It may be of interest to see when and why the words "Unless such failures are extremely remote" were inserted into the rule. Below you can see the rule as proposed in NPA 84-19, followed by the justification for the change that occurred in the final rule:
Proposal 3-46. The notice proposed several amendments to Sec. 29.927. Paragraph (c) is changed by revising and extending the rotor drive system lubrication failure test requirements for Category A rotorcraft and by clarifying the corresponding test requirements for Category B rotorcraft. Category A aircraft must have significant continued flight capability after a failure in order to optimize eventual landing opportunities. However, indefinite flight following the lubrication system failure is not expected. The changes to the Category B rotorcraft drive system lubrication failure test requirements are largely for clarification and are not substantive.
In NPA 84-19, issued on 10/10/84, this was the proposal:
(c) Lubrication system failure. For lubrication systems, the function of which is required for operation of the rotor drive system, the following apply:
(1) Category A. It must be shown by tests that each rotor drive system, where the probable failure of any element could result in the loss of lubricant, is capable of continued operation, although not necessarily without damage, for a period of at least 30 minutes at a torque and rotational speed prescribed by the applicant for continued flight, after indication to the flight crew of the loss of lubricant.
(2) Category B. It must be shown by tests that the rotor drive system is capable of operating under autorotative conditions, although not necessary without damage, for 15 minutes after indication to the flight crew of the loss of lubricant.
Following consultation, the Final Rules, Docket No. 24337, was issued on 08/26/88. The text had been amended with the following justification:
A commenter notes that paragraph (c), as proposed, could be interpreted to preclude credit for auxiliary lubrication systems or to require consideration of lubricant failures to self-lubricated bearings. This was not intended, and the wording of paragraphs (c)(1) and (c)(2) has been revised to eliminate this possible ambiguity.
The final rule being:
(c) Lubrication system failure. For lubrication systems required for proper operation of rotor drive systems, the following apply:
(1) Category A. Unless such failures are extremely remote, it must be shown by test that any failure which results in loss of lubricant in any normal use lubrication system will not prevent continued safe operation, although not necessarily without damage, at a torque and rotational speed prescribed by the applicant for continued flight, for at least 30 minutes after perception by the flight crew of the lubrication system failure or loss of lubricant.
(2) Category B. The requirements of Category A apply except that the rotor drive system need only be capable of operating under autorotative conditions for at least 15 minutes.