Engine accessory section diaphragm

This regulation requires air-cooled engines on Part 121 aircraft to have a diaphragm (barrier) that separates the engine's power section and exhaust system from the accessory compartment. This diaphragm must meet the fire-resistant material standards specified in § 121.247.

The purpose is fire protection. If a fire starts in the engine's power section or exhaust system, the diaphragm prevents it from spreading to the accessory compartment, which contains critical components like fuel pumps, hydraulic pumps, and electrical generators. Without this barrier, an engine fire could quickly damage these accessories and create additional hazards.

The regulation allows an exception: you can use "other means" to provide equivalent protection instead of a diaphragm, but you must demonstrate that the alternative provides the same level of fire isolation. In practice, most air-cooled engines use the required diaphragm as the standard solution.

*This is a plain-English summary for study only. The official 14 CFR text on this page is controlling — always read the current regulation and consult a CFI.*