NTSB Aircraft Accident Report: Trans World Airlines (TWA) Flight 800

OVERVIEW OF THE TWA FLIGHT 800 ACCIDENT
  • Event Date: July 17, 1996, at approximately 20312031 Eastern Daylight Time.
  • Aircraft: Boeing 747-131, Registration $N93119$.
  • Flight Route: John F. Kennedy International Airport ($JFK$), New York, to Charles DeGaulle International Airport ($CDG$), Paris, France.
  • Casualties: All 230230 people on board (2 pilots, 2 flight engineers, 14 flight attendants, and 212 passengers) were killed.
  • Probable Cause: An explosion of the Center Wing Fuel Tank ($CWT$) resulting from the ignition of a flammable fuel/air mixture.
  • Most Likely Ignition Source: A short circuit outside the tank that allowed excessive voltage to enter the CWT through the Fuel Quantity Indication System ($FQIS$) wiring.
CHRONOLOGY OF THE FLIGHT
  • Arrival at JFK: The aircraft arrived from Athens as TWA Flight 881 at 16311631. No operational abnormalities were reported by the incoming crew.
  • Ground Operations: The airplane remained at Gate 27 for about 2.52.5 hours. During this time, the Auxiliary Power Unit ($APU$) and two of the three air conditioning packs were operating.
  • Departure: The aircraft was pushed back at 20022002 and became airborne at 20192019.
  • Climb Phase:
    • At 2026:242026:24, the aircraft was assigned to maintain 13,00013,000 feet Mean Sea Level ($MSL$).
    • At 2029:152029:15, the Captain noted an erratic fuel flow indicator on engine number four (44).
    • At 2030:152030:15, the flight was cleared to climb to 15,00015,000 feet.
  • The Event:
    • At 2031:122031:12, Cockpit Voice Recorder ($CVR$) and Flight Data Recorder ($FDR$) recordings ended abruptly.
    • A "very loud sound" lasting 0.1170.117 seconds was recorded on all $CVR$ channels immediately before power loss.
    • Last secondary radar return occurred at 2031:122031:12.
AIRCRAFT TECHNICAL SPECIFICATIONS AND CONDITION
  • Boeing 747-100 Series: Manufactured in July 1971; had accumulated 93,30393,303 hours and 16,86916,869 flight cycles.
  • Engines: Four Pratt & Whitney $JT9D-7AH$ turbofan engines.
  • Center Wing Tank (CWT):
    • Capacity: 86,36386,363 pounds (12,89012,890 gallons).
    • State at Explosion: Contained approximately 300300 pounds (about 5050 gallons) of unusable fuel.
    • Located directly above the air conditioning pack bay.
  • Wiring System: Primarily $BMS13-42A$ Poly-X (aliphatic polyimide) insulated wire. This type of wire was found to become brittle and prone to cracking with age.
STRUCTURAL BREAKUP SEQUENCE
  • Red Zone (Early Separation): Includes the front spar of the $WCS$, Spanwise Beam 3 ($SWB3$), a large piece of the fuselage belly (LF6A), and the nose landing gear doors.
  • Yellow Zone (Mid-Sequence): Fuselage section 41 (nose) and forward section 42.
  • Green Zone (Late Sequence): Majority of the wreckage, including the aft fuselage, wings, and engines.
  • Breakup Initiation:
    1. Internal overpressure caused $SWB3$ to fracture at the upper chord and rotate forward.
    2. $SWB3$ impacted the front spar, leading to its failure.
    3. A large hole was blown out of the fuselage belly.
    4. The nose section separated about 33 to 55 seconds later.
    5. The main wreckage continued in crippled flight for approximately 4040 seconds before the wings separated and the remaining structure plummeted into the ocean.
IGNITION SOURCE ANALYSIS
  • Flammability: The $CWT$ was heated by the air conditioning packs to between 101∘F101^\circ F and 127∘F127^\circ F. At 13,80013,800 feet, the lower flammability limit ($LFL$) of Jet A is between a fuel/air mass ratio of 0.0320.032 to 0.0380.038.
  • Evaluated Sources:
    • External Factors: Lightning, meteorites, and missiles were ruled out due to lack of physical evidence (pitting, fragmentation patterns, or high-velocity impacts).
    • Internal Factors:
      • Static Electricity: Unlikely; maximum calculated discharge energy from unbonded clamps was 0.00950.0095 mJ, well below the 0.250.25 mJ $MIE$.
      • Fuel Pumps: Arcing in scavenge or jettison pumps was discounted as flame scanners/suppressors were intact or evidence was lacking.
      • Silver-Sulfide Deposits: Found on $FQIS$ components. These semiconductive deposits can provide a path for arcing even at low voltages.
      • Electrical Short Circuit: The most likely scenario. Aged $BMS13-42A$ wiring with cracked insulation could have allowed a high-voltage transient (e.g., from 115115V or 350350V lighting circuits) to short to the low-voltage (55V) $FQIS$ wiring.
WITNESS OBSERVATIONS
  • The "Streak of Light": Of 736736 witnesses, 258258 reported a streak of light resembling a flare or rocket.
  • Explanation: The NTSB concludes this was not a missile but the aircraft itself in crippled flight, trailing burning fuel from the breached $CWT$ after the initial explosion.
  • Psychological Factors: Witnesses lacked depth perception over open water, leading to the "equidistance tendency," making a distant falling object appear to be an ascending nearby flare.
SAFETY RECOMMENDATIONS AND REVISIONS
  • Fuel Tank Flammability: The NTSB urged the $FAA$ to require nitrogen-inerting systems to displace oxygen in fuel tanks (Safety Recommendation $A-96-174$).
  • Wiring Standards: Recommended more rigorous inspection of aged wiring and improved standards for separating high-voltage and low-voltage circuits (Safety Recommendation $A-98-38$).
  • Design Change: Prompted the proposed amendment of 1414 $CFR$ $25.981$ to require that the fuel tank thermal environment be as safe as a wing tank in new designs.