Aspen Avionics has added an angle of attack (AOA) indicator to its Evolution primary flight and multifunction displays without the need to string new wires or plumbing, install any hardware or physically modify the airplane. The Evolution AOA is a software-only upgrade that uses Aspen’s ADAHRS to provide an algorithm-derived AOA indicator on the Aspen displays.
The Evolution AOA indicator can be displayed either on a half or one-third window on the Aspen Evolution multifunction display (MFD) or as a smaller indicator on the upper area on the primary flight display (PFD), but in all cases, the AOA is in the pilot’s normal instrument scan. The AOA automatically shows up when the airplane is flying within the AOA indicator envelope, so at cruise speeds the AOA indicator disappears.
Aspen has not yet announced pricing for the Evolution AOA upgrade but expects to shortly. The AOA software upgrade must be done by an Aspen dealer and takes just a few minutes to accomplish via the Aspen display’s SD card reader. Following the software upgrade, a calibration flight must be flown. The upgrade should be available in July.
“We’re quite excited about this,” said Aspen president and CEO John Uczekaj, “because of loss of control accidents and the lack of use of angle of attack indicators and lack of understanding [of the benefits]. We really want to push prevention of loss of control through the avionics as an integral part of improving safety.”
Another issue is that even in airplanes equipped with AOA systems, many pilots never receive proper training because some flight instructors are not themselves trained in the devices’ use. “We hope to excite the training community as well,” he said. “They can’t ignore it and will have to address it with the student because it’s right there. It’ll force the training community to deal with [AOA].”
While there are currently no FAA certification standards for a derived AOA system, Aspen plans to obtain certification for the Evolution AOA. “We are going to have it certified,” Uczekaj said. “We believe strongly that the certification process and procedures in place for [certification] of the Aspen PFD and MFD were very valid. The fundamentals of certification do improve safety. We will go through the normal certification process. There are no specific guidelines for certifying a derived AOA, but we hope to set a precedent for others to follow.”
Aspen developed the Evolution AOA in collaboration with the Italian Space Agency’s CIRA (Italian Centre for Aerospace Research). “They developed the algorithm,” he said, “and we developed the display. It’s the first product from that [collaboration]. “We think it’s going to take angle of attack into the general aviation community in a very meaningful fashion.”
Flying the AOA
James Buck, Aspen Avionics director of flight operations, demonstrated the Evolution AOA during a flight in a company Cirrus SR22 at Fort Worth Alliance Airport in Texas. Because there was a convective Sigmet for thunderstorms nearby, we were unable to fly elsewhere to try stalls or other low-speed maneuvers but remained in the Alliance traffic pattern. The Cirrus is equipped with Aspen’s Evolution 2500 multi-display system, with an MFD1000 on the right, PFD1000 in the center and MFD500 on the left, all in the pilot’s instrument panel.
The AOA indicator is colored with bands, ranging from blue at the bottom to green then yellow then red at the top. The color bands purposely don’t correspond to specific flight regimes or 1.3 VSO (dirty stall speed) because the idea isn’t to fly the AOA indicator but to fly normally and use the AOA as a supplement, Buck explained. The center of the green band is optimum, and if the AOA shows yellow, the pilot should lower the nose and if needed add power to return to the green band. “We don’t want people to feel comfortable in the yellow,” he said. The yellow/red margin indicates that the airplane is near the stall speed, and in some installations, this corresponds to the stall warning. “They’ve got to see that yellow as, ‘Hey, reduce that AOA.’ We don’t want to desensitize people to yellow. And we’re not advocating that you fly in the yellow. If it’s there, get back to the optimum AOA [green].”
Buck likes to set his MFD1000 on the right side with the AOA in the upper right corner, where it easily falls in his peripheral vision when looking outside the windshield. This mirrors his experience flying with AOA in Beechcraft T-34Cs in the Marine Corps, where he was an instructor pilot. “In the T-34 it was kind of up in the same area,” he said. “When I was scanning outside, I could see the AOA in my periphery. The whole point of AOA is not to stare but be able to put it in your peripheral scan.”
The Evolution AOA has two indicator “needles” that float up and down through the color bands, depending on the angle of attack being flown. The left side shows the AOA with flaps up, and the right side shows flaps-down AOA. The idea is that the pilot can instantly see the energy state of the airplane in either configuration. For example, when it’s necessary to fly a go-around, the flaps-down AOA needle might be in the green band, while the flaps-up needle will be yellow or red. This tells the pilot that retracting the flaps will quickly put the airplane into an AOA that could be dangerous. “You’ve always got both [indicators] so you know where you’re at, clean or dirty,” Buck explained. “Coming down an approach with full flaps, then you’ve got to wave off and start yanking back [on the controls], if you go flaps up the needle is red, so if you raise the flaps you’re already stalled. It’s very predictive.”
I flew the Cirrus around the pattern two times and was able to see how the Evolution AOA works. When banking at slower speeds, the AOA needles moved up the colors from blue to green, showing how the angle of attack increases during a turn. When turning from base to final, the flaps-down needle moved from the green range into the yellow, and I had to lower the nose to return to green. On final approach in winds that gusted from seven to 17 knots, the AOA quickly highlighted the energy state of the Cirrus and helped me adjust pitch and power to keep the AOA in the green range.
“The best place [to demonstrate AOA benefits] is that approach turn,” said Buck. “That’s where people get in trouble, they get wrapped up, look around, their hands follow their [gaze], and as it’s brought in tighter, the needles go right up into the yellow. We’re trying to reduce mishaps during approach turns.”