In a November 17 letter to aviation fuel distributors, ExxonMobil U.S. General Aviation Operations Manager Martin Tippl wrote: "ExxonMobil Aviation does not support or endorse the supply of jet fuel to aircraft powered by diesel engines." The letter was circulated as a PDF by the Thielert Engine operators' user group, which called the policy "a shot across the bow for owners of diesel-powered aircraft." The ExxonMobil letter also included an Indemnity Agreement releasing ExxonMobil from any liability resulting from the sale of jet fuel to diesel operators. The agreement is to be signed by suppliers and returned to ExxonMobil. Tippl cited ExxonMobil research into the issue that revealed concerns over the following aspects of using jet fuel in diesel engines: Ignition quality-the minimum cetane for airworthiness has not been established, and cetane is not measured as part of jet fuel specification. Freezing point-without the friction heating of fuel that high-speed jets enjoy, there is danger in slower piston aircraft of fuel freezing. Lubricity-diesel engines rely on fuel to supply some of their lubrication needs, and standards for how much jet fuel provides for these needs have not been established. Though the letter does not expressly forbid fueling diesel-powered aircraft with jet fuel, it does state expressly, "No fueling of diesel engine aircraft with jet fuel may be performed without a valid indemnity agreement signed by the customer in place."