Description

On 28 October 2013, a BAe Jetstream 31 (SE-FVP) being operated by Avies on a scheduled domestic passenger flight from Kärdla to Tallinn Estonia veered off the side of the 1520 metre-long runway 14 after take-off power was set for departure in normal day visibility. The aircraft was observed to continue a right turn through more than 180° on the grass before slowing down and turning to the left and onto the apron where it stopped and the 17 passengers were disembarked.

Investigation

An Investigation was carried out by the Estonian Safety Investigation Bureau. The parameters available on the FDR were assessed to be irrelevant to the Investigation and it was not removed. It was discovered that data on the 2 hour CVR had been intentionally erased after the event but were successfully recovered and a subsequent sound spectrum analysis proved important to the Investigation.

It was noted that the 56 year-old Captain, who was PF for the investigated take-off, had accumulated 14,500 total flying hours which include 530 hours on type. The 49 year-old First Officer had 2,500 total flying total hours of which 2,000 hours were on type.

It was established that when the final item on the After Start Checklist 'Start Locks' which refers to the disengagement of the propeller start locks was read out by the First Officer, "the Captain did not reply to that item". During the subsequent taxi from the apron to the beginning of runway 14, both pilots stated that they had "not noticed any abnormalities". They stated that on completion of the Before Take-off Checklist, take-off power had been immediately applied, upon which "the aircraft turned sharply to the right and the crew could not control the directional movement of the aircraft with nose wheel steering or rudder". The Captain reported that he had retarded the power levers to idle and tried unsuccessfully to correct the direction of travel using the rudder. He had eventually regained directional control and had decided to continue "over the grass to the apron where the passengers were disembarked". After this had been completed, the crew had started the engines again, taxied back to the runway and tested the aircraft engines, propellers and systems without detecting any abnormalities. It was noted that "during […] interview, the crew reported “strange noises” coming from the right engine on engine start and starting locks disengagement" but that no evidence was subsequently found of any malfunction.

After careful inspections and some testing, no evidence of any compromise to full aircraft airworthiness before or after the excursion was found and in the concurrent absence of any adverse weather or runway conditions which might have contributed to the event, the remainder of the Investigation was "focused on crew coordination and training aspects".

It was noted that the aircraft is powered by two Honeywell TPE331-10UF engines which drive four bladed propellers. The engine has a two stage centrifugal compressor and a three stage axial flow turbine. Propeller pitch change is achieved by using boosted engine oil pressure to drive the blades towards fine pitch / reverse and spring pressure assisted with counterweights to drive the blades to coarse pitch / feather. After engine shut-down, this system results in a tendency for spring pressure to coarsen blade pitch so in order to prevent this from happening and thereby reduce the load on the engine starter motor, the blades are held at zero pitch for engine start-up by starting locks, which are automatically turned on shutdown and must be disengaged by pilot action after engine start.

This disengagement needs a combination of an engine RPM greater than 28% (at least 450 rpm) and application of enough hydraulic pressure to the fine side of the pitch control piston to move it forward. This is achieved by moving the power levers to the REVERSE to increase engine torque. It was noted that the aircraft manufacturer's 'Normal Procedures' for taxiing require a “Take off torque” Check to be performed to verify the position of the starting locks but that "there was no procedure in the Operator's 'Taxi' or 'Before Take-Off' Check Lists to verify the status of the start locks before take off power was applied". It was also noted that with only one propeller off the start lock, it would have been possible to taxi the aircraft normally "at all operating weights within the ground range".

It was clear from a sound spectrum analysis of the CVR data that, although the Captain had not replied to the challenge 'Start Locks' at the end of the After Start Checks, there had been "some pilot action" which had been intended to disengage them. It was noted that "while the crew went through the After Start Check List, the engines had been running at approximately 72% (equivalent to 76Hz on the CVR) and that just after the 'Start Locks' call, both engines had briefly reached 81% (equivalent to 86Hz) and 80% (equivalent to 85Hz) one after the other during the check". However, although the RPM applied had been enough to overcome the elastic forces of the start lock springs, the power levers had not been moved all the way to the REVERSE position and held there until the engine torque had increased. The right hand propeller had remained on the starting lock but the left hand one had been released. The Captain confirmed that the there was no flight deck indication of start latch position and that this asymmetric condition went unnoticed. He also advised that torque indications were not checked when seeking to disengage the start locks and neither pilot was familiar with the design of the start lock system or its operation.

Data on the CVR showed that take off power was set 20 seconds after completion of the Before Take-off Checklist. Two seconds after the rpm began to increase, the aircraft had veered sharply to the right due to the asymmetrical power to the propellers. The First Officer had immediately called a stop and the crew had "eventually" regained directional control of the aircraft and decided that in view of the extent of the excursion, they would continue taxiing over grass to the apron.

The formal statement of Conclusions included the following:

• The pilots did not perform the propeller lock disengagement procedure correctly.
• The crew did not try to stop the aircraft exiting the runway by applying the brakes.
• The crew erased the CVR recordings after this Serious Incident.

The Cause of the Serious Incident was formally determined as "incorrect pilot action on propeller lock disengagement procedure - the throttle levers were not pulled fully to the REVERSE position until torque had begun to increase, leaving the RH propeller starting latches in the engaged position with this condition unnoticed by the pilots and resulting in asymmetric thrust when applying take off power".

Two Contributory Factors were also identified:

1. Neither of the pilots tried to stop the aircraft exiting the runway by applying the brakes.
2. The fact that there is no indication in the cockpit with regards to the position of starting latches has caused pilots to develop and use multiple unofficial procedures to determine the condition of a propeller.

One Safety Recommendation was made as a result of the Investigation as follows:

• that Avies should, when operating aircraft with single acting propellers with starting lock systems, develop and implement procedures to determine starting lock position and to stabilise the aircraft before applying take off power.

The Final Report was completed during 2014 and made available in English in January 2017.

Related Articles

• Runway Excursion
• Checklists - Purpose and Use
• Standard Operating Procedures (SOPs)
• Cockpit Voice Recorder (CVR)
• Airworthiness