Engine Trend Monitoring: A Source-Based Owner Workflow
A conservative piston-engine trend workflow built around calibrated instruments, repeatable data, approved limits and timely maintenance review.
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Updated July 16, 2026
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Engine trend monitoring is the disciplined comparison of repeatable engine data over time. Its purpose is to identify changes that deserve attention, not to turn a pilot into a remote diagnostician.
The FAA's active Advisory Circular 20-105C provides a practical baseline for reciprocating-engine monitoring. It stresses calibrated instruments, consistent data collection and comparison with manufacturer recommendations.
This page is an owner workflow, not maintenance data for a specific engine. The aircraft flight manual or pilot's operating handbook (POH), engine operator's manual, approved supplements and authorised maintenance advice control.
Quick answer: what should an owner track?
Track a small set of parameters under repeatable conditions:
- engine RPM;
- manifold pressure, where applicable;
- oil pressure and oil temperature;
- cylinder-head temperature (CHT);
- exhaust-gas temperature (EGT);
- fuel quantity and fuel flow where installed;
- oil added and hours flown;
- ambient conditions, altitude, power and mixture setting; and
- maintenance events or instrument changes that affect the baseline.
Compare the trend with the exact manufacturer's limits and normal history. A change is a reason to review the data and involve an appropriately authorised maintainer; it is not a stand-alone diagnosis.
1. Start with instrument accuracy
FAA AC 20-105C says a trend programme is only as good as the information collected and analysed. Before building a baseline, the owner or operator should ensure the relevant RPM, oil-pressure, oil-temperature, CHT, EGT, fuel and manifold-pressure instruments are tested for accuracy.
Lycoming likewise states that leaning recommendations are based on calibrated instrumentation and recommends annual calibration of relevant engine instruments.
This matters because a stable sensor error can create a false baseline, while a drifting probe can look like an engine change. Record:
- instrument make and model;
- whether it is primary or advisory;
- probe or transducer changes;
- calibration or accuracy-check date; and
- firmware or configuration changes that alter displayed or logged values.
When hardware changes, do not splice the old and new traces together without a note.
2. Define a repeatable data point
Trend data becomes harder to interpret when every sample comes from a different phase of flight. Choose a condition permitted by the aircraft and engine instructions, then record enough context to reproduce it.
A practical log might include:
| Field | Record |
|---|---|
| Date and tach/flight time | |
| Time since oil or maintenance event | |
| Outside-air temperature and pressure altitude | |
| Flight phase and stabilisation time | |
| RPM and manifold pressure | |
| Mixture method or setting | |
| Oil pressure and temperature | |
| CHT by cylinder | |
| EGT by cylinder | |
| Fuel flow, if installed | |
| Oil added since last entry | |
| Abnormal indications, roughness or maintenance notes |
The correct sampling point varies by aircraft. Do not create a power setting that conflicts with the POH merely to make the spreadsheet tidy.
3. Watch the pattern, not an internet threshold
Generic CHT, EGT and oil-pressure numbers often circulate without an engine model, probe location, power setting or approved limit. They are not a safe substitute for the applicable manual.
Use three layers:
- Limit: Is any parameter outside an approved operating range or accompanied by a warning?
- Change: Has a parameter moved materially from its established baseline under comparable conditions?
- Context: Did weather, power, mixture, maintenance or instrument configuration change at the same time?
One EGT or CHT trace can have several possible causes. Avoid declaring a blocked injector, valve problem or cylinder defect from a generic pattern alone. Preserve the data and let a maintainer combine it with the engine history, inspection findings and approved troubleshooting information.
4. Treat oil use as a separate trend
Lycoming describes oil consumption as an important engine-health trend and recommends recording the oil used. It notes that a noticeable change deserves investigation with the other maintenance evidence.
For an owner log, record:
- oil type and grade;
- quantity added;
- tach or flight time between additions;
- oil-change date and interval;
- filter or screen inspection result; and
- oil-analysis result, if used.
Do not compare “a quart” between aircraft without comparing hours, operating practice, fill level and the model-specific maximum consumption information. Oil analysis is one input, not a release-to-service decision.
5. Preserve raw data before changing the airplane
Many digital monitors can export flight data. Follow the applicable pilot guide and retain the raw file before reformatting it. A useful evidence packet for a maintainer includes:
- the raw export;
- the flight or tach-time reference;
- the conditions and power setting;
- a note of recent maintenance or probe changes;
- the relevant POH or engine-manual limits; and
- the exact concern without a pre-selected diagnosis.
J.P. Instruments publishes pilot guides and download instructions for its EDM products. Other monitors require their own approved manuals and software.
6. Use a conservative response ladder
When a parameter changes:
- compare it with the approved limit and any cockpit warning;
- check whether the data point is comparable and the sensor is credible;
- preserve the raw data and operating context;
- review the aircraft and engine instructions;
- contact an appropriately authorised maintainer; and
- do not operate when a limitation, warning, abnormal engine behaviour or maintenance determination says otherwise.
Do not repeat a potentially unsafe condition merely to obtain a cleaner graph. Trend monitoring is meant to support reliability and timely review, not justify continuing an abnormal operation.
Part 91 and Part 135 are not the same programme
AC 20-105C offers generic owner/operator charts and notes that some Part 135 operations have specific engine trend-monitoring requirements under 14 CFR 135.421(c). A personal tracking sheet is not automatically an approved commercial programme.
Operators should place the process inside their applicable maintenance and operational system, with the required roles, procedures and record control.
What changed in this revision
The former page supplied universal temperature spreads, diagnostic conclusions, cost claims and product rankings without retained model-specific evidence. Those statements were removed.
The revised page keeps the useful search intent while narrowing the advice: calibrate, standardise, record, compare with approved material and escalate changes to maintenance.
