Back to the Basics

Reciprocating engine theory and operation is essential in AMT training

Looking back 40 years, I am thankful for having had the opportunity to work on aircraft that used large radial engines. My introduction to the R-2800 radial engine was during my enlistment in the Navy. It was not until my enrollment at an aircraft mechanic school that I was actually schooled in the theory and operation of reciprocating engines. General aviation, the sport pilot category, and unmanned vehicles all require technicians who are able to troubleshoot and repair reciprocating engines.

According to the General Aviation Manufacturers Association (GAMA) there are an estimated 157,123 aircraft powered by reciprocating engines certified by the FAA in the United States — from sport pilot, to homebuilt, to old War Birds still flying.

At the same time, many in the aviation industry predict a shortage of skilled AMTs due to the retirement of the Vietnam generation — the very technicians who were focused on reciprocating engines as the mainstay of their training. There are an estimated 5,000 airports in the United States available for general aviation flight operations, and this number is growing. These factors illustrate the need for trained technicians to fill the void.

As an instructor at Redstone College, I see students come through this facility who want to focus on the new technology — the leading-edge technology — which is great. But we stress the importance of learning basic reciprocating engine theory as an essential piece of training for today’s AMT. With the number of reciprocating engines still in operation, this training is critical, and also helps build a foundation for training on some of the more modern technology.


Reciprocating engine theory: the foundation for inspections, troubleshooting, and repair

Training today’s AMT on reciprocating engine theory and operations is necessary to maintain the fleet of aircraft still using these engines, which operate using the same theory as the more common opposed engine designs.

This style of engine has been used since the Wright brothers’ first flight. Engine design prior to and during the early stages of World War I were quite rudimentary as compared to the later designs developed prior to and during World War II.

In our Airframe and Powerplant (A&P) program at Redstone College, we start students out with basic physics, covering Newton’s Laws of Physics and other foundational theories. We then move on to Theory of Operation, starting out with basics such as work power, horsepower, force, etc. to set the stage for basic engine theory, starting with the Otto Cycle. The Otto Cycle is a four-event cycle, intake, compression, ignition, and exhaust. It is the most popular style.

The current day A&P student must be able to fully understand in detail the intake, compression, power, and exhaust stroke for engine theory of operation.


Understanding the relationship between air density and power stroke

Reciprocating engine theory is an excellent introduction to turbine engine theory. Part 147 school students are tasked with understanding cooling, induction, fuel metering, ignition, and exhaust systems as applicable to reciprocating engine operation.

A&P students must learn about the relationship between engine power production and the density of the air. All aspects of flight are determined by the amount of air available for wings to generate lift, and engines to develop power. Students learn that atmospheric pressure, altitude, barometric pressure, temperature, and humidity all determine the density of the air.

It is equally important that the students understand the proper procedures when leaning the fuel and air mixture ratios at altitude. As the training advances, the lessons explain the difference between a naturally aspirated engine, and a mechanically aspirated engine. In addition, the students learn that constant speed propellers and supercharged or turbocharged engines require additional instrumentation. These designs require a manifold absolute pressure (MAP) gauge. During these lessons the students learn about density altitude (DA), both high- and low-density altitude. Low DA enables the engine to produce more power, and then the airfoil surfaces can generate more lift. The opposite occurs during operations that have high DA.

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