Training Tip: The power of preparation

The trainees have a lot in common—for example, both have taken their training in 160-horsepower Cessna 172s—but there are differences between their aircraft, as they discover as they thumb through their pilot’s operating handbooks. One student pilot has been training in a 1985 Cessna 172P that is powered by a 160-hp carbureted Lycoming O-320 engine. The other student, who flies a later-model Cessna 172R, checks the powerplant (engine) description and notes that it is a 160-hp fuel-injected Lycoming O-360 engine.

Digging into the matter further, they read that the engine in the Cessna 172R is “derated” from 180 hp to produce 160 hp at 2,400 rpm in the 1990s-era Skyhawk.

Examining the meaning of “derated,” the students find a definition: “A derated engine is typically a piston engine that is limited by the manufacturer to develop less power than it was originally designed to produce.”

Can you think of reasons why a manufacturer would derate an engine, and how an engine’s derating is accomplished?

For the why of derating, one reason might be to extend the engine’s TBO (time before overhaul—another term to learn!) as Robinson Helicopter Co. did with the big Lycoming O-540 engine in its R44 Cadet model. Here’s another example: An engine that could be rated at 210 hp is used as standard equipment on a 1970s-era Cessna Skyhawk-family airplane, the Hawk XP, rated at 195 hp. The lower horsepower rating means a Hawk XP pilot does not need extra training as required to fly an airplane of more than 200 hp.

As for the how of derating, when the Cessna 172R debuted, AOPA reported that derating “was a simple affair—Cessna simply bolted a higher-pitch, 75-inch-diameter prop on the 172R, which redlines the engine at 2,400 rpm.”

Note that changing an engine’s horsepower rating isn’t a one-way street. To restore the 172R engine to 180 hp, Cessna simply had to change the prop “to a size and pitch that allows the engine to spin up to 2,700 rpm,” the article said.