News
The Cirrus Vision personal/business jet has been grounded by the FAA with an airworthiness directive related to the aircraft’s angle of attack vane. The Airworthiness Directive issued Thursday 18 April indicated that “the aircraft’s stall warning and protection system (SWPS) or Electronic Stability & Protection (ESP) system engaged when not appropriate.” The incidents lead to stall warnings for the crew and activation of the stick shaker or stick pusher despite the aircraft maintaining adequate airspeed and angle of attack.
The FAA indicated that unintended activation of the stall protection systems could result in excessive nose down attitude and concluded that “the noted condition presents an immediate danger to pilots and passengers of Cirrus Design Corporation Model SF-50 airplanes because an un-commanded pitch down may be difficult to recover from in some flight regimes with potential fatal consequences.”
Analysis
Cirrus two days prior to the AD issued a mandatory Service Bulletin to replace the AoA vane with a modified part. The problem apparently resulted from improper torquing of screws mounting the potentiometer shaft to the AoA vane shaft that could result in inaccurate readings. The FAA felt the problem serious enough to change the five flight hour window of the service bulletin to an immediate grounding.
Fortunately, in three reported incidents, pilots were able to stop the automatic commands by following emergency procedures. Coming shortly after the MCAS problems experienced on the Boeing 737MAX, albeit with different sensors, the FAA took little time to ground the airplane.
Cirrus has developed an FAA-approved corrective action and revised the emergency procedures in the airplane flight manual. Unlike the MCAS on the Boeing 737MAX, a Cirrus pilot can easily override the ESP system with control inputs.
Insight
After losing face with the international airworthiness community, the FAA is giving greater scrutiny to every potential problem that emerges and taking swift action to regain the mantle of leadership it once had. After being last to ground the 737MAX, the FAA is appropriately stepping up its game.
But in this case, with fewer airplanes operating (99 in the US), many fewer flight hours than the 737MAX, and the ability for pilots to overcome the fault through control inputs, this appears to be an issue that could have been solved by the mandatory service bulletin issued by Cirrus, and a potential overreach by the FAA.
Angle of attack indicators that have failed have produced two regulatory groundings this year, as they can be inputs to control systems that could have consequential results from inaccurate data. In the case of Boeing, the software had fatal consequences for 346 souls. At Cirrus, fortunately pilots are able to overcome the forces of a system that fails and still land the plane safely. While erring on the side of caution is always good, the rapid AD action clearly shows that the FAA is now paying close attention to an issue it earlier ignored, contributing to 157 additional fatalities and losing the trust of the international aviation community.
