On 31 January 2005, an ATR 42-300 being operated by Danish Air Transport on a scheduled passenger flight from Bergen to Florø in day Visual Meteorological Conditions (VMC) encountered pitch control difficulties during rotation and subsequent climb and after declaring an emergency made a successful return to land on the departure runway seven minutes later. None of the 25 occupants were injured and the only damage found was to the elevator and its leading edge fairings.
An Investigation was carried out by the Accident Investigation Board Norway (AIBN). It was noted that the aircraft was making its fourth flight of he day with the same flight crew. It was established that the take-off roll acceleration had been normal but that during rotation, the aircraft commander as PF had needed to apply “excess force” on the control column to get the aircraft airborne. Once airborne, it became clear that the elevator was not working as it should and full elevator deflection was necessary to maintain normal pitch for the initial climb. For a period, the First Officer assisted physically with elevator control and both flight crew informed the investigators that it was extremely demanding to maintain control of the aircraft with the response to pitch control inputs unevenly and "jerky". An Emergency was declared to ATC one minute after take off with the aircraft about 3 NM north of the airport and approaching 2,000 feet altitude. The control problems were considered by the Flight Crew to have diminished somewhat once the aircraft levelled out and turned back towards the airport and a successful visual return was accomplished.
The Investigation considered that the damage to the leading edge fairings and the flange on the elevator main beam had resulted from abnormal movement when the elevator hinges became loose. It was also considered that the self-locking nuts on the centre and outboard elevator hinges “cannot have been tightened with the required torque, as they are not exposed to (any rotational) forces that could cause them to loosen.” It was noted that maintenance documentation for the aircraft indicated that these nuts had not been touched since the aircraft had been repainted in 1999 which “indicates that the installation carried out in France in 1999 was not according to specification” and that “it is most probable that the two nuts were only applied/tightened finger tight, without this being discovered.”
Confirmation that the nuts on the centre and outboard bolts had been loose for a long time was considered to be the findings during the investigation of:
- Wear on the surface treatment of both bolts indicating that they had rotated and wandered sideways in the hole
- Corrosion in the centre and outboard bolt holes indicating that there had been a loose connection allowing moisture ingress
The Investigation concluded that:
“The control problems experienced by the crew during take-off and the rest of the flight began when the outboard of the three hinge bolts that attach the right elevator to the stabiliser loosened and fell out. As the centre bolt had fallen out at an earlier point in time without being discovered, the elevator was hanging in place only attached by the inboard hinge. The self-locking nuts that should hold the centre and outboard hinge bolts in place cannot have been tightened with the required torque.”
It was also noted that:
- The elevator hinges are only visible during a DVI which is only performed at 8 year intervals and does not specify special attention to the bolts/nuts/bearings in the hinges.
- The relevant DVI was performed in February 2003 without discovering the incorrect assembly.
- The bolt in the centre hinge assembly had, at an earlier point in time, probably several months before the accident, fallen out of the hinge and into a space in the elevator structure.
- After the bolt in the outboard hinge assembly fell out during take-off, the elevator was only attached to the stabiliser by the inboard hinge.
- Since the outer part of the elevator was then hanging below the stabiliser, it was difficult to maintain normal control of the aircraft.
- Electrical Power was not disconnected from the 30 minute CVR promptly after landing and so recordings from the period in question were lost.
Overall, it was noted that:
“a maintenance error was introduced onto an aircraft during maintenance work on a safety-critical system. The error was not discovered during the double inspection immediately following installation, later maintenance work, various inspections, checks or pre-flight check of elevator function. Several safety barriers seem to have failed in turn, so that one single error was allowed to develop into a serious situation involving control problems during flight. Such circumstantial factors as position of the centre of gravity, weather conditions and the fact that the aircraft did not receive consequential damage when the elevator detached, all contributed to it being possible for the crew to handle the situation and carry out a controlled landing.”
Two Safety Recommendations were issued as a result of this Investigation, both to JAA / European Aviation Safety Agency (EASA), who were recommended to:
- Consider whether the regulations should be amended in order that systems that are critical to safety are double checked following maintenance work. Special consideration should be made as to whether the manufacturer should be given a responsibility on this matter. (12/2006)
- Consider whether the regulations (Appendix 1 JAR OPS 1.1045 pt. 11) should specify that procedures must be drawn up for preservation of data from flight and cockpit voice recorders and included in operation manuals, so that the JAR OPS 1.160 requirements are better adhered to. (13/2006)
The Final Report was submitted in April 2006 and may be seen at SKYbrary bookshelf: AIBN SL RAP.: 8/2006