Deep Stall Aerodynamics

In October 22, 1963, a prototype of the British Aircraft Corporation One-Eleven (BAC 1-11) short-haul jet airliner, registration G-ASHG, crashed near the village of Chicklade in southwest England. The aircraft was evaluating stall characteristics at varying center of gravity locations when the flight crew found the flight controls unresponsive after entering a stable stall and the aircraft struck the ground at a wings level attitude with a high rate of descent and little forward speed. All aboard died in the crash.

The 1-11 was one of the second-generation of jet airliners-others being the Douglas DC-9 and Boeing 727-featuring aft-mounted engines, swept wings and all-moving T-tail horizontal stabilizers. Post-crash investigation concluded the prototype 1-11 had experienced an unrecoverable deep stall in which the wake of the stalled wing covered the high-mounted horizontal stabilizer, thus blanking the elevator controls and preventing normal recovery techniques.

While the deep-stall phenomenon (or “super stall” as it had been known) was identified at least 10 years earlier during testing of the delta-wing Gloster Javelin, not much was understood about its causes or effects. Investigation into the 1-11s deep-stall crash contributed a great deal to the aerodynamic understanding of the phenomenon, and caused quite a stir when it was discovered that each of the cutting edge aerodynamic features of the new generation of airliners contributed to the phenomenon in some way.

While most of us never have the opportunity to fly a T-tailed jet transport, deep stalls can and do affect other aircraft. Thankfully, most general aviation aircraft are aerodynamically boring for the most part, and thats actually a good thing. Exceptions, however, can be found, perhaps at your local airport.

Before we begin a discussion of deep stalls, a quick trip back to private pilot ground school is in order, to review the basics. Just about any private pilot will quickly (err, should quickly) tell you an aerodynamic stall is a condition in which the wings actual angle of attack exceeds its critical angle of attack, causing oncoming air to no longer flow smoothly over the wing. As one result, the wing loses lift. Thats good for starters, but lets dig a little deeper this time, stopping short of involving funky Greek letters.

To better understand stalls in general, and deep stalls in particular, its important to examine the effects of wing design and angle of attack on the wings center of pressure (also called the center of lift). This is the point at which the lifting force acts; in other words, its the up arrow on the typical four-forces diagram. The center of pressure is typically located at one-third to one-half of the wings chord, and is balanced by the down force provided by the horizontal stabilizer. (Stick with me here, itll get more interesting, I promise.)

For aerodynamic reasons not being addressed here, as the angle of attack increases, the center of pressure shifts forward. As the wing stalls at its critical angle of attack, the center of pressure goes to zero. Presuming timely recovery is effected and the wings angle of attack decreases, the center of pressure then shifts rearward with the decrease. However, in typical straight-or tapered-wing piston singles and twins, there really isnt a very wide range over which the center of pressure can shift. The result? What we consider “normal” stall behavior. The concept that the center of pressure shifts forward with increasing angles of attack is the first key point to keep in mind for later.

During private pilot training, your instructor probably pointed out the controls became increasingly sluggish during stall entry. Thats because the stall begins at the wing root and progresses outward toward the ailerons, with the intention of keeping them flying as long as possible. Thats usually the result of engineers designing washout into the wing; the angle of incidence at the root is greater than at the wingtip, causing the stall to begin at the root. The concept that patterns of stall progression differ for various wing designs is the second key point contributing to deep stalls.

Anyone looking at the corporate jets and airliners flying these days cant help but notice they look quite a bit different than garden variety piston singles and twins. These aircraft have features not typically found on light general aviation aircraft such as swept wings, T-tails (while some light aircraft have them, they are the minority) and aft-mounted engines. These aircraft are designed for high-speed and high-altitude flight, and would not make very good trainers. On the flip side, a Piper Cherokee isnt exactly the most suitable all-weather transport aircraft, either.

While the swept wing has numerous aerodynamic advantages for high-speed flight, it is characterized by very poor stall characteristics. Due to the effect of spanwise flow, less air flows over the wing at the wingtips than at the wing root, thereby causing the stall to begin at the wingtips and resulting in loss of aileron control. Additionally, because the turbulent airflow separating off of the ailerons will not strike the horizontal stabilizer, no buffet occurs to give pilots the aerodynamic stall warning commonly found on lighter aircraft when some of the turbulent airflow separating from the wing root strikes the tail. Thats what produces buffeting with which we all should be familiar.

Another more sinister stall characteristic of swept-wing aircraft results from the forward shifting of the center of pressure, as discussed above. While the wings of straight-winged light aircraft allow the center of pressure to shift only slightly with changes in angle of attack, the range the center of pressure is able to shift on a swept-wing aircraft is far greater, leading to a pronounced nose-up tendency during stalls. This is a very bad thing because if left unchecked, the nose-up tendency can pull the aircraft deeper into the stall, leading to extremely high angles of attack.

There were a variety of aerodynamic benefits that led designers to choose to build aircraft with T-tails as opposed to conventional tails, such as permitting a longer effective distance between the wing and the tail increasing the tails moment, thus allowing a smaller, lighter surface providing the same amount of downforce as a larger tail with a shorter moment. By using a T-tail, the space toward the rear of the fuselage is freed up to be used to mount engines, which has its own aerodynamic advantages.

Despite designers having the best of intentions, when you put all of these factors together you get an aerodynamic perfect storm. Should a swept-wing aircraft be allowed to stall, there will typically be no aerodynamic warning, and the aircraft will become longitudinally unstable. As one result, the nose will continue to rise, deepening the stall. With a conventional tail design, the turbulent air separating off the wing would trail mostly above the tail, whereas with a T-tail design the tailplane bears the complete load.

This turbulent wake interferes with the smooth flow of air over the tailplane, limiting its effectiveness (remember, the nose is rising uncontrollably at this point, and the wingtips are already stalled, thus rendering the ailerons ineffective), eventually blanking it entirely. At high angles of attack, the engine nacelles have a similar effect on the tailplane. This is known as a “locked-in deep stall,” and its a sure-fire way to ruin a pilots day.

The aircraft features mentioned above are found in many of the most popular transport aircraft designs in recent history, so how come they arent falling out of the air left and right? Deep stalls have been and continue to be a serious concern in aircraft design, and a number of design changes have emerged over the years to prevent deep stalls from occurring.

A direct result of the crash of the BAC 1-11 prototype discussed above was the incorporation of a “stick pusher,” which later became a common feature on most similar airliners, among other design changes. Stick pushers operate by monitoring an angle-of-attack sensor and forcing the yoke forward when it becomes dangerously close to exceeding the critical angle of attack. A stick shaker physically vibrating the yoke to give pilots unmistakeable warning of an impending stall is typically found on transport aircraft which would otherwise have no natural stall warning.

A fairly commonplace aerodynamic modification is the incorporation of ventral strakes on aircraft, such as the delta fins found on Learjets, the Piaggio Avanti and Diamonds D-Jet. These primarily aid in dampening dutch roll tendencies, minimizing or eliminating the need for yaw dampers. A secondary benefit is they induce a strong nose-down force at high angles of attack, preventing the aircraft from entering a deep stall. In addition, some more exotic devices such as parachutes and small rockets have been installed in the tails of aircraft known or thought to be susceptible to deep stalls in an attempt to physically force the tail upward and remove it from the wake of the wing should it experience a deep stall.

Despite ongoing aerodynamic research, wind-tunnel testing, and computer modeling, deep stalls continue to plague new aircraft designs from time to time. A Canadair CRJ100 Regional Jet, C-FCRJ, on a test flight out of Wichita, Kan. crashed on July 26, 1993, after entering an unrecoverable deep stall. More recently, a deep stall resulted in the crash of Pulkovo Flight 612, a Russian Tu-154, registration number RA-85185, near Donetsk, Ukraine, on August 22, 2006. Data shows that the Tupolevs descent from FL330 to impact took 210 seconds, close to 9500 ft/min.

It is generally thought that non-swept wing aircraft are not susceptible to deep stalls unless they feature a T-tail, and that swept wing aircraft can be susceptible to deep stalls regardless of their tail position. Combining a swept wing with a T-tail presents the greatest potential for entry into a locked-in deep stall. Most modern aircraft designs are concerned more with equipping the aircraft with safeguards such as stick pushers or AOA limiters which prevent the aircraft from stalling thereby preventing deep stalls from occurring. However, the fundamental aerodynamics of the aircraft could still allow such an event to occur should the safeguards fail.

This has been the whirlwind survey course of stalls and deep stalls painted with broad strokes by someone who is certainly not an aeronautical engineer. Just whatever you do, please dont find a CFI and ask him to go show you deep stalls, I dont think either of you would enjoy it much.