Confined Spaces: Aircraft fuel tank work poses unusual risks

Oct. 1, 2001

Confined Spaces

Aircraft fuel tank work poses unusual risks

By David D. Wagner and Roger J. Wells

October 2001

David Wagner is responsible for all phases of product management for Industrial Scientific Corporation’s portable and fixed system product lines. Based in Oakdale, Pa., he earned his B.S. in Electrical Engineering from The Pennsylvania State University. Roger J. Wells is the Industrial Safety Coordinator for Aircraft Mechanics Fraternal Association Local 19, Hartsfield International Airport, Atlanta, GA. Prior to assuming his safety responsibilities, he spent many hot summer days and cold winter evenings inside the bellies of aircraft fuel tanks. It’s a space only experienced spelunkers could enjoy — dark, cramped and fraught with danger. But while the payoff for cave crawling enthusiasts is visual splendor, for aircraft maintenance technicians, it’s what they can’t see within an aircraft fuel tank that’s most deadly: potentially hazardous atmospheric conditions. For technicians, entry into aircraft fuel tanks is a unique and difficult form of work. It’s required during major maintenance and to rectify emergent problems, such as fuel leaks, sensor replacement, fuel boost pump failure, wiring trouble or other problems, or to inspect engine struts and structural damage to wings.
Because of its constricted environs, working within a fuel tank presents exceptional challenges. Within a DC-9, for example, the largest fuel cell is beneath the fuselage. It measures about 2-1/2 ft. high, 4-1/2 ft. wide and 4-1/2 ft. long. Because an average size man cannot sit upright without bumping his head, he must cross his legs in order to enter. Once inside, the worker may navigate through four separate compartments, carefully maneuvering through baffle holes to reach those compartments. He also must be mindful of his movements, as he must follow the same motions to exit. Wing tanks are even smaller and require the ability of a contortionist to enter, legs first, with a partner supporting the entrant’s trunk. Many tank spaces become progressively smaller until a technician is forced to lie on his stomach or back, with just inches of headroom.Confined Space Rules of Entry According to statistics compiled by California State University, Fullerton, about 2.1 million workers enter confined spaces every year. Thirty-nine of those workers never make it out alive. Another 6,000 workers are injured. Most confined space fatalities and injuries occur because employees are unaware of the inherent dangers, and are not properly equipped to handle hazardous situations. The Occupational Health and Safety Administration (OSHA) has established guidelines for operating within a confined space. According to OSHA’s rule, 29 CFR Part 1910.146, a confined space is "an area large enough to bodily enter and perform work, has limited means of entry or exit and is not designed for continuous human occupancy." Additionally, OSHA requires that permits be obtained before entering certain types of spaces. A permit-required confined space has one or more of the following characteristics: • Contains, or has a known potential to contain, a hazardous atmosphere. • Contains material with potential for engulfment. • Has an internal configuration such that an entrant could be trapped or asphyxiated in inwardly converging walls, or a floor that slopes and tapers to smaller sections. • Contains any other recognized serious safety or health hazard. Confined space entry training is an important factor in OSHA regulations and applies to all employees — including supervisors — who are required to enter a space with restricted entry or exit, and which has the potential of accumulating dangerous gases or reduced oxygen levels. OSHA regulation 1910.146 requires that all employees who enter restricted spaces receive training and information on the following topics: • Expected duties of the attendant, authorized entrant and entry supervisor • Contents, location and availability of the organization’s confined space entry plan • Atmospheric conditions • Entry/exit access • Engulfment conditions • Specific confined space entry procedures • Operating and rescue procedures • Confined spaces entry permit forms and authorization • Test equipment procedure and calibration/maintenance schedule Employees must be trained prior to their initial assignments into confined spaces, before a change in assigned duties and before changes in operations or when deviations occur in procedures. Records of confined space locations and testing results should be maintained for five years as part of the overall program. Canceled permits must be maintained for at least one year as part of the standard’s requirement for an ongoing review process. While OSHA regulations clearly outline training required by law, keep in mind that these are minimum requirements. Always be aware of confined spaces to be entered and the operations going on in and around those spaces. For more information, and for a sample confined spaces program, visit the OSHA confined spaces section at its Web site: Or, refer to the following publications: Permit-Required Confined Spaces, Final Rule; OSHA, 29 CFR Part 1910.146; Federal Register 63: 66018-66036, December 1, 1998. A Guide to Safety in Confined Spaces (NIOSH Publication Number 87-113), July 1987. Safety Requirements for Confined Spaces, American National Standards Institute, Z117.1-1989.

Yet, getting inside and moving about fuel tanks may be the least of workers’ worries. They also face a host of health and safety hazards, most notably oxygen deficiency, flammability, explosion and the toxic effects of fuel vapors.

Atmospheric conditions
Although fuel tanks can vary considerably from one aircraft type to the next, all of them have a limited volume. And, therefore, even a relatively small amount of a hazardous substance inside one of these confined spaces can create significant levels of flammable or toxic vapor. Naturally, the leading problem is jet-fuel vapor concentration. Commercial carrier jet fuels (Jet A and Jet A-1) are kerosene-based and often are mixed with other compounds, such as naphthalene, although refineries may blend other available hydrocarbons to meet jet-fuel specifications.
Jet fuels, of course, are flammable, and may ignite given certain ambient conditions — primarily, temperature and vapor concentration. An environmentally unsafe or explosive vapor concentration is present when a fuel vapor reaches a certain level known as the lower flammable limit (LFL) or lower explosive limit (LEL). These limits usually are expressed as a percentage of volume. Different types of jet fuels can have different LELs. For instance, 100 percent LEL of Jet A is 6,000 parts per million (ppm), while 100 percent LEL of JP-4 is 13,000 ppm.
Another potential problem within the tanks is oxygen deficiency. Oxygen deficiency often is caused by physical displacement of the oxygen in a space. Normal ambient air contains an oxygen concentration of 20.8 percent of volume. At oxygen-deficient levels (19.5 percent of volume and below), a person will begin to exhibit signs of oxygen starvation, including headache, nausea, drowsiness and slurred speech. At steadily decreasing oxygen concentrations more severe reactions occur and ultimately, death by asphyxiation is possible.
Other volatile organic compounds (VOCs) used in integral fuel-tank work, such as cleaning solvents, sealants and lubricants, can be even more harmful than jet fuel, and may mix with other compounds present to create significant and immediate health problems.

Maintenance Crew Preparation and Training
Clearly, fuel tanks can be a perilous working environment, and one that requires extreme caution.
Because of these potentially hazardous characteristics, the Occupational Health and Safety Administration (OSHA) classifies an aircraft fuel tank as a "permit-required confined space." Operations within them are governed by OSHA’s confined space standard, 29 CFR Part 1910.146 (see sidebar on pg. 41). Other state and local laws also may apply.
The most important factor in preventing injury during fuel tank work is a properly trained and equipped entry crew. OSHA’s standard spells out the necessary precautions for safe entry and operations. However, the hazards of fuel tank work aren’t limited to the person inside; the National Institute for Occupational Safety and Health (NIOSH) reports more than 60 percent of all confined space fatalities occur among would-be rescuers. Therefore, OSHA mandates confined space training for all team members — entry workers, standby attendants and supervisors — including:
• Continuous voice communications practices
• Safety equipment usage, including proper use of gas detection instruments
• Ventilation and air monitoring techniques
• Use of intrinsically safe tools, equipment, lighting
• Emergency response plans (including entrant self-evacuation, attendant-order evacuation and unresponsive tank-entrant rescue).
To aid in your entry and rescue plan, here are five crucial steps you should follow for safe fuel tank operations:
1. Effectively prepare the fuel tanks. This may include:
• Electrically ground and defuel the airplane.
• Prepare fire protection equipment.
• Deactivate associated airplane systems, including fueling/defueling and fuel transfer systems.
2. Ensure adequate ventilation prior to and during maintenance operations.
Proper venting is the single-most important method of controlling fire, explosion and toxic hazards. Continuously pushing fresh air into a fuel tank helps reduce the chance of fire and explosion by preventing a fuel vapor concentration from reaching its LFL. Fresh air helps to dilute toxic chemical concentration and reduce risks of VOC compound exposure and oxygen deficiency.
One practice recommended for conducting tank ventilation is the push-pull technique. First, open an upstream "push" access hole. Next, open a downstream "pull" hole. Then locate a blower at the push hole to force fresh air into the tank. Exhaust equipment can be located at the pull hole to supplement tank airflow.
Additionally, to prevent oxygen displacement inside of the fuel tank, any pneumatic tools used during maintenance operations must be powered by compressed air only, not by nitrogen or other inert gas.
Northwest Airlines’ Rescue Squad Volunteers Are First Responders to Fuel Tank Emergencies
Atlanta’s Hartsfield International Airport is home to the world’s largest DC-9-specific maintenance facility, one belonging to Northwest Airlines. More than 1,500 mechanics work there on approximately 120 aircraft each year, ranging from light maintenance to wingtip-to-wingtip system overhauls.
Safety is not just a compliance issue at the Atlanta site; it’s a human issue for both NWA, Aircraft Mechanics Fraternal Association Local 19 and its workers, as evidenced by its Partners in Safety program. Labor and management realize the human and economic factors at play in an effective safety program. Each year, mechanics complete refresher training on rescue, hazard communications, fire extinguishment, and protection for hearing, respiratory and blood-borne pathogens.
Perhaps nothing best exemplifies the facility’s safety commitment more than NWA’s Rescue Squad.
The Rescue Squad consists of 80 highly trained, certified volunteer mechanics. Five-person teams are on-site and on-call with pagers and radios around the clock. They are the first responders to victims trapped within fuel tanks.
The field mechanics initiated the Rescue Squad four years ago, with the support, encouragement and guidance from NWA’s St. Paul headquarters, the AMFA, Canadian Fall Group and Hartsfield’s Airport Fire Department. While anticipating Federal requirements for dedicated rescue teams, the mechanics were motivated more by a drive to take care of their own — especially after news swept through the ranks that a 37-year-old United Airlines mechanic had died while repairing a fuel leak inside a Boeing 767.
The NWA Rescue Squad tabbed a handcart to station its dedicated emergency response equipment, including air-supplied respirators, gas-detection instruments, communications gear and other safety tools. Total cost: about $5,000.
Then, squad leaders established a training curriculum and began intensive instruction and drilling in confined space and emergency rescue procedures. All training and quarterly drills are conducted during regular work shifts with the full support of NWA staff, and in conjunction with the Hartsfield Fire Department.
During these realistic drills, the squad’s average response time, from first alert to placing a "victim" on deck is less than 14 minutes. From rescue entry to evacuation, it’s just four minutes.
Thankfully, the volunteers have never been pressed into a life-or-death rescue—an obvious tribute to the preparedness of NWA’s mechanics.

3. Monitor the entry point for atmospheric hazards.
After venting, use gas detection instruments to assess the entry point for oxygen, flammability and toxic vapors levels. The instruments used should be intrinsically safe, fitted with visual and audible alarms and provide continuous monitoring of oxygen and combustible levels. Ideally, the instruments should be calibrated to Jet A-1 vapor, which will provide a more sensitive reading than those calibrated to pentane.
However, because obtaining an accurate jet fuel vapor sample at a known concentration is difficult, using a correlation factor supplied by the manufacturer and a calibration gas such as pentane is sufficient. Some instruments also come equipped with an intrinsically safe sampling pump and Teflon® tubing to minimize the absorption effect of fuel vapors. These accessories help ensure highly accurate readings at the crucial point of entry. With a built-in datalogger, the instrument can also calculate the short-term exposure and time-weighted average, which can be downloaded to a personal computer. The datalogging features are useful because more data can be collected for different situations (e.g., in different temperatures for different types of aircraft), which would facilitate a future revision or audit of the fuel tank entry program.
4. Prepare for entry.
After the entry point atmosphere is tested and found to be in the acceptable range, a confined space permit may be completed and posted at the space’s entrance. Only then should maintenance personnel enter the space. Necessary equipment for the entrant and standby attendant may include:
• Head/Eye/Hearing protection
• Supplied air respirator
• Portable gas detector attached to the worker’s belt to perform continuous space monitoring
• Emergency response equipment
• Multi-channel voice communications and alarm system between the workers inside and out. (Given the space constraints of fuel tanks, an integrated system, combining air supply, communications and ground wire in a single "umbilical" lifeline, is preferred.)
5. Once a worker is inside, continuously monitor the fuel tank atmosphere.
Both the sentry and the technician inside should have instruments activated as long as the worker is within the fuel tank. This will alert crewmembers immediately of any atmospheric changes.
Also, it’s important to remember that, because the vapor density of gases may differ, testing must be performed at varying heights to ensure full atmospheric evaluation is completed. Some gases are heavier than air and tend to collect at the bottom of a confined space, while others are lighter than air and tend to collect in higher concentrations near the top of a space. Also, the physical structure of some tanks can create dead air spaces, with the small openings between tank sections inhibiting airflow. Gases tend to pocket in these areas.
To ensure proper gas readings, fit the gas detector with an extendible probe. This will ensure accurate air sampling approximately four feet in the direction of travel and extending out to each side.
So, be mindful of instrument readings. If, for example, there is a change in oxygen readout without a change in one of the other readings, it could be a sign that there probably could be some condition present that is capable of displacing or changing the oxygen level.
While entry into aircraft fuel tanks is necessary for inspection and maintenance, it is a job that can pose significant personal health risks to untrained aircraft maintenance technicians. Through effective worker education, proper training and the correct use of safety equipment and instrumentation, workers can mitigate the risks involved.