Aviation Infiltration

Confined space entry in aircraft assembly and maintenance is fraught with unique hazards, such as jet fuel.

DURING the majority of inspections and maintenance performed on aircraft fuel tanks, personnel must enter the interior of the tanks. This type of entry is defined as confined space entry and is regulated by the Occupational Safety and Health Administration. In these spaces, personnel can be potentially exposed to dangers such as oxygen deficiency and enrichment, explosive gases, and toxic effects from fuels and maintenance chemicals.

The aircraft industry faces many unique concerns that other industries do not encounter. The need for specialized sampling equipment, the presence of chemicals that can kill gas sensors, and the extremely limited entrance dimensions can affect emergency exit and recovery situations.

The confusion as to which organization, OSHA or the Federal Aviation Administration, has jurisdiction over confined space entry in aircraft was alleviated in August 2000, when a memorandum of understanding between OSHA and the FAA was signed giving OSHA jurisdiction. The memorandum can be found on OSHA's Web site. The FAA remains responsible for the safety of the aircraft while OSHA establishes rules for the safety of the personnel entering the confined space.

OSHA defines a confined space by the following criteria:

  • It is large enough for a worker to enter
  • It is not designed for continuous worker occupancy
  • It has limited openings for entry and exit

Confined spaced entry may or may not require a permit. A permit is required under any of the following conditions:

  • It is a hazardous atmosphere (known or potential)
  • Material with the potential for engulfment is present
  • The space has inwardly sloping walls or dangerously sloping floors
  • The space contains any other serious safety hazard

By OSHA's definition, aircraft fuel tank entry falls into the permit-required confined space (PRCS) entry requirements; as stated in 29 CFR Part 1910.146, employers must:

  • Identify Confined Space hazard areas
  • Inform employees by posting signs where feasible
  • Prevent entry by unauthorized persons
  • Establish procedures and practices to allow safe entry (Permit system)
  • Train employees
  • Provide required equipment
  • Control hazards where possible through engineering or work practices
  • Ensure procedures and equipment necessary for rescue
  • Protect entrants from external hazards
  • Enforce established procedures

The Dangers Within
Known hazards in typical confined space entry are oxygen deficiency or enrichment and combustible gases. Hazards specific to fuel tank entry come from the toxic and combustible effects from jet fuel and from some maintenance compounds.

Confined space entry in aircraft fuel tanks can be extremely dangerous. Conditions can change very rapidly due to the limited space in the work areas. Entry and exit holes may be as small as one foot by two feet. During the maintenance of these areas, small amounts of jet fuel can become vapor, which is toxic and combustible and poses a serious hazard to personnel.

In a confined space there is the chance that oxygen levels can become depleted through oxidation or from displacement of another gas. The typical concentration of oxygen in the environment is 20.9%. When oxygen levels drop to 19.5% to 12%, judgment is impaired, and personnel may experience an increased pulse and fatigue. If levels drop further, from 12% to 6%, fatigue will worsen and nausea and vomiting will occur. In the final stages of deprivation, when levels are between 6% and 0%, convulsion and cardiac arrest can occur, resulting in death. When oxygen levels fall below 19.5%, it is widely accepted that action should be taken.


Oxygen level

29 CFR 1910.146 (PRCS)

< 19.5%

29 CFR 1910.134 (Respiratory Protection)

< 19.5%1

ANSI Z117.1-1995 (Confined Spaces)

< 19.5%

ACGIH (Threshold Limit Value Booklet)


1. Oxygen content below 16% at sea level is considered IDLH (Immediately Dangerous to Life or Health)--oxygen deficient.

Increased levels of oxygen dramatically promote combustion. Many standards (including 29 CFR 1910.146) specify 23.5% as an oxygen-enriched environment. Other codes (such as 29 CFR 1915 and NFPA guidelines) are more stringent. A more conservative approach is to use 22.5% as a hazardous condition threshold.

Jet Fuels
The most prevalent hazard in aircraft confined space entry comes from the fuel that powers the aircraft. The common types of aircraft fuel are Jet A-1, Jet A, Jet B, JP4, JP5, JP8, and aviation gas. Jet fuel presents both combustible and toxic hazards in confined space entry.

Hazardous exposure levels came into effect in 2003. The American Conference of Governmental Industrial Hygienists (ACGIH) lists the threshold limit values (TLVs) for jet fuel as an eight-hour time-weighted average (TWA) of 200 mg/m3 (total aerosol and vapor). This converts to approximately 30 parts per million (ppm) when expressed in isobutylene units. Typical gas monitors that employ photoionization detector (PID) technology are calibrated to isobutylene.

Combustible lower explosive limit (LEL) sensors are typically displayed in %LEL readings. The LEL is the minimum concentration of a combustible gas or vapor in air that will ignite if a source of ignition is present. The upper explosive limit (UEL) is the maximum concentration in air that will support combustion. The range between the LEL and UEL of a combustible gas or liquid is called the flammability range. Concentrations within the flammability range will burn or explode if a source of ignition is present.

Different jet fuels can have different LEL and UEL levels. For example, 100% LEL of Jet A is equal to approximately 6,000 ppm. In the past it was believed that LEL sensors could be sufficient to detect for the hazards from jet fuels. As new toxic regulations have come into place and more is learned about LEL sensors, it has been determined that they are not capable of detecting hydrocarbons like jet fuel at low enough levels to be useful for toxic-level detection.

ACGIH standards reference an eight-hour TWA for jet fuel of approximately 30 ppm. The LEL concentration for Jet A is 0.6% (6,000 ppm). If the combustible sensor alarm is set at 10% LEL, with a properly calibrated instrument, it would take a concentration of 0.10 X 6,000 ppm = 600 ppm to trigger an alarm. Even if the alarm is set to 5% LEL, it still would require a concentration of 300 ppm to trigger the alarm. These values are considerably larger than the TLV for jet fuel.

PID technology is the preferred detection method because of the low-level ppm measurement. The ultraviolet (UV)-based technology is also immune to poisons that can kill the LEL sensor. Not only is there a concern from chemicals like silicone that can show up in the maintenance of aircraft, but tetraethyl lead can occur in jet fuel and aviation gas; these compounds can render the sensor completely ineffective. There are various other chemicals used in aircraft maintenance such as sealants, solvents, and cleaning agents that have their own TLVs. Some specific compounds such as methyl ethyl ketone, toluene, and xylene can be monitored efficiently using PID technology.

Gas Detector Technology Improvements
For the aviation industry, a typical portable gas detector is configured with an O2, LEL, and PID sensor combination. Improvements in technology have allowed these detectors now to be small enough to be brought directly into the fuel tank during the maintenance or inspection process. This has allowed an added amount of safety as workers can now have a gas detector close to their breathing zone and not just rely on external sampling as the sole safety factor.

Despite the advancements made in gas detector technology and the portability of gas detectors, sampling equipment is still important. Special equipment consideration must be made for the sampling tubing and probes used in aircraft fuel tank confined space entry. Devices made from polycarbonate plastic can degrade when exposed to jet fuel fumes. There is also the concern that tygon-type sample tubing can absorb fumes from the jet fuel, causing the sample to never get to the gas detector. Teflon® or Teflon-lined tubing can be used as an alternative because it will not absorb the gas.

Safety Considerations
Confined space entry in aircraft fuel tanks is hazardous and requires several considerations. There must be an adequate level of ventilation and the ability to increase ventilation during the confined space entry. To decrease the chance of explosive or toxic environments, ventilation can be increased. Oxygen deficiency can also be overcome with increased ventilation.

Maintenance equipment is another safety consideration. All equipment that enters the confined space should be classified as intrinsically safe so there is no chance that an ignition source can be created. Respiratory equipment should be present under certain circumstances. If there is a concern from chemical hazards or oxygen deficiency, breathing apparatus should be brought into the confined space.

Constant communication between the entrant and the crew assigned to monitor the confined space is important for worker safety. If hazardous conditions change or the entrant becomes unresponsive, a rescue scenario should be initiated.

Protecting workers in aircraft fuel tanks during confined space entry requires knowledge of the hazards that are present. Adherence to procedures and proper use of equipment will provide a safe working environment.

This article appeared in the February 2007 issue of Occupational Health & Safety.

This article originally appeared in the February 2007 issue of Occupational Health & Safety.

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