David Learmount/LONDON
Running Boeing 737 fuel boost pumps with no fuel passing through them could cause a fuel tank explosion, the US Federal Aviation Administration has warned. Linking it with the 3 March Thai Airways International 737-400 centre wing fuel tank explosion, the FAA has issued an emergency airworthiness directive (AD) banning the "extended dry operation" of the pumps.
Although the inquiry into the Thai disaster is continuing, two areas of investigation have been highlighted by the US National Transportation Safety Board (NTSB) because the same conditions applied at the time of a Philippine Airlines (PAL) 737-300 tank explosion in May 1990 (Flight International, 27 March-2 April).
One was that the centre wing fuel boost pumps were running when the tank was empty and the engines were not running, which is contrary to the practice laid down in the 737 flight manual; the other is that the aircraft air conditioner had been running continuously in the hot weather during the 40min since the Thai aircraft's previous flight. The NTSB notes that "air conditioning packs...are located directly beneath the centre wing tank and generate heat when they are running." The precise ignition source for the PAL explosion was never determined for certain, but the FAA AD notes that "the centre tank fuel pumps were operating dry at the time of that explosion".
The FAA explains the risk as: "Extended dry pump operation can result in overheating and excessive wear of the pump bearings and consequent contact between the rotating and non-rotating parts of the pumps. Both overheating of the bearings and contact between rotating and non-rotating parts have the potential to create an ignition source in the form of hot surfaces or sparks [which] can create a flame front that can reach the fuel tank and cause a fuel tank explosion."
Two weeks ago Boeing "reminded" all 737 operators that the fuel boost pumps should not be running when a tank is empty or the engines are not operating.
Source: Flight International
