VOLUME 3 General technical administration
CHAPTER 64 MAINTENANCE TIME LIMITATIONS
Section 2 Evaluate and Approve a Part
Seats or Less) Operator’s Proposed Engine Time In Service Interval Extension, Oil Analysis Program, and Engine Trend Monitoring Program
3-5085 PROGRAM TRACKING AND REPORTING SUBSYSTEM (PTRS) ACTIVITY CODES.
A. Maintenance: 3351.
B. Avionics: None.
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
part 91 subpart K
(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.
3-5088 OIL ANALYSIS AND ENGINE TREND MONITORING PROGRAMS.
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
NOTE: The current edition of Advisory Circular (AC)
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.
3-5089 ENGINE TIME IN SERVICE INTERVAL EXTENSIONS.
A. Engine Manufacturers’ Time In Service Intervals. Engine manufacturers usually list time in service intervals
in their Service Bulletins (SB), Service Instructions, or Service Information Letters. 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.
91K Operators. For part
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.
3-5090 PROGRAMS FOR INTERVAL EXTENSIONS.
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.
2) The certificate-holding district office (CHDO) 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.
B. Best Practices for an Operator’s Time in Service Interval Extension Program. The CHDO 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
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 Operating Handbook (POH) or Aircraft Flight Manual (AFM) can make a big difference in the reliability of
16) Vibration Analysis data.
17) Any boroscope inspection findings and propeller balancing data/results.
3-5091 REFERENCES, FORMS, AND JOB AIDS.
A. References (current editions):
· Title 14 CFR Parts
Reciprocating Engine Power-Loss Accident Prevention and Trend Monitoring; and
Part 135: Additional Maintenance Requirements for Aircraft Type Certificated for Nine or Less Passenger Seats.
B. Forms. None.
C. Job Aids. None.
3-5092 PROCESS A PROPOSED TIME IN SERVICE INTERVAL EXTENSION.
A. Determine the Length of the Proposed Time In Service Interval Extension.
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 CHDO 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.
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 CHDO with adequate information to justify all
aspects of the proposed time in service interval extension.
B. Evaluate the Proposed Time In Service Interval Extension.
1) If the operator wants a time in service interval extension, the operator will submit a request in writing
to the CHDO.
2) The CHDO should collaborate with the regional specialist 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, Service Instructions,
Service Letters, 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.
· Any other data deemed necessary by the CHDO 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 (ALIs), and Maintenance Review Board Report (MRBR) limitations.
4) Time in service interval extensions do not authorize extensions for life-limited parts, ALIs, or ADs.
5) The CHDO should coordinate with the appropriate AEG/Aircraft Certification Office (ACO) regarding time in
service interval extensions to calendar time.
6) The CHDO 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 CHDO 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
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, Service Instructions, Service Letters, or Recommendations. Research the engine
manufacturer’s SBs, service instructions, and service letters 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), in‑depth 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
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.
r) Continuing Analysis and Surveillance System (CASS) Reports. The CHDO 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.
C. Analyze Findings.The CHDO should consult with the regional specialists, appropriate AEG, and/or appropriate
ACO with any concerns or findings during the analysis/review. The ASI will inform their CHDO management of any findings or concerns during
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
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
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.
2) Approval. If the CHDO 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/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
· 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.
· Any additional limitations/restrictions deemed
appropriate by the approving CHDO 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.
· The approved time in service interval extension
package will be retained by the operator as long as the extension is in effect. The approving CHDO will retain a copy of the approval in the
operator’s certificate management file.
3-5094 FUTURE ACTIVITIES.
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.