Sets Additional Cert Requirements, Guidelines On Fuel

Recognizing that interest in diesel engines within the general aviation community has grown appreciably in the past several years, the Federal Aviation Administration has issued a draft memorandum to clarify the certification requirements for 14 CFR part 33, type certification of diesel reciprocating engines.

The memorandum, available for download as a Word file at the FMI link below, provides compliance interpretations the agency says are necessary to accommodate the unique design features of a diesel reciprocating engine. It also identifies areas of regulatory compliance that may require equivalent level of safety findings (ELOS) or special conditions (SC) to address design features of diesel engines not envisioned when part 33 was created.

Despite the generalized guidelines, the FAA also states appropriate ELOS or special conditions for each engine model must be determined on a case-by-case basis, in accordance with part 21, §§ 21.16, 21.17, and part 11.

"Interest in diesel aircraft engines has recently been renewed in response to the demand for engines that do not require leaded fuel for operation," the FAA states. "Two recent diesel engines have been certified in Europe and validated by the FAA. Several other diesel reciprocating engines are in development in both the US and in Europe."

"There are many new and novel issues to be addressed as modern diesel engine technology is integrated into these aircraft engines," the agency adds.

Areas of compliance specific to diesel engines -- referred to as compression ignition, or CI, engines -- include evaluation of a CI engine's high-pressure fuel system, and ELOS determinations for fuel flow and power level angles to be used in computing engine operating limitations.

The traditional method for computing operating limits involves measuring airflow into the engine through manifold air pressure (MAP) sensors -- a system that may not provide accurate readings for CI engines, as diesels do not meter airflow into the engine the same way a conventional, or spark ignition (SI) engine does.

The memorandum also specifies guidelines for aviation-grade diesel fuels. The FAA anticipates that in most cases, operators of planes with CI engines will use either Jet-A, or automotive diesel fuel in their engines.

However, aviation jet fuel specifications such as ASTM International D1655 do not include criteria for control of cetane number, because this is not a critical performance parameter for turbine engines. In diesel engines, however, the cetane number is a big deal, as it characterizes the ignition capability of a diesel fuel, and is a critical parameter for assessing the acceptability of a fuel for a particular engine design. Using the appropriate cetane rated fuel in a diesel engine is critical to developing the appropriate power. 

Therefore, the FAA mandates that a CI engine applicant will be required to provide an analysis that shows operation with jet fuel with an inadequate cetane number will not create an unsafe condition. The agency also says the applicant's analysis should consider the minimum cetane requirement of their CI engine design, the probability that commercially available fuel may have a cetane number below that minimum, and the consequences of operating with a fuel with a cetane number that is below that minimum.

The FAA also specifies CI engines must meet similar standards for icing protection as turbine engines, as diesel fuel absorbs more water than regular avgas does.

FMI: Read The FAA's Full Guidelines