The FAA did not consider lightweight inerting systems for commercial jets until the 1996 crash of TWA Flight 800. The crash was blamed on an explosion in the center wing fuel tank of the Boeing 747 used in the flight. This tank is normally used only on very long flights, and little fuel was present in the tank at the time of the explosion. A small amount of fuel in a tank is more dangerous than a large amount, since heat entering the fuel tank with residual fuel causes the fuel to increase in temperature faster and evaporate. This causes the ullage fuel-to-air ratio to increase rapidly and exceed the lower flammability limit. A large quantity of fuel (high mass loading) in the fuel tank can retain the heat energy and slow the fuel evaporation rate. The explosion of a Thai Airways International Boeing 737 in 2001 and a Philippine Airlines 737 in 1990 also occurred in a tank that had residual fuel. The above three explosions occurred on a warm day, in the center wing tank (CWT) that is within the contours of the fuselage. These fuel tanks are located in the vicinity of external equipment that inadvertently heats the fuel tanks. The National Transportation Safety Board's (NTSB) final report on the crash of the TWA 747 concluded "The fuel air vapor in the ullage of the TWA flight 800 CWT was flammable at the time of the accident". NTSB identified "Elimination of Explosive Mixture in Fuel tanks in Transport Category Aircraft" as Number 1 item on its Most Wanted List in 1997.