Working inside aircraft fuel tanks is very unpleasant and, if not done properly, very risky. Yet "tank diving" is a common and frequent maintenance task. Although technology exists to speed up safety processes required for tank diving, regulatory roadblocks are preventing its commercial use.

The conventional method for making tank diving safe is air purging of the jet-fuel fumes—or volatile organic compounds (VOC)—by blowing air into tanks for many hours. A company called NanoVapor proposes and briefly deployed an alternative method that coats tank surfaces with tiny chemical particles, which takes much less time. However, NanoVapor cannot be used by U.S. airlines or shops until the FAA approves it. NanoVapor has been evaluated for more than a decade, but the FAA declines to state its conclusions or indicate when it might do so, says the company.

The two biggest dangers in tank diving are ignitable fuel vapors and breathing these vapors, according to Gary Peterson, a former American Airlines mechanic who now heads the airline division of the Transport Workers Union. These dangers are countered by several means:  lengthy air purging of tanks to reduce vapors, mechanics entering with respirators and meters that measure vapor concentrations, and having several mechanics stand by tanks in case the diver gets in trouble.

Sentinel Occupational Safety offers a new wearable device that senses health, breathing conditions, behavior and location. Peterson sees advantages in this tool for tank divers, but cautions that it should never replace stand-by mechanics.

Tank entries are necessary during heavy checks and when a tank needs inspection or repair, for example, in the case of leaks. With fuel smells penetrating skins and tasks done in confined spaces, the work is very unpleasant. “Most people don’t want to do it,” Peterson says.

Aviation Week spoke to two major airline mechanics who have been tank diving for nearly four decades and know all the problems. “You can forget how you came in and get a panic attack,” says one mechanic. “And the temperature can be 80-110 degrees.”

The mechanic’s carrier sends divers into fuel tanks with respirators that use charcoal filters when VOCs are below 200 parts per million, and with respirators and outside air supplies when VOCs are up to 700 parts per million. Both feel safe as long as rules are followed. “I’d rather work in tanks than in aircraft cabins, with all those germs,” jokes one mechanic.

Nevertheless, most tank diving is now outsourced. For example, STS Aviation Services will use 100 "tank tigers" to enter more than 1,300 tanks this year, according to Senior Vice President Craig Rose. Tigers enter tanks without any respirators when VOCs are below 35 parts per million, but do not enter at all when VOCs are above 700 parts per million.

In Europe, companies like BOSA provide similar tank-diving services.

Preconditions for tank diving thus include sufficient oxygen and VOC levels that are safe from fires and safe for breathing. Fire safety requires VOCs less than 1,400 parts per million, and mechanics might theoretically enter such tanks with respirators and external air supplies. Breathing safety is reached with Jet A-1 VOCs below 160 parts per million. Boeing recommends respirators with charcoal filters at this level.

But individual airlines set their own VOC limits, usually ranging from 35-700 parts per million. Different limits imply different air purging times. For example, Southwest Airlines blows air for 8-12 hr. into its Boeing 737 tanks. Air purging, also called degassing, takes longer for larger jets, delaying work in checks and grounding planes on the line.

NanoVapor injects a tiny mix of chemicals and water vapor to suppress VOCs to 100 parts per million—safe for charcoal-filtered respiration without any external air supply—in a narrowbody tank in what it says is as little as 13 min., compared with more than seven hours required for air purging. It says this suppression also keeps VOCS low long after use, compared with air purging, which must continue to prevent VOCs from evaporating off tank surfaces.

With no objection from a local FAA field office, the technique was used by Polar Air Cargo to reduce out-of-service time on six Boeing 747s from 2002-07. It caused no airframe or engine problems, according to both Maintenance Director David Yankana and IAI-Bedek, which overhauled the aircraft’s Pratt & Whitney engines. However, the FAA changed its approval process.

By 2012, a committee of American Society for Testing and Materials (ASTM) was formed to advise the FAA on new fuels and fuel additives. Though NanoVapor was not an additive, its use was referred to this committee, which originally included Boeing, Airbus, Embraer, Honeywell, Rolls-Royce, Pratt & Whitney and GE.

In succeeding years, NanoVapor commissioned tests and submitted results to the ASTM committee arguing the safety of its solution, culminating in a 200-page report in October 2019. New ASTM committee-member Dassault raised questions, including one about the solubility of NanoVapor in fuel. If not soluble, the solution might cause engine problems.

NanoVapor responded by citing lab tests showing its chemicals are fully soluble in Jet A fuel. Using data from Delta Air Lines, the company estimated its solution would be used, on average, only once every two years per aircraft and at most three times a year for a particular aircraft. Moreover, vapors are injected by a computer-controlled dosing pump to ensure correct amounts of chemicals are inserted, a safeguard NanoVapor says is backed up by an independent fuel test.

NanoVapor countered Dassault’s concerns about acids in its technology through commissioned lab tests showing no change in the acid number of fuel after NanoVapor was added. Similarly, these lab tests showed that NanoVapor does not alter fuel flammability. And tests on 737s and 747s indicated that NanoVapor does not leave liquid water in tanks, the removal of which might offset its time-saving features.

Finally, lab emulsification tests demonstrated the 0.5% concentration of NanoVapor in water could not dislodge existing deposits in tanks, which might lead to downstream fuel problems.

A Boeing engineer raised concerns, similar to Dassault’s solubility question, based on reports that very large chemical polymers had appeared in aircraft fuel tanks, entering into engines and in one case gumming up two Airbus A330 engines. The suspected source was superabsorbent polymers in airport fueling system filters, which have now been phased out.

NanoVapor CEO Fernando Lehrer noted these filter polymers had molecular weights ranging from 30,000-60,000 atomic mass units, or 20-50 times NanoVapor's molecular weight. Further, he says the filter polymers were designed to be insoluble in jet fuel, while NanoVapor molecules are soluble in jet fuels at up to 50 times the amount used in a typical degassing event. Lehrer also says only 0.5 grams, a quarter of the initial two grams of suppressant, remain in the tank after each degassing.

Even without such loss of suppressant, Lehrer says a hundred NanoVapor degassings over 50 years of aircraft life would be necessary to reach the molecular weight of the filter polymers, so he insists the degassing alternative is safe.

Air purging also appears safe if rules are followed, but it is usually slow. Lehrer says NanoVapor saves aircraft downtime and, thus, money. He argues that saving time might discourage any corner-cutting when managers are eager to get aircraft in service. A FedEx mechanic recently lost consciousness temporarily in a fuel tank. It is unclear whether this was due to violation of safety rules, as FedEx will not discuss the incident.

As of the publication of this article the FAA, Boeing, ASTM, GE, Pratt & Whitney and Rolls-Royce declined to state their opinions of NanoVapor or say when they might do so.