Braathens Regional RJ1H at Malmo on Sep 29th 2016, stick shaker and stick pusher activated on departure
Last Update: September 12, 2017 / 16:45:18 GMT/Zulu time
Sweden's Statens Haverikommission (SHK) opened an investigation into the occurrence.
On Sep 12th 2017 the SHK released their final report concluding the probable causes of the serious incident were:
The serious incident was caused by the mix up of test instructions for installation of “Vane assembly” and “Airflow sensor”, which led to a prescribed functional test was not performed and the fault in the airflow sensor was not detected.
- The different component names Vane assembly and Airflow sensor enhance the risk of confusion between tasks.
- The interruptions during the change of the airflow sensor were a stress factor which increased the risk of mistakes.
- Re-inspection after replacement of airflow sensor was not performed.
The SHK reported that during pre-flight inspections a damage at the left airflow sensor was detected. The sensor was replaced which caused a delay of 30 minutes.
After engine start the crew spotted that the circuit breaker for the left airflow sensor heating was out, pushed the breaker back in and no further errors occurred.
Upon becoming airborne the stick shaker activated, the captain (51, ATPL, 12,479 hours total, 10,000 hours on type), pilot monitoring, immediately identified that speed and attitude were correct and decided the first officer (24, CPL, 1,160 hours total, 280 hours on type), pilot flying, should continue to fly the aircraft. The first officer also identified speed and attitude were correct, together with the knowledge that the left airflow sensor had been replaced both crew identified the stick shaker activation to be false.
The aircraft was climbing through about 660 feet MSL in cloud when the stick pusher activated. The first officer opposed the stick pusher in order to prevent the aircraft from descending, however, the aircraft levelled off at 660 feet MSL and the first officer asked the captain for assistance, the stabilizer trim was used to counter act the stick pusher and reduce the steering load. The crew did not perform the memory item to inhibit the stick pusher, however, inhibited the stick pusher while reading the associated checklist for stick shaker and stick pusher activation. The system was shut down and the problems ended.
After aving brough the situation under control the crew advised ATC and requested to return to Malmo.
The SHK reported the stick shaker activated at 72 meters/236 feet above sea level.
THe SHK reported following the occurrence the left hand air flow sensor was removed from the aircraft and sent for examination. A mechanical check including x-ray did not reveal any anomaly, however, it was noted "that the vane shaft gear clamp was tight but was not oriented per the drawing. In addition, the two potentiometer shaft gear clamps were tight but not oriented correctly. One of the potentiometer clamps was interfering with the spring that connects the two potentiometer gears when the vane was moved to the corresponding stop. This resulted in the spring stretching, and returning the vane gear to a neutral position when the vane was let go. When the spring was removed, the vane shaft appeared to balance normally."
The unit was found to be 45-50 degrees out of specification on all angles. After calibration the unit operated normally.
The SHK analysed:
Shortly after take-off, when the main wheels were in the air, the stall warning system was activated. By immediately checking the speed and attitude of the aircraft, along with the awareness that an airflow sensor was replaced before the flight, the flight crew could immediately identify the stall warnings as false.
Deactivation of stick shake is not a memory item, which can explain that circuit breakers E5 and E6 were not pulled immediately.
However, deactivation of the stick push is a memory item which in the checklist specifies that both buttons for IDNT 1 and 2 are to be pushed. This was done when the emergency checklists were read. The explanation for this is likely to be found in that the commander previously inhibited IDNT 1 while IDNT 2 did not indicate a fault, which means that the correct action in the arising situation does not appears natural, to deactivate the system. In addition, malfunctions in the stall warning systems are not normally trained in the simulator.
With respect to the replacement of the airflow sensor the SHK analysed:
When replacing the airflow sensor, wrong tests were used after installation.
Instead of the airflow sensor test, the vane assembly test was used. The term vane assembly leads to the fact that it covers several parts and not just the vane itself. This condition, as well as when the tasks lacks clear boundaries in the manual, can explain why wrong test instructions were used.
The repeated interruptions during the installation have also been a disturbance factor, which probably contributed to the incorrect execution of the task and that the re-inspection was not performed.
The sensor screen test that was performed with approved result could have been approved because it was under stress and that the values were misinterpreted. The only thing that separates the values is a minus sign in front of the full-range value in the end positions.
With respect to the faulty sensor the SHK analysed:
Testing and troubleshooting at the manufacturer showed that the clamps of the potentiometers and the clamp of the vane shaft were tight but not orientated according to the drawing. In addition, one of the potentiometer clamps was interfering with the spring that connects the two potentiometer gears when the vane was moved to the corresponding.
This indicates that the sensor at some point been incorrectly assembled or manipulated. In the absence of tampering or warranty seals, it is impossible to determine if the device has been delivered incorrectly or if someone has manipulated it at a later stage.
It is likely that the device had not been installed if a broken tampering or warranty seal had been found.
This article is published under license from Avherald.com. © of text by Avherald.com.
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