The Crash of Flight 592 - Part I

Behind an oxygen mask designed to save lives during emergencies in aircraft, submarines and space shifts, there is a chemical oxygen generator which activates upon depressurization of occupied compartments. Although useful to preserve lives in critical situations, these devices may have harmful effects and cause major tragedies.

One of the most important accidents involving dangerous goods in the last 15 years was the crash of a Douglas DC 9-32, operated by the company Valujet Airlines, into the Everglades, a National Park located South of the Florida peninsula, United States, in 1996.

Flight 592 left Miami towards Atlanta, but 6 minutes after take-off, at a height of 3200 m, the black box recorded an unidentified sound followed by the words “What was that?” uttered by the captain of the flight. After 15 seconds, the black box recorded the words “Fire! Fire! Fire!” coming from the passenger cabin. Less than four minutes after that, the aircraft crashed into the Everglades with 105 passengers and 5 crew members. There were no survivors.

In accordance with the U.S. National Transportation Safety Board (NTSB), the accident was caused by a fire in the front cargo deck. The fire was caused by the activation of chemical oxygen generators which, at that time, were transported as “Company materials,” or COMAT. The unidentified sound first recorded was the explosion of a tire in the deck, adjacent to the oxygen generators.

It was impossible to duly determine the moment on which the devices activated, but according to the investigation carried out by the NTSB, it is very likely that activation occurred after loading the cargo in the aircraft cargo deck (still on ground, where there is a great number of abrupt movements that may affect the goods within their packages), or else during take-off.

According to the evidences analyzed, the packages containing the chemical oxygen generators were located exactly in the area where the aircraft showed more significant structural damages. Thus, it was possible to rule out any failure of the electrical system as ignition source. The electrical system was also damaged by the fire and the lack of it played an important role in the crash of the aircraft.

During the investigation, wrongly-packed generators were discovered among the debris from the accident. Many of them lacked the regulatory safety caps to prevent accidental activation (which was what finally occurred). All this reinforced the theory of the activation of the oxygen generators.

The relevant cargo involved 5 boxes containing generators allegedly emptied at Sabre Tech's facilities. Sabre Tech was the company retained by the airline to check and maintain aircraft. One of the main causes of the accident was the failure by the maintenance service provider to identify the unspent chemical generators and to properly package them for air transportation, according to the investigation carried out by the NTSB (1).

The Chemical Oxygen Generators

Chemical oxygen generators are devices which involve a reaction of thermal decomposition of potassium chlorate, which is contained in a cylindrical metallic shell, mixed with barium peroxide and potassium perchlorate (the oxidizer core), and a combustible material that is oxidated, in general, iron powder.

The thermal decomposition of potassium chlorate produces oxygen and sodium chlorate. The activation mechanism involves some important safety-related aspects with respect to this device:

• The combustion produced within the chemical oxygen generator is an exothermic reaction, i.e., it releases heat. This implies an increase in temperature, which may reach up to 200-260 ºC in the surface of the generator, and up to 600 ºC in the core, depending on the bibliography considered (1)(2). While potassium chlorate decompose, part of the oxygen generated reacts with the combustiblematerial and produces more heat, which, in turn, will be used to feed the potassium chlorate decomposition reaction. Thus, the reaction maintains and continues (see article Runaway Reaction - Accident in T2).

• Although the potassium chlorate decomposition releases energy, it is necessary to supply energy to the system in order to activate and maintain the reaction. The activation may be achieved electrically or by an explosive or friction device. It should be mentioned that heat sources from fire may also supply the energy necessary to trigger the reaction. In the case of the chemical oxygen generators involved in the tragedy of flight 592, they used an explosive to start the reaction: a combination of lead styphnate and tetracene.

• The device activates when removing a retaining pin connected to the relevant mask through a wire or cord. The retaining pin holds a spring that contains a firing pin. The removal of the retaining pin (for instance, when the passenger pulls the mask during the emergency) activates the firing pin, which hits the percussion cap containing the explosive and, thus, starts the reaction. As an additional security measure, the percussion cap is protected with a safety cap to prevent the firing pin from accidentally hitting.

With this, chemical oxygen generators may be the source of fire or else help the propagation of fire by the action of the oxygen they generate.

This makes them a trap if loaded as cargo in decks without ventilation, as in the case of the crash of flight 592. In fact, the materials were in a Class D cargo compartment, designed to extinguish fire by lack of oxygen (for that reason there was no ventilation in that deck). This type of cargo compartment did not require any kind of smoke detection system until the time of the accident. After that, the NTSB determined that the existence of that system could have helped prevent the accident.

There was no smoke detection system or ventilation system: it was the ideal environment for an oxidizing substance (or for a device such as this generator, which releases an oxidizing substance as oxygen) to act at will and at large.

Disposal of Removed Chemical Generators

Specifically, these devices have a useful life of about 12 years, according to the manufacturer. Consequently, airlines must replace them periodically, following the manufacturer's instructions.

Please note that discarded devices may not be transported by aircraft (3). The new ones may only be transported in cargo aircraft (after the crash of flight 592).

The useful life is more related to the device functionality than to its safety. With time, the solid mix may crack. This would stop the chemical reaction that generates oxygen if the generator is activated. Consequently, oxygen generators must be replaced regularly to ensure that they may properly function in the event of an emergency during a flight.

The replacement should involve a scheduled activation of the chemical oxygen generators which are removed from the circuit, as well as appropriately handling spent generators as dangerous waste, given that the soluble barium salts in the mixture after the reaction may be considered toxic substances (1).

This task is usually outsourced through a service company. However, as with any outsourcing, the question arises as to how to control or monitor the tasks performed by the companies retained to carry out such delicate operations, even more taking into account that such tasks are carried out in facilities not belonging to the company itself. The investigation detected that one of the root causes of the accident was the contractor involved, Sabre Tech. But another major cause was detected: the airline should have monitored the tasks entrusted to the third party, but failed to do so.

Although chemical oxygen generators removed from aircraft may not be transported by air, remaining on the land does not imply that they are less dangerous. As we have already mentioned, in any facility where such generators are handled it must be taken into account that, if they are not deliberately activated and, thus, unloaded, they involve the same risks as if they were in the deck of an aircraft.

The example of Apex

A clear example occurred on October 5, 2006, about 10 years after the accident of Valujet DC 9-32. In this case, the inhabitants of the City of Apex, North Carolina, USA, were affected. There was a fire in a facility for the treatment and disposal of dangerous waste, belonging to the company EQ Industrial Services. The investigators of the Chemical Safety Board (CSB) also found among the debris some hints indicating, as in the Valujet aircraft, that some chemical oxygen generators had not been activated or spent (4).

Although the causes of the fire were never determined, the report issued by the CSB (6) concluded that the oxygen generators very probably contributed to the rapid propagation of the fire, as they were activated during the accident, whether by the heat or by the flame itself.

By chance, the devices had been sent from other aircraft maintenance facilities, after being removed from service. The problem was that in the aircraft maintenance facility the oxygen generators were not activated and spent, and when entering in EQ's facilities, they were identified as “oxidizing waste” in the relevant waste manifest (5).

At EQ's facilities there was waste posing different types of risks. Among such waste, there were flammable solvents stored in several 200 liters drums near the oxygen generators. These drums exploded when they were reached by the accident and threw balls of fire hundreds of meters away.

This situation mobilized the near-by community of Apex (approximately 3,500 people), and the evacuation was coordinated immediately from that city; this prevented any casualties (6).

The accident was serious, but not as much as the crash of flight 592. It occurred in a waste treatment plant with few individuals inside and with time and resources to protect the more exposed groups. There were no casualties. Only about 30 individuals had to receive medical assistance for breathing problems derived from the accident. EQ's facilities were totally destroyed.

The other accident occurred during a flight. There were no survivors. 110 people died.

The two accidents had something in common: they involved chemical oxygen generators which had to be deliberately activated in a controlled environment before their final disposal. In both cases, the generators came from aircraft maintenance facilities.

The consequences of the lack of care with this type of cargo may be suffered anywhere; to a great extent, the time and place depend on luck. Thus, in the case of dangerous goods, in particular, chemical oxygen generators, if you leave something to chance, you lose. And you will not be the only one suffering a loss.

(1) Aircraft Accident Report, In-Flight Fire and Impact with Terrain, ValuJet Airlines, Flight 592, DC-9-32, N904VJ, Everglades, Near Miami, Florida, May 11, 1996, Report No. NTSB/AAR-97/06(PB97-910406), National Transportation Safety Board (NTSB), August 1997

(2) Response of Aircraft Oxygen Generators Exposed to Elevated Temperatures. Report No. DOT/FAA/AR-TN03/35, Department of Transport

(3) Dangerous Goods Regulations – International Air Transport Association (IATA), 51 ed.

(4) Safety Advisory. Dangers of Unspent Aircraft Oxygen Generators. Chemical Safety Board. Report No. 2007-I-NC-01-SA..USA.

(5) In USA, the Dangerous Waste Manifest is the form Uniform Hazardous Waste Manifest, OMB No. 2050-0039.

(6) Case Study. Fire and Community Evacuation in Apex, North Carolina. Chemical Safety Board (CSB). Report No. 2007-01-I-NC, April 2008.

Translated by Camila Rufino, Acredited Translator