Installed on most complex aircraft engines, a propeller governor is a device used to regulate engine revolutions per minute (RPM) by adjusting a propeller's blade angle. The RPM, which usually ranges between 2,000 and 2,500 on most modern piston-engine aircraft, is set manually by the pilot via a control in the cockpit. Through the use of the aircraft's oil supply, the propeller governor uses an internal piston to apply oil pressure to the aircraft's propeller, which adjusts the blade's pitch. Adjusting the pitch of an aircraft's propeller blade allows the pilot to maximize an engine's performance in different phases of flight, such as takeoff, climb, cruise and descent.
When an aircraft is taking off, it is in the pilot's best interest to maximize the aircraft's performance to shorten the length of runway necessary for the takeoff roll. The checklist performed just before takeoff requires a pilot to put the RPM control in the maximum position. A propeller governor's job is to adjust the propeller's blade angle to displace the maximum amount of air per rotation, therefore increasing the engine's efficiency. After takeoff, a high RPM setting will allow the pilot to gain the maximum amount of altitude in the shortest distance, aiding in the aircraft's avoidance of obstacles.
With a high RPM setting, however, the engine must work harder and burn more fuel. Obviously, this is a negative consequence for the pilot once he gets to the cruise portion of his flight. Unless he is in a rush and only covering a short distance, the pilot will need to lower the aircraft's RPM. When he brings back the RPM setting, the propeller governor lowers the propeller's blade angle, which decreases the load on the engine. In response, engine fuel burn decreases. The pilot consults his operating manual and finds the optimum RPM setting, allowing him to travel the maximum distance with the minimum amount of fuel burn.
Propeller governors are only found on complex aircraft engines. Basic, less expensive aircraft engines without a propeller governor are unable to adjust a blade's angle, resulting in lost efficiency. The downside to an aircraft equipped with a propeller governor is that it involves more moving parts, which is an increased opportunity for failure along with increased maintenance costs. Operation of an aircraft with a propeller governor also requires additional training for the pilot and a complex endorsement issued by a properly qualified flight instructor.