NPA 2017-07

Rotorcraft gearbox loss of lubrication

The objective of this NPA is to address a safety issue related to the ability of Category A rotorcraft to continue safe flight for an extended duration after suffering a loss of oil from a gearbox that is reliant on a pressurised lubrication system to provide lubrication and cooling of rotating components.

The specific objective is to reduce the level of risk associated with loss of lubrication of rotorcraft gearboxes and to implement recommendations arising from the Joint Certification Team (JCT) review of rotorcraft gearbox certification specifications (CSs). This aims to both reduce the potential for lubrication system failures from occurring and to mitigate the consequences of any failure.

The objective shall be achieved by improving the safety assessment of pressurised lubrication systems, and by improving the certification and development testing specifications for the ‘loss of lubrication’ condition in order to substantiate a maximum period of continued operation which can be included in the rotorcraft flight manual (RFM) emergency procedures. More specifically, this NPA proposes to amend CS 29.917(a) to include rotor drive system gearbox lubrication systems in the definition of the rotor drive system. This means that these lubrication systems will be considered to be within the scope of the design assessment of 29.917(b). As the design assessment CS currently addresses the risk of single hazardous and catastrophic failures, additional material has been added to complement the Federal Aviation Administration (FAA) Advisory Circular (AC) 29-2C, supporting 29.917(b), specifically in the domain of lubrication systems. CS 29.927(c) on ‘loss of lubrication’ has been completely revised and replaced by a more objective-based CS that requires substantiation of the gearbox ability to continue safe operation (for at least 30 minutes) after a loss of lubrication to be followed by a safe landing. This is supported by substantial changes to the associated acceptable means of compliance (AMC). Finally, CS 29.1521 has also been amended to include an additional power plant limitation that describes how the RFM emergency procedures should reflect the test evidence relating to a loss of lubrication.

The proposed changes are expected to provide an increase in the safety level of rotorcraft operations.