attitudes and in extreme weather conditions. Standards  of powerplant reliability are agreed upon by the Federal  Aviation Administration (FAA), the engine manufacturer,  and the airframe manufacturer. The engine manufacturer  ensures the reliability of the product by design, research,  and testing. Close control of manufacturing and assembly  procedures is maintained, and each engine is tested before  it leaves the factory. It is necessary to calculate the horsepower for each speed of an  aircraft, since the horsepower varies with speed. Therefore, it  is not practical to try to rate or compare the output of a turbine  engine on a horsepower basis. The aircraft engine operates  at a relatively high percentage of its maximum power output  throughout its service life. The aircraft engine is at full power  output whenever a takeoff is made. It may hold this power  for a period of time up to the limits set by the manufacturer.  The engine is seldom held at a maximum power for more than  2 minutes, and usually not that long. Within a few seconds  after lift-off, the power is reduced to a power that is used for  climbing and that can be maintained for longer periods of  time. After the aircraft has climbed to cruising altitude, the  power of the engine(s) is further reduced to a cruise power  which can be maintained for the duration of the flight. Durability is the amount of engine life obtained while  maintaining the desired reliability. The fact that an engine  has successfully completed its type or proof test indicates  that it can be operated in a normal manner over a long period  before requiring overhaul. However, no definite time interval  between overhauls is specified or implied in the engine  rating. The time between overhauls (TBO) varies with the  operating conditions, such as engine temperatures, amount  of time the engine is operated at high-power settings, and the  maintenance received. Recommended TBOs are specified by  the engine manufacturer. If the weight of an engine per brake horsepower (called  the specific weight of the engine) is decreased, the useful  load that an aircraft can carry and the performance of the  aircraft obviously are increased. Every excess pound of  weight carried by an aircraft engine reduces its performance.  Tremendous improvement in reducing the weight of the  aircraft engine through improved design and metallurgy  has resulted in reciprocating engines with a much improved  power-to-weight ratio (specific weight). Reliability and durability are built into the engine by the  manufacturer, but the continued reliability of the engine is  determined by the maintenance, overhaul, and operating  personnel. Careful maintenance and overhaul methods,  thorough periodical and preflight inspections, and strict  observance of the operating limits established by the engine  manufacturer make engine failure a rare occurrence. The basic parameter for describing the fuel economy of  aircraft engines is usually specific fuel consumption. Specific  fuel consumption for gas turbines is the fuel flow measured  in (lb/hr) divided by thrust (lb), and for reciprocating engines  the fuel flow (lb/hr) divided by brake horsepower. These are  called thrust-specific fuel consumption and brake-specific  fuel consumption, respectively. Equivalent specific fuel  consumption is used for the turboprop engine and is the fuel  flow in pounds per hour divided by a turboprop’s equivalent  shaft horsepower. Comparisons can be made between the  various engines on a specific fuel consumption basis. At  low speed, the reciprocating and turboprop engines have  better economy than the pure turbojet or turbofan engines.  However, at high speed, because of losses in propeller  efficiency, the reciprocating or turboprop engine’s efficiency  becomes limited above 400 mph less than that of the turbofan.  Operating flexibility is the ability of an engine to run  smoothly and give desired performance at all speeds from  idling to full-power output. The aircraft engine must also  function efficiently through all the variations in atmospheric  conditions encountered in widespread operations. To affect proper streamlining and balancing of an aircraft, the  shape and size of the engine must be as compact as possible.  In single-engine aircraft, the shape and size of the engine also  affect the view of the pilot, making a smaller engine better  from this standpoint, in addition to reducing the drag created  by a large frontal area. Weight limitations, naturally, are closely related to the  compactness requirement. The more elongated and spread  out an engine is, the more difficult it becomes to keep the  specific weight within the allowable limits. Durability and reliability are usually considered identical  factors since it is difficult to mention one without including  the other. Simply put, reliability is measured as the mean time  between failures, while durability is measured as the mean  time between overhauls. More specifically, an aircraft engine is reliable when it can  perform at the specified ratings in widely varying flight  Engine specific weight and specific fuel consumption were  discussed in the previous paragraphs, but for certain design  requirements, the final powerplant selection may be based  1-3 Powerplant Selection Durability and Reliability Compactness Operating Flexibility Fuel Economy