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.
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.
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 torqueing.
As a result the aircraft might display spurious indications based on erroneous angle-of-attack data, including abnormal low-speed information.
Such false indications might result in unintended automatic flight-control activation and excessive nose-down attitude. The pilot could experience difficulty controlling the aircraft and there is a risk of possible collision with terrain, the FAA warns.
“The noted condition presents an immediate danger to pilots and passengers of [the SF50] because an uncommanded pitch down may be difficult to recover from in some flight regimes, with potential fatal consequences,” it adds.
It has barred operators from further flights until the suspect Aerosonic sensors are replaced with a different model.
Inappropriate engagement of the stall-warning system could result in audio alarms and stick-shaker activation, followed by possible underspeed protection engagement or activation of the stick-pusher. The pilot would also see colour-coded airspeed indications displaying the stall band, regardless of the actual indicated airspeed.
The latest of the three SF50 incidents leading to the FAA action occurred this month, while two others have taken place since November last year.
One event resulted in the aircraft, while under manual control, activating several downward pitch commands as well as stall-warning alerts and the stick-shaker. Another operator reported stall-warning and stick-pusher failure in flight, while a third had stall-warning and stick-shaker activation, and related failures, during descent.
Cirrus acknowledges that one of its company pilots experienced engagement of the stall-warning system “when not appropriate” during a flight at altitude in April.
“The pilot followed the published airplane flight manual procedures and landed the aircraft safely,” it states. “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.”
While a Cirrus service bulletin issued on 16 April, covering replacement of the sensor, allowed 5h time in service beforehand, the FAA says that even this short period “does not mitigate” the unsafe condition, and it has ordered the sensors replaced before further flight.
Operators will be allowed, however, to fly affected aircraft to a location where the modification work can be performed, subjected to specific procedures.
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.
In addition to its 16 April mandatory service bulletin Cirrus had also issued two service advisories over the preceding eight days.
“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.”