A general view of the intake and blade design of an open rotor: Snecma website

Call them unducted fans (UDFs) as they were termed in the 80s, or by the newer term of open rotor engines, they are still around as seriously funded concepts, and they continue to suffer from one problem.

They don’t meet current international engine safety certification standards.

If a UDF or open rotor disintegrated on an airliner in flight it would crash, and with the crash caused by the tail being cut off or the wing and fuselage shredded it would be an unsurvivable accident. Which is what made some of the reports of the new full scale demonstrator engine being designed and built for flight testing on an A340 by SNECMA somewhat disconcerting.

In this report earlier this month in Flightglobal  Snecma research and technology director Pierre Guillaume is quoted as saying that he expects “extensive discussions” with the relevant authorities as current engine-approval standards – for example regarding fan-blade failure – will have to be changed for the open-rotor architecture.

Since when has it been even thinkable that design rules that would prevent massive loss of life ought to be waived for the latest version of very old plans for an engine design which promises very commendable savings in fuel consumption?

Surely less safety is hardly the way to advance mass air transport. That would be as questionable as granting exemptions to the certification of, for arguments sake, of some of the rules applying to heavy duty lithium ion batteries because an aircraft designer convinced a compliant and less than vigilant regulatory team in the FAA that there was absolutely no chance they would ever overheat.  Not even the FAA would do that, would it?

The jet engines used in airliners are always contained or ducted. The design rules are formulated to prevent ‘uncontained’ engine failures, but as a Rolls Royce powered Qantas A380, performing QF32,  demonstrated near Singapore in November 2010, they don’t always succeed under unforseen circumstances. That jet was saved by the brilliant and intuitive teamwork of experienced pilots trained well over and above the minimum standards that constitute world’s best practice in airline regulation these days, but that is to digress.

Uncontained engine failures have occurred in various types of airlines down the years, and they can have serious consequences, even though the design rules ought to be credited with reducing the frequency of such events.  Indeed contained failures which are less uncommon have repeatedly demonstrated the wisdom of such standards.

Points to keep in mind as the lure of less fuel consumption in open rotor designs rises in the PR campaigns of various engine makers is that these proposals are not about a standard jet power plant minus a shroud at all.

They are, like the UDF demonstrators flown on MD-80s and a 727 in the late ’80s, about two sets of stubby vanes rotating (contra rotating) in opposite directions at trans sonic velocity at their tips in order to increase the ratio by which air by passes the combustion chambers of the jet engine core which turns them.

The energy and complexity of open rotor designs is much more than that of conventional (and comparatively tiny) propeller equipped commuter or general aviation aircraft.

Up to this time, cutting off tails to one side, the main problems of open rotors has been noise, make that acoustic energy so intense it can degrade adjacent sections of the engine, wing or fuselage even when working normally. The design in those earlier times was also altitude limited to lower heights than potentially available to shrouded or ducted engines.

However it must be noted that Snecma (the French half of CFM) and Rolls-Royce which also has ambitions for open-rotors,  have variously insisted that the noise is getting beaten, hopefully not by ‘reforming’ regulatory definitions of noise problems.

The core challenge appears to be if the absolutely, totally, unconditionally impossible blade failure or ‘ejection’ occurs, hundreds of people will die.

Massaging this with PR softening up campaigns, or indeed, not even mentioning the issue in media reports, has already begun.

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