8900.1 CHG 440



Section 2  Evaluate and Approve a Part 135 or 91K (Nine Seats or Less) Operator’s Proposed Engine Time-In-Service Interval Extension, Oil Analysis Program, and Engine Trend Monitoring Program


A.    Maintenance: 3351.

B.    Avionics: None.

3-5086    OBJECTIVE.

A.    Purpose. This section provides guidance for an aviation safety inspector (ASI) with a maintenance specialty on how to evaluate the following:

    Proposed engine time-in-service interval extensions,

    Oil analysis programs, and

    Engine trend monitoring programs.

B.    Applicable Engines/Aircraft. The guidance in this section applies to reciprocating and turbine‑engine aircraft operated under Title 14 of the Code of Federal Regulations (14 CFR) part 135, § 135.411(a)(1) or part 91 subpart K (part 91K) (nine seats or less).

3-5087    ENGINE TIME-IN-SERVICE INTERVALS. The engine manufacturer establishes the recommended time-in-service interval, which is an estimated number of hours or hours and cycles (turbines) an engine can safely and reliably operate without exceeding the overhaul service wear limits. Some engine manufacturers refer to recommended time-in-service intervals as time between overhaul (TBO) intervals.


A.    Oil Analysis Programs.

1)    Many operators use oil analysis as a tool for evaluating the health of their engines. Oil sample analysis and oil filter inspections work hand-in-hand to provide the operator with an indication of developing mechanical problems. Although an oil analysis does not identify all developing engine problems, it can provide information indicating that parts are not wearing normally.
2)    The following are examples of typical sources of wear.
a)    The typical sources of wear for reciprocating engines include worn bearings, crankshafts, cylinder walls, oil pump gears, piston pin bushings, piston rings, push rods, rocker arms, valve guides, and valve springs.
b)    The typical sources of wear for turbine engines include worn bearings, bearing seals and retainers, bearing housings, constant speed drives, oil pump gears, and gearbox castings.
3)    When an oil analysis report shows abnormal readings, the operator (or its maintenance provider) should determine the problem and take the appropriate corrective action.

B.    Engine Trend Monitoring Programs.

1)    An engine trend monitoring program is a continuous data collection system that periodically records and analyzes selected engine readings, indications, or conditions. As a result, this program helps to detect deterioration of an engine’s performance due to wear/malfunction of engine components and accessories.
2)    An effective engine trend monitoring program consists of four key procedures:
a)    Data collection,
b)    Data processing,
c)    Analysis, and
d)    Management alerts.
3)    Systems for an engine trend monitoring program differ between reciprocating engines and turbine engines.
a)    Reciprocating Engines. A system for a reciprocating-engine records and analyzes a number of engine-operating indications. (The system may also record other engine conditions.) An engine trend monitoring program for reciprocating engines should address at least three engine areas for monitoring:

1.    Engine case components,

2.    Cylinder assemblies, and

3.    Accessories.

NOTE:  Advisory Circular (AC) 20-105, Reciprocating Engine Power-Loss Accident Prevention and Trend Monitoring, contains additional information about engine trend monitoring programs for reciprocating-engine aircraft.

b)    Turbine Engines. Engine performance data is collected at least once each flying day after the engine parameters stabilize in cruise flight. Generally, there are three types of deviations that warrant further investigation:

1.    Slow gradual changes over several readings,

2.    Rapid changes over a few readings, and

3.    Sudden changes between two readings.

C.    Evaluate the Operator’s Oil Analysis and Engine Trend Monitoring Programs.

1)    If the operator has an oil analysis and/or an engine trend monitoring program, verify the written policy and procedures are adequate for operator’s type of operation.
2)    The operator should make the reports from its oil analysis and engine trend monitoring programs readily available to the Federal Aviation Administration (FAA) for review.
3)    When an oil analysis report shows abnormal wear readings, the ASI should verify that the operator (or its maintenance provider) has taken the appropriate corrective action. Repeat oil analysis reports showing abnormal wear may indicate a problem with the operator’s engine maintenance program.
4)    The ASI should review engine trend monitoring reports for abnormalities that would indicate a problem with the operator’s engines. When an engine trend monitoring report indicates that a problem exists, the operator (or their maintenance provider) should determine the problem and take the appropriate corrective action which may include modifying the current maintenance program.


A.    Engine Manufacturers’ Time-In-Service Intervals. Engine manufacturers usually list time-in-service intervals in their Service Bulletins (SB), service instructions (SI), or Service Information Letters (SIL). Some engine manufacturers allow for time-in-service interval extensions based on how an operator operates and maintains its engines. However, other engine manufacturers remain silent with regards to such extensions.

B.    Part 135 and 91K Operators. For part 135 and 91K (nine seats or less) operators, operation specification (OpSpec) D101 and management specifications (MSpec) MD101 lists the time-in-service intervals. These time-in-service periods are typically the same as the engine manufacturer’s recommended time‑in-service interval periods.

C.    Basis for Time-In-Service Interval Extensions. The FAA may allow a time-in-service interval extension if the operator can extend the useful life of an engine without compromising safety. The ASI should base any time-in-service interval extensions on an operator’s:

    Adequate in-service reliability,

    Proper justification, and

    Risk analysis.


A.    Programs for Time-In-Service Interval Extensions. For the purposes of this chapter, a time‑in‑service interval extension program is defined as an operator’s documented policies and procedures to maintain its engines in an airworthy condition so they can extend the useful life of those engines. The operator should include the time-in-service extension program in its Policy and Procedures Manual (PPM).

1)    The operator should follow any specific instructions available from the engine manufacturer in order to operate and maintain engines in a manner that warrants a time-in-service interval extension.
Indicates new/changed information.
2)    The responsible Flight Standards office should recommend that the operator develop a time‑in‑service interval extension program. A time-in-service interval extension program would monitor the health of an engine from the last overhaul through the time-in-service interval extension. The time-in-service interval extension program should be able to determine an engine’s condition so that the operator can remove the engine from service prior to failure.
Indicates new/changed information.

B.    Best Practices for an Operator’s Time-In-Service Interval Extension Program. The responsible Flight Standards office should recommend that the operator’s time-in-service interval extension program include policies and procedures for performing and documenting best practices. An operator may include the following best practices in its time-in-service interval extension program:

1)    Engine trend monitoring.
2)    Engine oil analysis.
3)    Record of the oil consumption for each engine.
4)    Accomplish oil changes at frequent intervals (reciprocating engines).
5)    Inspect oil filter elements/ oil filter debris analysis.
6)    Accomplish cylinder compression checks at frequent intervals (reciprocating engines).
7)    Inspect baffle conditions to ensure proper engine cooling (reciprocating engines).
8)    Inspect the engine case for condition.
9)    Inspect engine components for security and condition.
10)    Ground run an engine at scheduled intervals to determine satisfactory performance of powerplant systems and static power output.
11)    Use a reputable engine maintenance provider to perform engine overhauls. Many operators seeking fleet-wide time-in-service interval extensions use the engine manufacturer or a single source as their engine maintenance provider.
12)    Verify the quality of the maintenance provider’s engine maintenance and overhaul performance. Any replacement parts identified in the appropriate engine manufacturer’s SB should be replaced at the overhaul or at the appropriate maintenance task interval.
13)    All engine accessories should be overhauled or replaced per the manufacturer’s recommendations. Accessories play a very important part in the life of an engine.
14)    Request engine overhaul teardown reports that show dimensional checks and wear of critical parts. Prior to teardown, the operator, seeking the time-in-service interval extension should have operated the engine to within 5 percent of the current approved time-in-service interval.
15)    Flightcrew training, if a properly trained flightcrew operates the engine, it may help to extend the TBO. Continued compliance with the pilot’s operating handbook (POH) or Aircraft Flight Manual (AFM) can make a big difference in the reliability of the engine.
16)    Vibration Analysis data.
17)    Any borescope inspection findings and propeller balancing data/results.


A.    References (current editions):

    Title 14 CFR Parts 91K and 135;

    AC 20-105, Reciprocating Engine Power-Loss Accident Prevention and Trend Monitoring; and

    AC 135-7, Part 135: Additional Maintenance Requirements for Aircraft Type Certificated for Nine or Less Passenger Seats.

B.    Forms. None.

C.    Job Aids. None.


A.    Determine the Length of the Proposed Time-In-Service Interval Extension.

Indicates new/changed information.
1)    An ASI and the operator should collaborate with each other to determine a reasonable length of the time-in-service interval extension. The length of the extension should allow for the operator to extend the useful life of its engines without compromising the safety of the flying public. The responsible Flight Standards office can reiterate to the operator that the responsibility and liability for operating the engine during a time‑in‑service interval extension lies with the operator itself.
Indicates new/changed information.
2)    The length of the time-in-service interval extension may vary depending on the operator’s particular operation. For example, a time-in-service interval extension could be a one-time extension of 25 flight-hours for one engine, or a fleet-wide extension of several hundred hours per engine. It’s the operator’s responsibility to provide the responsible Flight Standards office with adequate information to justify all aspects of the proposed time-in-service interval extension.

B.    Evaluate the Proposed Time-In-Service Interval Extension.

Indicates new/changed information.
1)    If the operator wants a time-in-service interval extension, the operator will submit a request in writing to the responsible Flight Standards office.
2)    The responsible Flight Standards office should collaborate with AFS-300 and/or appropriate Aircraft Evaluation Group (AEG) if the ASI requires assistance to evaluate a time-in-service interval extension. The AEG, if necessary, would coordinate with Aircraft Certification Service (AIR) for any additional assistance.
3)    The operator will submit any documentation applicable to possible justification for the proposal such as:

    An engine manufacturer’s SBs, SIs, Service Letters (SL), or other recommendations that discuss time‑in‑service interval extensions.

    Past Operating Experience (OE) to include oil analysis reports, engine trend monitoring reports, and maintenance history of the applicable engine(s).

    Overhaul teardown reports from the operator’s engine fleet that show dimensional checks and condition of critical parts. The number of teardown reports that the FAA may request depends on the size and complexity of the operator’s aircraft fleet.

    Recommendations from the operator’s engine maintenance provider as to the recommended details of any possible time-in-service interval extensions.

Indicates new/changed information.

    Any other data deemed necessary by the responsible Flight Standards office or AEG/Engine Directorate that is needed to substantiate the time-in-service interval extension.

    To include reliability reports, Service Difficulty Reports (SDR), engine utilization reports, oil consumption reports, Airworthiness Directives (AD) records, life-limited parts, airworthiness limitation items (ALI), and Maintenance Review Board Report (MRBR) limitations.

4)    Time-in-service interval extensions do not authorize extensions for life-limited parts, ALIs, or ADs.
Indicates new/changed information.
5)    The responsible Flight Standards office should coordinate with the appropriate AEG/responsible Aircraft Certification Service office regarding time-in-service interval extensions to calendar-time.
6)    The responsible Flight Standards office should review any information relevant to the proposed time-in-service interval extension. Depending on the type of engine (i.e., turbine or reciprocating), the items that the responsible Flight Standards office should review may include:
a)    Mechanical Interruption Summary Reports (MISR). Review previous MISR to detect trends or irregularities. This may indicate problem areas in maintenance procedures, operational procedures, or the operator’s training with regards to the reliability of its engines.
b)    SDRs. Query the SDR database for information on the component the operator wants to extend. A high number of reports, failures, or other deficiencies may be a reason to reject a time-in-service interval extension.
c)    Aircraft/Engine Utilization Reports. The monthly aircraft/engine utilization report provides data for entry into the utilization system database. This utilization system database is one of several data repositories that interface with the Safety Performance Analysis System (SPAS).
d)    Type Certificate Data Sheets (TCDS). Review the appropriate current TCDS for any information relating to time extensions or restrictions. The data sheet will also indicate life limits or reference the manual where life limits are located, if applicable.
e)    Engine Manufacturer’s SBs, SIs, SLs, or Recommendations. Research the engine manufacturer’s SBs, SIs, and SLs for time-in-service interval extension information. These documents may also include special operating considerations. The operator must comply with certain inspections and other criteria in the service documents in order to propose a time-in-service interval extension.
f)    Oil Analysis Reports. Review the operator’s oil analysis reports for abnormal wear and recommendations from the lab for follow-up action. Repeat abnormal wear reports may indicate a problem with the operator’s engine maintenance program.
g)    Trend Monitoring Reports. If the operator has a trend monitoring program, review it for abnormalities that would indicate a problem with the operator’s engine maintenance program.
h)    Engine Overhaul Teardown Reports. The operator should provide current (last overhaul), indepth teardown reports showing recorded dimensional checks and the condition of critical parts. The operator should have operated the engines that it chose for the teardown to within 5 percent of the currently approved time-in-service interval. The number of teardown reports that the FAA may request depends on the size and complexity of the operator’s aircraft fleet.
i)    Recommendations and/or Nonconcurrence from the Engine Maintenance Provider. The operator’s engine maintenance provider is in a position to help determine the length of the proposed time‑in‑service interval extension. This is the Maintenance Organization (MO) that disassembled, inspected, performed dimensional checks of critical parts, and completed the overhaul of the engine.
j)    Engine Maintenance History. Review the operator’s past engine maintenance history for early engine removals from service, early overhauls, repeat maintenance actions, cylinder changes, compression checks, oil filter inspections, and static power output engine runs. Verify the engine received other maintenance actions, including the installation of replacement parts (identified in the appropriate engine manufacturer’s SBs) that should be replaced at the engine’s overhaul.
k)    Oil Consumption History. This is the history of oil consumption throughout the engine’s operation since its last overhaul.
l)    AD Records. Some ADs may restrict operating an engine past the recommended time-in-service interval.
m)    Life-limited Parts and ALIs. Review the status of any applicable life-limited parts and ALIs.
n)    MRBR. If the operator has a transport category aircraft, the MRBR (if applicable) may contain pertinent information about the engine.
o)    Operator’s Violation History. If the operator has a history of maintenance related violations, this may be a factor when analyzing the risk of the proposed time-in-service interval extension.
p)    Operator’s Accident, Incident, and Occurrence History. If the operator has a history of accidents, incidents, or occurrences related to maintenance issues, this may also be a factor when analyzing the proposed time-in-service interval extension.
q)    SPAS. Use SPAS to review the operator’s PTRS history for items that may indicate the operator’s safety culture and its compliance with regulations. An ASI can identify the operator’s specific areas that may present a greater risk and then analyze safety critical performance measures.
Indicates new/changed information.
r)    Continuing Analysis and Surveillance System (CASS) Reports. The responsible Flight Standards office should review CASS reports (for those operators who have a CASS program) for any trends involving the operation of the operator’s engines or powerplant systems.
Indicates new/changed information.

C.    Analyze Findings. The responsible Flight Standards office should consult with AFS-300, appropriate AEG, and/or the responsible Aircraft Certification Service office with any concerns or findings during the analysis/review. The ASI will inform their responsible Flight Standards office’s management of any findings or concerns during the analysis/review.

D.    Perform a Risk Analysis and Determine Approval/Rejection. Perform a risk analysis to determine if the operator can extend the useful life of the engine(s) without compromising safety.

1)    If the risk analysis determines that the operator’s safety culture or operating practices present an unacceptable safety risk, an ASI should reject the time-in-service interval extension.
2)    If the risk analysis determines that any accessed risk may be accepted without further action, and the operator can provide service with the highest possible degree of safety, an ASI may approve the time‑in‑service interval extension.

NOTE:  An ASI should evaluate each proposed time-in-service interval extension for risk by using Safety Management System (SMS) principles, as applicable, in the current edition of AC 120-92, Safety Management Systems for Aviation Service Providers.

E.    Debrief the Operator. Debrief the operator by discussing the results of the evaluation of the proposed time-in-service interval extension, oil analysis program, and engine trend monitoring program. Also discuss any risks identified during the risk analysis.

3-5093    TASK OUTCOMES.

A.    Complete the PTRS Record.

B.    Complete the Task. Completing this task results in one of the following:

1)    Rejection. If an ASI determines the proposed time-in-service interval extension, engine trend monitoring program, or oil analysis program is unacceptable, that ASI will notify the operator by letter that the FAA rejects its proposal or program. The letter should include the reasons for the rejection. Also, the ASI should return the proposed time-in-service interval extension documentation to the operator.
Indicates new/changed information.
2)    Approval. If the responsible Flight Standards office determines that the proposed time‑in‑service interval extension and/or engine trend monitoring and/or oil analysis program is acceptable, they will accomplish the following:

    Update OpSpec D101/D102 or MSpec MD101/MD102 to reflect the revised time-in-service interval and reference the operator’s document that contains the TBO time-in-service extension and/or program, including (if exists) the engine oil analysis/trend monitoring program.

    If for a particular serial number engine of a particular make, model, and series (M/M/S) permanent TBO extension, identify the engine by serial number and under the Make/Model column and reference the operator documents.

    Notify the operator by letter that the FAA approves its proposed time-in-service interval extension and/or engine trend monitoring program with oil analysis.

    Approval letters should include the maximum allowable time-in-service, hourly/calendar/cycles extension in terms of aircraft and engine values.

    A statement that the extension is not transferrable.

    A statement that any change to the operating procedures, environmental, utilization or maintenance program as of the approval, may result in the termination of the extension.

Indicates new/changed information.

    Any additional limitations/restrictions deemed appropriate by the approving responsible Flight Standards office such as: all ADs will be accomplished when due; the engine remains installed at respective position on aircraft and is not removed for any other reason; all scheduled/unscheduled inspections are accomplished when due; engine continues to perform within manufacturers minimum standards, etc.

    For Approved Aircraft Inspection Program (AAIP) operators, verify their submission is adequate and approve it by conventional established means for any AAIP revision.

Indicates new/changed information.

    The approved time-in-service interval extension package will be retained by the operator as long as the extension is in effect. The approving responsible Flight Standards office will retain a copy of the approval in the operator’s certificate management file.


A.    Periodic Checks. Periodically review the operator’s engine trend monitoring and oil analysis reports.

B.    Continual Monitoring. Monitor the operator’s engine maintenance program through normal surveillance.

RESERVED. Paragraphs 3-5095 through 3-5130.