Confined space entry in aircraft assembly and maintenance is fraught with unique hazards, such as jet fuel.
- By Shane McEwan
- Feb 01, 2007
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)
with the potential for engulfment is present
space has inwardly sloping walls or dangerously sloping floors
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:
Confined Space hazard areas
employees by posting signs where feasible
entry by unauthorized persons
procedures and practices to allow safe entry (Permit system)
hazards where possible through engineering or work practices
procedures and equipment necessary for rescue
entrants from external hazards
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
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.
29 CFR 1910.146 (PRCS)
29 CFR 1910.134 (Respiratory Protection)
ANSI Z117.1-1995 (Confined Spaces)
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
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
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
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
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.
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.