is that the FAA says we must install them. The other three are based on airframe limitations, not stall characteristics. We need to know when it’s safe to put the gear down and lower the flaps and, additionally, what VNE — never exceed speed — is. Nothing on the airspeed indicator will tell you in absolute terms when you are about to stall. Think of it like driving down the road using only your tachometer to tell you how fast you’re going. You can do it, but it’s an imprecise measurement — and when it comes to measuring the reserve of lift we have remaining before the wing will stall, imprecision is exactly what we want to avoid.
Angle-of-attack indexers, like this one from Alpha Systems, indicate when the airplane is flying too slow or too fast on approach.|
The Birth of Stall Warning
Randy Greene understands this principle as well as anybody. His father, Dr. Leonard Greene, invented the stall-warning indicator (as well as the stick shaker, autothrottle, wind shear warning and more than 200 other things).
Today, Randy Greene is president of Safe Flight Instrument Corp., founded by his father in 1946 in White Plains, New York, to make and sell lifesaving gear for aircraft. The company has produced hundreds of thousands of stall-warning and AOA devices for airplanes. The stick shaker is another stall-related lifesaving invention pioneered by Safe Flight. It physically shakes the control column as a pre-stall warning meant to simulate stall buffet. Stick pushers, meanwhile, physically lower the nose to unload the wing and keep an airplane flying. All of these technologies have the same purpose: avoiding stalls and saving lives.
Not surprisingly, Safe Flight is also a leading manufacturer of AOA indicators. The technology behind its indicators relies on lift detectors and transducers, speed control systems, and swept-vane and paddle-vane AOA sensors. They are useful not only at speeds that correspond to stalls but also at speeds about 40 percent above this margin, where they can be used to maximize range. After all, if you can perfectly measure lift, you can also gain the maximum benefit from that measurement.
Greene demonstrated Safe Flight’s AOA indicator for us in the company’s Beech Baron. Integrated with the stall-warning tab on the wing, the indicator provides a direct measurement of lift. To put the indicator to the test, Greene flew an approach at exactly 1.3 times Vso into the 1,981-foot runway at Andover Aeroflex Airport in New Jersey. We were down and stopped with 500 feet of pavement to spare. “An extra 5 knots means taking up 500 more feet of runway,” he said. “Going into an airport like this in a Baron, that’s important.”
As you already know, a stall occurs when the critical angle of attack is reached, not when we go below a certain airspeed. We also know that lift increases as angle of attack increases — to a point. At a certain angle of attack — about 15 degrees in most light airplanes — half or more of the lift abruptly disappears. At the same time, drag increases dramatically. High bank angles can be extremely dangerous, because the increase in load factor increases the stall speed — it is what’s known as an accelerated stall.
At approach speeds, an AOA indicator can be used to give instantaneous fast-slow indications that tell the pilot whether he or she is flying at the optimal target airspeed. Being too fast on approach can be just as dangerous as being too slow. Many flight students are taught to respect high wing loading and the corresponding increase in stall speed by keeping their speed up when maneuvering close to the ground, such as during the base-to-final turn. That leads to wide base legs, shallow turns and high airspeeds, all of which aren’t necessarily dangerous practices, but they ignore what our true aim should be: flying at the optimal approach airspeed for the conditions, including load factor, weight, altitude, flap setting and even the application of engine power.
Autopilots, such as the DFC90 from Avidyne, can take control of the airplane to prevent a stall.|
For the past 17 years Mark Korin has been delivering this message to anybody who would stop and listen as they strolled past his booth at Sun ’n Fun or Oshkosh. Korin is a design engineer, who in 1996 founded Alpha Systems, a maker of AOA indicators for light airplanes that utilize basic cone-style sensors that measure pressure differential to determine angle of attack. As long as the probe is installed in undisturbed air a minimum of about two feet outside the propeller arc, the system can give pilots an accurate indication of lift reserve that is unaltered by aircraft weight, temperature or altitude. Once the system has been calibrated in flight with the press of a button to the “optimum alpha angle,” the pilot can use the indicator in the cockpit to see at a glance whether he’s flying too fast, too slow or at the optimal approach airspeed.
The cost for that increased margin of safety is surprisingly affordable. Alpha Systems sells 12 versions of its mechanical and electronic AOA indicators, with prices ranging from $1,000 for the most basic setup to $2,200 at the high end. Best of all, the company received a letter from the FAA’s Small Airplane Directorate permitting installations of its systems in existing inspection-plate locations as “minor alterations” that do not require a supplemental type certificate.
The Cirrus cuffed wing is designed to improve low-speed handling by incorporating a slightly lower angle of incidence on the outer portion of the wing.|
The FAA itself is helping to spread the word about the value of angle-of-attack information. This spring the agency announced it is streamlining the approval process for AOA indicators. The agency is also addressing the need for AOA information in the cockpit with its proposed rewrite of Part 23 aircraft certification standards. The General Aviation Joint Steering Committee (GAJSC), co-chaired by the FAA and AOPA, recently released a 148-page report that addresses loss-of-control accidents in the approach-and-landing phase, which the group notes were responsible for 40 percent of fatal general aviation accidents from 2001 through 2010. The report’s authors said they want general aviation to “embrace to the fullest extent” AOA indicators.
Spin-Resistant Design
Icon Aircraft is one of the first light-airplane makers to announce it will include AOA indicators as standard equipment. The company has also made the claim that its A5 light sport amphibian has been designed and tested to meet Part 23 spin-resistance standards.
The earliest attempts to create a spin-resistant airplane date back to the early days of flight, well before World War II. Most famously, the Ercoupe was developed in an attempt to be safer than other light airplanes by being less susceptible to spins. Through simple means, such as limiting control-surface deflection and CG range, the airplane was certified as “characteristically incapable of spinning” by the Civil Aeronautics Administration (the FAA’s predecessor). However, to achieve this, the Ercoupe’s rudder was mechanically linked with its ailerons, preventing the pilot from actively controlling aircraft yaw.
The Cirrus SR20 and SR22 and Cessna Corvalis employ a cuffed wing design to advance stall- and spin-resistance characteristics, although they do not meet the Part 23 spin-resistance standards. There are a variety of other ways wings can be made stall-resistant. The most common are leading-edge devices, such as slats or Krueger flaps, or enlarged or drooped leading edges. These have the effect of either forcing more air up over the wing to fend off the forward flow or of making the trip around the leading edge less strenuous, so that the air has more energy left when it encounters the burbles of an oncoming stall. Vortex generators, meanwhile, delay airflow separation and aerodynamic stalling, thereby improving the effectiveness of wings and control surfaces.
Avionics makers have been attempting to use autopilot technology to prevent stall/spin accidents in light airplanes. We all know that multimillion-dollar fly-by-wire jets are incapable of stalling because the computers that physically move the control surfaces refuse to allow it. Pull the sidestick in a Falcon 7X all the way aft and hold it there and the nose will point skyward until angle of attack increases and the stall approaches. As if by magic, the nose will drop and the air flowing over the wing will stay safely affixed. If you fail to apply power, you might still have the misfortune of running into the ground, but not with the wing in a stalled condition.
Garmin and Avidyne have achieved something similar, but without the multimillion-dollar price tag. Garmin’s Electronic Stability and Protection (ESP) system is electronic monitoring and “exceedance-correcting” technology for the G1000 through G5000 integrated avionics systems that assists the pilot in maintaining a safe, flight-stable condition, while preventing the onset of stalls and spins, steep spirals or other loss-of-control scenarios. Garmin’s ESP functions independently of the autopilot, operating in the background whenever the pilot is hand-flying the airplane. The system can gently nudge the controls back toward stable flight whenever pitch, roll or high-speed deviations exceed recommended limits. The technology will disengage when the airplane returns to normal flight.
Similar in philosophy, the Avidyne DFC90/100 autopilot can assist pilots in coping with an upset through the use of automatic-recovery algorithms. A handy “Straight & Level” button can be pressed at any time to immediately return the airplane to wings-level flight. If it enters an unusual attitude, the autopilot servos will automatically recover, again with a nudge of the yoke.
Stalls in light airplanes are not in themselves perilous. But they can have dangerous and often deadly consequences if they occur when we’re caught unaware. Thankfully, there is an abundance of technology, from AOA indicators to wing and airframe design and even electronic minders, that can help keep us safe. We should strive to understand the technology, in terms of both the benefits and the limitations, and use all of it as appropriate.
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