Investigation Summary
The investigation into Flight 143 focused on the ignition source within the Center Wing Tank (CWT). The tank had not been fueled since March 9, 1990, leaving a large ullage (empty space) filled with volatile fuel vapors. The high ground temperature in Manila, combined with the heat generated by the air conditioning packs located directly beneath the CWT, raised the temperature of the fuel vapors above their flash point. Investigators found no evidence of a bomb or incendiary device, leading to the conclusion that an electrical fault was the most likely ignition source. Specifically, the airline had installed logo lights after delivery, which involved routing additional wiring through the fuel tank vapor seals. The NTSB identified chafing in wiring looms, fuel boost pumps, and float switches as potential points of electrical arcing. This accident was a precursor to the similar CWT explosion of TWA Flight 800 in 1996, which eventually led to significant industry-wide changes in fuel tank safety and inerting systems.
Final Conclusions
The explosion was caused by the ignition of fuel vapors in the center wing tank. The vapors were heated to a volatile state by the prolonged operation of the air conditioning packs on the ground in high ambient temperatures. The ignition source was determined to be damaged or chafed electrical wiring within the tank. The accident resulted in the first hull loss of a Boeing 737-300 and the first for the entire Boeing 737 Classic (-300/-400/-500) series.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
Center Wing Fuel Tank (CWT)
A fuel storage tank located in the fuselage between the wings. In this incident, the tank contained volatile vapors that were heated by external sources.
Air Conditioning Packs
Units that provide cooled air to the cabin. On the Boeing 737, these are located directly beneath the center wing tank and generate significant heat during operation.
Fuel Tank Wiring
Electrical looms for boost pumps and float switches. Chafing in these wires was identified as the likely source of the spark that ignited the fuel vapors.
Interesting Facts
- 01The center wing fuel tank had not been refilled for approximately two months prior to the accident.
- 02The accident was the first hull loss of a Boeing 737 Classic (-300) series aircraft.
- 03Ambient air temperature at the time of the explosion was approximately 35°C (95°F).
- 04Investigators found no evidence of explosive devices or sabotage.
- 05The NTSB later linked the mechanics of this accident to the loss of TWA Flight 800 and Thai Airways Flight 114.
- 06The aircraft, MSN 24466/1771, was manufactured in 1989.
Safety Actions & Advisories
NTSB Safety Recommendations A-90-100/103
The NTSB recommended that the FAA issue an Airworthiness Directive requiring inspections of fuel boost pumps, float switches, and wiring looms for signs of chafing.
FAA Fuel Tank Flammability Reduction Rule
Following subsequent similar accidents, the FAA eventually mandated in 2008 that passenger aircraft be fitted with technology to reduce fuel tank flammability, such as nitrogen-inerting systems.
Known Controversies & Unanswered Questions
FAA Delay in Regulatory Action
Reference ↗Despite NTSB recommendations following the 1990 Philippine Airlines accident regarding fuel tank wiring inspections, the FAA did not mandate significant design changes to prevent center wing tank ignition until after the TWA 800 and Thai Airways 114 accidents.