Part 91.7(a) - "No person may operate a civil aircraft unless it is in an airworthy condition."

Part 91.403(a) - "The owner or operator of an aircraft is primarily responsible for maintaining that aircraft in an airworthy condition,…"

All right, so what does "Airworthy" mean? The official definition is: "Conforms to the type design and is in a condition for safe operation." When there have been major repairs or alterations then it is necessary to have the FAA Form 337 documenting the basis for approval of those repairs or alterations. In effect, Form 337’s become a supplemental type certificate when approved.

Part 91.405 MAINTENANCE REQUIRED
Each owner or operator of an aircraft -

1. Shall have that aircraft inspected…and shall between required inspections, except as provided in paragraph (c ) of this section, have discrepancies repaired as prescribed in part 43 of this chapter;
2. Shall ensure that maintenance personnel make appropriate entries…;
3. Shall have any inoperative instrument or item of equipment, permitted to be inoperative by para. 91.213(d)(2) of this part, repaired, replaced, removed, or inspected at the next required inspection; and
4. When listed discrepancies include inoperative instruments or equipment, shall ensure that a placard has been installed as required by para. 43.11 of this part.

Now it appears that there is some relief, for inoperative instruments and equipment, contained in part 91.213(d)(2), so let’s take a look at it.

Part 91.213 INOPERATIVE INSTRUMENTS AND EQUIPMENT
Except as provided in paragraph (d) of this section, no person may take off an aircraft with inoperative instruments or equipment installed unless the following conditions are met:

1. An approved Minimum Equipment List exists for that aircraft.
2. The aircraft has within it a Letter of Authorization, issued by the FAA Flight Standards District Office having jurisdiction…
3. The approved MEL must -
   1. Be prepared in accordance with the limitations specified in paragraph b) of this section: and
   2. Provide for the operation of the aircraft with the instruments and equipment in an inoperable condition.
4. The aircraft records available to the pilot must include an entry describing the inoperable instruments and equipment.
5. The aircraft is operated under all applicable conditions and limitations contained in the Minimum Equipment list and the letter authorizing the use of the list.

Part 91.213 (c ), as referenced in para. (d), deals with persons authorized to use an MEL under parts 121, 125, or 135.

Now let’s get to the section that concerns us, that is, 91.213 (d).

Part 91.213 (d) - "Except for operations conducted in accordance with paragraphs (a) or (c ) of this section, a person may take off an aircraft in operations conducted under this part with inoperative instruments and equipment without an approved minimum equipment list provided -"

We’ll paraphrase from here on.
The aircraft has to be a rotorcraft, non-turbine powered airplane, glider, or lighter than air aircraft for which a master MEL has not been developed, or a small rotorcraft, non-turbine powered small airplane, glider, or lighter than air aircraft for which a master MEL has been developed. Now if there is no MEL for the aircraft, the following conditions must also be met:

The inoperative instruments and equipment cannot be a part of the VFR-day type certification instruments and equipment prescribed in the applicable airworthiness regulations under which the aircraft was type certificated. They cannot be required equipment on the aircraft equipment list, or on the Kind of Operations Equipment List for the kind of flight operation to be conducted. It cannot be required by para. 91.205 or any other rule of this part for the specific kind of flight being conducted, or required to be operational by an Airworthiness Directive.

Once it is determined that the inoperative instrument or equipment meets the above criteria, then the following needs to be accomplished:

The inoperative instrument or equipment needs to be removed from the aircraft, the cockpit control placarded, and a maintenance record entry accomplished in accordance with Part 43.9, or deactivated and placarded "INOPERATIVE". If it is deactivated and it involves maintenance to accomplish, it must be accomplished and recorded in accordance with Part 43.

A determination must be made by a properly rated and certificated pilot or mechanic that the inoperative instrument or equipment does not constitute a hazard to the aircraft.

If the above criteria are met, then the aircraft is considered to be in a properly altered condition acceptable to the FAA.

The bottom line is, you can operate with inoperative instruments and equipment if the following conditions are met:

1. It is in a rotorcraft, non-turbine powered airplane, or lighter-than-air aircraft.
2. It is conducted under Part 91 flight rules.
3. The aircraft doesn’t already have an Approved MEL under any flight rule.
4. The inoperative instrument or equipment isn’t required by the certification basis, regulation, equipment lists, or Airworthiness Directive.
5. The inoperative instrument or equipment is removed from the aircraft or deactivated.
6. The cockpit control and/or equipment/instrument is placarded "Inoperative".
7. A maintenance record entry is accomplished by an appropriately rated airman in accordance with Part 43.
8. A determination is made, by an appropriately rated pilot or mechanic, that the inoperative instrument or equipment does not present a safety hazard to the aircraft.
9. The inoperative instrument or item of equipment shall be repaired, replaced, removed, or inspected at the next required inspection.