In a document leaked to Flight International which responds to the fears of a member of the public, a senior UK Department for Transport official states: "There is no reason to suspect the test is anything other than safe. The risks are understood and managed and the flight is comparable to many other R&D flights."
Flight International has also learned that the London Heathrow to Amsterdam trip, to take place by the end of February with an empty aircraft and limited crew, will use a 20% blend of algae-derived biofuel in one of the aircraft's four General Electric CF6-80C2 engines, indicating that biofuel will produce the energy powering the equivalent of about 20km (11nm) of the 370km journey.
Discussing post-test safety, the DfT official says the selected engine, which is due for major overhaul at KLM Engineering & Maintenance, will be stripped and inspected for any evidence of degradation.
"It has been agreed that, as a precaution, the fuel system will undergo a further flushing through to remove any residual biofuel," he says, adding that the test has involved the European Aviation Safety Agency together with the Dutch and UK CAAs, which have now conducted a full operational and technical pre-flight assessment.
Virgin has worked with Boeing and engine manufacturer GE Aviation on the initiative since last April, but has declined to specify the fuel, saying only that it is "truly sustainable", not competing with food and fresh water resources. However, GE revealed in August that there were three candidates, each using different sources and processes.
Discussing the project, the DfT official mentions the significant testing and bench research of the fuel's waxing, aromatic and calorific qualities, and adds: "But obviously, at some stage, it has to fly."
He says checks have ensured test procedures have been documented correctly and regulations complied with. "The test involves isolating one engine with an independent fuel system, so that if any problems did occur with that engine, the other three can ensure the continued safe take-off and landing, irrespective of when it might be shut down." The other power units would be fed purely with conventional fuel, he says.
Two development areas given particular attention are matching the much lower aromatic fraction of biofuel to petroleum-based fuels, which can swell the neoprene seals in fuel lines and pumps, increasing the chance of leakage.
"The industry does not believe this is a difficult problem to solve, but it does make interoperability of the two fuels more difficult," says the official. "With more research, we may determine that a range of alternative fuel feedstocks produces a satisfactory fuel."
Turning to concern that the lower freezing point of biofuels makes them less suited to long flights in polar regions, the official says: "They are considered safe for shorter flights where the heat soak is less severe and where the fuel is less likely to be pumped to the wing-tips for balance and trim. Ideally, tank heaters would be used to offer the flexibility to use the fuel globally, but for safety reasons these are undesirable in case of overheating type failure."
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