Synchroscope Accelerometers Stall Warning and Angle of Attack (AOA)Indicators 10-33 The simplest form of AOA indicator is a stall warning device  that does not have a gauge located in the cockpit. It uses an  aural tone to warn of an impending stall due to an increase  in AOA. This is done by placing a reed in a cavity just aft of  An aircraft’s angle of attack (AOA) is the angle formed  between the wing cord centerline and the relative wind. At  a certain angle, airflow over the wing surfaces is insufficient  to create enough lift to keep the aircraft flying, and a stall  occurs. An instrument that monitors the AOA allows the pilot  to avoid such a condition.  Electric and digital accelerometers also exist. Solid-state  sensors are employed, such as piezoelectric crystalline  devices. In these instruments, when an accelerating force is  applied, the amount of resistance, current flow, or capacitance  changes in direct relationship to the size of the force. Micro- electric signals integrate well with digital computers designed  to process and display information in the cockpit. Simple accelerometers are mechanical, direct-reading  instruments calibrated to indicate force in Gs. One G is  equal to one times the force of gravity. The dial face of an  An accelerometer is an instrument that measures acceleration.  It is used to monitor the forces acting upon an airframe.  Accelerometers are also used in inertial reference navigation  systems. The installation of accelerometers is usually limited  to high-performance and aerobatic aircraft.  One use of synchroscope involve designating one of the  engines as a master engine. The rpm of the other engine(s) is  always compared to the rpm of this master engine. The dial face  of the synchroscope indicator looks like  Figure 10-59 . “Slow”  and “fast” represent the other engine’s rpm relative to the  master engine, and the pilot makes adjustments accordingly.  The accelerometer operates on the principle of inertia. A mass,  or weight, inside is free to slide along a shaft in response to  the slightest acceleration force. When a maneuver creates  an accelerating force, the aircraft and instrument move, but  inertia causes the weight to stay at rest in space. As the shaft slides through the weight, the relative position of the weight  on the shaft changes. This position corresponds to the force  experienced. Through a series of pulleys, springs, and shafts,  the pointers are moved on the dial to indicate the relative  strength of the acceleration force. [ Figure 10-60]  Forces can  act upon an airframe along the three axes of flight. Single  and multi-axis accelerometers are available, although most  cockpit gauges are of the single-axis type. Inertial reference  navigation systems make use of multi-axis accelerometers  to continuously, mathematically calculate the location of the  aircraft in a three dimensional plane.  If both engines are operating at exactly the same speed, the  synchroscope motor does not operate. If one engine operates  faster than the other, its tach generator signal causes the  synchroscope motor to turn in a given direction. Should the  speed of the other engine then become greater than that of  the first engine, the signal from its tach generator causes the  synchroscope motor to reverse itself and turn in the opposite  direction. The pilot makes adjustments to steady the pointer  so it does not move.  The synchroscope is an instrument that indicates whether two  or more rotating devices, such as engines, are synchronized.  Since synchroscopes compare rpm, they utilize the output  from tachometer generators. The instrument consists of a  small electric motor that receives electrical current from the  generators of both engines. Current from the faster running  engine controls the direction in which the synchroscope  motor rotates. accelerometer is scaled to show positive and negative forces.  When an aircraft initiates a rapid climb, positive G force  tends to push one back into one’s seat. Initiating a rapid  decent causes a force in the opposite direction, resulting in  a negative G force. Most accelerometers have three pointers. One is continuously  indicating the acceleration force experienced. The other two  contain ratcheting devices. The positive G pointer follows  the continuous pointer and stay at the location on the dial  where the maximum positive force is indicated. The negative  G pointer does the same for negative forces experienced.  Both max force pointers can be reset with a knob on the  instrument face.  Figure 10-59.  This synchroscope indicates the relative speed of the  slave engine to the master.  Synchroscope S L $_{O}$W F A S T