What does TPE331 engine rigging mean, and does it mean the same thing to pilots and mechanics?
Pilots describe what they see and feel in flight, while mechanics translate those symptoms into a technical problem. A pilot may report that an engine is “not making power,” while maintenance may determine that the engine is actually fuel limited.
Operators may use “rigging” in different ways. Some mean the physical geometry of rods and levers connecting engine components to one another and the aircraft to the engine. Others use it more broadly to describe aircraft performance, engine indications, and lever symmetry.
Turbo Commanders can add another layer of confusion because 690A and 690B models may use either Bendix or Woodward fuel control systems. Operational characteristics are similar, but corrective action at the FCU can be very different.
The following terms can help clarify communication between pilots and mechanics.
Fuel Limited
The engine cannot reach rated torque or its temperature limit with full power lever travel because it is not receiving enough fuel flow. This is different from an engine that is truly unable to make rated power.
Torque Limited
The engine has reached maximum rated horsepower. Fuel and temperature margin may remain, but torque limits prevent additional power. This is common in colder conditions.
Temperature Limited
The engine has reached its rated temperature limit. Advancing the power lever further would overtemp the engine. If minimum takeoff power has not been reached, stop.
Acceptable Rated Takeoff Power: Indicated vs. Actual
Rated takeoff power is the minimum required power for the day, based on ambient conditions and POH limits. The horsepower indication is not always correct, so suspected indication discrepancies should first be checked against the engine’s DSC data in the logbook. True horsepower output versus indicated horsepower must then be confirmed through torque calibration using the Lebow.
Power Lever Symmetry
The first question is where the lever split occurs: takeoff, cruise, landing, or reverse. That tells maintenance where to begin.
Some FCU adjustments affect others, so the order of adjustment matters. The location of the split also helps determine whether the correction is static or dynamic.
For example, if the aircraft yaws on short final or during flare, the issue may require a dynamic adjustment, such as flight idle fuel flow, or a static rigging correction, such as verifying that the power lever attachment at the engine is set to the required 40 degrees. This is why accurate wording matters when describing the problem.
These terms are only the starting point. High-utilization operators, especially those with multiple aircraft, often work through rigging issues by necessity and repetition. Lower-utilization Part 91 pilots may live with the same symptoms longer because the aircraft has gradually taught them what feels “normal.” That may hold until a sale, checkride, 150-hour inspection, or handling issue brings the problem into view.
If pilots and mechanics can agree on a common language for faults and defects, they have a better starting point for deciding what is acceptable for safety of flight and ease of operation. Not everyone needs to know how to replace a turbine wheel, but everyone involved should understand the basic terms well enough to communicate clearly.
Thirty years ago, roughly 6,000 aircraft operated with TPE331 engines, and as many as 50 engine shops had technicians with deep experience on the motor. Today, the aircraft are older, many experienced technicians have retired, and fewer mechanics have regular exposure to the engine. That is why precise language matters.
Mark Richardson
Managing Director
Time Flies Aviation
35 years of TPE331 experience
