ValuJet Flight 592

A Miami departure with five boxes of aircraft parts

On 11 May 1996, ValuJet flight 592 was scheduled to take passengers from Miami International Airport to Atlanta. Its aircraft, a DC-9-32 registered N904VJ, had arrived from Atlanta with the same crew at about 13:10. The inbound flight had been delayed by maintenance involving a hydraulic pump circuit breaker. The return flight had been scheduled to leave Miami at 13:00. Its planned cruise altitude was 35,000 feet, with an estimated flight time of one hour and 32 minutes.

The aircraft carried 105 passengers, two pilots and three flight attendants. Passenger records listed 104 passengers; a four-year-old child aboard was missing from the manifest and the weight and balance form. The aircraft was within its takeoff weight and center of gravity limits. Conditions around Miami were suitable for visual flight, and the NTSB found that weather played no part in the accident.

Baggage, mail and company-owned material filled the cargo holds. The company material included three tires and five cardboard boxes described on a shipping ticket as empty oxygen canisters. A lead ramp agent showed that ticket to the first officer and obtained approval to load the shipment in the forward compartment. The agent recalled no discussion of the oxygen-canister entry. The boxes actually contained unexpended chemical oxygen generators removed from other aircraft during maintenance.

Generators removed without protective caps

Earlier in 1996, ValuJet had acquired two MD-82s and an MD-83 and sent them to SabreTech's Miami facility for modifications and maintenance. Inspection showed that most oxygen generators on the two MD-82s had passed their permitted service life. The remaining generators on those aircraft were approaching that limit, so ValuJet directed SabreTech to replace them all. The removed generators were accumulated in boxes and eventually moved into a holding area for ValuJet property.

A chemical oxygen generator supplies emergency breathing oxygen through a reaction inside its metal casing. Pulling a retaining pin releases a spring-loaded mechanism that strikes a percussion cap and starts the reaction. The process releases both oxygen and substantial heat. Manufacturer tests typically produced exterior shell temperatures of 450 to 500 degrees Fahrenheit. N904VJ used a different passenger oxygen arrangement; the generators aboard flight 592 were cargo from the MD-80 maintenance work.

The maintenance instructions required a safety cap over the initiation mechanism of an unexpended generator after removal. Those caps prevent mechanical activation of the percussion cap. SabreTech personnel signed maintenance cards indicating completion, but the caps were never installed. Some mechanics had raised the need for caps with supervisors, and an inspector signed a final inspection block after being told the issue would be handled in shipping and stores. He did not verify that it had been done.

The shipment reaches the forward hold

SabreTech had no formal written requirement for someone moving parts to the shipping holding area to tell shipping employees what the items were or whether they were hazardous. During a cleanup before a prospective customer's visit, a stock clerk offered to prepare the boxes for shipment to Atlanta. He rearranged the generators on their sides, end to end, and added bubble wrap above them. He then sealed the boxes and attached address and company-material labels describing the contents as aircraft parts.

The clerk had not been told what the generators were or their condition. He associated their green parts tags with unserviceable equipment and assumed that the canisters were empty. A receiving clerk prepared the shipping ticket from the description he was given. On 11 May, a driver delivered the five boxes and three tires to the ValuJet ramp, where a ramp agent signed for them.

Inside the forward cargo compartment, baggage and mail were followed by the tires and generator boxes. The agent who loaded them recalled boxes placed atop a large tire around a smaller tire, with another large tire leaning over the shipment. He felt objects moving inside one box and heard a clink. The boxes lacked hazardous materials markings. ValuJet was not authorized to carry hazardous materials, but the NTSB found that the ramp agents' and flightcrew's acceptance was reasonable given the information they received.

The first warnings come during the climb

Flight 592 was pushed back shortly before 13:40 and began taxiing toward runway 9L at about 13:44. Air traffic control cleared it for takeoff at 14:03:24, and its takeoff roll began about 10 seconds later. The captain flew while the first officer handled radio communications. The crew completed its checklists and takeoff briefing, and the NTSB assessed their performance before the emergency as appropriate.

At 14:10:03, while the aircraft climbed through about 10,634 feet, the cockpit voice recorder captured a sound. At the same time, recorded airspeed and pressure altitude briefly fell and then returned to normal. The NTSB considered these changes consistent with a pressure pulse caused by an inflated tire rupturing after fire had burned through part of its sidewall. Within seconds, the captain reported electrical trouble. At 14:10:22 she said that they needed to return to Miami. Shouts of fire from the cabin followed.

At 14:10:31 the first officer requested an immediate return. The aircraft reached its maximum recorded altitude of about 10,879 feet and began descending. The controller supplied a left turn and a descent clearance, then further headings and lower altitudes. At 14:11:12 a flight attendant reported that the cabin was completely on fire. At 14:11:37 the first officer asked for the closest available airport; he then requested radar vectors after the controller described a return to Miami's runway 12.

Heat damages the aircraft during the return

The first emergency vectors took the aircraft through a wide left turn. For roughly 80 seconds after the cabin shouts began, it continued northwestward, away from Miami. Meanwhile, the pilots faced several developing malfunctions. Right-engine thrust fell toward idle while the left engine remained at high power. The NTSB concluded that fire damage had prevented the crew from reducing left-engine thrust. The crew needed continuing control inputs to oppose the resulting tendency to roll right.

Pitch behavior also changed. Recorded vertical acceleration increased while the control column had not moved, followed by a forward control input that reduced it. The aircraft briefly leveled around 9,500 feet. The NTSB associated the sequence of electrical, engine-control and pitch-control problems with progressive fire damage. The cockpit-cabin interphone was inoperative, requiring the cabin crew to communicate by opening the cockpit door. The investigators found only a small amount of smoke had entered the cockpit before the last recorded flightcrew speech at 14:11:38.

The flight data recorder stopped at 14:12:48, but radar continued recording the aircraft. At about 14:12:58 it entered a steep left turn and rapid descent. Its descent rate averaged about 12,000 feet per minute over the next 32 seconds. It subsequently turned toward Miami and reduced that descent. Those movements led investigators to conclude that the pilots maintained at least partial control until close to 14:13:34. Eight seconds later, the aircraft struck the Everglades with its right wing down and nose down. The impact was unsurvivable.

An oxygen-fed fire defeats the cargo protection

Recovered wreckage showed the most severe fire damage near the forward compartment ceiling above the generator shipment. Investigators found that caps were missing, several retaining-pin lanyards were unsecured, and the packaging allowed generators to strike neighboring mechanisms or dislodge pins. The NTSB concluded that activation of one or more generators started the fire. It placed the likely activation between the beginning of loading and the takeoff roll; the exact timing remained uncertain.

The class D cargo compartment depended on a liner and limited ventilation to contain a fire and restrict its oxygen supply. It had neither smoke detection nor a suppression system, and regulations required neither. The oxygen generators supplied fresh oxygen within that enclosed space. The liner, cabin airflow and initially oxygen-rich combustion delayed noticeable smoke reaching the occupants. By the time the crew recognized the fire, heat had escaped the compartment and was damaging surrounding wiring and controls.

Fire tests supported the ability of generators packed in cardboard and bubble wrap to ignite a severe fire. In the final test, with generators, a pressurized tire and luggage, the measured ceiling temperature exceeded 3,200 degrees Fahrenheit about 11.5 minutes after activation. Test conditions and the accident shipment differed. The investigation could not reconstruct their exact arrangements. For the final loss of control, the NTSB favored flight control failure caused by extreme heat and structural collapse, while allowing possible crew incapacitation by smoke or heat in the last seven seconds.

Failures in handling, oversight and fire requirements

The final report identified SabreTech's preparation, packaging and identification failures, ValuJet's inadequate oversight of contract maintenance, and the FAA's failure to require class D smoke detection and suppression as probable causes. ValuJet had assigned technical representatives and audited SabreTech, but its ongoing quality assurance failed to correct the missing caps, false signoffs and inadequate handling procedures. FAA surveillance of the airline had given insufficient attention to the heavy maintenance performed by contractors.

The NTSB adopted its final report on 19 August 1997. It recommended faster rulemaking for cargo smoke detection and suppression, depletion of expired generators before transport, clearer disposal instructions and inspector signoffs on maintenance cards, and hazardous materials training for airline and contractor maintenance personnel. It also called for equal FAA surveillance of maintenance performed by airlines and contractors, functioning cockpit-cabin interphones, and improvements to smoke goggles and emergency guidance. The report recorded the FAA's June 1997 proposal to require class D detection and suppression equipment.