With all the current hype about Angle of Attack indicators, a casual observer might think the concept was just discovered. But as any pilot with even a basic knowledge of history knows, it’s been a hot topic numerous times over the years, as far back as Wolfgang Langewiesche’s classic book Stick and Rudder. Today, almost every turbine aircraft has an AOA instrument. It is definitely not a new idea.
The latest push has centred on bringing this technology to all general aviation aircraft (GA), as we saw at AERO SA. It started with the FAA, who declared affordable AOA instruments one of its most-wanted safety improvements. AOPA’s Air Safety Institute has enthusiastically supported the cause, too.
Angle-of-attack indicators provide a visual representation of how much lift is being generated by your wings for a given airspeed. This is incredibly useful because speed alone is not a reliable parameter to avoid a stall. That's because aircraft can stall at any speed, as long as they exceed their critical angle-of-attack.
You could think of AOA indicators as instruments for stall margin awareness. In short, AOA indicators measure the current AOA of the aircraft in comparison to the aircraft's critical AOA. For a given configuration, an aircraft will always stall at the same AOA, called the critical angle-of-attack.
The critical AOA does NOT change with:
Centre of Gravity
How do you turn and maintain altitude at the same time? You need to increase the total amount of lift your wing is producing. And to do that, you need to pull back on the yoke, which increases the angle-of-attack that your wing is flying at. This part is important, because when you increase your angle-of-attack, you get closer to the critical angle-of-attack, which is the point when your wing stalls.
Without an AOA indicator, the AOA is "invisible" to pilots. In certain configurations and attitudes, you might not realise you're approaching a stall, as we have seen over and over in many GA accidents.
We won't dive into the specific system components or measurements used to calculate AOA, because each variant system has its own parameters. In corporate, airline, and military flying, you'll often see "vane style" AOA indicators. They show direct indications of AOA but must be mounted in a position with clean airflow.
"Vane style" AOA
In general aviation, you'll commonly see "pressure-derived" AOA indicators. Most of these measure pressure differences at the pitot tube to determine an accurate AOA. While extremely popular and affordable, most of these models don't calculate AOA based on various flap configurations. However, they're still a very useful tool. Some of the newest glass-cockpit systems provide AOA data including flap configuration changes.
"Pressure-derived" AOA probe
Installation of an AoA system may aid in preventing loss of control accidents. Manufacturers have requested a streamlined method of design and production approval for supplemental systems. Since these systems provide only supplemental information to the pilot and are not required by regulation, the FAA has developed a simple approval process, hopefully the SACAA will follow suite.
AOA Indicators however are not a miracle cure – Pilots must still use their common sense.
Some AOA advocates point to the Asiana 214 crash in San Francisco as an example of the type of accident that could be prevented with such an instrument. Remember, pilots have to look at the instrument for it to be of any use. In the Asiana case, the crew managed to ignore the airspeed indicator for a long time, even as it eventually showed them being almost 30 knots slower than they should have been. Another instrument may have been ignored as well and in that case, it wouldn’t have made a difference.
Besides looking at the instrument, pilots have to know how to react to its indications. For the GA pilot struggling to log 25 hours in a year, the physical stick and rudder skills may be more important than the recognition skills.
An AOA instrument also won’t help the idiot who buzzes his girlfriend’s house at 20 ft and pulls up at 3 Gs. Neither will it prevent the over-gross take-off on a hot day that eventually results in a stall. These scenarios are both depressingly common and get grouped under the “loss of control” heading, when they should be in a pure “pilot error” group.
There is the school of thought that believe we don’t have an instrument problem; we have a stick and rudder skills problem. Instead of spending a lot of money on new instruments, they advocate teaching pilots how to maintain the proper airspeed on final.
As with everything in GA the AOA indicator has its pros and cons, they are available at a reasonable cost. The choice is yours!!