Boeing Co 747-300, HL7470

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.