Aircraft Maintenance Technology

OCT 2018

The aircraft maintenance professional's source for technological advancements, maintenance alerts, news, articles, events, and careers

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Page 18 of 63 19 Today, most fuel uplifted into commercial aircraft flows through a filtration system known as a full flow monitor, which is designed to remove both water and solids. The water is removed through absorption by what the industry refers to as super absorbent polymer (SAP). More specifically, sodium polyacrylate. “When sodium polyacrylate (or SAP), which is the same stuff that goes into diapers, reaches its saturation point with water, it literally stops the flow of fuel,” states John Leonard, refined fuels market manager for Facet Filtration. “It’s designed to prevent any water from getting into the fuel being put in the aircraft.” However, under a variety of conditions, these “Pampers for the fueling industry,” as Leonard calls them, can leak small quantities of crystal-like gel, which can move into the aircraft during fueling. There have been eight documented events where this has happened and caused operability issues for the aircraft. The most serious of these incidents occurred in April 2010 in Surabaya, where both engines on an Airbus A330 lost thrust control after becoming gummed up with SAP. There were 309 passengers and a crew of 13 on board the Cathay Pacific flight. The crew successfully landed the plane at almost twice the normal landing speed. Fifty-seven passengers were injured in this event. Since then, there have been seven similar incidents involving engine issues allegedly linked to fuel contamination from filter monitors. The International Air Transport Association (IATA) convened a special interest group in May 2014 to investigate these incidents and determine if there was a correlation between fuel control unit and/or hydro-mechanical unit operability issues and the presence of SAP. Participants in this group included representatives from airframe and engine OEMs, aviation fuel filter manufacturers, airlines, airline associations and the Energy Institute (EI). The group determined there was no guarantee that SAP will not pass downstream of the filter monitor elements when in service. Tom Muzik, business unit manager of Parker Velcon, sat on this committee. He states their investigation found “SAP filters do shed SAP in some amount — some more than others. But they do shed SAP, and they shed more SAP as differential pressure increases,” he says. “Therein lies the rub, we know that SAP at some level causes an issue, but it would take billions of dollars and years of research to figure out how much is too much.” In an industry where safety is paramount, an unknown like this is unacceptable. As such, engine manufacturers have reduced their allowable SAP tolerance from 50 parts per billion to zero. And, the special interest group decided, “We have to come up with an alternative solution that does not involve this particular polymer because of its propensity — under certain conditions — to go downstream,” Leonard says. Differential Pressure Difference The IATA group first recommended standardizing the differential pressure requirement for fueling. When a filter monitor becomes saturated and stops or slows the flow of fuel, there is a spike in differential pressure. This pressure is monitored by differential pressure gauges, such as those from Gammon Technical Products. Differential pressure monitors were in place in the Surabaya incident, but an industry expert, who wishes to remain anonymous, reports “no one looked at them, believing there was a problem with the hardware — not the fuel. But what had happened is the operator had gotten a lot of water into the fueling system. The filters stopped it, but they kept struggling to get flow. This eventually damaged the filters and pushed water and filter media into the aircraft.” In the U.S., aviation differential pressure cannot exceed 15 psi. However, in other countries up to ©ISTOCKPHOTO.COM/OLLO

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