FAA's Special Conditions For Alternate Technology

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In a torrent of regulatory verbosity last week, the FAA issued five diesel-related notices of proposed special conditions dealing with its concerns about putting Jet-A-powered engines in planes designed to use 100 LL. It detailed the many and varied factors that those seeking diesel STCs from the two main manufacturers will have to consider and prove not to be a threat to the safety of the aircraft. For instance, diesels tend to vibrate more (or differently) and the FAA wants to ensure the vibration (including the shaking that would result from one cylinder shutting down) isn't beyond the tolerances of the airplane. It's also concerned about the effect of diesel on fuel-system components, the adaptation of wiring and other systems, fuel grades, fuel filters and a host of other technicalities. There are also requirements to protect the full authority digital engine controls (FADEC) from electrical interference. What isn't clear (to us, anyway) is the effect of the issuance of the documents. It seems likely that the engine manufacturers have anticipated, and maybe even consulted with the FAA, on the technical details of this kind of engine swap and the documentation is an important step in moving the STC approval process forward. Since the comment period expires July 14, we'll know soon enough.