FAA Grounds Cirrus SF50 Jet over AoA Vane Issue

The FAA's decision to ground the Cirrus SF50 Vision Jet with an emergency Airworthiness Directive (AD) in April came as something of a surprise, although owners say Cirrus had already told them of potential issues with their aircraft's angle-of-attack (AoA) sensors and had established a plan to remedy the problem.

The April 18 emergency AD immediately grounded all SF50 Vision Jets in the U.S.—approximately 100 in-service aircraft, and another 10-12 yet to be delivered—except for approved ferry flights to a Cirrus-authorized service center for replacement of the aircraft’s AoA sensor assembly.

The AD states that erroneous AoA data could engage the SF50's Garmin-sourced Envelope Stability and Protection (ESP) system that acts upon the elevator pitch servo to lower the jet’s nose if AoA input increases to the point where the aircraft’s stall warning and protection systems (SWPS) activate. The situation may arise while either hand-flying at lower altitudes or higher altitudes with the autopilot engaged.

According to a Cirrus spokesperson, a company pilot first encountered an inadvertent SWPS activation “at altitude” in early April. Subsequent research by Cirrus linked that issue to a customer-reported occurrence in November 2018, which led the company to issue two service advisories to customers, on April 8 and April 12, followed by mandatory service bulletin (MSB) SB5X-34-03 on April 16 calling for replacement of the AoA sensor assembly within five flight hours.

Three SF50 pilots told AIN the planemaker informed them when the issue first surfaced through direct communications and posts to the company's Vision Jet owners and operators web portal.

"They described the problem as an issue with the retaining collar behind the mounting plate that holds the [AoA] vane within the bearing," explained Jay Jolley, who took delivery of SF50 S/N 29 in December 2017. "The service advisory included a video demonstrating how to check the vane during preflight by using your thumb and index finger to gently pull on it, similar to removing a key from a lock. Any movement [of the vane] meant a failure condition."

The advisories also detailed the emergency procedure should pilots encounter a stall warning crew alert system (CAS) message and activation of the SF50’s stick shaker and/or stick pusher in otherwise normal flight. "It was a matter of pushing the autopilot disconnect button on the side stick, followed by pulling the autopilot servo circuit breaker," Jolley said. "If you’re wearing a watch on your left arm, that C/B would be right next to the dial when holding the stick."

Owners Say Cirrus Working Quickly To Fix Their Jets

Despite Cirrus’s actions to address the AoA sensor issue, the FAA opted soon after to publish the emergency AD stating “the unsafe condition described previously [in the MSB] is likely to exist or develop in other products of the same type design.” The agency added the condition “presents an immediate danger to pilots and passengers.”

The FAA’s quick issuance of the emergency AD may have been driven by the apparent similarity to the current grounding of Boeing’s 737 Max commercial airliner, in which erroneous AoA sensor inputs also appear to have triggered that aircraft’s maneuvering characteristics augmentation system (MCAS) leading to two fatal accidents in Indonesia and Ethiopia. No accidents or injuries have been linked to the SF50 AoA sensor issue.

The SF50 AD identified the probable cause as “a quality escape” by air data probe manufacturer Aerosonic, in which “[t]wo set screws that secure the potentiometer shaft to the AOA vane shaft may have improper torquing and no application of thread locker (Loctite) to secure the two set screws."

Owners state the repair involves removing the original AoA vane assembly and returning it to Aerosonic for modification, with corrected replacement parts returned for installation by Cirrus personnel. Another SF50 pilot, who flies for the aircraft’s owner, stated operators were concerned initially that replacement parts would be in short supply following release of the MSB.

"However, it soon became clear that things were happening really fast and Cirrus told us we would likely see an AD,” she continued. “That followed a few hours later.” That pilot added Cirrus contacted her the following Monday, April 22, to arrange repairs.

“They said they could be at [our] hangar that day to remove the AoA and send it out, but by that time I'd already lined up a ferry permit to take the aircraft to Colorado for its previously-scheduled annual [inspection]," she continued. "They pulled the AoA assembly as soon as I arrived.”

The replacement process also unfolded at a fairly swift pace for Jolley, whose aircraft was already at a Cirrus service facility to address an unrelated service bulletin while he traveled out of the country. "Cirrus delivered the replacement part yesterday, it was installed today [April 25] and should be test flown tomorrow," he said. Jolley later flew his plane home the evening of April 26.

Dane Jasper, owner of SF50 S/N 18, said his aircraft had its AoA sensor replaced April 30 by a California service center. “I just got a call 30 minutes ago stating they were heading out for the test flight,” he added. “The [vane] removal and replacement must be performed by Cirrus personnel.

“I've heard from owners whose planes aren't already at a maintenance facility that Cirrus is coming to them to handle repairs,” Jasper continued. "All companies go through crises, and Cirrus has moved quickly and mobilized a lot of resources to get this dealt with. They've been very helpful in making it an easy process.”

Jolley also gave Cirrus a “thumbs up” for its response to the AD. “No one signs up for this, but they’ve done a good job communicating a very difficult message,” he concluded.