After any maintenance, repair, or overhaul work on a reciprocating aircraft engine, the technician's job is not finished when the last bolt is torqued. A carefully executed engine run-up and power check is the final, essential verification step that confirms every system is functioning within acceptable limits before the aircraft is returned to service. Think of it as the engine's final exam — one that the Aviation Maintenance Technician (AMT) administers and evaluates using instruments, sound, and procedure rather than paper and pencil.
Understanding how to conduct a proper post-maintenance run-up, what values to look for, and which discrepancies demand immediate attention is a core competency for any powerplant technician. It is also a topic the FAA tests directly, because an improperly returned-to-service engine represents a serious flight safety risk. This article walks through the full process, grounded in FAA guidance and standard maintenance practice.
Purpose of the Post-Maintenance Run-Up
A post-maintenance run-up accomplishes several objectives simultaneously. First, it allows the engine to reach normal operating temperature, which is necessary before most performance checks are meaningful. Oil viscosity, clearances between metal parts, and fuel vaporization all change significantly between a cold start and a fully warmed engine. Second, it surfaces any leaks, loose fittings, or assembly errors that only become apparent under operating pressures and temperatures. A fuel line fitting that appears tight at atmospheric pressure may weep fuel once the engine-driven fuel pump is producing full pressure. Third, it provides a functional check of every system the maintenance work may have affected — or inadvertently disturbed.
The scope of the run-up should always be proportional to the scope of the maintenance. A top-overhaul or cylinder replacement demands a more thorough and extended run-in procedure than a simple magneto timing adjustment. Major overhauls typically require a multi-stage ground run sequence, sometimes followed by a test flight, before the aircraft is released to the customer. In every case, the guiding principle is the same: do not release an aircraft to service with any unresolved discrepancy that was discovered during the run-up.
Preparation Before Starting
Safe run-up preparation begins well before the starter engages. The technician should position the aircraft on a firm, level surface pointed away from hangars, parked aircraft, and personnel. The propeller arc must be clear in all directions — wash and debris thrown by the propeller at high power settings can cause injury and damage. Wheel chocks should be placed, and on high-power checks, tie-downs or a ground anchor may be appropriate for heavier aircraft.
Before the first start, verify that oil quantity is correct and that all oil filler caps, dipstick caps, and drain plugs are properly secured. Check that all cowling panels and access doors that are installed for the run are properly latched. Confirm that fuel sumps have been drained and checked for contamination, and that fuel selector position is correct. Review the maintenance records for the work just completed and have the applicable manufacturer's maintenance manual on hand so that run-up limits and sequences are followed exactly.
Initial Start and Warm-Up
After a start, monitor oil pressure immediately. On most reciprocating engines, oil pressure should begin to rise within 30 seconds of start in warm conditions; in cold weather it may take slightly longer, but if oil pressure does not indicate within the time limit specified in the engine manufacturer's data, the engine must be shut down immediately and the cause investigated. Running an engine without oil pressure — even briefly — can cause catastrophic bearing damage.
Allow the engine to warm up at a low power setting, typically 1,000–1,200 RPM, until oil temperature reaches the bottom of the normal operating range (the green arc) and oil pressure stabilizes within the normal range. Attempting a power check on a cold engine produces inaccurate results and risks shock-loading cold, tight components.
The Magneto Check
The magneto check is one of the most important steps in any post-maintenance run-up and is especially critical after work involving ignition components. To perform the check, increase engine RPM to the value specified by the manufacturer — commonly in the range of 1,700–1,800 RPM for many general aviation engines, though the exact value varies by engine model. With RPM stabilized, switch from BOTH to LEFT magneto only, note the RPM drop, return to BOTH, allow RPM to stabilize, then switch to RIGHT magneto only, note the drop, and return to BOTH.
The FAA and engine manufacturers establish limits for two key values:
- Maximum RPM drop on either single magneto: Typically no more than 125 RPM (exact limits vary by manufacturer — always consult the applicable manual).
- Maximum differential (spread) between left and right magneto drop: Typically no more than 50 RPM difference between the two drops.
A drop that exceeds the maximum limit suggests a timing problem, fouled plugs, defective ignition leads, or an internal magneto fault. A large differential between the two magnetos indicates that one side of the ignition system is weaker than the other, even if both are within absolute limits — this asymmetry can be a precursor to in-flight engine roughness. After any magneto maintenance, a check that shows zero RPM drop on a particular magneto is not a good sign; it often indicates that magneto is not actually switching off, suggesting a grounding lead (P-lead) fault. If either magneto continues to fire when switched to OFF, the magneto is considered unsafe because it can fire when the engine is being hand-turned on the ground.
Power Check and Engine Instrument Verification
After the magneto check, advance the throttle smoothly to full power (wide-open throttle) while monitoring the tachometer. For a fixed-pitch propeller installation, compare the static RPM achieved against the aircraft or engine manufacturer's published maximum static RPM. For a constant-speed propeller installation, set the propeller control to full fine (high RPM) and verify that the engine reaches rated takeoff RPM and that manifold pressure corresponds to ambient conditions minus any induction losses.
During the full-power check, scan all engine instruments simultaneously:
- Oil pressure: Must remain in the green arc throughout. A drop at high power can indicate an oil system restriction, pump wear, or an internal leak.
- Oil temperature: Should be in the normal operating range. Overheating at high power with the aircraft stationary is expected to a degree (ground cooling is less effective than in-flight cooling), but a rapid rise toward the red line demands investigation.
- Cylinder head temperature (CHT): Should not approach the red line during a brief ground run. Prolonged full-power ground operation must be avoided for this reason.
- Fuel pressure: Must remain within limits specified by the manufacturer. Low fuel pressure at high power can indicate a pump problem or a partially restricted fuel line.
- Exhaust Gas Temperature (EGT): Where equipped, check that EGT indications are reasonable and relatively balanced across cylinders, especially after cylinder work.
After verifying full-power readings, reduce power to idle and confirm that the engine idles smoothly at the manufacturer-specified idle RPM (often around 600–700 RPM for many light aircraft engines). A rough or unstable idle after carburetor or fuel system maintenance indicates the need for further adjustment. Finally, perform a throttle acceleration check — advancing from idle to full power briskly — to confirm there is no stumbling or hesitation, which can signal carburetor accelerator pump issues or fuel metering problems.
Leak Check After Run-Up
Immediately after shutdown while the engine is still warm, perform a thorough visual inspection for leaks. Oil, fuel, and exhaust leaks are far easier to detect on a warm engine. Check all oil line fittings, the oil filter, drain plugs, push rod housings, and the area around any cylinders that were disturbed. Inspect the entire induction system for fuel staining that could indicate a leaking fuel injector or carburetor fitting. Examine exhaust flanges and slip joints, since improper installation of exhaust components can cause exhaust leaks that are a carbon monoxide hazard and a fire risk.
Why This Matters for Safety and Airworthiness
14 CFR Part 43 requires that maintenance be performed in accordance with manufacturer's instructions and FAA-approved data, and the AMT signing off the return-to-service endorsement in the maintenance record is certifying that the aircraft is in airworthy condition. A skipped or abbreviated run-up undermines that certification. Statistically, the period immediately following maintenance is a period of elevated risk for mechanical problems — an engine that was reassembled with a subtle error may show no symptom during a quick idle but will reveal the problem under a proper power check. The run-up is the technician's last opportunity to catch that error on the ground rather than during flight.
Key Numbers and Rules
- Oil pressure must rise within 30 seconds of start (warm conditions); shut down if pressure does not appear within manufacturer's specified time limit.
- Magneto RPM drop: typically no more than 125 RPM on a single magneto (verify exact limit in the applicable engine manual).
- Magneto differential: typically no more than 50 RPM between left and right drops.
- Zero drop on a magneto during the check often means the P-lead is open (magneto not grounding properly — a safety hazard).
- All engine instrument readings must fall within the green arc (normal operating range) at the appropriate power setting.
- Leak inspection must be performed after the run-up while the engine is warm.
- Return to service is prohibited if any discrepancy found during the run-up remains unresolved.
Common Test Traps
- Zero magneto drop does not mean the magneto is perfect. Students often interpret zero drop as a passing grade, but it almost always means the magneto is not grounding — the P-lead is faulty — making the magneto live even when switched to ground (OFF). This is a dangerous condition.
- Confusing the two magneto limits. There is a maximum drop limit for each individual magneto AND a separate maximum differential between the two drops. An engine can pass the individual drop limit but fail the differential limit — both criteria must be met.
- Running the full-power check too long on the ground. Without airflow through the cowling, CHT rises rapidly. Sustained full-power ground operation can overheat cylinders and distort valves. Keep high-power ground runs as brief as practicable.
- Performing the run-up before the engine reaches operating temperature. Instrument readings taken on a cold engine are not valid for comparison against normal operating limits and can miss real problems that only appear at operating viscosity and pressure.
- Skipping the post-run-up leak inspection. Many leaks only manifest under operating pressure and temperature. A visual check before the run-up does not substitute for an inspection immediately after shutdown while the engine is warm.