Stick Sense: Questioning AOA Indicators in Light Airplanes
Stick Sense: Questioning AOA Indicators in Light Airplanes
Angle of Attack is a pivotal component of flight. Also known as AOA, alpha, or α, it is the relationship between the chord line of a wing and the relative wind. Simplified, it’s the angle between the wings and the flight path.
Indicators that measure and display AOA have long been a standard in certain realms of aviation. Until recently, that did not include light airplanes.
Such indicators are now marketed and sold for light airplanes, mainly with the promise of increased safety. But for most general aviation flying, they’re simply an expensive Band-Aid on a more significant issue.
AOA is Life
AOA is perpetually relevant to a pilot in every phase of flight. There is never a situation in which AOA does not matter.
Most crucially, airfoils have an AOA limit, called the Critical AOA, beyond which smooth airflow over the wings separates from the surface of the airfoil — becoming turbulent and drastically increasing drag while lift decreases. This airflow separation is a stall and has long been, and continues to be, a leading cause of General Aviation accidents.
A longstanding joke in the flying community is that pilots “pull back to make the houses smaller and pull back hard to make them bigger.” It’s morbidly accurate, and avoiding unintentional stalls—and recovering from them—is a critical piloting skill. To do this, pilots must be aware of their AOA.
Pilots Can Intuit AOA
Fortunately, an airplane provides a plethora of real-time information about AOA that doesn’t require anything more than our eyes (outside), our ears, our ass, and our brains—maybe not in that order.
For many pilots, myself included, intuitively managing AOA is one of the most enjoyable parts of flying airplanes, whether it’s a Piper Cub or a Boeing 737.
We can see the difference between attitude and flight path. In a climb, for example, the nose is pointed at one angle (e.g. 5º) but the airplane climbs at a shallower angle (e.g. 2º). Most light trainers don’t have the excess thrust to climb with zero AOA like a fighter jet can. Recognizing the gap between attitude and flight path is an observation of AOA and requires nothing more than looking outside.
Pull back slowly, and the flight controls become mushy and require larger inputs. This indicates decreasing speed more than increasing AOA, but it helps tell us to keep the AOA in mind. Similarly, we can hear the airflow get quieter as we slow. If we pull back further, airflow will begin to separate, and we’ll feel buffeting as turbulent air hits the wing. That is a direct indication of AOA.
Or, if we pull back abruptly in flight, we can achieve that same buffeting even at higher speeds. The airflow is faster, but the angle is still too steep.
Notice that these examples discuss pulling back the controls. The stick or yoke is also a direct indication of AOA. Fascinatingly, an airplane with a simple manual cable-and-pulley flight control system (yes Cirrus drivers, this includes you 😉 ) will achieve, for a given configuration, the same AOA at the same stick position, regardless of airspeed. In other words, if you land with full flaps at critical AOA, the stick is in the same position as when you practice a landing stall at altitude.
That stick position, by the way, is somewhere very close to one’s lap. So, if you’re in a turn from base to final and find yourself compelled to pull the stick back into your crotch, you may want to think again.
While stick position directly indicates AOA, be mindful that the control pressures at that stick position can vary wildly depending on airspeed and trim setting.
Pilots can recognize immediate changes in AOA by intuitively synthesizing all of this information in flight without looking at a single instrument.
The Odd Rise of AOA Indicators
There are valid reasons to install expensive AOA indicators in specific aircraft and for particular missions. A direct AOA indication (or advanced flight control systems, or both) is important in an airplane where stick position doesn’t directly correlate with AOA. They also have profound value for fast-paced, high-stress, disorienting operations requiring high-G and high-AOA maneuvers to aerodynamically slow the airplane after a fast approach to the airport. An F/A-18 pilot landing on an aircraft carrier at night has an excellent reason to have an immediate indication of AOA.
Transport category airplanes like the McDonnell Douglass DC-9 use a stick shaker to alert the pilot of an excessively high AOA. This mechanism shakes the yoke to get the pilots’ attention. Some systems will even push the yoke forward if a pilot doesn’t respond quickly to a high AOA situation. However, in routine flight, crews rarely have additional AOA information beyond what is available in a light airplane. It still comes down to flight path, attitude, stick position, and buffeting.
Ironically, AOA indicators in light airplanes are often marketed as a technology to allow GA pilots to “do it like the professionals.” Many experienced professionals have never used one.
Part of this argument involves a case for flying more efficiently by referencing direct AOA. As weight and center of gravity change, the AOA remains a valuable reference without any corrections. While technically accurate, this is more of a case for larger airplanes. Changes in weight and center of gravity in light airplanes have a relatively minor impact on handling characteristics and speeds so long as they remain within the manufacturer’s envelope. Likewise, any efficiency gains will be negligible for an average pilot and will likely come nowhere near offsetting the cost of the device and installation.
In short, companies marketing AOA indicators for light airplanes promote applications in aircraft and operations where the equipment has negligible value. And they’re expensive. Worst of all, they’re a Band-Aid on the general issue of a lack of training and proficiency.
Why Rely on an Inferior Method?
The flight path, stick position, equipment, and aerodynamic factors provide ample information for discerning pilots to approximate their AOA. Even in larger airliners, this remains the case. Those airplanes have AOA measuring devices and distribute that information to various systems, but rarely do they present a visible AOA indication, and the pilots fly approaches using the same information as they would in a Cessna 172.
Instruments can and do fail. Why should we encourage pilots to become increasingly fixated on an instrument that can fail when the same information is readily available to tell at all times with just their peripheral vision and brain?
The key to AOA safety is integrating awareness into pilots’ natural monitoring processes rather than relying on a specific instrument. Accomplishing that is not easy per se, but it’s entirely feasible. The greatest obstacle in teaching most pilots this skill is an unfounded belief that it’s impossible or unsafe. The approach to teaching this subject is a version of the age-old “teach a man to fish” parable. Should we equip airplanes with AOA indicators or simply teach pilots to recognize, anticipate, and manage AOA better?
Simply giving better flight training negates the primary value of an expensive AOA indicator. Better training is the authentic way to “do it like the professionals.”
The Real Solution to AOA Safety: Better Training
Pilots considering purchasing an AOA indicator should consider the advanced training they could receive for the same cost. The advantage of training is that it stays with the pilot, not the airplane.
On the low end of prices (not even including installation), it’s at least 10 hours of aerobatic training from an experienced instructor.
Training is also the difference between a pilot who reacts to a blinking multi-colored light and an aviator who already knows what that light will read ten seconds from now.
This training can take many forms, but the priority should be on teaching pilots to naturally intuit AOA through flight path, stick position, and other sensory clues, but with minimal emphasis on flight instruments. Training should include accelerated stalls, spins, falling leaf maneuvers, slow flight, and approaches to landings with the instrument panel fully covered. Pilots should be pros at recognizing an approach to stall, understanding the conditions by which pilots often inadvertently enter fatal stalls-spins, and understanding to what degree they can fly the airplane in a stalled or near-stalled condition. And they should instinctively recover from high AOA conditions.
Pilots will ultimately have an intuitive understanding of AOA that stays with them, even as the airplanes change. Their knowledge will enable them to put an airplane where they want it when they want it there, with minimal reference inside the cockpit.
The end result is a safer, more situationally aware, far more comfortable pilot who has a lot more fun! Pilots, their passengers, families, and the greater flying community deserve this experience.
AOA Indicators Aren’t the Solution You Seek
While AOA indicators provide valuable information, they provide nothing that pilots can’t intuit with proper training. There’s nothing wrong with wanting to spend money on the latest, “greatest” technology, but make no mistake that the problems AOA indicators profess to solve are better—and more affordably—solved in other ways.