Pilots need to be comfortable with their understanding of this important FAR, but the minimum equipment list concept seems to complicate the situation unnecessarily.
Regarding inoperable instruments, the basic premise is that all of an aircraft's equipment and instruments must work before takeoff unless, by FAR 91.213(d), the pilot can determine that the inoperable item is not needed according to specific criteria. The exception would be if the aircraft has a minimum equipment list FAA approved for that aircraft by N number. The pilot would then follow a specific checklist of individually named items allowed to be inoperative without having to rely on pilot judgment.
Sounds simple enough, right? We hope so, but let's work through the details for a review.
Small rotorcraft, non-turbine-powered small airplanes, gliders, and lighter-than-air aircraft can fly with inoperative instruments and equipment as long as the item(s) are not in one of the following four regulatory areas:
The inoperative instrument or equipment must be placarded as inoperative and either deactivated or removed from the aircraft. The pilot can placard the item as long as no maintenance is required to deactivate the item; i.e., if no mechanical work, other than what the pilot can perform under preventive maintenance, is required or a circuit breaker can be pulled. If a mechanic is required to make the deactivation, a logbook entry is required in addition to the placard. Either the pilot or a mechanic must thereby make the determination that the inoperative instrument or equipment does not constitute a hazard to the aircraft.
Let's also note that the use of FAR 91.213(d) does not apply to large rotorcraft (over 12,500 pounds), large (over 12,500 pounds) non-turbine airplanes, and any turbine aircraft. These aircraft must either have everything working or be operating under a specific minimum equipment list (MEL).
Now let's talk minimum equipment lists.
An MEL is defined in Advisory Circular 91-67 as a precise listing of instruments, equipment, and procedures that allows an aircraft to be operated under specific conditions with inoperative equipment. An MEL is part of an FAA authorization that allows for the operation of an aircraft from other than the provisions of FAR 91.213 that regulates inoperative instruments and equipment that we just reviewed. FAA flight standards district offices have listings of generic MELs for various models of aircraft; these are called master MELs. An operator or pilot can get a copy of the master MEL for his model aircraft and customize it according to his particular instruments and equipment. With FAA detailed review and approval of his document by N number, an FAA letter of authorization (LOA) can be granted, giving the authority to operate under the provisions of the MEL.
The value of an MEL lies in the fact that pilot judgment is removed; this can be comforting to the commercial operator who wants every one of his pilots and/or renters to make the same decision but frustrating to the individual without options. As one example, let's say the artificial horizon is to be required on all VFR flights.
You can probably see that the pilot operating under FAR 91.213 instead of an MEL would have the flexibility to decide for himself about the artificial horizon on a VFR day. Although he may agree that at 1,500 and 3 miles he wants the artificial horizon, at clear and 20, he would hate to be restricted from flying, as the pilot under the MEL in this case would be. It's difficult to think of every scenario ahead of time to write it into an MEL.
Possible value of the MEL to the private operator lies in the safety of already having something on paper ahead of time so that his judgment and research skills couldn't be questioned by the FAA.
Our suggestion to owners and operators is that they should create a file containing the original certification minimum equipment data, a copy of the aircraft's type certificate document, and a photocopy of FAR 91.205 (day/night/VFR/IFR equipment). These will all photocopy to a few 8.5 x 11-inch pages that could be easily kept in the aircraft. AOPA can help you pull these together.