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Stalls and Spins

Stall/Spin Awareness: Part I

An evolving concept

BY RICH STOWELL (From Flight Training, October 1993.)

Few topics in aviation have been shrouded in as much mystery as the stall/spin phenomenon. Considerable misinformation, largely rooted in fear, persists even today in spite of a growing body of scientific and empirical knowledge about the subject. Let's begin this three-part series on stall/spin awareness with a look at some of the noteworthy events that have shaped our understanding of, and training policies regarding, stalls and spins.

England

September, 1911: Fred Raynham becomes the first pilot in recorded history to recover from an inadvertent spin (called "spiral dives" back then). Unfortunately, Raynham is unable to recall his recovery actions. While it now seems unlikely that he actually spun his airplane, the event triggers the idea that spins may be controllable.

August 1912: Wilfred Parke inadvertently enters a left spin during a spiral glide, in front of a host of witnesses. Parke's first reaction is to add full power, pull the elevator control fully aft, and press the rudder all the way to the left. These inputs do not stop the rotation. In the process, Parke feels himself being pushed to the right, away from the direction of the spin. He releases the stick to steady himself in his seat and applies full right rudder. The airplane promptly recovers to a level flight attitude 50 feet above the ground! The event becomes known as "Parkes Dive."

Parke is the first to identify application of opposite rudder as the key to spin recovery. The experience also highlights the fact that spin recovery actions are contrary to a pilot's natural instincts; therefore, they must be a learned response.

June, 1914: Harry Hawker stuns the aviation community by being the first to deliberately spin an airplane. Following an argument, Hawker climbs aloft to prove the theorized recovery characteristics of his airplane. He intentionally enters a spin but is unable to recover because he freezes on the controls. The airplane crashes. Incredibly, Hawker emerges unscathed from the wreckage. He repairs the airplane, climbs aloft again, and deliberately enters another spin! This time, he enters the controls and the airplane recovers. Intentional spinning becomes a widely used evasive maneuver during World War I.

1916-1917: Scientific spin research is undertaken at Farnborough. The spin recovery actions that evolve from these early investigations include switching off the engine, neutralizing the ailerons, moving the elevator forward, and centering the rudder (not the current recovery procedure).

Back in America

September 1917: The United States formally includes spins during advanced training for U.S. Army pilots. This requirement eventually evolves into mandatory spin training for all pilot applicants.

1919: The National Advisory Committee for Aeronautics (NACA) begins a scientific spin test program at Langley, Va., using a Curtiss Jenny.

January 1936: A technical paper based on years of spin research at Langley is published by test pilot W.H. McAvoy. The paper details the NACA Recovery Procedure. The recommended recovery actions, assuming idle power and neutral ailerons, include applying full opposite rudder against the spin, followed with brisk forward movement of the elevator control one quarter of a turn later (getting closer to today's recovery procedures.

1945-1948: General aviation booms following World War II. Thousands of military pilots return home to civil pursuits, including light airplane flying. A large influx of surplus military aircraft, many of which possess high wing and power loadings, flood the civilian aviation market. Manufacturers promote airplanes as an alternative to automobiles. These elements merge in an environment that is largely unstructured and unprepared for rapid expansion. Stall/spin accidents account for almost 50 percent of general aviation fatalities during this period.

June 1949: Responding to the high fatality rate associated with stall/spin accidents, as well as mounting pressure from manufacturers to broaden the appeal of light airplane flying, the Civil Aeronautics Agency (CAA) enacts Civil Aviation Regulation (CAR) Amendment 20-3. The amendment is intended to improve safety by eliminating mandatory spin training in favor of stall avoidance training and by providing manufacturers with an incentive to build spin-resistant or spin-proof aircraft.

May 1962: Spin-resistant and spin-proof airplanes envisioned CAR Amendment 20-3 do not evolve. Instead, manufacturers move toward higher-performance designs that have greater useful loads spread over wider center of gravity ranges. This trend makes it increasingly difficult to satisfy spin recovery requirements for aircraft certification. As a result, CAR Amendment 3-7 is adopted, deleting spin testing, as well as spin recovery capability, for normal category aircraft certification. At one time, airplanes weighing less than 4,000 pounds had to demonstrate recovery from six-turn spins in one and one-half additional turns by simply releasing the controls. Amendment 3-7 permits single-engine normal category designs, to be certificated if they demonstrate recovery from one-turn abused stalls in one additional turn following the application of normal recovery controls. Twin-engine airplanes do not have to satisfy this or any other spin or abused stall recovery requirement.

An unsettling disparity between pilot training and the spin characteristics of their airplanes evolves. Formal spin education linked with inherent spin recovery capability evaporates. Instead, new pilots, possessing virtually no experience with spins or spin recovery, are permitted to fly airplanes that are capable of spinning and, because it has never been demonstrated in certification tests, potentially unrecoverable beyond one turn in a spin. Moreover, the control inputs needed to recover these airplanes within their one-turn margins of safety do not have to be taught to the pilots flying them!

September 1972: The National Transportation Safety Board (NTSB) publishes "Special Study - General Aviation Stall/Spin Accidents, 1967-1969." The number of fatalities attributed to stall/spins is about 25 percent during this period, down from nearly 50 percent 20 years earlier. It is unclear, however, whether this improvement is due to the elimination of spin training in 1949, improvements in airplane design and pilot training methods, or a combination thereof.

The NTSB offers the FAA nine recommendations to try to further reduce the number of stall/spin fatalities. Some of these include exploring "innovation in ground and flight training curricula," evaluating "the feasibility of requiring at least minimal spin training of all pilot applicants," and advising pilots "to guard against the occurrence of a stall/spin accident subsequent to an engine failure."

1975-1976: A year-long study, "General Aviation Pilot Stall Awareness Training Study," is performed for the FAA's Systems Research and Development Branch. Members of MIT's Department of Aeronautics and Astronautics and other researchers determine weaknesses in current stall/spin training. They also develop and evaluate an experimental stall/spin training increment.

Information driving this landmark study includes data showing that the median experience of pilots involved in stall/spin accidents is 400 hours. Pilots with more than 1,000 hours of flight time account for one-third of all stall/spin accidents. The common thread linking such accidents is pilot distraction followed by an unintentional stall, particularly during critical flight operations close to the ground. The study focuses on improving situational awareness, educating pilots about the factors contributing to stall/spins, and providing training aimed specifically at avoiding unintentional stalls.

A survey of then-current civil pilot training programs identifies several inadequacies in stall/spin education. For instance, the FAA's Flight Training Handbook has fewer than two pages devoted to spins. Private pilot written exams (in 1975-76) contain no questions on spins or scenarios that may lead to stalls/spins. Ground instruction manuals lack information on stall/spin accidents. Stalls are treated as intentional flight maneuvers and are often related to low airspeed rather than a high angle of attack. FAA safety literature lacks sufficient stall/spin information as well.

Two formal flight schools are chosen for the survey to ensure similar experience levels between volunteers and to maintain tight control over scheduling. The schools use different training aircraft - Piper PA-28 Warriors at one, Cessna 150's at the other - providing a better representation of a national stall/spin training system. Hour-long evaluation flights are performed with each pilot to calibrate the study. Another evaluation flight follows the experimental instruction increment. The number of accidental stalls, unintentional spins, and evaluator "takeovers" are recorded on each flight.

Four test groups are formed. Group 1 is the control group and receives no additional instruction beyond the normal training. Group 2 receives an extra 2-hour ground training increment, consisting of stall/spin lectures, movies, and quizzes.

Group 3 receives extra ground training and 2 hours of additional flight training on stall/spin avoidance, which includes slow flight, stalls after takeoff, engine failures in climbs followed by 180-degree turns, cross-controlled stalls in gliding turns, approach-to-landing stalls and go-arounds, oscillation stalls, and incipient spin recoveries. Group 4 receives the same ground and flight training as Group 3, plus an additional 30 minutes of intentional spins with recoveries.

A statistical analysis of the test data shows additional ground instruction (Group 2) to be effective in reducing the number of accidental stalls and spins. The extra stall and slow-flight instruction (Group 3) is not sufficiently different from normal training to influence the number of accidental stalls. But the extra instruction positively reduces the number of unintentional spins and evaluator takeovers. The full spin training increment (Group 4) is the most effective in preventing unintentional spins and evaluator takeovers.

1977-1987: NASA conducts full-scale spin tests of four different light airplanes represented in the general aviation fleet. Researchers verify the relative effectiveness of the NACA Recovery Procedure published in 1936, but cannot identify a recovery input that always stops the spin. Of the one turn spins investigated, only one aircraft fails to recover as required by FAR Part 23, including those spins that eventually develop into unrecoverable flat spins beyond one turn.

April 1991: The NTSB's 1972 recommendations, the MIT report published in 1973, and the striking results documented in the 1976 stall awareness study shift momentum toward improved stall/spin education. Stall avoidance training alone is no longer an acceptable standard. FAR Amendment No. 90 and Advisory Circular AC 61-67B, Stall and Spin Awareness Training, form a cohesive stall/spin training increment that draws on the research conducted 15 years earlier. Even FAA written exams now include questions on spins.

Pilots at all levels of certification must receive specific ground instruction in stall awareness, spin entry, spins, and spin recovery techniques. Flight training must address stall avoidance at slow airspeeds, with realistic distractions, including departure stalls, engine failures in climbs followed by 180-degree gliding turns, cross-controlled stalls in gliding turns, power-off stalls, stalls during go-arounds, and elevator trim stalls. Actual incipient and full-spin training is reserved for certificated flight instructor applicants only, who now must demonstrate instructional competency in this area.

The debate continues

Despite scientific evidence to the contrary and regulatory changes to address stall/spins in greater detail, some spin training opponents still cling to stall avoidance training as an adequate standard.

The FAA's own research quantified the relative superiority of stall/spin training over stall avoidance training. The introduction of incipient and full spins into flight training curricula unmistakably reduced inadvertent stalls and spins - the root of most stall/spin accidents.

The current state of flight training, however, makes mandatory spin training for all pilot applicants impractical. There simply are not enough suitable training aircraft approved for spins, or qualified spin instructors, to safely implement a national spin training policy. Even so, the stall/spin awareness concept adopted in 1991 is a step in the right direction. Propagating unfounded myths about spins and creating stall/spin phobias in pilots has been superseded by legitimate education.

Next month, we'll investigate the aerodynamics of stalled flight. We'll discuss factors that affect stall behavior and highlight techniques that foster favorable stall recovery characteristics. We'll do the same for spins in the December issue.