In-Flight Test Procedures - Image

Regular In-Flight Test Procedures You Should Perform

Implement These In-Flight Test Procedures to Ensure Your Aircraft Engine is in Top Shape.

In-flight test procedures are an excellent way to verify if your aircraft engine is functioning optimally. The best test results are typically achieved when you use an engine monitor unit set to the highest sample rate possible. While it’s possible to perform these in-flight test procedures without an engine monitor unit, it can be more challenging and you may not obtain the high-fidelity data that you’re seeking. We recommend upgrading to a reliable engine monitor unit early in your aircraft ownership journey if your plane isn’t equipped with one yet.

Rotax Engines

These in-flight test procedures can also be performed on Rotax engines, with the exception of the GAMI lean test. This is because Rotax engines are modern, state-of-the-art internal combustion engines that don’t require manual leaning.

Preparation and Execution

Though not particularly difficult, in-flight test procedures may be unfamiliar to many pilots, so it’s important to take your time. Consider bringing along a second pilot to assist with the procedures. Always ensure a comprehensive pre-flight briefing covering all the procedures is conducted on the ground. Also, clearly designate the pilot-in-command to prevent any misunderstandings.

Mixture Distribution Test

It’s advisable to carry out the mixture distribution test every twelve months or after every 100 hours of flight time, and anytime you suspect any irregularities with the engine. The purpose of this in-flight test procedure is to examine the mixture distribution and detect issues like dirty or (partially) clogged fuel nozzles, improperly-sized fuel nozzles, intake valve problems, induction leaks, and other engine anomalies that can cause uneven mixtures among cylinders. It also checks the requirements for GAMIjector® fuel injectors.

General Aviation Modifications, Inc.

Our partner, General Aviation Modifications, Inc. (GAMI), has long promoted the mixture distribution test, which is why it’s often referred to as the GAMI lean test. This test quantifies the balance of the fuel/air ratio in the engine, a value known as the GAMI Spread and measured in gallons-per-hour (GPH). The GAMI Spread is calculated by determining the total engine fuel flow at which each cylinder reaches peak exhaust gas temperature (EGT), then subtracting the lowest flow rate from the highest.

GAMI Lean Test

While implementing these in-flight test procedures, GAMI strongly recommends the presence of a safety pilot. GAMI also suggests keeping the cowl flaps open, if so equipped, and conducting the test at 65% power. It may be necessary to use a lower power setting to keep the cylinder head temperatures (CHT) below 400°F and/or the turbine inlet temperature (TIT) beneath your max continuous redline (Note: All turbochargers permit short-term exceedance of the TIT limit for leaning purposes – usually one minute or less).

GAMI Lean Test Methods

There are three effective methods for performing the GAMI lean test. The following methods assume you have some means to measure the EGT on each cylinder and the total engine fuel flow (this can be digital or analog). We recommend the download method with the shorthand method as an alternative.

    • Download Method
    • Shorthand Method
    • Longhand Method

Perform the GAMI lean test procedure by setting up the airplane in normal cruise – utilise your autopilot if available.

IMPORTANT: Cruise power setting for this test should be 65% or less at wide open throttle (WOT) to avoid excessive CHTs and stress on the cylinder assemblies.

GAMI Lean Test - Download Method

Implement the following procedure (JPI users – we obtain better data if you’re not in Lean Find mode):

Setting Up for the Test
  • For normally aspirated engines: Climb to an altitude that allows 65% power or less at WOT.
  • For turbo engines: After selecting a manifold pressure (MAP) and RPM representing 65% power or less, climb to the altitude where ambient air pressure nearly equals your chosen MAP. (Assume ambient sea level pressure of ~29.92 drops 1” for every 1000 ft of altitude.)
  • Begin with a rich mixture of at least 100°F rich of peak (ROP).
Performing the Mixture Control Sweep
  • Retard the mixture control as slowly and steadily as possible until the onset of roughness. Try to do this slowly enough that it takes no less than 3-4 minutes to transition from the rich mixture to the lean one. The slower and smoother you move the mixture control, the better. Note the fuel flow in GPH or PPH. For turbos that do not have automatic waste gates, strive to maintain your target MAP as you lean the mixture by adjusting MAP as necessary.
  • Reverse the process, enriching very slowly until you get back to about 100°F ROP. Then reverse again and lean to your previously noted GPH or PPH lean point. Again, attempt to do this slowly enough that it takes 3-4 minutes to perform the sweep while maintaining your target MAP.
  • Repeat this rich-to-lean-to-rich mixture sweep procedure, preferably three times. (This allows us to obtain an average of several sweeps, providing more accurate information than one sweep alone and enabling us to discard sweeps with poor data e.g., moving too quickly through peak.)

NOTE: There’s no added value in leaning beyond the onset of roughness to the point of engine stumble or flame-out.

Transitioning to In-Flight Lean of Peak Magnetos Check
  • On your last rich-to-lean sweep, while still on the lean side, don’t simply return to a cruise ROP or lean of peak (LOP) cruise setting, but instead transition to the in-flight lean of peak magnetos check before making any changes. (The reason for this is we want to precisely determine how LOP the engine is when the in-flight lean of peak magnetos check is performed.)

GAMI Lean Test - Shorthand Method

Most engine monitors allow you to view all your EGTs simultaneously in a graphical format as individual columns. As you gradually adjust the mixture from rich to lean, those columns will rise as each individual EGT approaches its peak. The columns will then fall as each cylinder reaches its peak EGT and becomes lean of peak. Some monitors simplify determining peak EGT by inverting the EGT columns, making them flash, or changing the bar colour when a particular cylinder peaks. Implement the following procedure:

  • Begin with a full-rich mixture and record the exact total engine fuel flow (to the nearest 0.1 GPH or 1 PPH) and EGT for each cylinder.
  • Lean very slowly until the first cylinder reaches peak EGT. Note which cylinder has peaked, the exact total engine fuel flow, and peak EGT for that cylinder.
  • Continue in this manner until you’ve recorded the total engine fuel flow and peak EGT of each cylinder in the order they reached peak.
  • Reverse the process, enriching the mixture very slowly and note which cylinder has peaked, the exact total engine fuel flow, and peak EGT for that cylinder.
  • Repeat this rich-to-lean-to-rich mixture sweep procedure multiple times to ensure reliable results.

GAMI Lean Test - Longhand Method

This method is more comprehensive but more time-consuming than the shorthand method. With this method, you record each EGT (and optionally each CHT) at small increments of fuel flow adjustment from around 2 GPH rich of the first peak EGT to a point lean of the last peak EGT. Aim for these lines of data to be in the smallest practical fuel flow increments – 0.2 to 0.3 GPH works well. Also, try to use the smallest EGT resolution possible. For many monitors, this means 1°F, though for some monitors 5°F or 10°F is the smallest. An 1°C resolution works fine, too.

The monitor’s “lean find” or “lean assist” function may be necessary to find the 2 GPH rich of peak starting point. The reason for starting 2 GPH rich of peak is to avoid either collecting more data than necessary or starting the data collection too late to capture sufficient information.

NOTE: Do not lean each cylinder individually. Start at a fuel flow where all EGTs are ROP, and lean in small increments until all EGTs are LOP, recording the EGT of all cylinders at each increment.

GAMI Lean Test - Results

You can calculate the GAMI Spread and Lean Range once you have completed the in-flight test procedures.

    • GAMI Spread: Total Engine Fuel Flow (Last cylinder to reach peak EGT) – Total Engine Fuel Flow (First cylinder to reach peak EGT) in GPH
    • Lean Range: Full-rich EGT – peak EGT in °F (for each cylinder)

Typically, the GAMI Spread must be under 1 GPH for the engine to operate smoothly when lean-of-peak (LOP). Most engines are considered to have a good fuel/air ratio balance if the GAMI Spread is less than 0.5 GPH.

Upgrade to GAMIjector® Fuel Injectors

We recommend installing GAMIjector® or TurboGAMIjector® when the GAMI Spread is more than 1 GPH and other issues like dirty or (partially) clogged fuel nozzles, improperly-sized fuel nozzles, intake valve problems, or induction leaks have been ruled out.

Fine-Tune Your GAMIjector® Fuel Injectors

Additionally, we can fine-tune your GAMIjector® or TurboGAMIjector® when the GAMI Spread is more than 0.5 GPH (0.7 GPH for Continental engines) and other issues like dirty or (partially) clogged fuel nozzles, improperly-sized fuel nozzles, intake valve problems, or induction leaks have been ruled out.

Observe Your Lean Range

The lean range of each cylinder should typically be around 250 to 300°F. If any cylinder has a substantially lower lean range than the others, it could indicate a clogged fuel nozzle or induction leak. This in turn could result in excessively lean operation at take-off and potentially lead to overheating or detonation.

Ignition Stress Test

Regularly performing the ignition stress test (preferably every second or third flight) and any time you suspect anomalies with the engine is advisable. The purpose of this in-flight test procedure is to evaluate your ignition system under demanding conditions, in contrast to the pre-take-off magnetos check done on the ground. As a lean mixture is more challenging to ignite than a rich one, an in-flight lean of peak magnetos check truly evaluates your ignition system’s performance.

In-Flight Lean Of Peak Magnetos Check

The in-flight lean of peak magnetos check is a diagnostic tool for your ignition system, used to identify potential issues with magnetos, ignition harnesses, spark plugs, or ignition timing. A lean mixture can reveal a weak plug that might be concealed in a richer mixture. Our objective is to select a mixture that will uncover weak sparks without causing them.

To perform the in-flight lean of peak magnetos check, set up the airplane in normal cruise mode – using autopilot if available. Follow the steps below:

Setting Up for the Test
  • Start by conducting at least one GAMI Lean sweep as outlined above from ROP to LOP. Alternatively, at the end of your multiple GAMI Lean sweeps from ROP to LOP, continue to lean to 50º LOP if possible, or to the onset of roughness. If you have leaned to the point of initial roughness, slowly enrich just enough to revert to a smoothly running engine. (We do not recommend exceeding 50º LOP as this is the maximum LOP you should ever need to run, and there’s no value in maintaining an unnecessarily lean operation.)
  • Activate the “normalise mode” on your engine monitor. This sets all the EGT bars to mid-scale and heightens the sensitivity of the bar-graph display.
Perform Magneto Switch Cycling
  • If your aircraft has a key or rotary style magneto switch (found in most singles), cycle through BOTH-LEFT-BOTH-RIGHT-BOTH, letting the mag switch remain in each of these positions for at least 10 engine monitor sample times or a minimum of 30 seconds, including in the BOTH position. (If your sample interval is 6 seconds, that will be a full minute; if your sample interval is 1 second, then adhere to the minimum of 30 seconds.)
  • If your aircraft has individual magneto toggle switches (common in most twins), follow the same procedure but with individual magneto switches.

NOTE: It’s normal for engines to run slightly rougher on one magneto than on two, but the roughness should not be excessively uncomfortable. Please report the level of perceived roughness during each phase of the test. (e.g. smooth, moderate, severe).

NOTE: If the engine stalls while switching to single mag operation, reduce the mixture to idle cut off, switch to the other mag, then slowly increase the mixture until the engine restarts.

NOTE: For turbocharged engines, TIT may rise to or exceed the red-line during single-magneto operation. This is typical and not harmful for the short duration of the flight test.

NOTE: RAM engines or Experimental engines with uneven or split timed magneto timing and unconventional spark plug wiring may respond differently. If your engine is a RAM engine or has uneven mag timing, please provide relevant details.

NOTE: For engines with Electronic Ignition, follow the same LEFT-BOTH-RIGHT order, but remember to inform us which magneto is conventional and which one is electronic, the type of electronic mag, and any unconventional wiring details.

In-Flight Lean Of Peak Magnetos Check - Results

A healthy ignition system should cause all EGT bars to rise by 50 to 100°F when you switch to single-magneto operation.

The rise may not be uniform; it’s perfectly normal for even-numbered cylinders to rise more than odd-numbered cylinders, and vice versa. What’s important is that all EGT bars rise and remain stable at their elevated levels.

During single-magneto operation, you may notice a slight loss of power and an increase in roughness. However, this roughness should not be alarming.

Induction Leak Test

You should perform the induction leak test whenever you suspect a MAP or mixture distribution anomaly. The purpose of the induction leak test is to examine your induction system for leaks. The most suitable scenario for this test is level cruise flight at approximately 5000 feet MSL.

Induction Leak Test

This test comprises two parts: a high-MAP test and a low-MAP test.

  • For the high-MAP test, begin with relatively high power settings – wide-open throttle for normally aspirated engines, or MAP equal to outside ambient pressure for turbocharged engines – and a full-rich mixture. (Assume that the ambient sea level pressure of approximately 29.92 drops 1” with every 1000 MSL of altitude.) Record the EGT for each cylinder.
  • For the low-MAP test, decrease MP by about 10 inches and once again record the EGT for each cylinder.

Ignore the absolute EGT values. Instead, calculate the change in EGT (“delta”) for each cylinder between the high-MAP and low-MAP tests. Ideally, the amount of EGT change should be roughly the same for all cylinders. If one cylinder (or two adjacent cylinders) shows significantly less change than the others, this may indicate an induction system leak affecting that cylinder (or those adjacent cylinders).

Induction Leak Test - Results

Here’s the principle behind this test: During the high-MAP test, the induction manifold pressure is very close to outside ambient pressure, meaning any induction leak will have minimal or no impact on engine operation. However, during the low-MAP test, the manifold pressure is considerably lower than the outside ambient (by about 10 inches), so any induction leak will cause the affected cylinder(s) to run significantly leaner than the others, resulting in a smaller drop in EGT than the others.


While the in-flight test procedures may initially appear complex and challenging, once you have gone through them once, their significance becomes apparent. They aren’t as difficult as they might seem and the benefits they offer in terms of ensuring the health and efficiency of your aircraft engine are invaluable.

Regular Testing for Optimal Performance

By regularly conducting tests such as the mixture distribution test, ignition stress test, and induction leak test, you can identify and address potential issues before they escalate. These tests allow you to keep a close eye on your engine’s performance, ensuring it operates at peak efficiency.

Consider GAMIjector® for Fuel/Air Mixture Balance

If your GAMI spread is not optimal, consider implementing GAMIjector®. These precision fuel injectors can balance the fuel/air mixture across all cylinders, promoting even combustion, enhancing engine performance, and potentially extending engine life.

Benefits of Operating Lean of Peak

Once your aircraft engine is running smoothly, you can start operating lean of peak. This operation is not only more efficient in terms of fuel consumption but also contributes significantly to the longevity of your engine. Operating lean of peak can reduce wear and tear on the engine, extending its useful life and saving you significant maintenance costs in the long run.

The Significance of In-Flight Testing

In conclusion, these in-flight test procedures are an integral part of your aircraft’s maintenance routine. Despite their apparent complexity, the understanding and insight they provide into the health of your engine make them an essential tool in the hands of any serious aircraft owner or operator. Stay diligent with these tests and your aircraft will reward you with reliable service and excellent performance.

Downloadable Test Procedures and Test Cards

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About Quest Aeronautics

Quest Aeronautics is a state-certified engineering office for aviation, dedicated to shaping the future of general aviation by providing innovative and cost-effective solutions to enhance aircraft performance and operations. With a focus on CS/FAR-23 and experimental/amateur-built (E/A-B) aircraft, Quest Aeronautics provides a range of services including flight testing, aircraft operations and maintenance consulting, high-quality aviation products, and tailored support for E/A-B projects. Collaborating with industry-leading partners, Quest Aeronautics is committed to delivering unparalleled support and expertise to individuals and organisations in the general aviation market.

About Author

Sebastian, the founder of Quest Aeronautics, is a driven and enthusiastic individual with a passion for aviation. Before delving into aviation, he gained valuable experience as a chemical process engineer and laboratory technician. Sebastian holds a Master of Science in Engineering and a commercial pilot licence, with several fixed-wing aircraft ratings under his belt. He has also completed an introduction course for fixed-wing performance and flying qualities flight testing at the National Test Pilot School in Mojave, CA and is compliance verification engineer for flight.