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

Event Date: July 17, 1996, at approximately 2031 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 230 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.

Arrival at JFK: The aircraft arrived from Athens as TWA Flight 881 at 1631. No operational abnormalities were reported by the incoming crew.
Ground Operations: The airplane remained at Gate 27 for about 2.5 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 2002 and became airborne at 2019.
Climb Phase:
- At 2026:24, the aircraft was assigned to maintain 13,000 feet Mean Sea Level ($MSL$).
- At 2029:15, the Captain noted an erratic fuel flow indicator on engine number four (4).
- At 2030:15, the flight was cleared to climb to 15,000 feet.
The Event:
- At 2031:12, Cockpit Voice Recorder ($CVR$) and Flight Data Recorder ($FDR$) recordings ended abruptly.
- A "very loud sound" lasting 0.117 seconds was recorded on all $CVR$ channels immediately before power loss.
- Last secondary radar return occurred at 2031:12.

Boeing 747-100 Series: Manufactured in July 1971; had accumulated 93,303 hours and 16,869 flight cycles.
Engines: Four Pratt & Whitney $JT9D-7AH$ turbofan engines.
Center Wing Tank (CWT):
- Capacity: 86,363 pounds (12,890 gallons).
- State at Explosion: Contained approximately 300 pounds (about 50 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.

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 3 to 5 seconds later.
5. The main wreckage continued in crippled flight for approximately 40 seconds before the wings separated and the remaining structure plummeted into the ocean.

Flammability: The $CWT$ was heated by the air conditioning packs to between 101^\circ F and 127^\circ F. At 13,800 feet, the lower flammability limit ($LFL$) of Jet A is between a fuel/air mass ratio of 0.032 to 0.038.
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 0.0095 mJ, well below the 0.25 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 115V or 350V lighting circuits) to short to the low-voltage (5V) $FQIS$ wiring.

The "Streak of Light": Of 736 witnesses, 258 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.

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 14 $CFR$ $25.981$ to require that the fuel tank thermal environment be as safe as a wing tank in new designs.