In July of 1996, TWA Flight 800 exploded in midair soon after leaving New York’s JFK International Airport. After an enormous salvage and reconstruction effort and years of investigation, the National Transportation Safety Board determined that the primary cause of this disaster, which killed all passengers and crew members aboard, was an explosion of the Boeing 747’s center wing fuel tank.

Since then, the Federal Aviation Administration issued Airworthiness Directive 64 FR 4959 No. 2102/02/99, a portion of a federal program that mandated corrective design and operational measures. The FAA’s AD requires proper management and inspection of fuel tanks, flame arrestor and pressure relief valve installations, and modifications designed to provide shielding and separation of fuel system wiring.

Helpful guidelines

The Occupational Safety and Health Administration (OSHA) is involved with aircraft personnel safety, as aircraft fuel tanks are covered under 29 CFR Part 1910.146, OSHA’s standard for confined space entry. OSHA’s requirements for atmospheric testing include evaluation testing of confined space “using equipment of sufficient sensitivity and specificity to identify and evaluate any hazardous atmospheres that may exist or arise.” Oxygen testing is to be performed first, followed by combustible gas testing and lastly, toxic gas testing.

In addition, the Naval Air Warfare Center in NAVAIR 01-1A-35 (wing tank procedure guidelines) states that “personnel shall not enter fuel cells which contain flammable atmospheres above 10% of the LEL or which are Immediately Dangerous to Life and Health (IDLH) except in case of extreme emergency such as a rescue effort.”

Another guideline concerns JP-8, a common jet fuel that reaches 100% LEL at 6% by volume. U.S. Air Force Technical Order 1-1-3, Inspection and Repair of Aircraft Integral Tanks and Fuel Cells, states that “a conservative estimate of JP-8 LEL is 0.6% or 6,000 ppm. The concentration of JP-8 vapor must be below 600 ppm (10% LEL of JP-8) before tank entry is authorized.”

The need for instruments

Compliance with regulations, rules and other wing fuel tank-related procedures necessitates the use of gas detection instruments. Challenges remain due to the complexities of jet fuel composition, which can vary from one manufacturer to another. Aircraft technicians must frequently enter center wing fuel tanks (which are generally large enough for one person) to perform maintenance and inspections. Workers must be properly equipped for such hazardous duties, as combustible vapors can quickly accumulate in small confined spaces. Oxygen level monitoring is also critical in such potentially explosive environments.

Jet fuels (blends of hydrocarbons) emit volatile organic compounds (VOCs). Other sources of VOCs are the chemical ingredients of lubricants, solvents and sealants necessary for aircraft maintenance, but which may also put workers at risk for toxic vapor exposure. Concern for exposure to toxic gases necessitates proper detection of hydrocarbons and oxygen in wing fuel tank applications.

Detection of VOCs is well-suited to photoionization detectors, or PIDs. Chemical compounds are ionized via an ultraviolet lamp, and the measured concentration is promptly displayed by the instrument in parts-per-million (ppm). The ability to detect very low levels of combustible gas concentrations is crucial, as some combustible gases reach their lower explosive limit (LEL) well before their concentration reaches 1% by volume, the equivalent of 10,000 ppm. PID technology for this application is superior to older combustible sensor technology developed for methane detection using catalytic beads. Typical catalytic bead or “LEL” sensors cannot reliably detect the VOCs found in jet fuel due to the fuel’s higher flashpoint, as these sensors are generally limited to vapors with flashpoints of 100°F or less.

Jet A, Jet A-1 and JP-8 are the most common of jet fuels and have flashpoints ranging from 110-150°F depending upon the manufacturer’s blend. Concentrations of combustible gases also must reach at least 1% LEL to be detectable by catalytic bead sensors and are therefore not sensitive enough for measuring jet fuel vapors.

PID monitoring

A strong option for wing fuel tank gas detection is a compact portable instrument that can perform PID monitoring, along with oxygen and toxic sensors incorporated as well. More detection specificity is offered by instruments capable of using both 10.6 eV and 9.8 eV PID lamps for VOC monitoring. A given instrument’s versatility, ease of use, simultaneous multigas monitoring, durable housing and reliability offer the user a wide range of gas detection options.

Wing fuel tank maintenance is inherently a hazardous occupation; however aircraft technicians may now opt for a personal monitor for comprehensive gas detection, to help protect and keep them safe.