Inside an aircraft fuel tank

July 9, 2009
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Aircraft maintenance checks are required after a certain number of flight hours. Some of these periodic inspections are done overnight or at an airport gate, while others require the use of a hangar and may put the aircraft out of service for some time.

Part of the maintenance routine requires a technician to inspect and modify the aircraft fuel tanks and their associated systems. To do this, inspection and maintenance personnel have to physically enter the fuel tank where many environmental hazards exist.

This type of entry is defined as a permit-required confined space entry and is regulated by OSHA. The Federal Aviation Administration is responsible for the safety of the aircraft, while OSHA establishes rules for the safety of personnel entering the confined space.

Assess the danger
In the confined space of a fuel tank, the most common hazard is the jet fuel. The residual fuel and fumes are both toxic and flammable; they can ignite under certain temperatures and vapor concentrations. Further hazards come from cleaning solvents, sealants, lubricants and other chemicals used in the maintenance work. To minimize a portion of the risk, a toxic gas monitor is required during regular repair and maintenance.

To help prevent injuries, the operator and repair station maintenance organizations must develop procedures for identifying, controlling and/or eliminating these hazards. Developing successful procedures requires an understanding of the four aspects of a safe fuel tank entry program:
  • 1. Fuel tank hazards
  • 2. Pre-entry planning
  • 3. Tank-entry requirements
  • 4. Emergency response and rescue planning
1 — Fuel tank hazards

Personnel working in or around aircraft fuel tanks are exposed to both chemical and physical hazards. The most obvious chemical hazard is from the jet fuel itself.

Jet fuel is a flammable liquid that can be ignited given certain ambient conditions (primarily temperature and vapor concentration). A hazard is present when fuel vapor concentrations reach a level known as the lower explosive limit (LEL). These limits are usually expressed as a percentage by volume.

In addition to its flammability risks, jet fuel is also an irritant and poses a toxic threat. At high concentrations, jet fuel and other hydrocarbons can affect the nervous system, causing headaches and dizziness. Long-term effects can include chronic health problems such as liver and kidney damage.

Oxygen levels are also a major concern in a typical fuel tank entry. The levels can become depleted through oxidation or from displacement caused by other gases. The normal concentration of oxygen in air is about 21 percent. At oxygen-deficient levels (19.5 percent and below), a person will begin to exhibit signs of oxygen starvation. Symptoms include headache, impaired judgment, nausea and drowsiness. At increasingly lower oxygen concentrations, more severe reactions occur and death by asphyxiation is very possible.

For the aviation industry, a typical portable gas detector will be configured to measure these hazards with O2, LEL and photoionization (PID) sensors. The physical characteristics of fuel tanks also pose hazards. Entries into most aircraft fuel tanks are through an oblong hole less than two-feet long and one-foot wide. These limited entrance dimensions can seriously affect emergency exit and recovery situations.

Interior dimensions vary considerably, but most fuel tanks have ribs and walls that separate different bays in each tank. A small amount of a chemical or fuel inside one of these bays can create significant levels of flammable or toxic vapors and depleted oxygen levels.

2 — Pre-entry planning

Several steps must be completed before maintenance personnel can enter an aircraft fuel tank. These include defueling and grounding the aircraft, deactivating associated aircraft systems, having adequate fire and personal protection equipment available and ensuring a safe atmosphere for maintenance personnel to enter.

Ventilation is the most effective method of controlling fire, explosion and toxic hazards in an open fuel tank. The more fresh air in the tank, the safer the environment. Maintenance personnel should not enter a fuel tank until it has been adequately ventilated.

In addition, vapor concentrations must be properly monitored. To determine if the tank is suitable for entry, the internal atmosphere should be continuously checked. Use an approved gas detection device to measure levels of oxygen, flammable vapor and toxic vapor concentrations.

3 — Tank-entry requirements

A properly trained and equipped maintenance crew is the most important factor in preventing injury during fuel tank work. A typical fuel tank entry crew includes an entry supervisor, standby attendant and the entry personnel. Each crew member must be able to recognize potential hazards and evacuate the tank if conditions deteriorate.

The entry crew must individually and collectively be aware of the following requirements: continuous voice communications, proper respiratory protection, proper ventilation, continuous air monitoring and the proper use of electrically powered equipment.

4 — Emergency response and rescue planning

Fuel tank work must also address what to do in emergency situations. If specific procedures are not developed, an emergency may result in severe injury or death.

At least once a year, a preventative emergency evacuation drill should be conducted. This will ensure that all parties (aircraft maintenance personnel, local fire, rescue and police departments, etc.) are able to react in an appropriate and timely manner.

Create a safe work environment
Although absolutely necessary for inspection and modification, aircraft fuel tank entry is hazardous and must be approached with caution. But this work does not have to place maintenance personnel at risk. Effective preparation and training are the answers.

Providing a safe work environment involves identifying potential hazards, developing control measures, training personnel in specific procedures and providing appropriate equipment. This process can help aircraft operators and repair station maintenance organizations ensure the safety of fuel tank personnel.

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