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The Alpha (and Omega) - How a Small Angle Can Make a Huge Difference

Source: FAA Safety Briefing, May/June 2014.
By Tom Hoffmann

Unlike some of my classmates at the time, I actually have some fairly fond memories of high school geometry. In particular, I enjoyed breaking out the compass and protractor to measure, draw, and dissect angles. Long ago familiar terms like transversal, supplementary, complementary, and alternate-exterior are fun to rehash in my mind. Lucky for me geometry followed me into my flying career and become an important element to understanding aerodynamics and unlocking some of the mysteries of flight. Wing dihedral, angle of incidence, and the effect of aerodynamic forces are all examples of how geometric principles govern the way we fly. Then there’s the “alpha” angle — the all-important angle of attack which every student pilot learns early on is an aerodynamic threshold that deserves the utmost respect.

Simply put, the angle of attack is the angle between an aircraft’s wing and the oncoming air. If this angle becomes too great in flight, the wing will be unable to produce lift and the aircraft will stall. Not good. Most general aviation pilots rely on airspeed and the piercing whine of the stall warning horn to avoid getting themselves into a stall situation. However, another stall warning device that has long been available — but not without a sizeable effort and cost to install — is the angle of attack (AOA) indicator.

These supplementary devices are designed to alert pilots of a high angle-of-attack condition before a stall occurs, either with a visual or aural warning, or both. AOA systems provide an added layer of safety due to a more reliable indication of airflow towards the wing than an airspeed indicator can provide, regardless of gross weight, G-loading, or turbulence. And now, thanks to a revised FAA policy for producing and installing these devices, there’s good news for those who were previously put off by the prohibitive cost and red tape.

So what’s changed for AOA installations? Under the new policy announced February 5, 2014, manufacturers can now build the AOA indicator system according to standards from the American Society for Testing and Materials (ASTM). They then apply for FAA approval for the design via a letter certifying that the equipment meets ASTM standards and was produced under required quality systems. That means manufacturers no longer have to go through the full Technical Standard Order certification process to have an AOA device approved. The FAA’s Chicago Aircraft Certification Office will process all applications to ensure consistent interpretation of the policy.

“This represents a drastic change for the FAA,” says Craig Holmes, Aviation Safety Inspector with FAA’s Aircraft Certification Service, referring to the manner in which this new streamlined policy was implemented. “The new guidance will allow us to significantly speed up the application and approval process and should help encourage owners to equip their aircraft with this important safety device.”

There are a few important items to keep in mind with this new policy, however. First, it applies only to supplemental AOA systems — not those required for type certification of the aircraft. Second, it is limited to those systems installed in U.S.-registered aircraft, excluding commuter and transport category airplanes. The guidance also stipulates that no operational credit can be taken for such items as reduced approach speeds and shorter landing distances.

While the use of AOA systems is an effective means of reducing loss of control accidents, their effectiveness can be limited by how much proficiency an operator has gained with a particular device.

“Given the lack of available training on certain AOA systems, I recommend going up with a qualified instructor and testing it out thoroughly,” says Holmes. “With an instructor by your side, you’ll be able to monitor precisely how your AOA device reacts during stalls and other maneuvers.”

Regardless of your take on geometry, I’m sure you’ll appreciate the FAA’s new “angle” on improving safety for GA.

Tom Hoffmann is the managing editor of FAA Safety Briefing. He is a commercial pilot and holds an A&P certificate.

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