Air travel safety regulations require passengers to have access to an immediate supply of breathable oxygen if cabin pressure is lost at high altitudes. Above a certain elevation, the partial pressure of oxygen in the air becomes too low to sustain consciousness and human function. To meet this need for a rapid, localized oxygen source, commercial airliners employ small, self-contained chemical generators. These devices produce a reliable flow of oxygen quickly and consistently for every passenger station.
The Chemistry of Oxygen Generation
The production of emergency oxygen relies on the controlled thermal decomposition of an alkali metal chlorate, most commonly sodium chlorate (NaClO3). When heated, this compound breaks down to release pure oxygen gas and a stable salt residue. The core reaction is 2NaClO3 -> 2NaCl + 3O2, yielding sodium chloride and oxygen gas.
Sodium chlorate is pressed into a solid chemical block, often called an oxygen candle, and mixed with other components. A metal powder, such as iron, is incorporated as a fuel source. Initial heat causes the iron to react, providing the thermal energy necessary to begin the chlorate decomposition.
This process is highly exothermic, releasing significant heat as a byproduct necessary for the design. The heat generated is sufficient to sustain the reaction once initiated, creating a self-propagating chemical chain. Temperatures inside the generator canister can exceed 250°C (500°F) during operation.
The intense heat ensures the reaction continues until all chemical material is spent, guaranteeing a consistent flow of oxygen. The solid residue is the non-toxic, stable sodium chloride salt. The entire chemical assembly is a compact, solid-state system that remains inert until activated.
Integration into Aircraft Emergency Systems
The chemical oxygen generator is housed inside a metal canister concealed within the overhead service unit above passenger seats. This canister is connected by tubing to the oxygen masks stored in the same compartment. Mask panels automatically release when the cabin altitude climbs to approximately 14,000 feet, a level unsafe for unassisted breathing.
Once the mask drops, a passenger must pull down sharply to initiate the oxygen flow. This action pulls a lanyard connected to a retaining pin, which releases a spring-loaded firing pin. The firing pin strikes a percussion cap, igniting a small explosive charge that generates the high heat necessary to start the chemical decomposition.
The generator begins producing oxygen almost instantaneously upon activation and continues until the chemical components are exhausted. Oxygen is routed through the tubing to the masks for 12 to 20 minutes. This duration provides sufficient oxygen while pilots execute an emergency descent to a safer altitude, generally below 10,000 feet.
A single generator often supplies oxygen for an entire group of masks, such as those for a row of three or four seats. Since the chemical reaction is self-sustaining and cannot be turned off once started, the oxygen supply is guaranteed for the full duration. This design prioritizes an uninterrupted, limited-time supply for a large number of people in a sudden emergency.
Engineering Rationale for Chemical Generators
The decision to use chemical oxygen generators in the passenger cabin, rather than traditional compressed gas cylinders, is based on distinct engineering and logistical advantages. Chemical generators offer superior volumetric efficiency, meaning they store a greater mass of oxygen per unit of volume compared to gaseous systems. This compact nature is a significant benefit in the space-constrained cabin environment.
Weight savings are a substantial factor, as these solid-state units are lighter than the heavy, high-pressure tanks required for equivalent oxygen gas. Reducing aircraft weight contributes directly to fuel efficiency and lower operating costs. The localized generators also eliminate the need for extensive plumbing to route high-pressure oxygen throughout the cabin.
Chemical generators require minimal maintenance because they are inert until activation. Unlike compressed gas systems, they do not require periodic checks for leaks, pressure degradation, or cylinder fatigue. This makes them a reliable emergency system that maintains integrity for years without the constant inspection schedule of a pressurized storage unit.