The FAA has grounded Cirrus SF50 Vision Jet over Angle of Attack Issues
Operators of Cirrus Aircraft SF50 Vision Jets have been ordered urgently to replace angle-of-attack sensors, after three incidents in which stall-protection systems improperly activated before it can fly again.
The US FAA has warned that activation of the stall-warning or the electronic stability protection systems could occur even if the aircraft is flying at a suitable angle-of-attack with sufficient airspeed.
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.
The agency noted that in each incident, the plane attempted to automatically pitch the nose of the plane downward while alerting the pilot of an impending stall. Fortunately, the pilots in each incident were able to stop the automatic commands by following emergency procedures.
Investigation of the incidents has identified a “quality escape” during assembly of the Aerosonic angle-of-attack sensor as the probable cause, says the FAA, adding that screws securing the potentiometer shaft to the angle-of-attack vane shaft might have improper torquing.
As a result, the aircraft might display spurious indications based on erroneous angle-of-attack data, including abnormal low-speed information.
The control issue has been attributed to a production defect in the jet's AOA sensor. As a result, the sensor can give faulty readings that may 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," the FAA said.
"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 potentially fatal consequences," the FAA said in the directive.
Cirrus says that having investigated the matter, it has identified a hardware issue – not a software problem – with the angle-of-attack sensor. It also stresses that “no accidents have resulted” from the issue.
“Our [angle-of-attack] hardware supplier is now producing corrected AOA hardware sensors which are beginning to ship to operators now,” the company points out. “These new corrected AOA hardware sensors will be installed on fielded aircraft and new aircraft deliveries.”