Agency Notes Three Incidents Of ESP System Engaging When Not Appropriate

The FAA has issued an emergency AD for Cirrus Model SF50 (Vision Jet) airplanes following three reports of the stall warning and protection system (SWPS) or Electronic Stability & Protection (ESP) System engaging when not appropriate, with the first incident occuring in November 2018 and the latest in April 2019. The SWPS or ESP systems may engage even when sufficient airspeed and proper angle of attack (AOA) exists for normal flight.

This AD requires replacing the AOA sensors with improved AOA sensors using the instructions in SB5X-34-03 before further flight.

The SWPS includes the stall warning alarm, stick shaker, and stick pusher. The ESP includes under speed protection (USP). The SWPS system engaging inappropriately could potentially result in a STALL WARNING crew alert (CAS) message activation, accompanied by an audio alarm and stick shaker activation, followed possibly by either low speed ESP/USP engaging, and/or the stick pusher engaging. The pilot will also observe the dynamic and color-coded (Red) airspeed awareness ranges displaying the stall band, regardless of actual indicated airspeed.

The information below presents information on each incident.

• While the airplane was under manual pilot control, the airplane activated several downward pitch commands coincident with stall warning, stick shaker, and several associated alerts. The pilot reported “AOA FAIL” and “STICK PUSHER FAIL CAS” messages preceding the pitch command. The pilot was able to stop the automatic pitch commands by pressing and holding the autopilot disconnect button in accordance with the emergency procedure in the airplane flight manual and safely landed at his destination.
• The operator reported stall warning and stick pusher failure in flight.
• The airspeed indicator went red and the stall warning and stick shaker were heard and felt while on descent. The autopilot was disengaged with the same results. The system settled with stick pusher fail, stall warning fail, and low speed awareness (LSA) fail under the airspeed. The pilot hand flew the approach and had no VREF indicator but AOA appeared to be operating normally.

Cirrus and Aerosonic (manufacturer of the technical standard order AOA sensor) have identified the probable root cause as an AOA sensor malfunction due to a quality escape in the assembly of the AOA sensor at Aerosonic. Two set screws that secure the potentiometer shaft to the AOA vane shaft may have improper torqueing and no application of thread locker (Loctite) to secure the two set screws. The AOA sensor with this quality escape is labeled with part number 4677-03.

Potential erroneous AOA derived indications may occur before, during, and after unintended automatic control system engagement. These indications include an abnormal appearing low speed red band or VREF green donut presented on the airspeed tape. Failed indications or intermittent indication may result in one or more of the following:

• 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 reviewed Cirrus Design Corporation SF50 Service Bulletin Number: SB5X-34-03, dated April 16, 2019 (SB5X-34-03). The service information provides instructions for replacing the AOA sensor with an improved flight sensor.

The agency is issuing this AD after evaluating all the relevant information and determined the unsafe condition described previously is likely to exist or develop in other products of the same type design. 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 fatal consequences. The "before further flight" compliance time and need to replace the AOA sensors due to the potential fatal consequences does not allow for prior notice and opportunity to comment for the public.

(Source: FAA. Image from file)

FMI: Emergency AD