the maximum altitude at which, in standard atmosphere, it  is possible to maintain, at a specified rotational speed, a  specified power or a specified manifold pressure. A critical altitude exists for every possible power setting  below the maximum operating ceiling. If the aircraft is  flown above this altitude without a corresponding change in  the power setting, the waste gate is automatically driven to  the fully closed position in an effort to maintain a constant  power output. Thus, the waste gate is almost fully open at sea  level and continues to move toward the closed position as the  aircraft climbs, in order to maintain the preselected manifold  pressure setting. When the waste gate is fully closed (leaving  only a small clearance to prevent sticking), the manifold  pressure begins to drop if the aircraft continues to climb. If  a higher power setting cannot be selected, the turbocharger’s  critical altitude has been reached. Beyond this altitude, the  power output continues to decrease. senses air pressure upstream from the throttle; the other side  samples pressure on the cylinder side of the throttle valve.  [Figure 3-18]  At the “wide open” throttle position when the  density controller controls the waste gate, the pressure across  the differential pressure controller diaphragm is at a minimum  and the controller spring holds the bleed valve closed. At “part  throttle” position, the air differential is increased, opening  the bleed valve to bleed oil to the engine crankcase and  reposition the waste gate piston. Thus, the two controllers  operate independently to control turbocharger operation at all  positions of the throttle. Without the overriding function of the  differential pressure controller during part-throttle operation,  the density controller would position the waste gate valve for  maximum power. The differential pressure controller reduces  injector entrance pressure and continually repositions the  valve over the whole operating range of the engine. The position of the waste gate valve, which determines power  output, is controlled by oil pressure. Engine oil pressure acts  on a piston in the waste gate assembly, which is connected  by linkage to the waste gate valve. When oil pressure is  increased on the piston, the waste gate valve moves toward  the closed position, and engine output power increases.  Conversely, when the oil pressure is decreased, the waste  gate valve moves toward the open position, and output power  is decreased as described earlier. The differential pressure controller reduces the unstable  condition known as “bootstrapping” during part-throttle  operation. Bootstrapping is an indication of unregulated power  change that results in the continual drift of manifold pressure.  This condition can be illustrated by considering the operation  of a system when the waste gate is fully closed. During this  time, the differential pressure controller is not modulating the  waste gate valve position. Any slight change in power caused  by a change in temperature or rpm fluctuation is magnified  and results in manifold pressure change since the slight change  causes a change in the amount of exhaust gas flowing to the  turbine. Any change in exhaust gas flow to the turbine causes  a change in power output and is reflected in manifold pressure  indications. Bootstrapping, then, is an undesirable cycle of  turbocharging events causing the manifold pressure to drift  in an attempt to reach a state of equilibrium. The position of the piston attached to the waste gate valve is  dependent on bleed oil, which controls the engine oil pressure  applied to the top of the piston. Oil is returned to the engine  crankcase through two control devices, the density controller  and the differential pressure controller. These two controllers,  acting independently, determine how much oil is bled back to  the crankcase and establishes the oil pressure on the piston. The density controller is designed to limit the manifold  pressure below the turbocharger’s critical altitude and regulates  bleed oil only at the full throttle position. The pressure- and  temperature-sensing bellows of the density controller react to  pressure and temperature changes between the fuel injector  inlet and the turbocharger compressor. The bellows, filled  with dry nitrogen, maintain a constant density by allowing the  pressure to increase as the temperature increases. Movement  of the bellows repositions the bleed valve, causing a change  in the quantity of bleed oil, which changes the oil pressure on  top of the waste gate piston.  [Figure 3-18] The differential pressure controller functions during all  positions of the waste gate valve other than the fully open  position, which is controlled by the density controller. One  side of the diaphragm in the differential pressure controller  The differential pressure controller is essential to smooth  functioning of the automatically controlled turbocharger,  since it reduces bootstrapping by reducing the time required  to bring a system into equilibrium. There is still extra  throttle sensitivity with a turbocharged engine than with a  naturally aspirated engine. Rapid movement of the throttle  can cause a certain amount of manifold pressure drift in a  turbocharged engine. Less severe than bootstrapping, this  condition is called overshoot. While overshoot is not a  dangerous condition, it can be a source of concern to the  Bootstrapping is sometimes confused with the condition  known as overboost, but bootstrapping is not a condition that  is detrimental to engine life. An overboost condition is one in  which manifold pressure exceeds the limits prescribed for a  particular engine and can cause serious damage. A pressure  relief valve when used in some systems, set slightly in excess  of maximum deck pressure, is provided to prevent damaging  over boost in the event of a system malfunction. 3-14