This executive summary is provided by Ronald J. Weaver President of AVStar Fuel Systems in response to the FAA Airworthiness Concern Sheet issued on December 16, 2025, and is intended to place that notice within the proper historical and technical context for both Cessna 172R and 172S aircraft and Piper Archer III, Pilot 100i equipped with the Lycoming IO-360-L2A engine.

Although we are focusing primarily on the Cessna 172R and 172S aircraft, the fuel vapor issue also applies to Piper.  Piper Aircraft offers a fuel return kit (see Service Spares Letter number 504) for high operation temperatures (Piper Aircraft part numbers 88679-706 and -707) to return fuel to tank. Since 2022, Piper Archer III and the Pilot 100i, new aircraft being produced have the return to tank fuel system.

Since the introduction of the Cessna 172R and 172S equipped with the Lycoming IO-360-L2A engine, manufacturers and regulators have published extensive guidance addressing engine operation at low power and idle, particularly during hot ambient conditions and extended ground operations. These characteristics have been well recognized and documented within the Cessna 172R and 172S fleets for decades.

The historical record reflects a measured, multiyear progression of operational guidance, configuration refinements, and regulatory actions intended to address idle sensitivity and fuel vapor formation. Importantly, these publications consistently describe the behavior as a function of operating environment and conditions, not the result of a specific fuel system component or manufacturer.

As documented in the service publications, fuel injected engines operating at low power, low fuel flow, and elevated temperatures can be more susceptible to fuel vapor formation. When fuel vapor forms, it can result in vapor entrapment within the fuel stream. This vapor entrapment can momentarily influence how the fuel metering regulator responds, resulting in transient fuel delivery disturbances. This behavior occurs regardless of fuel servo manufacturer and is inherent to the operating environment rather than a defect in the fuel servo itself.

This susceptibility increases during extended ground operations, prolonged idling, and certain flight training profiles common to the Cessna 172R and 172S. Recognizing these operating environments led to the development of specific pilot procedures, idle and mixture verification requirements, and system enhancements aimed at improving consistency during low power operation.

These actions were implemented across the fleet as part of a broader industry response and were well established years before AVStar Fuel Systems became a manufacturer of aircraft fuel systems.

For owners who wish to review the complete historical documentation including all referenced service bulletins, service instructions, FAA publications, and supporting materials, a consolidated compilation is available at the link below:

Engine Idle & Fuel Vapor in Cessna 172R/172S Historical Compilation of Service Publications

*Photo courtesy of Jack Fleetwood (www.JackFleetwood.com).