The Grumman Widgeon stalled at 200 feet above the water, with only a small buffet from the wing to signal the event. The pilot, cognizant that the amphibian’s design excluded any kind of stall warning device, constantly monitored his airspeed – especially when low – to mitigate the risk of a stall. And yet, even with the airspeed indicating above-stall, a classic stall/spin event was unfolding. The pilot was quick to apply power and lower the nose, but his actions came too late, the altitude was just too low. The Widgeon impacted the water, left wing first, with such force that a post-crash fire ensued. The pilot was seriously injured, along with one of the two passengers. The airplane eventually sank and was never recovered.
This unfortunate accelerated-stall accident occurred during a photo shoot of a sailboat race. The physics in play that day apply to many loss-of-control accidents, one of the leading causes of GA fatalities according to the General Aviation Joint Steering Committee. How can this be? From day one of their training, students are drilled on the disastrous outcomes of stall/spin accidents, and how to prevent them. Furthermore, stall recognition-and-recovery is a big part of recurrent training, especially in Part 142 schools. And still, these accidents continue to occur.
The explanation is largely rooted in an incomplete understanding of the dynamic nature of lift required to maintain controlled flight. While airspeed is a significant factor, it alone is a relatively poor predictor of lift. And yet, airspeed is the primary metric that most GA pilots lean on to assess their margin and predict how close the airplane is to stalling. To more precisely determine how much of the wing’s lifting capacity is consumed, the wing area, weight, temperature and altitude (for establishing air density), load factor, and center of gravity must all be included in the calculation.
A much better indicator of lift capacity is the Angle of Attack (AOA), the angle between the wing chord and the relative wind. This angle precisely correlates with the coefficient of lift (CL), the inverse of which is the lift capacity remaining. Knowing the lift capacity remaining means knowing how close the airplane is to a stall at any time (CL Max). As CL increases toward CL Max, the lift capacity remaining decreases, reducing the margin of safety. At CL Max all the available lift capacity is consumed and the airplane is at risk for a stall, potentially leading to a loss of control event. Conventional AOA instruments, typical of airliners, business jets, and military airplanes, monitor such conditions as air pressure, temperature, weight, CG, and load factor, and often employ a complicated exterior vane sensor that literally measures the angle. These systems work well, but they are expensive and difficult to retrofit, largely the two reasons they have not enjoyed much success in the single-engine piston and twin markets.
A New Day For AOA
Fortunately, change is in the wind. A new series of GA-targeted AOA products has recently emerged, including the latest innovation from Aspen Avionics. Unique among the lower-cost offerings, the Aspen AOA software integrates into the Evolution EFD 1000 PFD and MFD and requires no vanes, special pressure sensors, or component displays. Instead, the system leans on airspeed, temperature, altitude, pitch, and acceleration information provided by the EFD 1000’s ADC and AHRS, along with some simple weight and V-speed parameters unique to the aircraft. And rather than measuring AOA to determine CL, the Aspen product calculates the coefficient directly. Ironically, angle of attack, in the sense of relative wind and wing chord, does not play a direct role in the calculation. This 100% software upgrade retails for around $1,900 – a bargain compared to the big airplane products. But the question begs, does the system really work?
In pursuit of an answer, we purchased the software upgrade for the installation in our 1944 G44 Widgeon. We chose the Widgeon not only to redeem its namesake but because, like most airplanes built before 1950, the Widgeon has no stall warning device whatsoever. Furthermore, the airplane, a complex, heavy piston twin amphibian with a symmetrical wing and flaps, promised to test the full envelope of support offered by Aspen. Our Widgeon already benefitted from a single Aspen Evolution PFD, so, in theory, the installation would require no more than a download and memory card reprogramming. In reality, the installation process required starting with a version of Evolution base software that was compatible with the unlocking software used to enable the AOA functionality. It turns out some versions (ironically, the newer ones) were not recognized by the unlocking software, a problem that has since been corrected. Once this was sorted out, our avionics technician was able to successfully complete the installation.
After installation comes calibration. First, information, including the aircraft’s “near” cruise speed (80% Vno), max gross weight, short field approach speed, basic empty weight, and calibration weight (the weight of the aircraft as loaded) is entered into the system. Next, the pilot must fly three different aircraft configuration/airspeed scenarios. Each scenario requires the pilot to hold airspeed, vertical speed, pitch, and roll precisely for up to 2.5 minutes. Exceeding any one of the tolerances invalidates the calibration. For example, the speed target is +/ – 3 mph, and the pitch cannot vary by more than one degree. (Most autopilots cannot achieve this level of precision.) I found the calibration process to be challenging, especially during the final step when the airplane has the flaps and gear extended. On our particular day, the airplane was loaded with CG forward, so hand-flying and holding a target speed just above stall, while complying with the tolerances, demanded concentration and strength. Performing the calibration on a calm day is a must.
Following calibration, the Aspen Avionics AOA manual instructs the pilot to configure the airplane for landing, flaps and gear down, and perform a power-off stall to test the system. As the airplane slows, the pilot observes a graphic on the EVO 1000 PFD (or EVO 2000 MFD). Two pointers travel vertically along a set of stacked colored bands that represent the CL range. The lower pointer indicates the CL when the airplane is dirty, the upper when it is clean. Starting at the bottom, the bands are colored blue (representing high remaining lift capacity), green, and then yellow. At the top, yellow with black hash marks represents low remaining lift capacity. As the airplane slows both pointers move upward, but with the airplane configured dirty, only the lower point indicates CL and proximity to a stall. It is this pointer that the pilot follows.
The second test is conducted with the airplane clean, this time keeping an eye on the upper pointer to determine CL. I queried Michael Studley, Aspen’s Director, Customer Service – Field Service Engineering, about the need for two pointers (confusing). He acknowledged that the two-pointer representation requires some practice, but affirmed that it is more accurate than competitor products that use one pointer and blend multiple configurations for their CL calculation. Also, according to Studley, the configuration that amphibians use to land on sea – wheels up, flaps down – changes the CL calculation minimally compared to land ops, so happily a third pointer is not necessary.
Using The System
The post-calibration tests ultimately proved that the system was working. Power-off stalls in the Widgeon while dirty are very honest. Applying a normal recovery, pitch down/full power, flaps to approach, positive rate, gear-up, can be accomplished with fewer than 200 feet of altitude. Even more impressive is the performance of Aspen’s branded AOA. Just prior to the stall, the lower pointer was near the top of the yellow hashed band, exactly where it should be. The same excellent performance was observed in the clean configuration, although remembering which pointer to follow took some effort.
We also flew the instrument in steep-banked turns to observe the effects of acceleration. Not wanting to cause a real loss-of-control event, we did not actually stall the airplane. Configured clean and with the airspeed considerably above published Vs, it was sufficient to experience the onset of buffets exactly coincident with the upper pointer reaching the hashed band zone.
The Aspen supports several modes of operation including “Auto”, where the indicator is only displayed on the PFD when the upper pointer (clean configuration) is approximately a quarter of the way up from the bottom of the green band. For airplanes equipped with their own MFD, the indicator can be enlarged for easier viewing. This is helpful for flying a safe speed or angle more precisely. And, while Aspen Avionics acknowledges that the device is not quantitative, the company’s recently-published promotional video suggests that it can be used to determine certain V-speeds to fly. For example, the best rate of climb may be accomplished with the upper pointer held exactly between the green and yellow bands. Still, not having an accompanying MFD in the Widgeon to display the larger indicator, it seems probable that holding the pointer so precisely would be difficult. I expect determining a bug speed and following the airspeed tape might be a more realistic answer. We did, however, discover that targeting the lower pointer between the green and yellow bands while on approach with the airplane configured to land nicely correlates with Vmc, making for a handy cross reference.
Aspen Avionics deserves a lot of credit for developing a practical and inexpensive AOA/CL device, one that functions well and supports a large number of airplane types. While I might wish for more features, including configuration sensitivity, audio output, a stick-shaker interface, and 1.3 X stall pointer bug on the airspeed tape, similar to what we have in our Citation CJ3, I am not sure I would be willing to pay the extra tens of thousands of dollars needed to accomplish the task in the smaller airplane. More to the point, knowing the available lift capacity remaining (CL) at all times is likely the best way avoid a loss control accident. And that is something the Aspen Avionics product does very well, indeed.