Search and Rescue (SAR)  AS365N3 Flying Control Disconnect (BFU Investigation: Northern HeliCopter D-HNHA)

On 1 June 2020 Northern HeliCopter Search and Rescue (SAR) Airbus Helicopters AS365N3 D-HNHA was involved in a serious incident at St. Peter-Ording Airfield in Schleswig-Holstein, Germany after a disconnect in the flight control system occurred.

Disconnected Flying Control on Northern HeliCopter SAR AS365N3 D-HNHA  (Credit: Operator via BFU)

Disconnected Flying Control on Northern HeliCopter SAR AS365N3 D-HNHA (Credit: Operator via BFU)

The Serious Incident

During take-off the helicopter entered a nose-up attitude, the tail contacted the ground and the helicopter touched-down hard according to the German Federal Bureau of Aircraft Accident Investigation (BFU).

In their preliminary investigation report the BFU explain that the four person crew were tasked to conduct a SAR mission on what was their second flight of the day:

During a shortened procedure, known as Scramble Take-off, the engines were started and the helicopter taxied to runway 07.

Among the items omitted in this shortened procedure are hydraulic control checks.

According to…the Pilot in Command (PIC), the co-pilot in the right-hand seat, who was also Pilot Flying (PF) during taxi, noticed that the pitch control’s position was unusually far forward and the helicopter needed an exceptional amount of thrust and still taxied slowly. Since there were no warning lights (limit light, caution or warning) and the rotor disc was in “normal” position, they therefore assumed a grinding wheel brake or being wrong about it [i.e. that this was normal] since they had recently been flying a different helicopter type.

Once they had reached runway 07, at about 1547 the PF increased thrust and the helicopter began to hover. It immediately began to pitch up, impacted the ground with the tail and touched down hard with the main landing gear.

The crew were uninjured but the helicopter was ‘”slightly damaged”.

Subsequently, the crew determined that forward and backward control inputs via the pitch control did not have any effect on the rotor disc’s position.

The engines were shut down while still on the runway.  The helicopter was towed back to the operator’s ramp.

…the asphalt runway only showed scrub marks of the tail skid.  The tail skid, which consisted of composite material, was torn and the metal protection at the end showed scrub marks.

Damaged Tailskid of Northern HeliCopter SAR AS365N3 D-HNHA  (Credit: Reibel via BFU)

Damaged Tailskid of Northern HeliCopter SAR AS365N3 D-HNHA (Credit: Reibel via BFU)

There were also isolated scrub marks on the lower tail fin fairing.

Northern HeliCopter SAR AS365N3 D-HNHA  In Service (Credit: Ichwarsnur CC BY-SA 4.0)

Northern HeliCopter SAR AS365N3 D-HNHA In Service (Credit: Ichwarsnur CC BY-SA 4.0)

The Safety Investigation

During the subsequent check in the hangar the crew determined that the connection of the left actuator to the swashplate was missing.

AS365N3 Flying Controls (Credit: EASA via BFU)

AS365N3 Flying Controls (Credit: EASA via BFU)

The missing bolt, two washers, and one crown nut were found on the gear box compartment below.

Nut and Bolt as Found on Northern HeliCopter SAR AS365N3 D-HNHA  (Credit: Operator via BFU)

Nut and Bolt as Found on Northern HeliCopter SAR AS365N3 D-HNHA (Credit: Operator via BFU)

The…actuators are located directly underneath the stationary part of the swashplate.

Normal Actuator to Swashplate Assembly of an AS365N3 (Credit: BFU)

Normal Actuator to Swashplate Assembly of an AS365N3 (Credit: BFU)

Each working piston of the actuators is fitted with a ball head. A 60 mm x 8 mm threaded bole serves as axis and connection with the swashplate.

Actual Bolt / Nyloc Nut (Upper) Northern HeliCopter SAR AS365N3 D-HNHA  and Exemplar Bolt / Nyloc Nut (Lower) (Credit: BFU)

Actual Bolt / Nyloc Nut (Upper) Northern HeliCopter SAR AS365N3 D-HNHA and Exemplar Bolt / Nyloc Nut (Lower) (Credit: BFU)

The bolt is kept in position with a nylon stop crown nut [i.e. castle-headed nyloc nut] and cotter pin.

The associated cotter pin was not found.

Exemplar Bolt / Nyloc Nut with Cotter Pin Fitted (Credit: BFU)

Exemplar Bolt / Nyloc Nut with Cotter Pin Fitted (Credit: BFU)

The investigators also noted that the bolt was lubricated.  The BFU show a Maintenance Manual illustration that is marked with a torque range and a ‘no lubrication’ symbol.

The bolt and the nyloc nut showed traces of wear. The thread on bolt and washer did not show any visible damage. The thread pitches were partially contaminated. When the nyloc nut was twisted on to the bolt it was possible to do so without any effort for the first 3.5 turns. The subsequent 4 turns, until the hole for the cotter pin became visible, more effort was required. But it was still possible [to turn] by hand, no tool was required. In this position the nyloc nut jammed in a way which should not allow loosening by vibrations or oscillations.

By comparison, a new nyloc nut could only be turned using a tool once the nylon stop application was reached.

The hole for the cotter pin and the 6 cut-outs of the nut showed signs of “contamination”.

Contamination Northern HeliCopter SAR AS365N3 D-HNHA (Credit: BFU)

Contamination Northern HeliCopter SAR AS365N3 D-HNHA (Credit: Operator via BFU)

Contamination Northern HeliCopter SAR AS365N3 D-HNHA (Credit: BFU)

Contamination Northern HeliCopter SAR AS365N3 D-HNHA (Credit: BFU)

Close Up of Lugs on Northern HeliCopter SAR AS365N3 D-HNHA  (Credit: BFU)

Close Up of Lugs on Northern HeliCopter SAR AS365N3 D-HNHA (Credit: BFU)

The BFU discuss the maintenance history:

Over the course of several weeks, until the middle of May 2020, extensive maintenance work was carried out on the helicopter and on 30 April 2020 a checkflight after maintenance conducted.

As part of the troubleshooting the leaking main gear box output to the rotor brake was fitted with a new seal.

To do this task, the left actuator was disconnected from the swashplate.  It was reconnected after.

According to the written statements of the mechanic, the examiner and the maintenance pilot provided to the BFU, the bolt fitting was properly executed, the cotter pin positioned and visually checked several times, before the helicopter was returned to the operator.

Between 18 May 2020, when the Part-145 [maintenance organisation] company had returned the helicopter, and the day of the occurrence, it had been operated 7 days and a total of 8:46 flight hours.

On 30 May 2020, the last 7-day/10-flight-hour inspection was conducted and the last release to service issued, i.e. 1:11 flight hours prior to the occurrence.

The mechanics, who performed the 7-day/10-flight-hour inspection, stated that they had not paid special attention to the connection of actuator and swashplate, which is located in a barely visible area and therefore they could not say if the nyloc nut and cotter pin had been present.

Checking the connection of the controls with the swashplate was not required as part of the pre-flight check. This area cannot be seen from the ground if faring and maintenance hatches are closed.

The helicopter was equipped with a Honeywell CVR/FDR Combi Recorder AR 602C.  The CVR data for occurrence had been overwritten because the recorder had remained powered for more than 60 minutes after the event. The FDR data is however available to the BFU.

The FDR data showed a maximum acceleration of about 1.5 g during the hard touch-down.

DHNHA SAR AS365N3 Northern HeliCopter fdr 1

 

Two acceleration peaks were determined during the occurrence flight compared with the engine start-up, taxiing and take-off of the previous flight on the same day.

DHNHA SAR AS365N3 Northern HeliCopter fdr 2

Usefully the 2004 helicopter had been retrofitted with an Appareo Vision 1000 unit.  It includes a cockpit camera, GPS-based flight data and background audio recording.

DHNHA SAR AS365N3 Northern HeliCopter appareo

This data has been provided to the BFU.

The BFU have not at this stage made any safety recommendations or drawn any conclusions.

The BFU will be continuing their investigation, and we will update this article when they publish their final report or other information emerges.

Safety Resources

Without prejudging the outcome of the investigation, the UK Air Accidents Investigation Branch (AAIB) has previously reported on an accident where the main rotor blade spindle fractured on AS332L G-PUMI in Aberdeen on Friday 13 October 2006.  Investigators identified that there was an…

Excessive clamping pre-load across the yoke, due to the tie bolt being torqued to the specified dry value in the presence of grease when it was reinstalled some 175 hours prior to failure of the yoke.

In July 2009 the manufacturer issued Service Bulletins which introduced a ‘wet’ assembly procedure, with new nuts, for the main rotor blade spindles.

They did also note flight loads were slightly higher than assumed during design and there were also two minor production anomalies in relation to a corner radius and shotpeening.

We have previously written:

Among our other maintenance related articles are:

Aerossurance worked with the Flight Safety Foundation (FSF) to create a Maintenance Observation Program (MOP) requirement for their contractible BARSOHO offshore helicopter Safety Performance Requirements to help learning about routine maintenance and then to initiate safety improvements:

mop

Aerossurance can provide practice guidance and specialist support to successfully implement a MOP.

Aerossurance will be running training workshops at the EHA European Rotors VTOL Show and Safety Conference in Cologne in November 2020 on a) Safety Culture and Leadership and b) Contracting Aviation Services: An Introduction to the Basics. european rotors 2020


Aerossurance has extensive air safety, operations, SAR, airworthiness, human factors, aviation regulation and safety analysis experience.  For practical aviation advice you can trust, contact us at: enquiries@aerossurance.com

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