Investigation Summary
The aircraft had arrived in Baltimore from San Juan earlier that day. During the flight from San Juan, the fuel load consisted of a mixture of Type A (Jet A) and Type B (JP-4) fuel. Type B fuel is more volatile and has a wider flammability range than Type A. The accident occurred during the subsequent short leg from Baltimore to Philadelphia. While holding at 5,000 feet near Elkton due to a squall line at Philadelphia, the aircraft was struck by lightning. The discharge ignited fuel vapors in the number 1 reserve tank, which then triggered secondary explosions in the center and right reserve tanks. The resulting structural failure caused the left wingtip to separate, rendering the aircraft uncontrollable. A nearby National Airlines flight witnessed the explosion and reported the aircraft descending in flames.
Final Conclusions
The Civil Aeronautics Board (CAB) concluded that the probable cause was the lightning-induced ignition of the fuel/air mixture in the number 1 reserve fuel tank. This ignition resulted in the explosive disintegration of the left outer wing and a total loss of control. The presence of volatile Type B fuel vapors in the tanks was a critical factor in the explosion.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
Fuel Reserve Tanks
Outboard tanks designed to hold extra fuel; the number 1 reserve tank was the site of the initial ignition.
Fuel Vent System
System designed to equalize pressure in tanks; it was identified as a potential path for flame propagation.
Static Discharge Wicks
Devices intended to dissipate static electricity; their installation was mandated following this accident.
Interesting Facts
- 01The aircraft was carrying a mixture of Type A (kerosene) and Type B (gasoline-kerosene) fuel.
- 02The lightning strike occurred while the aircraft was at or near the freezing level, where charge separation is most common.
- 03The explosion originated in the number 1 reserve fuel tank on the left wing.
- 04Wreckage was strewn over an area approximately four miles long and one mile wide.
- 05This was the first documented case of a lightning strike causing the crash of a commercial jet airliner.
Safety Actions & Advisories
Static Wick Mandate
The FAA mandated the installation of lightning discharge wicks on all commercial jet airliners to help dissipate electrical charges.
Fuel Tank Venting Improvements
Research was initiated into flame arrestors and improved venting to prevent external ignitions from reaching the interior of fuel tanks.
Known Controversies & Unanswered Questions
Lightning vs. Turbulence as Cause
Reference ↗Following the accident, there was significant industry skepticism regarding lightning's ability to destroy a modern jet. Some manufacturers and investigators initially proposed that severe turbulence, rather than lightning-induced ignition, was the primary cause of the structural failure. This led to a period of reservation in acknowledging lightning as a critical threat to fuel tank integrity.