A fuel spill of approximately 100 litres occurred during refuelling of the South African registered Boeing 747-312 aircraft on the international terminal apron at Perth airport. Eighty minutes later, the Aviation Rescue Fire Fighting Services (RFFS) observed fuel venting from the right wing of the aircraft as it commenced a take-off roll on runway 21, for a flight to Johannesburg, South Africa.
The fuel spill at the terminal was due to the refuel valve in the number-4 reserve tank failing to shut off. The tank became overfilled because of a faulty quantity indicator at the refuelling station panel. Fuel overflowed from the reserve tank into the fuel tank vent system and then spilt onto the apron from a ram air scoop located near the wingtip. The tank vent system vented the fuel tanks to atmosphere by a series of tubes from the fuel tanks, to a surge tank located in the outboard section of each wing. The surge tank vented to atmosphere through a flame arrester and the ram air scoop.
The aircraft refuelling system included an overfill protection system whereby if 75 mm of fuel overflowed into the surge tank, a float switch operated, which closed all refuel valves. The overfill protection system could be disabled by pulling the appropriate circuit breaker in the main equipment centre within the lower forward fuselage.
During the period from 7 December to 30 December 2001, the Perth RFFS attended six other fuel spills ranging from 20 to 100 litres from aircraft used by the same operator. Those spills involved five B747 aircraft (including ZS-SAJ) and were due to the failure of the refuel valves to shut off during refuelling operations.
In 1998, the United States Federal Aviation Administration (FAA) issued an airworthiness directive, AD 98-20-40, that required the replacement of fuel quantity indication system (FQIS) electrical wiring outside of the fuel tanks and surge tank on older versions of the B747 aircraft. The modification was intended to prevent arcing of the FQIS wiring or probes due to electrical transients induced by electromagnetic interference or electrical short circuit conditions.
The five aircraft involved in the fuel spills at Perth airport had been fitted with a transient suppression device that was approved as an alternate means of compliance with the airworthiness directive. The device had caused problems with the FQIS, including incorrect calibration of the refuelling system that resulted in the aircraft being loaded with an incorrect amount of fuel or the fuel tanks being overfilled.
The spillage of fuel from the tank vent system during the eight occurrences indicated that the overfill protection system circuit breaker was pulled during the refuelling operation to work around the calibration problem. The work around enabled refuelling operations to continue and prevent any delay to the departure of the aircraft. If some fuel remained in the surge tank after completion of refuelling operations, it could vent to atmosphere from the ram air scoop during the take-off roll, as was observed on 17 December 2001.
Following the fuel spills, the operator carried out rectification work on the five aircraft, including the checking and changing of fuel system components, the replacement of FQIS wiring harnesses, and the re-calibration of the system. The operator also implemented the local safety actions listed below and no further fuel spills were reported.
Local safety actions
As a result of the incidents, the operator implemented a program to replace the transient suppression devices fitted to all its B747 aircraft. The operator implemented the following safety actions to minimise the likelihood of fuel spills until all the B747 aircraft were fitted with the upgraded transient suppression device:
- Use of fuel tank quantities for fuelling operations that were less than the certificated maximum figures to ensure the tanks were not filled to maximum capacity;
- Pressurisation of the fuel manifold prior to commencing the fuelling operation in order to verify that no defects existed prior to opening any refuel valves;
- Manual closure of the refuel valves when a fuel tank was full;
- Cessation of the practice whereby circuit breakers were manually pulled to override the overfill protection system;
- Reducing refuelling pressure on reaching the required tank quantities;
- An increase from 60 minutes to 90 minutes in the period allocated for refuelling of aircraft to facilitate the manual operation of the refuelling system and to drain the surge tanks if required; and
- An audit during February 2002 of the refuelling operations at Perth by the operator's quality assurance and industrial safety staff.
|Date:||17 December 2001||Investigation status:||Completed|
|Time:||1202 hours WST|
|Release date:||13 August 2002||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Perth, WA|
|Departure time||1202 hours WST|
|Destination||Johannesburg, South Africa|