ENGINE OVERHAUL (OHL) AND REBUILT
Time between overhaul (abbreviated as TBO or TBOH) is the manufacturer’s recommended number of running hours or calendar time before an aircraft engine or other component requires overhaul.
On rotorcraft many components have recommended or mandatory TBOs, including main rotor blades, tail rotor blades and gearboxes.
For engines the time between overhauls is generally a function of the complexity of the engine and how it is used. Piston-based engines are much more complex than turbine-powered engines, and generally have TBOs on the order of 1,200 to 2,000 hours of running time. They tend toward the lower number if they are new designs, or include boosting options like a turbocharger. In comparison, jet engines and turboprops often have TBOs on the order of 3,000 to 5,000 hours.
Since overhauling needs the engine to be taken apart, it is typically expensive. The value of a used engine decreases if it is close to needing an overhaul, so used engines (and aircraft) typically list their time since overhaul or TSOH.
The TBO is a time ‘recommended’ by the manufacturer and, depending upon what rules the aircraft operates under, overhauling the engine at this time is not necessarily mandatory. For aircraft used non-commercially overhauls are not mandatory, but highly recommended. Likewise, overhaul at the recommended TBO does not guarantee that the engine will last that long.
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Commonly Used Engines
Lycoming Aero Engines
Lycoming is a major American manufacturer of aircraft engines. Headquartered in Williamsport, Pennsylvania, Lycoming produces a line of horizontally opposed, air-cooled, four-, six- and eight-cylinder engines including the only FAA-certified aerobatic and helicopter piston engines on the market.
The company has built more than 325,000 piston aircraft engines and powers more than half the world’s general aviation fleet, both rotary and fixed wing. The company is currently part of Textron’s Avco Corporation.
Continental Aero Engines
Continental Motors, Inc. is an aircraft engine manufacturer located at the Brookley Aeroplex in Mobile, Alabama, United States. It was originally spun off from automobile engine manufacturer Continental Motors Company in 1929 and owned by Teledyne Technologies until December 2010.
The company is now part of Aviation Industry Corporation of China, which is owned by the government of the People’s Republic of China.
Williams Aero Engines
The Williams FJ44 is a family of small, two-spool, turbofan engines produced by Williams International/Rolls-Royce for the light business jet market. Until the recent boom in the very light jet market, the FJ44 was one of the smallest turbofans available for civilian applications.
Although basically a Williams design, Rolls-Royce was brought into the project, at an early stage, to design, develop and manufacture an air-cooled high-pressure (HP) turbine for the engine. The FJ44 first flew on July 12, 1988 on the Scaled Composites/Beechcraft Triumph aircraft.
The Williams FJ33 is a smaller engine based on the basic FJ44 design.
Rotax Aero Engines
Rotax supplies aircraft engines for ultralight aircraft, light aircraft and unmanned aerial vehicles. Rotax engines designed specifically for light aircraft include both four-stroke and two-stroke models.
Current models are:
Rotax 912 series, four-stroke
Rotax 914 series, four-stroke
Rotax 915 series, four-stroke
Rotax 582 UL, two-stroke
The Rotax 914 is a turbo-charged, four-stroke, four-cylinder, horizontally opposed aircraft engine with air-cooled cylinders and water-cooled cylinder heads. It is designed and built by the Austrian company BRP-Powertrain, owned by BRP, as part of its Rotax brand. The engine commonly powers certified light aircraft, homebuilt aircraft, autogyros and military UAVs such as the MQ-1 Predator.
Pratt & Whitney Canada PT6
The Pratt & Whitney Canada PT6, produced by Pratt & Whitney Canada, is one of the most popular turboprop aircraft engines in history. The PT6 family is known for its reliability with an in-flight shutdown rate of 1 per 333000 hours since 1963, 1 per 651,126 hours over 12 months in 2016. Time between overhauls are between 3600 and 9000 hours and hot section inspections between 1800 and 2000 hrs.
In US military use, they are designated as T74 or T101. The main variant, the PT6A, is available in a wide variety of models, covering the power range between 580 and 920 shaft horsepower in the original series, and up to 1,940 shp (1,450 kW) in the “large” lines. The PT6B and PT6C are turboshaft variants for helicopters.
Garrett AiResearch Aero Engines
Garrett AiResearch was a manufacturer of turboprop engines and turbochargers, and a pioneer in numerous aerospace technologies. It was previously known as Aircraft Tool and Supply Company, Garrett Supply Company, AiResearch Manufacturing Company, or simply AiResearch.
In 1964, Garrett AiResearch merged with Signal Oil & Gas to form a company renamed in 1968 to Signal Companies, which in 1985 merged with Allied Corp. into AlliedSignal. In 1999 AlliedSignal acquired Honeywell and adopted the Honeywell name.
The Garrett TFE731 (now Honeywell TFE731) is a family of geared turbofan engines commonly used on business jet aircraft. Garrett AiResearch originally designed and built the engine, which due to mergers was later produced by AlliedSignal and now Honeywell Aerospace.
Since the engine was introduced in 1972, over 11,000 engines have been built, flying over 100 million flight-hours.
General Electric Aero Engines
GE Aviation, a subsidiary of General Electric, is headquartered in Evendale, Ohio, outside Cincinnati. GE Aviation is among the top aircraft engine suppliers, and offers engines for the majority of commercial aircraft.
GE Aviation is part of the General Electric conglomerate, which is one of the world’s largest corporations. The division operated under the name of General Electric Aircraft Engines (GEAE) until September 2005. GE Aviation’s main competitors in the engine market are Rolls-Royce and Pratt & Whitney. GE operates two joint ventures with Snecma of France, CFM International and CFM Materials.