Safe Flight AoA: Lift, Speed Control

The inventors of the stall warning bring critical low-speed awareness to a higher level with a new angle of attack system. FAA sign-off is tricky.

The FAA says that stall-related loss of control is responsible for approximately 40 percent of fatal accidents. As a result, the agency wants to streamline the certification process and bring angle of attack indicators to all Part 23 aircraft.

In a perfect world of avionics retrofitting, AoA systems would be considered a minor alteration. That’s hardly the case and part of the reason why AoA systems aren’t common in small certified aircraft.

The way we see it, if any company could succeed in bringing certified AoA systems to Part 23 aircraft, it’s the one that pioneered wing leading edge lift detection over 50 years ago and holds 135 patents that are spread out over a broad aircraft segment (they’ve developed 18 in the last five years). That company is Safe Flight Instrument Corporation, and they have a new AoA and speed control system to answer the FAA’s call.

We recently flew with the system and recognized additional benefits that could save you money in the long run by improving and making your landings more efficient.

Flying The Alpha
Safe Flight doesn’t want pilots to just think about AoA as a way to avoid stalls. It’s more about achieving consistent, stabilized speed control. While flying a safe margin above stall speed is foremost, AoA offers instantaneous speed trend information, which leads to consistent approach-to-landing flares. This is difficult to achieve by flying the pneumatic airspeed indicator alone, especially without regard for the aircraft’s gross weight and CG, plus other factors, including wind gusts, ground effect and wing loading.

AoA systems can also provide redundancy for backing up a traditional airspeed indicator. AoA-referenced speeds can improve touchdown accuracy, promote better directional control on rollout, and increase awareness of the effects of wing loading during maneuvering. Better yet, efficient landing speed can reduce wear on tires, brakes and landing gear.

All of these benefits were apparent to us when we flew the Safe Flight AoA and speed control system in the company Baron. In fact, after flying behind the system for a short time, the airspeed indicator became a secondary speed reference.

With increased sensitivity compared to an airspeed indicator, AoA indication is more useful closer to stall speed. On our demo flight, changing pitch angle close to stall speed didn’t create any indication on the airspeed indicator, but the AoA precisely reacted to even small changes in pitch. This makes the system useful not only for landings, but also for takeoff and maneuvering.

Stagnation Point
The backbone of the Safe Flight AoA and speed control system is a lift transducer—a direct replacement for the familiar lift detector—that’s used for stall warning. There’s also a lightweight remote speed control computer that outputs discrete data to an AoA display and optional speed indexer. This indexer is essentially a fast/slow indicator, providing instantaneous wing lift feedback with accuracy that’s unmatched by a traditional airspeed indicator.

The system weighs less than two pounds and complies with dynamic LAA indication for integration with PFD systems, including the one in Garmin-equipped Cirrus models.

Safe Flight told us that the location of the lift transducer is critical to accurate performance. It’s all about the airfoil’s stagnation point—the spot on the airfoil where the airflow slows to zero. Safe Flight noted that unlike another manufacturer’s AoA system, they won’t compromise with alternate mounting locations.

Since it’s critical to measure the location of the stagnation point, the lift transducer is mounted on the wing’s leading edge, at a position corresponding to one percent of the chord line, in reference to the underside of the wing. Measuring AoA in this location, says Safe Flight, provides accuracy for both stall and slow flight warning and is essential for precise speed control.

While Safe Flight’s AoA and speed control system can accommodate discrete inputs for flap and gear input, the location of the lift transducer at the stagnation point on the leading edge makes the interface unnecessary for all but a few aircraft. According to Safe Flight’s Randy Green, the stagnation point at the stall is usually the same with or without flaps.

That’s how we describe Safe Flight’s AoA and speed control cockpit hardware. The optional speed indexer made for consistently accurate landings in the Baron. Too fast, the yellow F annunciates. Get too slow, and the red S annunciates. The center green O (aka the donut) means you’re flying at exactly 1.3Vs. The indexer also makes it easy to recognize transition speeds.

You don’t focus your eyes on the fast/slow indexer, but instead—if it’s positioned properly—it remains in your peripheral view as you focus on the runway environment.

The two-inch AoA indicator, if a PFD isn’t being used, is best placed in the vicinity of the airspeed indicator. In the test aircraft, it was placed next to the round-gauge airspeed indicator, where the benefits of flying with an AoA were apparent. Where a pitot and static-referenced airspeed has lag, the AoA is instantaneous.

The range markings on the normalized AoA indicator are universal and designed for simple and intuitive interpretation. For example, the top of the red arc is 100 percent of available angle of attack—the point at which the aircraft will stall. The center of the white arc is 0.6 or 60 percent of available angle of attack, which is approach speed and 1.3Vs—an across-the-board number—no matter the wind gust, weight, or other aircraft factors.

Pathway To Certification
We covered AoA systems­—including the Alpha Systems platform—in the May 2012 issue of Aviation Consumer. As we reported, Alpha received a letter from the FAA stating that their system is a minor alteration and can be installed by any willing A&P and without an STC or 337 form. Sign it off in a logbook entry. That’s not the case with Safe Flight’s system.

Safe Flight’s Brad Bertele made it clear that there are major differences between the Alpha Systems AoA and his company’s speed control system. The installation of the Safe Flight AoA is a major alteration because of structural, flight characteristics and interface considerations. Appendix A to Part 43 of the regulations define modification to the wings as a major alteration.

The installation of the lift transducer requires cutting a hole in the leading edge of the wing. (Alpha Systems’ probe typically mounts in an inspection cover, not directly measuring AoA at the wing’s leading edge.) The Alpha system also earns its minor modification status because it doesn’t tie in to the flap system.

Moreover, since the Safe Flight AoA displays information to the pilot, Bertele noted that it requires a flight manual supplement—a document that was not approved by Safe Flight’s FSDO ASI—but instead passed along to the New York ACO, where it is currently being reviewed for approval.

The system is projected to sell for under $2000 and requires a voltage converter when installed in 14-volt aircraft. Safe Flight hopes to have certification in place in a few months but after talking with company engineers, we sensed their frustration with the delays in FAA certification. With over 700,000 lift sensors in service and Safe Flight’s proven track record for winning broad FAA approvals, adding AoA and speed control functions to enhance their existing lift detector technology seems trivial.

Additionally, we think this product answers the FAA’s call for better stall awareness. Contact:, 914-946-9500.

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Editor in Chief Larry Anglisano has been a staple at Aviation Consumer since 1995. An active land, sea and glider pilot, Larry has over 30 years’ experience as an avionics repairman and flight test pilot. He’s the editorial director overseeing sister publications Aviation Safety magazine, IFR magazine and is a regular contributor to KITPLANES magazine with his Avionics Bootcamp column.