FADEC-equipped Diesels Need More Electrical Back-Ups

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At AirVenture this year, I’ve seen numerous new aero turbo-diesel engines on display in various states of development. These engines run on Jet-A rather than AVGAS, a fuel that’s increasingly hard to find outside of the Americas and Europe, that costs $14 per gallon or more in certain parts of the world and that contains trace amounts of tetraethyl lead that potentially are harmful to the ecosystem. So, several general aviation engine manufacturers are rushing into a new era of turbo-diesel engine development, most of which are fitted with electrically powered full authority digital engine controls.

FAA and EASA regulators currently seem to be content to certify FADEC-equipped aviation turbo-diesels powered by single power generation sources and two or more battery back-ups. Diesel engine manufacturers say the design standard provides sufficient redundancy to keep FADECs powered for two or more hours in the event of an alternator failure, thereby sustaining engine operation while diverting to a landing facility. But, if the generator fails and you need to keep flying longer than the batteries last, the engine is going to quit when the FADEC loses electrical power.

That’s why I disagree with the single generator design philosophy for FADEC-equipped turbo-diesels. It contradicts aviation’s long standing principle of redundancy. Every critical system on an aircraft needs at least one back-up. That’s why AVGAS fueled piston engines have dual ignition systems. Aircraft also have back-up fuel boost pumps for their fuel injection systems and alternate landing gear extension systems for the same reason.

Imagine, for a moment, that you’re flying in hard IFR over the Rockies in an aircraft with a FADEC-equipped turbo-diesel engine and you experience alternator failure. How about if you’re flying over Greenland or between islands in the middle of an ocean? When the aircraft batteries run out of juice, chances are that you’re going to land in inhospitable terrain, on an ice cap or ditch into deep water.

In contrast to most of today’s aviation FADEC-equipped turbo-diesel piston engines, FADEC-equipped turbofan engines considerably more electrical redundancy, including two or more engine-driven generators or alternators, plus at least one self-energizing permanent magnet alternator on each engine that can power the FADECs. A notable exception is the Eclipse 500/550 that lacks PMAs. But, it has two engines and two generators that can power the FADECs, as well as two batteries.

So, I believe engine certification authorities need to revisit airworthiness standards for turbo-diesels. If the engine has a FADEC, it needs at least one additional, continuously available power generation source. That could be a generator or alternator powered by a second engine on a piston twin or a standby alternator or PMA on a single piston engine aircraft. Let’s not forget aviation’s time-proven principle of redundancy just because we’re mesmerized by the high-tech magic of next-generation turbo-diesels.

Discuss this Blog Entry 2

dan
on Jul 31, 2014

i agree. How about a manually deployable ram air turbine? Basically a bike generator with a little prop. Cheap, effective redundancy.

on Aug 5, 2014

"It contradicts aviation’s long standing principle of redundancy."
Redundancy is not a principle. It's one METHOD of achieving an OBJECTIVE. The objective is reliability, which is a cornerstone of safety.
Think about ETOPS. The whole scheme is a math-based probability study, made practicable by ensuring that a twin-engine vehicle can continue to fly using just one operative engine - as long as it keeps working.
Failure mode analyses conclude that the causes of contemporary multiple concurrent engine failures are unrelated to the count of engines (fuel contamination; bird ingestion; etc.). Thus, classic redundancy does nothing to elevate reliability.

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