AS355F1, G-XCEL, 2 December 2003

40 AAIB Bulletin: 7/2006 G-XCEL EW/C2003/12/01 ACCIDENT Aircraft Type and Registration: AS355F1, G-XCEL No & Type of Engines: 2 Rolls-Royce (Allison) 250-C20F turboshaft engines Year of Manufacture: 1 985 Date & Time (UTC): 2 December 2003 at 1438 hrs Location: Hurstbourne Tarrant, near Andover, Hampshire Type of Flight: Private Persons on Board: Crew - 1 Passengers - 2 Injuries: Crew - 1 (Fatal) Passengers - 2 (Fatal) Nature of Damage: Helicopter destroyed Commander’s Licence: Airline Transport Pilot’s Licence Commander’s Age: 51 years Commander’s Flying Experience: 7,800 hours (of which 1,322 were on type) Last 90 days - 81 hours Last 28 days - 18 hours Information Source: AAIB Field Investigation Synopsis The helicopter was engaged on a post-maintenance test‑flight following the fitment of a newly‑overhauled main rotor gearbox and combining gearbox. Eyewitnesses heard unusual noises coming from the helicopter before the tail boom apparently folded forward around the cabin. The helicopter then fell to the ground, catching fire on impact. All three occupants received fatal injuries. Examination showed that the two gearboxes and the main rotor had detached before impact. Subsequent investigation showed that the left freewheel showed clear evidence of slippage under load; the right freewheel also showed signs of slippage but not to the same extent. It is concluded that a series of freewheel slippages followed by aggressive re-engagements led to the structural failure. The reasons for the slippage however, cannot be proven conclusively. Although it was found that the rollers forming part of the freewheel mechanism had come from a manufactured batch that had been coated using an incorrect process, no laboratory testing could reproduce any greater tendency for such a coating to cause slippage. The helicopter manufacturer recorded five incidents of slippage under load, coinciding with the introduction of rollers from this batch. Satisfactory performance of the freewheels resumed following the removal from service of the incorrectly coated batch of rollers. 41 AAIB Bulletin: 7/2006 G-XCEL EW/C2003/12/01 History of the flight Maintenance work had been conducted on the helicopter requiring the pilot to spend two days completing engine ground run tests with the two engineers who had carried out the work. On the morning of the accident the pilot carried out a short flight to check that the helicopter’s handling was satisfactory and to examine the extent of a torque difference between the engines that had been identified during the previous ground runs. The two engineers were on board for the uneventful 17 minute flight. Adjustments were made to the helicopter, by one of the engineers, to rectify the difference in torque. About an hour after landing the helicopter, with the same pilot and engineers on board, departed for a further airtest. The purpose of the flight was to confirm that the adjustments to balance the engine torques had been successful. The air traffic controller’s log recorded that the helicopter departed Runway 07 at Thruxton at 1430 hrs. At the time there was a light easterly wind, with good visibility and a cloudbase about 1,200 ft above the airfield. The helicopter was seen to take off and appeared to be flying normally as it departed to the north-east. Recorded radar data, between 1431 hrs and 1433 hrs, indicated that the helicopter maintained a steady track to the north-east flying at an altitude of approximately 2,000 ft amsl and at a speed of approximately 120 kt. Witnesses, 8 nm from the airfield, saw the helicopter fly overhead and heard it making a loud, and unusual noise, described by one as “a loud screeching mechanical noise”. Another witness described seeing the whole helicopter shake. Witnesses then described seeing the tail of the helicopter fold forward against the side of the cabin, without fully separating, and the helicopter fall to the ground. There were variations in the witness accounts; some describing the tail folding to the right and others describing it folding to the left. One witness described seeing one of the main rotors “flip upwards” just before the tail folded. The helicopter fell to the ground on the ridge of a small hill and caught fire. The emergency services were quickly on the scene; however, all three occupants had been fatally injured in the impact. Helicopter description The Eurocopter (Aerospatiale) AS355 series of helicopters were derived from the AS350 Ecurreil (Squirrel) helicopter but were fitted with two turboshaft engines in place of the single engine fitted to the latter. Known in the UK as the ‘Twin Squirrel’, the first models were equipped with Rolls-Royce (Allison) Model 250‑C20 engines whilst later versions (AS355N) were fitted with Turbomeca Arrius engines. G-XCEL was fitted with Rolls-Royce engines. Of particular relevance to this accident is that fuel control in the Arrius engine is achieved by a full authority digital engine control unit (FADEC), whilst the Rolls-Royce engine uses a conventional hydro-mechanical system. Apart from the necessary changes to accommodate the different engine installations, the two helicopter models are essentially the same, particularly with respect to the main rotor transmission. The engines are mounted on the left and right sides of the main transmission deck and are referred to as left and right or No 1 and No 2 respectively. The power output from each engine, in both helicopter variants, can be trimmed so that the total torque required for flight can be shared equally between the engines. This is commonly known as ‘beep trim’ and is adjusted by the pilot using a rocker switch on the collective lever. On the AS355N models (fitted with Arrius engines) this is purely an electrical signal working through the two 42 AAIB Bulletin: 7/2006 G-XCEL EW/C2003/12/01 FADEC’s. However, the Rolls-Royce Allison engines use an electromechanical trim actuator to mechanically move the fuel control settings. In both variants this is co-ordinated so that, for example, if the pilot increases torque on the right engine by moving the switch to the right it not only increases the power output of that engine, but decreases power from the left engine. Under certain circumstances it is necessary to perform a ‘Power Assurance Check’ on each engine. In this case one engine is trimmed to its maximum (or until a limiting parameter is reached) and the other simultaneously trimmed down to deliberately induce a large torque imbalance between the engines. Aircraft and engine performance figures are noted and checked against manufacturer’s data in the Flight Manual. This is then repeated for the other engine. The Flight Manual includes graphs for performing the check either in-flight or on the ground, although it appears that the in-flight figures are more accurate. Engine overspeed Turbine engines can be subjected to an overspeed condition for various reasons, particularly in the free turbine application (see below). This can cause damage to the power turbine or even rupture of the turbine disc and consequent non-containment. In the case of the Arrius engine, the FADEC is programmed to completely shut down the engine at 115% Nf (power turbine rpm). However, in a twin-engine installation, if one engine has shut-down for any reason, overspeed protection is removed from the other engine and it is possible to burst the power turbine if a serious overspeed condition is also experienced on that engine. The Rolls-Royce Allison engine has no specific overspeed protection device, but the manufacturer stated that the normal governing function of the Power Turbine Governor is sufficient to prevent an overspeed burst of the turbine. Data was presented from a test on a Model 250 engine in which the load was abruptly removed whilst the power turbine was delivering 100% torque. The turbine accelerated rapidly to 142% Np before settling back to a steady state ‘

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AS355F1, G-XCEL, 2 December 2003

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