What happened
On January 10, 2011, a Boeing 737-300, registration F-GIXD, operated by Europe Airpost, was performing a positioning flight from Montpellier Méditerranée to Toulouse Blagnac. During the takeoff rotation, the aircraft's left-hand angle of attack (AOA) vane failed to provide accurate readings. This caused the Stall Management Computer to trigger the automatic deployment of the leading-edge slats to the full extension position. Simultaneously, the left-hand stick shaker activated.
The flight crew noted an erroneous speed indication on the pilot's Primary Flight Display (PFD), though no anomalies were present on the co-pilot's PFD, the standby instruments, or the engine indicators. The stick shaker lasted for approximately 12 seconds before ceasing as the AOA value returned to a normal range. After evaluating the systems, the crew elected to continue the flight to the destination without further incident.
The investigation
The investigation focused on the condition of the aircraft's sensors following recent maintenance. The aircraft had recently undergone painting operations at two different facilities. Analysis of the Quick Access Parameter Recorder (QAR) confirmed that the left AOA vane reading was frozen at 16.2 degrees from engine start until eight seconds after takeoff.
Physical examination of the left AOA vane revealed multiple layers of paint—specifically yellow, white, and yellow—accumulated on the movable base, the support plate, and the joint between the plate and the fuselage. Investigators determined that the mechanical movement of the vane was restricted by this buildup of white paint. The investigation also looked into the masking procedures used during the painting process, noting that while the maintenance manual (AMM) does not specify exact masking techniques, the practice of minimizing masking overlap to reduce retouching work likely contributed to the exposure of the sensor's moving parts.
Findings
- The stick shaker and automatic slat deployment were triggered by the blockage of the left-hand angle of attack vane.
- The blockage was caused by the accumulation of paint layers on the sensor's moving components.
- Inadequate masking during the painting process allowed paint to enter the gaps between the vane base and the fuselage.
- Post-painting inspections failed to detect the anomaly because the inspections focused on surface quality (scratches and dust) rather than the mechanical freedom of the sensor.
- There is a lack of standardized, manufacturer-provided procedures for protecting external sensors during specialized painting tasks.