On 15 March 2011, an ATR 72-200 on a non revenue positioning flight from Edinburgh to Paris CDG in night Visual Meteorological Conditions (VMC) with just the two pilots on board began to experience roll and directional control difficulties as the aircraft accelerated upon reaching the planned cruise altitude of FL230. A ‘PAN’ call was made to ATC and a return to Edinburgh was made with successful containment of the malfunctioning flying controls.
A Field Investigation was carried out by the UK AAIB. It was established that the incident flight was the first one after completion of a scheduled base “2 year Check" at a Part 145 maintenance facility which was within the same beneficial ownership as the aircraft operator. It was found that the Flight Data Recorder (FDR) data was “of such poor quality that the data was unreliable and therefore unusable” but the QAR also fitted was downloaded successfully. Since the 30 minute Cockpit Voice Recorder (CVR) had continued to run following the onset of problems, later recording had overwritten the initial part of the incident. It was noted that First Officer had been acting as PF for the incident flight and that the aircraft commander had accumulated the majority of his total flying experience on the ATR72.
It was established that the aircraft had appeared to respond normally to flying control inputs by the PF until the aircraft reached FL230 and speed reached 185 Indicated Airspeed. At this point, the crew felt the aircraft roll to the left by 5° to 10° and noticed that the slip ball was indicating fully right. The PF disengaged the AP and applied right rudder in an attempt to correct the sideslip and right aileron to correct the roll. He subsequently stated that the rudder pedals had felt unusually ‘spongy’ and that the aircraft had not responded to the rudder inputs made. Right bank of 15° to 20° was required to maintain a constant heading. Soon after regaining directional control, it was reported that a “FLT CTL” annunciation had appeared on the CAP (Crew Alert Panel) together with a “FLT CTL” fault light illumination on the overhead panel which indicated a fault with the rudder Travel Limitation Unit (TLU). In view of the control problems the aircraft commander requested radar vectors from ATC for a return to Edinburgh and subsequently declared a ‘PAN’. As there appeared to be very little control authority to make right turns, ATC were asked to give left turns only. The applicable QRH drill for a TLU abnormality was completed and a descent made at 180 KIAS in VMC onto an Instrument Landing System (ILS) approach to Runway 24 for a full flap landing. After 41 minutes airborne, touchdown was made just to the left of the runway centreline and the aircraft commander took control, applying reverse pitch. The aircraft had touched down on the right main wheel first and during deceleration, it diverged further to the left despite application of full right rudder until reducing speed allowed tiller steering input. The aircraft was then taxied clear of the runway and back to the engineering facility.
A visual inspection of the TLU found that one of the cams on the rudder rear quadrant shaft had been incorrectly installed, such that it was misaligned with the other cam. The maintenance organisation immediately suspended the approvals of the engineers concerned pending further investigation. Once the error context had been assessed, the AAIB issued Special Bulletin 1/2011 on 15 April 2011.
It was noted that the function of the TLU, which is installed on the rudder rear quadrant shaft (see diagram below showing the rudder control system) is to reduce the range of available rudder deflection at airspeeds above 185 knots so as to limit the structural loads on the rudder.
The ATR 72 Rudder Control System (reproduced from the Official Report)
It was ascertained that two of the operator’s aircraft had recently experienced significant delays at the Edinburgh maintenance facility, “a situation which had caused frustration within the maintenance organisation, the operator and at group level”. It was also noted that another of the operator’s aircraft was planned in for maintenance immediately following the incident aircraft and “the management at the maintenance organisation considered that another delayed aircraft would have been viewed as a major failure on their part and would result in loss of revenue if the following aircraft could not be accommodated". The organisation’s Accountable Manager in post at the time of the event stated that these factors directly influenced his actions in deciding to personally ‘direct’ an that an unapproved action was an appropriate response to a fault found in the TLU at a late stage in the Check. The subsequent absence of effective oversight of required procedures to complete work on the TLU, during which a cam was re-fitted incorrectly, as well as the failure to carry out both functional checks and an operational test of the re-assembled TLU were also considered indicative of systemic organisational deficiency under commercial pressure as a context for the ‘simple’ maintenance error which was the direct cause of the in flight control difficulty. The fact that it was possible to mis-fit the cam involved was considered a factor but overall was not the main concern of the Investigation.
The Conclusion of the Investigation was that:
“The incident was caused by the incorrect fitment of a cam on the rudder TLU mechanism which was not detected by maintenance personnel. This resulted in rudder control restriction which caused the aircraft to enter an uncommanded roll to the left when the airspeed increased above 185 knots. The required independent inspection of the work and the operational test of the TLU system were not carried out. Commercial pressure was identified as the most significant factor which influenced the decision to perform unapproved and unrecorded maintenance on the TLU system. A contributory factor was the design of the TLU cams, which allowed them to be installed in the incorrect orientation.”
A total of five Safety Recommendations were issued as a result of the Investigation. Three of these were issued in Special Bulletin S1/2011 as a result of the initial findings of a maintenance error cause during the Investigation and two further ones at the conclusion of the Investigation:
- It is recommended that ATR immediately informs all operators of ATR aircraft equipped with a Travel Limitation Unit that it is possible to install the cams on the rear rudder quadrant shaft in the incorrect orientation. [2011-10]
- It is recommended that ATR amends all relevant Aircraft Maintenance Manual tasks to include a warning to highlight that the cams on the rear rudder quadrant shaft can be installed incorrectly. [2011-11]
- It is recommended that ATR amends the Aircraft Maintenance Manual task ‘Operational Test of the Rudder Travel Limitation Unit’ to state that: (1) the test should be carried out for a minimum of 30 seconds and (2) should an asymmetric restriction of the rudder pedals be detected or if the FLT CTL light illuminates, further inspection of the TLU system should be conducted. [2011-12]
- It is recommended that the European Aviation Safety Agency require ATR to modify the cams on the rudder Travel Limitation Unit on all applicable aircraft, to reduce the risk of incorrect assembly. [2012-002]
It is recommended that ATR amend the ATR 72 QRH section 2.22 A to state that the green LO SPD light should illuminate after 30 seconds, when the rudder Travel Limitation Unit switch is manually selected to the LO SPD position. [2012-003]
It was noted that in response to the initial three recommendations, ATR had issued an AOM on 19 April 2011 advising operators of the incident and emphasising the importance of independent inspections after any maintenance is performed on a flight control system. ATR also updated the ATR72 AMM task ‘Removal and Installation of TLU Mechanism Assy’ to include a requirement to record the position of the right hand cam before removal and amended ATR 72 AMM task ‘Operational Test of the Rudder Travel Limiter Unit’ to reflect the intent of Safety Recommendation 2011-12.
A number of Safety Actions taken by the maintenance organisation involved, including the replacement of the JAR 145 Accountable Manager in post at the time of the event were also noted.
The Final Report of the Investigation AAIB Bulletin: 7/2012 EW/C2011/03/04 was issued on 12 July 2012.