The Reality Of Deep Stalls

My wife, Peggy, is a perfect example of a conscientious private pilot who'd just as soon never fly another stall---power-on, power-off, accelerated or any other kind---unless she's a few feet above the ground in a landing flare. She's done all the necessary book work, she understands what causes a stall, and she demonstrated the ability to recover successfully to her examiner last November. She hopes that's the last stall she'll ever have to fly.

That's not a surprise, and many pilots share her lack of enthusiasm for stalls. After all, would you be eager to fly your airplane at the absolute bottom of the performance envelope if you didn't have to? Unless you're a bush pilot or an aerobatic enthusiast, the answer is probably no.

There's one more stall that's the very worst kind, and you'd best hope you never encounter it. This is the dreaded Deep Stall, sometimes referred to as a Super Stall.

The good news is many general aviation designs are relatively immune to deep stalls. Deep stalls are most often associated with swept-wing, T-tailed airplanes, typically designs with wings mounted aft on the fuselage. Many airliners and corporate jets use this configuration, but some smaller aircraft also employ a T-tail. If these aircraft are flown into stall conditions at extreme angles of attack, they can enter a stabilized deep stall from which it may be difficult or impossible to recover.

Despite that introduction, there's nothing inherently sinister about T-tailed airplanes. T-tailed designs are no less stable or difficult to fly than airplanes fitted with conventional low tails or cruciform designs.

The normal goal with T-tails is to mount the elevator, whether it's contained in a stabilizer or an all-flying stabilator, up out of the wash from the fuselage and wings. This usually results in smoother, more effective elevator control. (In jets, another benefit of high tailing is expediting installation of engines on the aft fuselage.)

Contrary to popular belief, there are a significant number of general aviation models that fly beneath tails in T-formation. At one time back in the late '70s and early '80s, it was considered stylish to design the vertical tail with horizontal stabilizer mounted high across the top.

The obvious downside was higher bending loads on the vertical stab that dictated structural beef-up, often adding weight. Several manufacturers tried T-tails and liked the modern, updated look of the airplane, even if there was little aerodynamic benefit. That's not to suggest there aren't some very real advantages to T-tails, but just like winglets, stall strips and leading edge slots, they don't work on all airplanes.

Among the Pipers, T-tails included the Tomahawk, Arrow IV, Lance II and Seminole, plus the turboprop Cheyenne III and 400LS. (Today, the Seminole is the only Piper that remains T-configured.) Beech had the Skipper, Duchess and most of the King Air series; Diamond has the C1 Eclipse, DA40 Star and DA42 Twin Star; Piaggio has the P-180 Avanti; Pilatus has the PC-12 and, depending upon how you count them, you could add at least another half-dozen models to the list.

Canard-equipped airplanes that don't sport T-tails also may be susceptible to deep stalls by the very nature of their design. Models that fly behind canards typically enjoy a stall-immune existence---most of the time.

Designers configure the canard to meet the relative wind at a slightly higher angle of attack than the following wing. This means, by definition, the canard will always stall first, the nose will drop, and the wing will never be able to reach its critical AOA. Beware of the word "never." If the pilot does induce a high pitch attitude that stalls the wing, it may be extremely difficult to recover as the canard is even more deeply stalled.

Stall Aerodynamics
In order to understand deep stalls, we need to review normal stalls. As angle of attack increases, the wing assumes more and more lift, but it also begins to pick up additional drag. As AOA increases further, both lift and drag increase, but the drag curve begins to catch up with the lift curve as more airflow separates from the top surface of the wing and flow becomes progressively more turbulent. When drag finally exceeds lift and the airfoil reaches its critical angle of attack, the wing finally stalls and pitches forward. At least, that how it's supposed to work.

Most aircraft wings stall at or below 20 degrees angle of attack. Deep stalls can occur when the airfoil is forced into an attitude greater than its critical AOA. Depending upon wing location, this can place the T-tail in the airflow shadow of the wing, effectively blanking the tail and significantly reducing elevator effectiveness. With improper management, especially in certain CG situations, this can result in a stabilized deep stall. (It's also possible to experience a deep stall in a low-tail airplane, but that's an extremely rare event.)

Deep stalls can occur when pilot mismanagement or turbulent air forces the airplane into a severe nose-up attitude, and the elevator becomes totally ineffective. Once the airplane is in the embrace of a deep stall, there may be no way to force the nose down and fly the airplane out of the stalled condition.

One possible solution some pilots have used to facilitate recovery from a deep stall is to employ the ailerons to roll the airplane to knife edge and force the nose to pitch down sideways. In this manner, a pilot may be able to escape with a semi-normal stall recovery. Some pilots have also managed to recover by rocking the nose with what little elevator control remains until the angle of attack becomes so high that the nose finally falls through.

On multi-engine aircraft, asymmetric thrust may also succeed in breaking the airplane out of the deep stall.

Manufacturers have installed "stick shakers" and "stick pushers" to provide a semi-automatic means of recovery from such situations.

Deep stalls can be insidious, however. Until the age of voice and flight recorders that preserved the last few hours of every commercial flight, accident investigators were sometimes puzzled by crashes that seemed almost inexplicable. Airplanes would sometimes mush into the ground at high descent rates but with wings level and nose 20 to 30 degrees above the horizon. Most were in military or airline jets, usually fatal to all aboard, so there were no survivors left to describe what led up to the crash.

Four years ago, an Air France Airbus A330 crossing the South Atlantic in the middle of the night crashed into the ocean off the coast of Brazil in what many interpreted as a deep stall. The crash killed all 216 passengers and 12 crew, and the Airbus sank to the bottom of the Atlantic. For two years, investigators searched for the wreckage with a variety of submersibles, and by a stroke of what many searchers regarded as blind, dumb luck, finally found the black boxes in 13,000 feet of water.

Trouble was/is, the Air France accident was not a deep stall, despite the probable cause finding by the French investigation BEA team that the crew "executed inappropriate control inputs that destabilized the flight path, leading to an unrecoverable aerodynamic stall."

The key word here is "unrecoverable." Although the Airbus A330 was definitely in a fully stalled attitude when it hit the water, wings level and nose roughly 20 degrees above the horizon, the crew was receiving mixed signals from the airplane regarding airspeed and altitude, apparently the result of flying through a super-cold event that iced up all the airplane's pitot tubes.

The Airbus A330 entered a stall at 38,000 feet and descended in a nearly flat attitude for the next three minutes. Indeed, flight recorder data later suggested it was descending nearly vertically, but wings level in a slightly nose-up attitude when it hit the ocean at a vertical speed of roughly 11,000 fpm, about 120 knots straight down.

The full story of the accident is a tragedy of errors, but the bottom line was that the copilot held the airplane's side stick full back practically all the way to impact. Although the Airbus was in an extremely high angle of attack, it wasn't in a deep stall. It was doing exactly what it had been commanded to do. If the copilot had simply released back pressure!

If there's any aircraft designer on the planet who could figure a way to harness a deep stall to his advantage, it's Burt Rutan. If you followed the development of Rutan's SpaceShipOne to capture the Ansari X-Prize, you've seen a perfect example of how to tame and control a deep stall. Rutan's innovative "shuttlecock" concept is a brilliant solution to the problems of returning from space while successfully avoiding the heat and turbulence associated with re-entry.

The SpaceShipOne was deliberately designed by Rutan to return to Earth in a deep stall. The aircraft features an unusual feathering system that deflects the rear half of the wing and the tail booms up, forming a shuttlecock configuration that allows SpaceShipOne to descend back to Earth in more of a gentle mush rather than burning up the sky at extreme Mach numbers. While there's some speed buildup (Mach 2.9 on pilot Mike Melvill's flight in June 2004), it occurs at very high altitude where air pressure is so low that there's little frictional heating from outside airflow.

At 57,000 feet, the pilot streamlines the wings and tail back to the original trail configuration, and the aircraft becomes a normal glider again. For the Ansari prize, Rutan launched two flights in two weeks, both of which exceeded 100 km altitude and landed back at Mojave without power, just as the Space Shuttle landed at Kennedy Space Center in NASA's glory days. Rutan claimed the X-prize of $10 million, and his sponsor, Paul Allen, spent only about $20 million in the process.

For a more comprehensive examination of deep stalls, read Chapter 25 of Barry Schiff's excellent book Flying Wisdom: Proficient Pilot III published by ASA in Newcastle, Wash. Schiff is one of the smartest people I know on all things aviation. He's also a retired TWA captain, but don't hold that against him.