Crash of a Swearingen SA227AC Metro III in Aberdeen

Although the investigation was hampered by the lack of FDR data, which might have provided information on engine handling and behaviour, the available evidence from the crew and the initial examination of the aircraft at the accident site, pointed to a major loss of power in the right engine. As a result of the discovery of dead birds close to the point of lift-off on the runway, and a section of braided wire found near the start of the take-off roll, consideration was given as to whether the these had been factors in the accident. The braided wire was not identified as having originated from OY-BPH. There was conclusive evidence that the left engine, which had continued to run, had ingested parts of birds, but no such evidence was found with the right engine. Nevertheless, the crew were adamant that there was a power loss from the right engine and were not conscious of any power reduction from the left engine. The technical examination of the left engine and its propeller assembly revealed evidence of damage consistent with this unit delivering a high level of power at impact. The examination of the right engine and propeller revealed all damage to be consistent with a low, or no, power condition at impact, consistent with either a genuine loss of power or as a consequence of the commander pulling the 'Stop and Feather' control immediately before impact. However, an exhaustive examination of the right engine revealed no evidence of anything that could have caused a failure. Therefore, the items found on the runway were not considered to have been causal or contributory factors in the accident. The crew's recollection of the event included a low EGT indication, at 600°C. This suggested that either the engine might have flamed out, should they have only momentarily looked at the indication as the engine was cooling down, or that the engine was operating at reduced power for an undetermined reason. Flame out could have occurred due to a number of reasons, including, for example, water contamination of the fuel. However, analysis of the bulk supply samples proved negative and, moreover, there were no reports of contaminated fuel from other operators at Aberdeen Airport. Ingestion of ice or water could also have resulted in a flame-out, although this is considered unlikely due to the conditions not being conducive to the formation of engine icing, the lack of significant standing water on the runway, and the absence of heavy precipitation. However, it could not be completely discounted and, if flame-out had occurred, the non-incorporation of the FAA mandated auto-relight system would have reduced the possibility of an immediate relight. Approximately six months before the accident, the left engine had failed during a landing roll-out whilst the aircraft was being operated in Spain. This failure was never satisfactorily explained although, at one stage, the fuel cut-off valve came under suspicion. The valves were interchanged, according to the records, so that the unit that had been fitted to the left engine was installed on the right engine, at the time of the accident. Whilst a stray electrical signal causing the valve to close would certainly result in the engine flaming out, rumours that such events had occurred were not substantiated by the engine manufacturer. In summary, the left engine experienced a bird strike, but with no apparent power loss, and the extensive technical examination could not identify any reason for a loss of power on the right engine. Although an engine failure during takeoff after V1 is a serious emergency, the aircraft was at a relatively light weight and, even with an such a failure, the crew should have been able to fly OYBPH safely away. However, if other factors had been involved, the margins for safe flight would have become more critical. These other factors could have included incorrect operation of the NTS system and/or a failure of the feathering system on the right engine, a concurrent power loss from the left engine, or the crew not handling the emergency effectively. There is no doubt that the left engine had been producing power at impact, but a definite conclusion could not be made as to the blade pitch angles of the right propeller. From the evidence of the commander and the propeller examination by the manufacturer, it is probable that the right propeller was close to the feather position at ground impact. The functionality of the NTS and feathering system could not be determined but, as noted earlier, the pre-flight NTS check actually only ensured that oil pressure was available for this system, and did not check the operation of the whole system. With the evidence that only the left engine had ingested birds, there was a possibility that the left engine was not producing maximum power, although the crew considered it was operating normally. The commander could remember that he compared the engine EGT indications, once he had pushed both power levers forward, and recalled that the left EGT was indicating greater than 650°C with the 'Bypass' light on. He then retarded the left power lever until the 'Bypass' light went out, following which the EGT indicated about 630°C. This retardation of the power lever was not required, as the function of the 'Bypass' system is to reduce the fuel flow in order to keep the engine parameters within limits. This reported retardation could, however, have resulted in a lower engine power than was possible within the available limits, possibly with an associated reduction in EGT. While it remains a possibility that the left engine may have experienced a transient power reduction as a result of the bird ingestion, it is likely that the commander's action in retarding the left power lever resulted in a more significant reduction of power. To maintain straight flight, following an engine failure, it is vital to apply sufficient corrective rudder input to maintain the wings essentially level and minimise the drag due to sideslip. In the case of OYBPH, there was a constant turn to the right before ground impact. The commander's recollection was that left rudder had been applied by the co-pilot, but not to full deflection. The amount of rudder deflection required depends mainly on the airspeed and the difference in engine power but, with the wings not level, more deflection was obviously required, and was available. Greater use of rudder would have reduced the overall drag of the aircraft, with consequent improvement in the aircraft's performance. Furthermore, following an engine failure, the second segment climb requirement is to climb from 35 feet to 400 feet at V2 with the landing gear selected to up. The commander did not raise the gear because he did not observe a positive rate of climb. However, evidence from the FDR was that a maximum airspeed of 128 kt was achieved and, as this was some 19 kt higher than the V2 speed, it indicated that the aircraft had a capability to climb which was not being used. These last three factors may have combined to reduce the climb capability of the aircraft to zero and, in that situation, the decision to not raise the gear was correct. However, all these factors were influenced by inappropriate crew actions. Although both pilots had flown together before, the lack of adherence to JAR-OPS conversion requirements may have been partly responsible for their actions during the emergency.