AC 91-91

U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Maintaining Public Aircraft Date: 10/19/16 AC No: 91-91 Initiated by: AFS-300 Change:

Title 49 of the United States Code (49 U.S.C.) § 40102(a)(41) states that a public aircraft is (1) an aircraft used only by the United States (U.S.) government, or (2) an aircraft owned or operated by the government of a state, the District of Columbia, or a territory or possession of the United States, or a political subdivision of one of these governments. The Federal Aviation Administration (FAA) does not grant public aircraft status. Rather, the status of an aircraft is a function of its operator and type of operation, in accordance with (IAW) the applicable statutes.

As a result, although these operations must continue to comply with certain general operating rules, including those applicable to all aircraft in the National Airspace System (NAS), other civil certification and safety oversight regulations do not apply. Oversight of regulatory compliance of “public aircraft” is assumed by the Government entity while they have operational and maintenance control over the aircraft. The FAA enhances safety through certification of aircraft per Title 14 of the Code of Federal Regulations (14 CFR) part 21, and we encourage public aircraft operators to obtain an appropriate airworthiness certification. An operator makes an airworthiness certificate effective when the operator complies with all terms and conditions of the certificate. This advisory circular (AC) provides guidance that government entities can use if they choose to maintain their aircraft per 14 CFR regulations.

The information in this AC is based on proven maintenance practices that have been in use for many years. If not excluded by regulation all operational maintenance requirements that are required to operate in the NAS must be complied with; however, there are safety and economic benefits to maintaining continued airworthiness using information in this AC. The 14 CFR regulations specified in this AC are used to illustrate how the operator of a civil aircraft must comply with certain rules. Governmental agencies and private entities conducting operations with public aircraft are encouraged to comply, when practical with the regulations for the type of aircraft operated. Operators of such aircraft and the flying public will benefit from voluntary adherence to the safety standards established in the current regulations.

ORIGINAL SIGNED by /s/ John Barbagallo Deputy Director, Flight Standards Service

CONTENTS Paragraph Page

Chapter 1. General1-1

1.1. Purpose1-1

1.2. Applicability1-1

1.3. Background1-1

1.4. Advisory Circulars (current editions)1-1

1.5. Related CFRs1-2

Chapter 2. Aircraft Maintenance and Inspection2-1

2.1. Responsibility for Airworthiness2-1

2.2. Inspection Programs2-1

2.3. Aircraft Maintenance2-3

2.4. Major Repairs and Alterations2-4

2.5. Instructions for Continued Airworthiness2-4

2.6. Airworthiness Limitations2-5

2.7. Airworthiness Directives2-5

2.8. Spares2-5

2.9. Aircraft Records2-5

2.10. Personnel Training2-8

2.11 Return To Service2-8

Appendix A. DefinitionsA-1

Appendix B. AcronymsB-1

ii

CHAPTER 1. GENERAL

1.1 Purpose. This AC is intended to provide guidance for developing maintenance and inspection programs for aircraft operated as public aircraft. The recommendations in this AC were developed using industry best practices, depending on the aircraft being operated. The AC describes methods for maintaining aircraft in order to maximize both a safety and economic benefit.

1.2 Applicability. This AC applies to any person/owner/operator-known here as “operator”-who engages in the operation and maintenance of public aircraft as part of a governmental function.

1.3 Background. Title 49 U.S.C. § 44701 is the primary authority for 14 CFR regulations.

This section instructs the FAA Administrator to promote the safe flight of civil aircraft in air commerce through regulations and standards written to promote safety.

Title 49 U.S.C. § 40102 defines what can be considered a public aircraft. Title 49 U.S.C.

§ 40125, Qualifications for Public Aircraft Status, specifies criteria for operating as a public aircraft. Title 14 CFR part 91 specifies the requirements for operations in the National Airspace System (NAS) for all aircraft. In addition, it details those regulations applicable to civil aircraft and those applicable to all aircraft, which includes public aircraft.

Note: Simply stated, the FAA has statutory authority to regulate the operation and maintenance of civil aircraft used in air commerce. However, we have no statutory authority to regulate public aircraft, except for operations in the national airspace system. Oversight of the regulatory compliance of “public aircraft” is assumed by the government entity while they have operational and maintenance control the aircraft.

1.4 Advisory Circulars (current editions). The Regulatory and Guidance Library (RGL) is a set of searchable databases that contain regulatory guidance and aviation product information. The RGL contains certain Codes of Federal Regulation (CFR) and Special Federal Aviation Regulations (SFARs) in their current version as well as historical versions (http://rgl.faa.gov). Electronic copies can be downloaded at http://fsims.faa.gov/.

Select “Regulatory Guidance Library” in the left-hand column. The ACs listed below have additional information a public aircraft operator might find helpful:

• AC 00-1.1, Public Aircraft Operations.

• AC 20-62, Eligibility, Quality, and Identification of Aeronautical Replacement Parts.

• AC 20-77, Use of Manufacturers' Maintenance Manuals.

• AC 39-7, Airworthiness Directives.

• AC 43-4, Corrosion Control for Aircraft.

• AC 43-9, Maintenance Records.

• AC 43-12, Preventive Maintenance.

1-1

• AC 43.13-1, Acceptable Methods, Techniques, and Practices─Aircraft Inspection and Repair.

• AC 43.13-2, Acceptable Methods, Techniques, and Practices─Aircraft Alterations.

• AC 91-82, Fatigue Management Programs for In-Service Issues.

• AC 120-16, Air Carrier Maintenance Programs.

1.5 Related CFRs. The following sections of 14 CFR apply: Part 91, General Operating and Flight Rules.

1-2

CHAPTER 2. AIRCRAFT MAINTENANCE AND INSPECTION

2.1 Responsibility for Airworthiness. Title 14 CFR part 91 states that the owner/operator of a civil aircraft is primarily responsible for maintaining that aircraft in an airworthy condition, including compliance with FAA Airworthiness Directives (AD). We recommend that public aircraft operators use one of the inspection or maintenance programs specified in part 91, § 91.409, as described in chapter 2 and subsequent paragraphs. The safety benefits gained from following the regulations are derived from structured procedures for performing maintenance that will ensure the continued airworthiness of an aircraft. The economic benefit comes from the structured program itself, where expenditures can be planned in the most cost effective manner. This includes maintenance planning, obtaining replacement parts, and scheduling the aircraft for maintenance at repair facilities. Regardless of regulatory requirements, safety is paramount and you will enhance the safety of your operations by following the regulations.

2.2 Inspection Programs.

2.2.1 Program Details. Aircraft inspection programs detail inspection of the aircraft, engines, propellers, appliances, emergency equipment, and survival equipment. Operators may have the aircraft inspected per a manufacturer’s inspection program. The manufacturer’s program provides a comprehensive inspection program that can be tailored to the specific needs, while ensuring continued airworthiness. A manufacturer’s program also ensures continuity of the aircraft inspection program, especially if the operator uses the aircraft in both civil and public operations.

2.2.2 Service History. Operators may establish an aircraft inspection program based on service history and similar programs developed by owners/operators, especially for aircraft no longer supported by an aircraft manufacturer. An inspection program like this may be more suitable for an owner/operator of an ex-military aircraft, low-usage aircraft, or special operations aircraft, such as Forest Service smokejumpers or firefighting, where the aircraft may only be flying a few hundred hours a year. Such a program enables operators to tailor their program to suite the aircraft’s operational needs.

2.2.3 Inspection Program Development. Aircraft inspection program development requires an intimate knowledge of the airplane and its components. Operators need a sound knowledge and understanding of inspection procedures, techniques, and inspection system control. The entire aircraft should be covered by the program, and the inspection frequencies should be based on sound judgment and previous service experience with similar aircraft models. Most owners/operators find that adapting an existing program to their needs is most practical and offers the greatest economic benefit.

2.2.4 Inspection Program Types. Types of maintenance and inspection programs:

2.2.4.1 Small airplanes (12,500 pounds or less) (rotorcraft 6,000 pounds or less) must be inspected IAW § 91.409. Section 91.409 indicates you must perform at least an annual inspection. Part 43 contains those items, which should be 2-1

checked as part of an annual inspection. It also contains those items that are considered preventive maintenance. The items of inspection are usually recommended by the aircraft manufacturer. In lieu of this, the owner/operator may elect to utilize a progressive inspection, all inspection items are spread out to cover inspection of the entire aircraft within a 12-month period.

Progressive inspections are explained in § 91.409(d) when performed over the period of 1 year, progressive inspections would equal an annual inspection.

2.2.4.2 For small aircraft used in public operations that carry qualified non-crewmembers, the 100-hour annual inspection detailed in requirements of § 91.409(b).

2.2.4.3 Large civil airplanes (over 12,500 lbs.) (rotorcraft over 6,000 pounds max takeoff weights) and turbine powered rotorcraft are inspected IAW a different inspection programs than small aircraft the applicable rule is § 91.409(e).

Ex-military aircraft would also benefit from following this guidance. The following are options for large aircraft. We recommended that, owners/operators of a large aircraft, you use one of these programs:

1. A continuous airworthiness inspection program that may be part of the Continuous Airworthiness Maintenance Program (CAMP) of a 14 CFR part 121 or part 135 air carrier. This type of inspection program would include processes and procedures for inspections in a manual system. The manual system would provide instructions for:

• The performance of the inspections,

• Procedures on how the inspections are scheduled,

• Who may perform them,

• Return to service procedures,

• Procedures on how to handle unscheduled maintenance, and

• Subsequent aircraft return to service.

Note: Recordkeeping is an important part of this type of program.

These programs usually contain all instructions for continued airworthiness (ICA) elements published by the aircraft manufacturer.

2. An approved aircraft inspection program in use by part 135 operators. This program usually consists of a list of inspection tasks and associated intervals. The tasks are derived from a manufacturer’s recommendations and are comprehensive enough to ensure the airworthiness of the aircraft.

3. A current inspection program recommended by the aircraft manufacturer. In this case, operators use the inspection program published by the manufacturer. We also recommend in this case 2-2

that any program revisions be incorporated when the program is revised by the manufacturer.

4. An inspection program devised by the operator based on manufacturer’s recommendations or inspection programs in use by operators of similar aircraft. Operators may also use elements of the manufacturer’s recommended program, along with any inspections tasks you want to incorporate.

2.2.5 Inspection Program Recommendations. The aircraft inspection program should contain at least the following:

1. The instructions and procedures to conduct the inspection.

2. Necessary tests and checks.

3. Inspection items and areas of the airframe, engines, propellers, appliances, and emergency equipment.

4. A schedule for the inspections in terms of time in service of aircraft, engines, parts (life-limited parts), calendar time, number of airplane and engine cycles, or any combination of these.

5. Additional program content, such as:

• Special inspections for hard or overweight landings, turbulent air, extended out of service inspection, etc.;

• System for controlling life-limited parts;

• AD compliance control;

• System for recordkeeping, and

• Creation, approval, and recording of program revisions.

2.3 Aircraft Maintenance.

2.3.1 Types of Maintenance. Aircraft maintenance generally applies to the maintenance, preventive maintenance, and alterations, including inspections associated with specific products (i.e., airframes, engines, propellers, and appliances). Operators may tailor one of the programs described above for their particular use, depending on the environment and usage rates of the aircraft. By using one of these programs, operators will ensure the safety and continued airworthiness of the aircraft.

2.3.2 Approved Maintenance Personnel. If a public aircraft operator (PAO) is operating an aircraft that has an airworthiness certificate, it should be maintained by an FAA certificated personal or repair station and the maintenance and/or inspection must be documented IAW part 43 or 145.

2.3.2.1 If a PAO aircraft with an airworthiness certificate is not maintained IAW part 43, it cannot operate under civil use until all maintenance is documented and returned to service by a FAA certificated personal or repair station.

2-3

2.3.2.2 If the PAO is operating an aircraft that does not have an airworthiness certificate, then it does not have to be maintained by an FAA-certificated personal or repair station or meet the documentation requirements of parts 43 or 145. The requirements of part 43 do not restrict an FAA-certificated personal or repair station from maintaining a public aircraft that do not have an airworthiness certificate.

2.3.3 Scheduled and Unscheduled Maintenance. The maintenance performed may be scheduled or unscheduled maintenance.

2.3.3.1 Scheduled maintenance consists of maintenance tasks performed according to a maintenance schedule, and should include procedural instructions for the maintenance task and a method of recording the results of inspection checks, tests, and other maintenance. Operators may use the aircraft manufacturer’s job/task cards or develop their own. These should include procedural instructions for accomplishing maintenance and inspection tasks. Operators may also develop and use non-routine item job cards to document discrepancies found when performing maintenance and inspection tasks.

2.3.3.2 Unscheduled maintenance includes procedures, instructions, and standards for maintenance performed because of unscheduled or unforeseen circumstances.

A need for unscheduled maintenance may result from scheduled maintenance tasks, pilot reports, or unforeseen events such as hard or overweight landings, or ground damage.

2.4 Major Repairs and Alterations.

2.4.1 Data Required for Repairs and Alterations. Major repairs and major alterations performed on civil aircraft require the use of FAA-approved data. Some sources of FAA-approved data are type certificate (TC) data, Supplemental Type Certificate (STC) data, AD, most airframe structural repair manuals, especially concerning 14 CFR part 25 transport aircraft, data approved by an FAA Designated Engineering Representative (DER), and data approved by a Flight Standards Service (AFS) airworthiness inspector on FAA Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance). Using FAA-approved data ensures that the aircraft will continue to meets design specifications and continued structural integrity. For continuity of records, we encourage operators to use FAA-approved or FAA-accepted data for repair or alteration of their aircraft.

Accepted data is data that is supplied by the manufacturer or other FAA-accepted sources. For example: ACs, component maintenance manuals (CMM) and Operational Equipment Manufacturer (OEM) supplements.

2.4.2 Determining Level of Repairs or Alterations. For civil aircraft, operators must determine if a repair or alteration is major or minor, including maintenance performed on components. See Appendix A, Definitions, for more information. We recommend operators use part 43 to determine if a repair is major or minor. In either case, operators should use FAA-approved or FAA-accepted data to repair their aircraft. Aircraft used in 2-4

public operations are usually derived from type certificated (TC’d) aircraft and use of this type of data will ensure that the aircraft still meets its approved design standards.

2.5 Instructions for Continued Airworthiness (ICAs). ICAs are information developed per applicable airworthiness requirements, including applicable inspection tasks, intervals, methods, processes, and airworthiness limitations (AL), to maintain airworthiness throughout the aircraft’s operational life. Some aircraft are TC’d with ICAs, and these consist of airplane manufacturers’ manuals including an Aircraft Maintenance Manual (AMM), standard wiring practice manual (SWPM), the Manufacturer’s Maintenance Planning document (MPD), and Maintenance Review Board Reports (MRBR). These manuals contain information necessary for the continued airworthiness of an airplane.

Operators will find this information provided by the aircraft manufacturer is the most concise maintenance information for a particular make and model aircraft. Most civil aircraft manufactured prior to January 28, 1981, were not required to have ICAs for the airplane. Instead, the requirement was for an AMM. For those ex-military aircraft that are used in public operations, operators may use military technical orders (TOs) as a source of maintenance information.

2.6 Airworthiness Limitations (AL). Operators can locate mandatory inspections, replacement times and other maintenance specified in the Airworthiness Limitation Section (ALS) of a manufacturer’s maintenance manual or ICA. For aircraft TC’d prior to January 28, 1981, the ALs were contained in an FAA-approved section of the AMM.

Not all aircraft have ALs. An aircraft manufacturer’s inspection program should contain the ALs for that aircraft. Owner/operators of public aircraft are encouraged to comply with the aircraft’s ALs if they have them.

2.7 Airworthiness Directives (AD). ADs are issued by the FAA to notify aircraft owner/operators of unsafe conditions in aircraft, aircraft engines, propellers, and appliances and to require mandatory correction. ADs prescribe the corrective action, including inspection, repair, or alteration under which the product may continue to operate. Owner/operators of civil aircraft are required to comply with ADs issued by the FAA. In the event that an AD is issued to an aircraft, aircraft engines, propellers, or appliances in civil operation and that aircraft, aircraft engines, propellers, or appliances are likely to be operated, compliance with the AD will ensure a continued airworthy product. The owner/operators of public aircraft that have a standard airworthiness certificate are encouraged to comply with the requirements of all applicable ADs. In some cases, an AD cannot be accomplished or an improved process is developed to better achieve the intent of the AD. In this situation, an operator may apply for an alternative method of compliance (AMOC), which, when approved, would allow use of the differing procedures or techniques. The FAA Aircraft Certification Office (ACO) with responsibility for the TC’d of the aircraft approves these AMOCs. Compliance with all required ADs are mandatory prior to conducting civil use operations. Complying with ADs ensures the safety of the aircraft in public use and lessens the possibility of catastrophic failure that could lead to loss of life and property. Owner/operators of public aircraft can use the FAA’s RGL to search for ADs that may apply to their aircraft, engines, propellers, and appliances.

2-5

2.8 Spares. Components to be installed on a public aircraft that are maintained using the guidance in this AC should follow the requirements noted in the CFRs. Spare components should be repaired or overhauled at a facility such as a CRS or certificated air carrier. Owners/operators should use the component manufacturer’s recommended CMM when repairing or overhauling a component. In addition, eligibility for installation should be determined if the component is being installed on a TC’d airplane. The times and cycles on a particular component should be tracked in case the component is subject to a life limit or an AD.

2.9 Aircraft Records.

2.9.1 Recordkeeping. Title 14 CFR part 91 provides record keeping requirements for civil aircraft operators. Public aircraft owner/operators, while not required, are also encouraged to keep aircraft records. The information in this AC can be used as a guide to establishing a recordkeeping system. Aircraft records provide continuity of maintenance on an aircraft. The records can be used to plan future maintenance as well as show completion of past maintenance and to what criteria that maintenance was performed.

Records allow the owner/operator to ensure that proper data and procedures are used in maintaining the aircraft and to identify the person performing the maintenance. In addition, records allow owner/operators to track and identify components that are repaired or replaced and if an AD is issued, to determine applicability of that AD to their aircraft. For public aircraft, a recordkeeping system similar to that of a civil aircraft will facilitate transfer from public to civil operations and back. It should also provide for a seamless transfer of ownership from public to civil operators if the aircraft records are kept in a format similar to a civil aircraft operation. Maintenance records may be kept in any format that provides record continuity, includes required content, lends itself to addition of new entries, and provides for signature entry. Aircraft records should contain at least the following:

1. Record of maintenance for each aircraft, engine, propeller, rotor, and appliance of an aircraft. As a practical matter, many owner/operators find it advantageous to keep separate or individual records since it facilitates transfer of the record with the item when ownership changes.

2. The maintenance record entry should include a description of the work performed. The description should be in sufficient detail to permit a person unfamiliar with the work to understand what was done, and the methods and procedures used in doing it. A reference to the technical data used such as manufacturer’s manuals, service letters, service bulletins, work orders, FAA ACs and other data, which accurately describe what was done, or how it was done, should be referenced.

3. The maintenance record entry should contain the date the work was completed. This is normally the date upon which the work is approved for return to service.

4. The maintenance record entry should include the signature and certificate number of the person approving the work for return to service.

2-6

Note: Prior to a PAO aircraft being returned to civil operations a maintenance record entry is required for the completion of a conformity inspection, stating that all maintenance performed while under government contract meets all civil regulations. This record provides for a seamless transfer of ownership from public use to civil operation; a conformity inspection is required to ensure the aircraft meets all civil regulations.

2.9.2 Total Time In Service. The maintenance record entry should contain the total time in service for each airframe, engine, propeller, and appliance (as applicable). Time in service, with respect to maintenance records is that time from the moment an aircraft leaves the surface of the earth until it touches down at the next point of landing and is expressed in hours, cycles, or both.

2.9.3 Current Status of Life-Limited Parts. A current status of each life-limited part of each airframe, engine, propeller, and appliance that contain at least the following:

1. Time in service since new, expressed in the appropriate parameter (hours, cycles, and/or calendar time);

2. Time in service remaining to the specified life limit expressed in the appropriate parameter (hours, cycles, and/or calendar time);

3. The specified life limit expressed in the appropriate parameter (hours, cycles, and/or calendar time); and

4. A record of any action that alters the part’s life limit or changes the parameter of the life limit.

2.9.4 Time Since Last Overhaul. The time since last overhaul of all items installed on the aircraft that are required to be overhauled on a specified time basis include at least the following information:

1. An identification of the item that requires overhaul and its associated scheduled overhaul interval;

2. The time in service since the last overhaul was accomplished;

3. The time in service remaining until the next scheduled overhaul; and

4. The time in service when the next scheduled overhaul is due.

2.9.5 Current Inspection Status of the Aircraft. The current inspection status of the aircraft means a record that contains at least the following information:

1. The time in service since the last inspection required by the inspection program under which the aircraft is maintained.

2. The time in service remaining until the next required inspection under which the aircraft is maintained.

2-7

2.9.6 Current Status of Applicable Airworthiness Directives (AD). The current status of applicable ADs means a record that contains at least the following information:

1. Identification of the particular airframe, engine, propeller, appliance, or component to which the AD is applicable.

2. The AD number.

3. The date when the required action was accomplished and the time in service expressed in the parameter (hours, cycles, and/or calendar time).

4. If the requirement is recurring, the date when the next action is due, and the time in service expressed in the appropriate parameter (hours, cycles, and/or calendar time).

5. The method of compliance. With regard to an AD, method of compliance means a concise description of the action taken to comply with the requirements of the AD. If the AD, referenced manufacturer’s service bulletin, or other data permits the use of more than one method of compliance, the record should include a reference to the specific method of compliance used.

If the owner/operator uses an AMOC, the method of compliance means a description of this AMOC and a copy of the FAA approval.

2.9.7 Major Alterations and Major Repairs. A record of major alterations and repairs should be recorded in the aircraft maintenance records. These records should also be transferred when the aircraft is sold. The record may consist of FAA Form 337, which will contain the details of a repair or alteration. If an owner/operator is using an air carrier’s continuous airworthiness maintenance program to maintain the airplane, the air carrier’s internal documentation such as Engineering Orders, Repair Orders, and Repair Authorizations may be used to record the major repair or alteration.

2.10 Personnel Training. Under most circumstances, training programs for maintenance personnel are not required in part 91. Owner/operators of public aircraft can usually find training programs for their specific aircraft through the aircraft, engine or propeller manufacturer. They may also be able to contract with another operator or air carrier to provide training. Determining the training level of persons performing maintenance for an owner/operator of public use aircraft is essential in assuring safety and the airworthiness of the aircraft.

2.11 Return to Service. This is a term that is used in civil aircraft operations when an aircraft is put back into service after maintenance, preventive maintenance or alterations are performed. Prior to aircraft return to service the owner/operator should prepare an appropriate entry in the aircraft log. Part 43 prescribes the rules governing the maintenance of aircraft having a U.S. Airworthiness Certificate and must be followed if the aircraft has a U.S. Airworthiness Certificate. The FAA recommends that public use owner/operators utilize those persons for return to service of their aircraft.

2-8