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The pilot of the Boeing 747 requested descent from FL370. The air traffic controller denied the request because an adequate separation standard did not exist with following traffic at a lower level. The pilot then requested an immediate descent due to icing. The controller advised the pilot of the traffic and issued a clearance for the aircraft to descend. Consequently, there was a breakdown of separation although the pilot of the following aircraft advised that he had the Boeing 747 on radar.

The pilot of the Boeing 747 subsequently reported that the aircraft had been operating in light icing conditions for 20 minutes before it entered an area of warmer air. The aircraft consequently required an increase in thrust to compensate for the increased temperature, however the number four engine throttle lever did not respond to auto-throttle commands. Soon afterwards, the crew found that they were unable to move any of the throttle levers and had to descend the aircraft. After about 15 minutes in the descent, the crew regained control of the throttle levers and levelled the aircraft at FL290.

After the aircraft landed, a maintenance inspection was unable to identify any problems that might have resulted in restricted throttle lever movement. The cables were lubricated and the aircraft was returned to service.

The investigation established that there was a known problem associated with restricted movement in throttle levers, believed to be a result of moisture on throttle cables freezing to seals and fairleads. The aircraft manufacturer had issued Service Bulletin 747-76-2060, which strongly suggested that aircraft operators replace rigid throttle vapour seals and fairleads with new flexible seals and fairleads. The manufacturer further indicated that implementing the service bulletin would prevent restricted throttle lever movement due to ice accumulation on throttle cables. However, this service bulletin only applied to aircraft with line numbers from 001 to 584. The aircraft involved in this occurrence was line number 713, which had the modifications to the throttle cable system installed during manufacture. In order to establish why a modified aircraft had apparently experienced freezing of the throttle cable system, the manufacturer requested that the operator carry out further inspections of the throttle cable seals, fairleads and drain holes. The operator advised that these inspections did not identify any anomalies that could have contributed to the restricted throttle lever movement.

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