During the daily check of an A319, the maintenance crew checked the DOOR/OXY system page and noticed that the flight crew oxygen pressure was 1 450 psi instead of 1 500 psi, as per the Operator’s own requirement. They decided to perform an oxygen servicing task before the aircraft was dispatched.

During the servicing task, the Pressure Regulator Transmitter (PRT) between the oxygen servicing cart and the crew oxygen cylinder exploded and started a fire that propagated towards the oxygen cylinder. The fire at the PRT melted the inlet hose of the cylinder valve, which caused it to separate from the PRT. Pressurized oxygen gas rapidly discharged and injured a maintenance crew member’s face, causing skin and eye irritation. The second maintenance crew member managed to shut off the oxygen supply at the oxygen servicing cart. Other around the aircraft who heard the explosion reacted by shutting off the discharging flight crew oxygen cylinder, cutting power to the aircraft, and calling the emergency services. The maintenance crew member only suffered minor injuries.

Investigation revealed that the most probable sequence of actions that led to the fire was the following:

The crew oxygen cylinder valve was open and the pressure inside the PRT was around 1 450 psi.

The maintenance crew opened the shut-off valve of the oxygen bottle on the servicing cart, which provided a pressure of 2 000 psi upstream of the cart pressure regulator.

The cart regulator was then opened and set to a regulated pressure of 1 600 psi. At this stage, the flexible filling line was not pressurized, because the CHARGE/STOP valve was in a closed position (fig.1).

The maintenance crew then opened the CHARGE/STOP valve (set to the CHARGE position). This created a rapid pressurization inside the flexible filling line up to 1 600 psi, which caused the oxygen temperature to increase to approximately 800°C at the entry of the PRT. This phenomenon is called adiabatic compression. An O-ring located at the PRT inlet ignited at this temperature. The resulting fire propagated to the other PRT components and to the cylinder valve inlet, which separated from the PRT and released the oxygen from the cylinder (fig.2).

(fig.2) The sudden pressure build-up in the filling line led to a rapid increase of the oxygen temperature to 800°C. It triggered the ignition of an O-ring inside the PRT and the fire propagated to the cylinder valve.

Importance of following oxygen servicing instructions

The instructions for use of the servicing cart states that the CHARGE/STOP valve must be open before opening the cart pressure regulator. The risk of rapid pressurization was specifically mentioned and carefully following the instructions would have prevented the incident. When pressurizing an oxygen system or device, it is essential to always slowly open the valves and to control the slow pressurization of the system.

Oxygen is not a flammable gas but an excellent oxidizer, which is present at a concentration of approximately 21% in the ambient air. It is odorless, invisible, and difficult to detect.

When an oxidizer comes into contact with flammable materials in gas, liquid, or solid form, and if energy from a heat source or a spark is present, this starts a fire that burns until one of the three components of this fire triangle is removed (fig.3).

Some flammable material such as grease or oil can self-ignite in a pure oxygen atmosphere, but only at fairly high temperatures above 200°C. In an oxygen-enriched environment, those same materials can self-ignite at much lower temperatures depending on the level of oxygen concentration present. In that case, the fire will be of higher intensity and temperature. Even without sparks or an ignition source, large fires and explosions can occur.

Oxygen cylinders need regular servicing and they can either be refilled or replaced. There are specific precautions to follow for these tasks. They can only be performed by qualified personnel who are trained to work on oxygen systems and aware of the associated risks, especially fire risks. Only approved tools, materials, and procedures should be used.

The general precautions to take are available in the Airbus AMM/MP/AMP maintenance procedures dealing with oxygen systems.

Follow maintenance procedures and instructions of use

To prevent any oxygen leak during servicing, it is crucial to follow the AMM/AMP/MP procedures. It is also crucial to follow the operating instructions for the oxygen servicing cart and the instructions of use for the filling port adapter provided by the ground equipment manufacturer.

Keep track of your oxygen servicing

It is recommended to keep track of the frequency of the oxygen servicing especially if it often occurs before the scheduled maintenance as this could indicate a leak in the oxygen system.

Prevent heat coming from electrical discharge or electrical overheating

It is forbidden to use mobile phones during servicing and to perform oxygen servicing during thunderstorms. When refilling oxygen, it is mandatory to bond the aircraft to the oxygen servicing cart before connecting the refill valve (fig.5). This will ensure electrical continuity between the aircraft and the oxygen servicing cart and prevent the likelihood of sparks due to a potential difference between the aircraft and the cart. Airbus AMM/MP/AMP maintenance procedures do not require grounding (earthing) of the aircraft or of the oxygen servicing cart, but local authority regulations may request this.

Prevent rapid oxygen pressure build-up

The event described earlier highlights the need to handle oxygen valves with care to prevent rapid oxygen pressure build-up and ignition of flammable materials. Also if dust or debris are present in the system, for example, due to contaminated tooling, a fast oxygen flow could create sparks as a result of particle impact.

Respect a safety zone

As an industry standard, a 5-meter safety zone must be maintained around the oxygen filling port during refilling.

The importance of a clean work area, tools and servicing equipment

A clean work area, tools and servicing equipment should be ensured to prevent dust or debris entering into the oxygen system from the reception and storage of oxygen system components through to the final installation on the aircraft. Check that each person working on the oxygen systems, or performing oxygen servicing tasks, has clean hands, clean clothes, clean tools, and clean working areas. This will avoid oil or grease stains from coming into contact with oxygen gas. This will also avoid the presence of particles or other contaminants that could lead to a fire if combined with a heat source and oxygen.