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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.


Local Safety Action

The aircraft manufacturer subsequently published an In-service Activity Report, number 99-10 (17 September 1999), which included an article detailing the circumstances of this occurrence. The company was also reviewing the subject of in-flight throttle lever restriction events and may issue a follow-up report.

General details
Date: 18 April 1999 Investigation status: Completed 
Time: 0407 hours EST  
Location   (show map):185 km N KIMMI Investigation type: Occurrence Investigation 
State: Queensland  
Release date: 05 July 2000 Occurrence class: Technical 
Report status: Final Occurrence category: Incident 
 Highest injury level: None 
Aircraft 1 details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 747 
Aircraft registration: HL7470 
Serial number: 24194 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Nil 
Departure point:Seoul, KOREA
Destination:Sydney, NSW
Aircraft 2 details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 767 
Aircraft registration: HL7249 
Serial number: 26265 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Nil 
Departure point:Seoul, KOREA
Destination:Sydney, NSW
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Last update 13 May 2014