Another Airworthiness Issue with an AoA
The Vane is linked to Software
AD points to faulty parts
The Cirrus SF50 has had problem with its angle of attack (AoA) vane interacting with its stall warning and protection system (SWPS) or Electronic Stability & Protection (ESP). The FAA has issued an Emergency Airworthiness Directive (AD) grounding the aircraft until remedial actions are implemented. Though not given high profile in the AD, the pilot interface with the computer is surfacing as a relatively unprecedented and complex issue.
Historically, the FAA’s decision to ground an aircraft was based on a mechanical problem for which the catastrophic consequences are clear. The recent B737 MAX 8 airworthiness determinations involve software and the assessment of whether the problem justifies grounding is a new question. The analysis is further complicated when the potential failure includes the interaction between the pilot and the computer.
It was determined that the SF50 Vision remedy involved a repair of a faulty part; so, the issue of thresholds for grounding and then reauthorizing the airplane’s software as functioning to meet safety standards was not reached.
[this picture was copied from the AEROSONIC website, but it is not clear whether this AoA is installed in the SF50.]
“The FAA has issued an emergency Airworthiness Directive (AD) on the Cirrus SF50 Vision Jet, grounding the more than 110 in-service single-engine jets for immediate replacement of the aircraft’s angle of attack (AoA) vane. According to the emergency AD issued Thursday, Cirrus reported “the aircraft’s stall warning and protection system (SWPS) or Electronic Stability & Protection (ESP) System engag[ed] when not appropriate” in three incidents since November 2018, leading to a stall warning crew alert system (CAS) message and activation of the stick shaker and/or stick pusher despite the aircraft maintaining sufficient airspeed and AoA for normal flight.
Further, the FAA said unintended activation of the SF50’s stall protection systems could result in excessive nose-down attitude and difficulty in maintaining control of the aircraft. “The noted condition presents an immediate danger to pilots and passengers of Cirrus Design Corporation Model SF50 airplanes because an uncommanded pitch down may be difficult to recover from in some flight regimes with potential [sic] fatal consequences,” according to the agency.
The emergency AD came two days after Cirrus Aircraft issued a mandatory Service Bulletin (MSB) requiring immediate AoA vane replacement with a modified part from vane manufacturer Aerosonic after originally-installed components were found with improper torquing and retention of two set screws mounting the potentiometer shaft to the AoA vane shaft.
While the Cirrus MSB mandated that SF50 operators have the replacement AoA vanes installed within the next five flight hours, the emergency AD “requires such replacement before further flight,” although operators are able to obtain a special flight permit to a location where the replacement can be performed.”
Hopefully, the SF50 does not experience further issues, but careful attention to the frailties of artificial intelligence in airplane systems must be high. The decisions on the B737 MAX 8 and other similar airworthiness instances will define the parameters of when to ground an aircraft and when to resume flight in an intelligent, predictable manner.
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