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Factual Information

Kind of occurrence: accident

Date: 27. March 2001

Location: near Birkenfeld

Aircraft: helicopter

Manufacturer/type: Eurocopter SA 315B "Lama"

Injuries to persons: helicopter pilot suffered minor inju-ries

Material damage: helicopter destroyed

Other damage: field damage

History of the flight

With the a.m. helicopter external load flights for thepurpose of forest fertilising were conducted. During thelifting of the load for the fourth aerial work flight after arefuelling break - at that moment the lime containerhad already been lifted by about 2 m - the pilot noticeda push in the pedals of the tail rotor control. Immedi-ately afterwards the tail rotor control became ineffectiveand the helicopter started to turn counter-clockwiseabout the yawing axis. After several rotations the heli-copter touched the tops of several trees, crashed onthe ground and came to lie on the starboard side.

Investigation

The accident was investigated on the site by a field in-spector of the BFU. The following findings have beenmade:

Two of the three tail rotor blades had been broken, thethird blade had only been bend. The leading edges of

the blades did not show any imprints which would have

indicated rotation during the impact. The tail rotor driveshaft had been bent off shortly behind the cabin but didnot show any traces of high torsion strain. The tail rotorgear box was still filled with oil. It could be turnedsmoothly and was capable of transmitting torque. Thetail rotor control was not interrupted and was function-ing except for an impact induced rupture of the controlcable on the aft cable drum.

Most of the oil had leaked out of the main gear box atthe accident site. The oil showed a peculiar rustybrown discoloration. The oil filter as well as the mag-netic chip detector was free of chips. In the course ofthe investigation by the BFU a chemical analysis of theoil was not found to be necessary.

With rotating the main gear box manually it was foundthat the torque was no longer transmitted from themain rotor shaft to the tail rotor drive shaft. The outputflange on the main rotor gear box could be turnedsmoothly. As an internal damage to the main gear boxhad to be assumed the gear box was removed from

the helicopter and transported to Braunschweig to theBFU for further investigation.

During the disassembly of the gear box all parts insidethe housing were found to be covered by a reddishbrown greasy patch. The bevel gear on the main rotorshaft serving as a transmission element for the tail ro-tor drive could be rotated relative to the shaft. The con-nection in this area is realised via a multiple-splineshaft with a transition fit and a high axial preloading bymeans of a shaft nut. The latter could be screwed offmanually after removal of the locking wire.

The teeth on the main rotor shaft had been worn off tosuch a degree that the positive engagement with the

Bundesstelle frFlugunfalluntersuchung

Aircraft Accident Report 3X023-0/01September 2002

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bevel gear had been lost. The loss of material of theteeth was even over the whole circumference. Theconnection area was filled with sludge of a rusty colour.There were no visible chips or particles of the lost toothportions. However, plastics particles had been found inthe remaining tooth roots. During a subsequent analy-sis, the particles were found to be a bearing adhesive,

similar to the product known under the brand nameLoctite.

The manufacturer of the helicopter stated upon inquirythat according to the maintenance documents the useof a product such as Loctitewas not specified and thatthe product even was adverse to this function.

With regard on long term experience by similar applica-tions, the certified maintenance company, which hadperformed the last major overhauls of the gear boxfinds adhesives useful. The company stated furtherthat, Loctitehad been used only if there was light playin the spline shaft connection within the tolerances

specified by the manufacturer, and that, it had beenapplied to the mating faces beneath the spline shaft toachieve a better positioning of the pinion.

The manufacturer knows of a similar component fail-ure, which happened appr. 20 years ago. As a result ofthe investigation conducted at that time a special in-spection to be performed at regular intervals of 400(+20) hours in service is included in the maintenanceschedule of the SA 315B. During this inspection theangular play on the output flange of the gear box ismeasured. According to the existing inspection re-cords, the main gear box had been installed after304 hrs (TSO) into the accident helicopter. The inspec-tion of the angular play had been performed 208 hrsafter that installation and 186 hrs before the accident.

Analysis

As the technical investigations show the failure of aspline shaft connection in the main gear box hadcaused the tail rotor drive to fail. Such a technical fail-ure on a helicopter results in a loss of control which inhover flight cannot be coped with by the pilot.

The findings made on the spline shaft connection indi-

cate wear as a result of friction, in particular frictioncorrosion. A detailed tribological analysis of the dam-age had not been conducted. Friction causes at least aminimum relative motion between the contact surfacesand the damage evolution extends over a longer periodof time. Thus a form fitting without a sufficient perma-nent mechanical force transmission may fail.

The design of the spline shaft and the pinion provides atransition fit and conditionally no play. The tolerancesdue to the manufacturers documents can lead to alight clearance between pinion and sline shaft. In thiscase only the rather high preloading of the shaft nutwas left to prevent a relative motion in axial and incircumferential direction. The following factors may

have caused the wear in the connection:

1. The dimensions of the spline shaft and the pinionresulted in a clearance within the connection. The tran-sition fit by design is allowing that. Relative movementscould have been prevented either by a designed pres-sure fit or by assembling a matching pair.

2. The preloading due to the manufacturers directiveis not sufficient to prevent relative movement if clear-ance within the spline shaft connection exists.

3. The preloading is sufficient, however the shaft nuthad not been fastened by the required torque or con-tamination of the mating faces, e.g. by Loctite, had leadto axial play in the course of the operation.

It could not be clarified in the scope of the investigationin which way the play causing the friction wear wasproduced.

Wear of metal components in gear boxes is indicatedby chafing and corrosion products such as rust in theoil circuit. This kind of wear leads to a discoloration ofthe oil and forms a brown deposit on the magnetic chipdetector.

An assessment of the effectiveness of the special in-spection, i.e. measuring the radial play on the driveflange, which had been prescribed by the manufactureralready a long time ago, was not a subject of the inves-tigation.

ConclusionsFollowing the failure of the tail rotor drive the accidentwas inevitable. The cause of the wear leading to thecomponent failure could not be determined with suffi-cient certainty.

Safety Recommendations

The BFU is convinced that the continuous inspection ofthe gear box oil for rusty reddish contamination is theonly safe method to recognise wear. Following the in-vestigation the manufacturer published the Service

Alert Bulletin no. 05.99 providing a programme for con-tinuous supervision of the gear box by measurement ofangular play and oil checks. Besides the BFU recom-mends the daily oil check as the most effective prac-tice.

Investigator-in-charge Hasenfu / Leibe

field investigation Herzberg

The investigation has been conducted in compliance with the Law Relating tothe Investigation into Accidents and Incidents Associated with the Operation ofCivil Aircraft (Flugunfall-Untersuchungs-Gesetz - FlUUG) dated 26 August1998. According to this Law, the sole objective of the investigation shall be theprevention of future accidents and incidents. It is not the purpose of this activityto apportion blame or liability or to establish claims.

Herausgeber:

Bundesstelle frFlugunfalluntersuchung

Hermann-Blenk-Str. 1638108 Braunschweig

mail: box@bfu-web.dehttp:// www.bfu-web.de

Tel: 0 531 35 48 0Fax: 0 531 35 48 246