Every pilot-in-command or aviation manager responsible for the operation or maintenance of a civil aircraft has pondered the question of operating an aircraft with inoperative instruments and equipment.
In the U.S., both Federal Aviation Regulations Parts 91 and 135 say that no one “may take off an aircraft with inoperative instruments or equipment installed,” except under certain conditions.
For Part 135 operations, FAR 135.179(a) sets forth conditions for legally operating an aircraft with inoperative instruments or equipment.
For one, there must be an FAA-approved minimum equipment list (MEL) for the aircraft. For another, the Flight Standards District Office (FSDO) responsible for an operator’s certification must have issued Operations Specifications to that certificate holder that authorize operations in accordance with the approved MEL. The flight crew must have direct access at all times to all information in the MEL through printed or other FAA-approved means. Also, the aircraft must be operated in compliance with all applicable conditions, requirements and limitations contained in the MEL and Op Specs.
Often, MEL conditions set procedures to perform before an aircraft can be flown with the inoperative instrument or equipment. (For instance, the instrument must be placarded properly or the appropriate circuit breaker must be isolated with a collar to prevent it from being closed.)
Like many regulatory compliance matters, the devil is in the details. As an example, FAA Order 8900.1 says the easiest means of meeting the MEL direct access requirement “is for the Part 135 operator to carry the MEL aboard each aircraft.” The agency cautions that direct access does not include access through telephone or radio communications with maintenance or other personnel on the ground. But it’s possible that other forms of access may be acceptable.
Similarly, the general rule cited in Order 8900.1 says MEL use does not apply “to discrepancies or malfunctions that occur or are discovered during flight.” However, in a corollary statement, the FAA indicates that “once an aircraft moves under its own power, the flight crew must handle any equipment failure in accordance with the approved flight manual.”
Confusion or uncertainty can arise when the operator or pilot-in-command perceive, or encounter, conflicting regulatory requirements or priorities. It is not uncommon for pilots to distinguish between the operational requirements of Part 91 and the generally more stringent ones of Part 135. Nor is it uncommon for them to decide or act to conform to the more permissive requirements when circumstances permit. A pilot or aviation manager could misinterpret the effects of obtaining an FAA-approved MEL for a specific aircraft operated on a Part 135 certificate and occasionally fly the aircraft for Part 91 purposes.
However, a 1991 FAA interpretation of FAR 91.213(a) concluded that “a Part 135 air carrier operating under an approved MEL would be required to operate under that Part 135 MEL even when conducting operations under Part 91.”
A more subtle compliance “gotcha” is an advisory circular (AC 91-67) provision that the operator also must have on the aircraft “any technical manuals needed to accomplish operational or maintenance” procedures set forth in the MEL.
An operator will often prepare an MEL for a specific make and model by reproducing the FAA’s master minimum equipment list (MMEL). On that list, the remarks-or-exceptions column may state, “As required by 14 CFR,” for a particular instrument or equipment on the helicopter, such as the automatic deployable locator transmitter.
FAA policy says that term “is not used in MELs,” which “must contain the appropriate regulatory requirement and procedures supporting it.”
Pilots, mechanics and operators should always remember that MEL procedures, limitations and requirements (and compliance with them) do not relieve the operator from determining that an aircraft is safe for flight with certain instruments or equipment. The operator should always independently consider the effects of deactivating an inoperative instrument or equipment on the capabilities or performance of other instruments, equipment or appliances on the helicopter.