Boeing Co 747-312, ZS-SAJ

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.