Back to the Basics

Oct. 23, 2013
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 U.S. 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 five-event cycle, intake, compression, ignition, power, 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.

Crucial to understanding DA are the differences between pounds per square inch gauge pressure (psig), and pounds per square inch absolute pressure (psia).

One example of a differential pressure indication is an oil pressure gauge. The pressure of the oil system is that which is greater than atmospheric pressure. The MAP instrument is not a differential pressure gauge and provides the pilot a means of determining the actual density of the air in the engine's induction system. An engine equipped with an exhaust gas temperature (EGT) instrument, enables the operator a more accurate means for adjusting the fuel/air mixture ratio when leaning the mixture using the mixture control system.  

The A&P student is also introduced to the various mixture ratios used for engine operation. They learn that a 1:8 ratio is the richest mixture that will burn in a cylinder, and that a 1:18 ratio is the leaned mixture that can sustain flame propagation. Neither of these ratios is used for normal operation. Because an air-cooled engine has limitations at high power settings, the students learn that a 1:10 ratio – defined as takeoff mixture – is used for maximum power settings to provide additional internal cooling. Once the airplane achieves cruise speed, they learn that a 1:16 best economy ratio is used. A 1:12 ratio, best power, is used when the engine is equipped with an anti-detonation injection system. A stoichiometric mixture ratio is defined as chemically correct. This fuel/air mixture ratio, 1:15, is not commonly used due to the high temperatures created because all of the fuel and air is consumed during flame propagation in the cylinders assemblies.

Also included in the lessons are the reasons an aircraft reciprocating uses two spark plugs. They learn that using two magnetos for ignition provides a margin of safety should one system fail. Using two spark plugs ensures a more effective burning of the fuel/air change for increased power production.

Crucial to understanding the theory of operation for a gas piston engine centers upon the four strokes with an ignition event. The lessons in this area detail the Otto Cycle, used in the discussion of reciprocating engines. Students learn that the engines have a constant volume in the cylinders.

The students receive a detailed study about the intake, compression, power, and exhaust strokes used in reciprocating engines. Understanding the Otto Cycle and the events of a four-cycle engine are imperative for troubleshooting these engines. Simply stated, there are three items that the mechanic will use in troubleshooting: air, fuel, and ignition. A detailed communication with the pilot helps to determine if the engine needs repairs. Sometimes the pilot will report engine problems to the mechanic, but the error may exist in the manner by which the pilot conducts flight operations.

After learning the Otto Cycle, students will have a better understanding about turbine engine designs, which operate on the Brayton cycle – a constant-pressure engine. Both of these styles of engines are considered as air pumps.

The A&P student also learns about induction systems which could include the operation of super- and turbo-charger applications. Students learn that piston engine powered helicopters differ in operation than fixed wing versions. In a helicopter application all of the energy produced in the engine is needed to drive the helicopter’s transmissions, and they operate at the 1:10 mixture ratio continually, and do not have a cruise setting.

When written instructions aren’t enough

An integral part of basic reciprocal engine theory and training includes being able to read manuals and technical data.

The predominant engine manufacturers – Rotax, Lycoming, and Teledyne Continental Motors – continually refine their designs for increased reliability and safe operation. Each manufacturer, as mandated by the FAA, must provide written instructions for the inspection, troubleshooting, and repair of the engines.

Part 147 training is tied to regulations, and Part 43.13 tells us we must use everything from the manufacturer that is most current, including tools and instructions. Here at Redstone, one of our primary areas of focus is to get our students to be comfortable with reading, comprehending, and truly reliant on the manuals. Oftentimes, a new student doesn’t want to take the time to do this – it’s much more exciting to start digging in and taking things apart, but the foundation of all AMT expertise has to come from the ability to effectively use the manuals and instructions. This can be a challenge for some, because the manuals are often written in a passive voice, and can sometimes be left up to interpretation. Teaching students to understand and think analytically and critically about what they are reading will only stand to help them in every aspect of their career moving forward. Whether that is on the oral and practical exams, job interviews, or on the job, we see this as a critical skill.

Another reason we focus so specifically on the ability to read and understand written material is that it helps build confidence in the students. This is a highly regulated, high-stakes career with huge implications of a job not done right. Students can sometimes feel overwhelmed with the responsibility and pressure involved, and decide to leave the program. When we’ve focused heavily on comprehension and analytical skills, students feel more confident in their capabilities and are less likely to abandon this career path.

Back to the basics

It can be argued that the airline industry is the safest in the world. Those who accomplish the training and certification to become an AMT have a tremendous responsibility that provides great reward. We, as educators, have the responsibility to ensure that we are delivering the right training – not only the technical aspects, but the foundational and critical thinking skills as well – in order to hold our industry to the level of safety and standards we have today. Basic reciprocating engine theory is part of this foundational training, and we plan to continue to focus on this aspect as an important part of Redstone’s overall A&P program. The more thoroughly we can train tomorrow’s AMT, the safer we’ll all be in the sky. 

Steve Hankle has been an Airframe & Powerplant instructor at Redstone College in Denver for more than 12 years. After serving in the U.S. Navy, he graduated from Colorado Aero Tech in 1977 and spent 18 years as a helicopter maintenance technician at companies such as Airwest Helicopters, Four Corners Helicopters, and Pegasus Helicopters, Inc. He also spent five years as an instructor at Colorado Aero Tech. For more information visit