density (higher density altitude) decreases the power output  of the engine. As altitude changes, the position of the throttle  must be changed to maintain the same rpm. As altitude is  increased, the throttle must be opened further to indicate the  same rpm as at a lower altitude.  will increase or decrease as appropriate, with changes in  airspeed and propeller load. For example, once a specific  rpm has been selected, if aircraft speed decreases enough to  rotate the propeller blades until they contact the low pitch  stop, any further decrease in airspeed will cause engine rpm  to decrease the same way as if a fixed-pitch propeller were  installed. The same holds true when an aircraft equipped with  a constant-speed propeller accelerates to a faster airspeed. As  the aircraft accelerates, the propeller blade angle increases to  maintain the selected rpm until the high pitch stop is reached.  Once this occurs, the blade angle cannot increase any further  and engine rpm increases.  The adjustable-pitch propeller was the forerunner of the  constant-speed propeller. It is a propeller with blades whose  pitch can be adjusted on the ground with the engine not  running, but which cannot be adjusted in flight. It is also  referred to as a ground adjustable propeller. By the 1930s,  pioneer aviation inventors were laying the ground work for  automatic pitch-change mechanisms, which is why the term  sometimes refers to modern constant-speed propellers that  are adjustable in flight.  The first adjustable-pitch propeller systems provided only two  pitch settings: low and high. Today, most adjustable-pitch  propeller systems are capable of a range of pitch settings.  On aircraft equipped with a constant-speed propeller, power  output is controlled by the throttle and indicated by a manifold  pressure gauge. The gauge measures the absolute pressure of  the fuel-air mixture inside the intake manifold and is more  correctly a measure of manifold absolute pressure (MAP). At  a constant rpm and altitude, the amount of power produced  is directly related to the fuel-air mixture being delivered to  the combustion chamber. As the throttle setting is increased,  more fuel and air flows to the engine and MAP increases.  When the engine is not running, the manifold pressure gauge  indicates ambient air pressure (i.e., 29.92 inches mercury  (29.92 "Hg)). When the engine is started, the manifold  pressure indication decreases to a value less than ambient  pressure (i.e., idle at 12 "Hg). Engine failure or power loss  is indicated on the manifold gauge as an increase in manifold  pressure to a value corresponding to the ambient air pressure  at the altitude where the failure occurred.  [Figure 7-9]  A constant-speed propeller is a controllable-pitch propeller  whose pitch is automatically varied in flight by a governor  maintaining constant rpm despite varying air loads. It is the  most common type of adjustable-pitch propeller. The main  advantage of a constant-speed propeller is that it converts  a high percentage of brake horsepower (BHP) into thrust  horsepower (THP) over a wide range of rpm and airspeed  combinations. A constant-speed propeller is more efficient  than other propellers because it allows selection of the most  efficient engine rpm for the given conditions.  An aircraft with a constant-speed propeller has two controls:  the throttle and the propeller control. The throttle controls  power output, and the propeller control regulates engine  rpm. This regulates propeller rpm, which is registered on  the tachometer.  Once a specific rpm is selected, a governor automatically  adjusts the propeller blade angle as necessary to maintain  the selected rpm. For example, after setting the desired rpm  during cruising flight, an increase in airspeed or decrease in  propeller load causes the propeller blade angle to increase  as necessary to maintain the selected rpm. A reduction in  airspeed or increase in propeller load causes the propeller  blade angle to decrease.  The propeller’s constant-speed range, defined by the high  and low pitch stops, is the range of possible blade angles for  a constant-speed propeller. As long as the propeller blade  angle is within the constant-speed range and not against  either pitch stop, a constant engine rpm is maintained. If  the propeller blades contact a pitch stop, the engine rpm  The manifold pressure gauge is color coded to indicate the  engine’s operating range. The face of the manifold pressure  gauge contains a green arc to show the normal operating  range and a red radial line to indicate the upper limit of  manifold pressure.  Figure 7-9.  Engine power output is indicated on the manifold  pressure gauge.  7-6  Adjustable-Pitch Propeller  IN Hg  ALg.  MANIFOLD  PRESS  35 50 10 15 25 40 30 20 45 35  10  15  25  40  30  20  45 