Description

On 10 January 2011, a Boeing 737-300 being operated by Europe Airpost on a positioning flight from Montpelier to Toulouse with only the flight crew on board had just rotated for take off on runway 31R in day Visual Meteorological Conditions (VMC) when the leading edge slats extended from the Intermediate position to the Fully Extended position and the left stick shaker was activated. Initial climb proceeded and shortly afterwards, the slats returned to their previous position and the stick shaker activation stopped. It was decided to continue the planned flight which was uneventful thereafter.

Investigation

An Investigation was carried out by the French Bureau d'Enquêtes et d'Analyses (BEA). QAR data was used to inform the Investigation. It was found that the aircraft had been at Montpelier for external repainting at Latécoère Aeroservices, a Part 145 approved organisation. When the painting requirement changed and the contractor was unable to respond quickly, the aircraft operator decided to ferry the aircraft to another paint shop at Toulouse.

The aircraft commander, who had been PF, advised noticing an incorrect display of his PFD speed strip during the event but stated that he had not noticed any corresponding anomalies on the First Officer’s PFD or the SBY display, or on the engine displays.

Readout of the QAR data showed that at the time of rotation, the angle measured by the left angle of attack sensor did not correspond to the real angle of attack and had been fixed at 16.2º from engine start until 8 seconds after takeoff after which it had gradually returned to a correct position. As the measured AOA reduced through 14.6º, the theoretical threshold value for activation of the stick shaker, the stick shaker had ceased. It was noted that the values subsequently measured by the left AOA sensor during the remainder of the flight varied more slowly than those from previous flights.

The left AOA sensor was found to have successive layers of paint on the base of the mobile part, the attachment plate and the seal between the attachment plate and the aeroplane. After further examination, it was concluded that the blocking of the AOA sensor was probably due to the presence of white paint applied in Montpelier having intruded between the base of the mobile part of the vane, the fuselage and the attachment plate.

The links between the recorded AOA position, phase of flight, the automatic extension of the slats and stick shaker activation were reviewed. It was noted that: “at the time of rotation, the threshold for automatic slat extension is reduced from 19.8º to 16.2º to account for ground effect. As the left AOA sensor was stuck at 16.2º, the stall management computer (SMC) then activated the automatic extension of the slats to the fully extended position. The stick shaker activated in turn because its activation threshold is reduced during movement of the slats or flaps. Out of ground effect, the threshold for automatic extension returns to its initial value. The slats then returned to their intermediate position. The stick shaker stopped when the left AOA sensor became unstuck, probably due to aerodynamic forces, and the AOA value dropped below the activation threshold.” It was also noted that a maintenance message "AOA vane bent" had been recorded during the takeoff run. This provided an alert that the sensor was positioned more than 3º from its nominal position and is an automatic test carried out during take-off between 60 knots and 105 knots to indicate possible damage to the AOA sensor. Flight crew do not have access to this data.

Since the process of repainting the aircraft was evidently the origin of the occurrence in flight, a detailed review of the painting operation and safeguards included in it was undertaken. The fact that a contributing factor to a previous fatal accident investigated by the BEA: A320, vicinity Perpignan France, 2008 had also been actions taken in connection with the prior external repainting of the aircraft was noted.

It was noted that the Boeing 737 Aircraft Maintenance Manual was the high level reference document for painting operations. The sequential nature of painting operations was noted to involve two cycles of AOA sensor protection, the first is applied for stripping and cleaning and then removed and a second protection is applied for painting. It was noted that “the methods for masking the AOA sensors are not described in the AMM as they are considered to be basic steps. Nor are there any specific devices for masking these AOA sensors”.

It was found that whilst the protection for stripping involved protection which overlapped beyond the fuselage around the AOA sensor, the subsequent protection for painting was characterised by a “common practice” of masking the sensors to minimise overlap onto the fuselage skin so as to minimise any touch-up painting. This meant that only the base of the mobile AOA sensor was protected leaving the gaps between the sensor and the fuselage and between the fixed plate and the base of the mobile AOA sensor unmasked.

It was found that whilst Latécoère Aeroservices had a Part 66 certified mechanic making inspections as the work progressed in accordance with the regulatory oversight requirements of its Part 145 approval, no written procedure “describing masking methods for the critical elements in any precise manner” had been developed. In particular, the required checks “did not involve the freedom of movement of the AOA sensor, only the absence of any scratches or dust”.

It was noted that painting is quoted as an example of a “sub-contracted specialised activity” in the relevant EASA Part 145 AMC which allows it to be carried out by a non-approved organisation provided it is supervised under the quality system of contracting Part 145 organisation. It was also noted that Part 66 makes no provision for painters, who are considered as “specialised personnel”. Whilst these personnel must, under Part 145, be given human factors training in addition to their qualifications specific to painting, there is no requirement for any specific training to make them aware of the importance of protecting the sensors on which many aircraft systems depend. The French regulatory system for Part 145 organisations was found to consider activities such as painting to be “within the scope of maintenance activities” but to have “no reference text that establishes this point”.

The Conclusion of the Investigation was that:

“The activation of the stick shaker during aeroplane rotation was due to the blockage of the left AOA sensor by paint. The presence of paint was the result of inadequate masking of the AOA sensor during paint application. The post-painting inspection failed to detect this anomaly.

Basic rules alone are not enough to guarantee safe painting operations. In fact, the absence of any systematic procedure from aircraft manufacturers relating to the protection of external sensors during painting activities, or checks on their correct functioning at the end of the work, means that these operations do not necessarily take place in an appropriate and homogeneous manner in given workshops. In addition, the training requirements for specialized personnel do not ensure that they are aware of the potential impact of their activity on flight safety.”

It was further concluded in respect of aircraft operational safety that the extent to which aircraft manufacturers prescribe procedures for painting operations in the applicable AMM is very variable. It was considered that pollution of AOA sensors (or of external sensors more generally) associated with painting operations may cause unexpected behaviour of aeroplane systems, whose origin is not known (to) crew members, consequently diminishing flight safety.

Five Safety Recommendations were made as a result of the Investigation:

• that EASA ensure that manufacturers develop in their approved documentation some specific procedures for protection and verification of the external sensors during painting operations. [FRAN-2012-003]
• that EASA establish requirements for training in relation to the critical elements of aircraft for specialized maintenance personnel that do not hold a Part 66 licence. [FRAN-2012-004]
• that DGAC define painting work more precisely in an OSAC document, so that the oversight of painting activities is well-defined and homogeneous at the national level. [FRAN-2012-005]
• that DGAC ensure that maintenance organisations put in place appropriate checks throughout a painting operation. [FRAN-2012-006]
• that DGAC sensitise maintenance organisations to the importance of training for painters on the correct installation of protective devices during painting operations and the checks to be performed during a painting operation. [FRAN-2012-007]

The Final Report of the Investigation: BEA Incident Report was published in February 2012.

Further Reading

• Loss of Control
• Aerodynamic Stall Awareness and Avoidance
• Take Off Stall
• Unreliable Airspeed Indications
• Certification of Aircraft, Design and Production
• Aircraft Protection, during Washing and Painting - an article from Airbus magazine Safety First, Issue 17, January 2014
• Maintenance Briefing Notes: Invisible Protection by Airbus