The A340-600's technical log reported both fuel control monitoring computers (FCMC) had been reset during the previous sector. During preparation, the crew dealt with an FCMC2 and an FCMC1 failure, resetting both. Shortly after the 16:21 GMT take-off the ECAM advised "FCMC2 fault". The crew waited until cruise altitude before unsuccessfully trying to reset and had no more warnings.
Automatic fuel feed from the centre tank to the wing inner tanks stopped 3h into the flight, with 5,300kg (11,700lb) left. It was after trouble with the No 4 engine that the crew began manually commanding fuel transfer from the centre, trim and outer tanks to the inner tanks.
The AAIB says the two FCMC units have a master/slave relationship in which the "healthier" is the master. But if the master fails while the slave unit has a lower health status, the latter cannot take over the master role, so the system ceases to provide fuel management command or warning signals. In addition, the AAIB has determined, the system designed to provide low-fuel warnings cannot do so unless it detects a failure in both FCMCs.
Source: Flight International
