Diamond Aircraft is advising its customers to continue fueling their diesel-powered aircraft with jet fuel in spite of a recommendation from ExxonMobil to dealers to stop selling jet fuel to diesel aircraft owners. Diamond CEO Peter Maurer told AVweb he’s unaware of any service issues that might have led the oil giant to issue the letter sent to dealers last week. “It looks like they’ve come up with a solution to a problem that isn’t there,” Maurer said. He said he’s in the process of arranging a conference call with ExxonMobil officials to discuss the decision, which he said caught his company by surprise. “We’re frankly a little perplexed,” he said. Diamond is also sending a letter to customers outlining the company’s position. In the letter to fuel dealers, Martin Tippl, ExxonMobil’s U.S. General Aviation Operations Manager, says the company has determined that jet fuel may not meet the technical requirements for safe operation in piston engines and the company “does not support or endorse the supply of jet fuel to aircraft powered by diesel engines.” The letter is posted on the Thielert Owners Group Web site. Dealers are asked to sign an agreement not to pump jet fuel into diesels. There is a provision for diesel owners who insist on fueling with jet fuel, however. There’s a waiver they can sign indemnifying ExxonMobil from liability if something goes wrong. “We’re recommending [diesel aircraft owners] sign the waiver and continue using jet fuel,” Diamond’s Maurer said.
ExxonMobil technical folks have determined that jet fuel and its processing misses the mark as a diesel fuel on three counts. Jet fuels are not tested for cetane, which is the determining factor in the fuel’s ability to ignite. There’s also a fear that in extreme conditions jet fuel can cool to the point of becoming viscous and cause fuel-system problems. According to ExxonMobil, jet aircraft fly fast enough that air friction over the wings warms the fuel enough to prevent those problems. ExxonMobil also claims that jet fuel might not have enough lubricating qualities to protect fuel-system components. It also says the FAA is working with the fuel industry to determine if further certification action on diesel aircraft engines is required. Maurer noted that the Thielert engines in his company’s aircraft are specifically certified to use jet fuel only so he said he’s not sure where that statement came from or why, in general, ExxonMobil has taken the action. “We don’t know what was driving this at ExxonMobil,” he said. ExxonMobil did not immediately respond to AVweb‘s request for an interview.
