Image: Cirrus Aircraft
The FAA issued an emergency airworthiness directive (AD) for the Cirrus SF50 Vision Jet on Wednesday in response to three reports of the stall warning and protection system (SWPS) or Electronic Stability & Protection (ESP) System “engaging when not appropriate.” According to the AD, “Cirrus and Aerosonic (manufacturer of the technical standard order AOA [Angle of Attack] sensor) have identified the probable root cause as an AOA sensor malfunction due to a quality escape in the assembly of the AOA sensor at Aerosonic.” The AD (PDF) requires replacing existing AOA sensors with improved AOA sensors before further flight.
The ADnotes that the SWPS or ESP systems could engage even when the aircraft is operating with sufficient airspeed and at a proper AOA for normal flight, including activation of the stall warning alarm, stick shaker, stick pusher, under speed protection, and color-coded airspeed awareness ranges displaying the stall band, “regardless of actual indicated airspeed.” It warns that the potentially erroneous indications derived from a malfunctioning AOA sensor could result in “unintended automatic flight control activations; the flight crew having difficulty controlling the airplane; excessive nose-down attitude; and/or possible impact with terrain.”
“In early April of this year, one of our company pilots experienced the engagement of the Stall Warning and Protection System when not appropriate during a flight at altitude,” Cirrus said in a statement. “The pilot followed the published Airplane Flight Manual (AFM) procedures and landed the aircraft safely. Out of an abundance of caution, we immediately began working with the FAA and our internal teams to determine the root cause and began our operator communication process. With the benefit of detailed feedback from our company pilot about the April event, we were able to identify service histories that pointed to a probable similar occurrence in November.” The company issued Service Advisories on April 8 and 12 April 12 followed by a mandatory Service Bulletin on April 16 requiring that operators replace the AOA sensor. Cirrus emphasized that the root cause of the malfunction is an AOA sensor hardware issue, not a software problem.
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