duct either by using a movable restriction, such as a ramp  or wedge, inside the duct. Another system is some sort of a  variable airflow bypass arrangement, which extracts part of  the inlet airflow from the duct ahead of the engine. In some  cases, a combination of both systems is used. The third method is the use of a shock wave in the airstream.  A shock wave is a thin region of discontinuity in a flow of  air or gas, during which the speed, pressure, density, and  temperature of the air or gas undergo a sudden change.  Stronger shock waves produce larger changes in the  properties of the air or gas. A shock wave is willfully set up  in the supersonic flow of the air entering the duct, by means  of some restriction or small obstruction which automatically  protrudes into the duct at high flight Mach numbers. The  shock wave results in diffusion of the airflow, which, in turn,  decreases the velocity of the airflow. In at least one aircraft  installation, both the shock method and the variable-geometry  method of causing diffusion are used in combination. The  same device that changes the area of the duct also sets up a  shock wave that further reduces the speed of the incoming  air within the duct. The amount of change in duct area and  the magnitude of the shock are varied automatically with the  airspeed of the aircraft. To prevent the engine from readily ingesting any items that  can be drawn in the intake, a compressor inlet screen is  sometimes placed across the engine air inlet at some location  along the inlet duct. Engines that incorporate inlet screens,  such as turboprops  [Figure 3-28]  and APUs  [Figure 3-29]  are  not as vulnerable to FOD. The advantages and disadvantages  of a screen vary. If the engine is readily subjected to internal  damage, as would be the case for an engine having an axial  compressor fitted with aluminum compressor blades, an  inlet screen is almost a necessity. Screens, however, add  appreciably to inlet duct pressure loss and are very susceptible  to icing. Failure due to fatigue is also a problem. A failed screen can sometimes cause more damage than no screen  at all. In some instances, inlet screens are made retractable  and may be withdrawn from the airstream after takeoff or  whenever icing conditions prevail. Such screens are subject  to mechanical failure and add both weight and bulk to  the installation. In large turbofan engines having steel or  titanium compressor (fan) blades, which do not damage  easily, the disadvantages of compressor screens outweigh  the advantages, so they are not generally used. A bellmouth inlet is usually installed on an engine  undergoing testing in a test cell.  [Figure 3-30]  It is generally  equipped with probes that, with the use of instruments, can  measure intake temperature and pressure (total and static).  [Figure 3-31]  During testing, it is important that the outside  static air is allowed to flow into the engine with as little  resistance as possible. The bellmouth is attached to the  Figure 3-29.  An example of an inlet screen on an APU.  Figure 3-28.  An example of a turboprop engine that incorporates  inlet screens.  Figure 3-30.  A bellmouth inlet used during system tests.  Bellmouth Compressor Inlets Compressor Inlet Screens 3-20