used in training, this saves much electric starter and battery  wear. High oil pressure from the propeller governor recharges  the accumulator just moments after engine rotation begins. POH describes crossfeed limitations and procedures that vary  significantly among multiengine airplanes. Many multiengine airplanes have a propeller synchronizer  (prop sync) installed to eliminate the annoying “drumming”  or “beat” of propellers whose rpm are close, but not precisely  the same. To use prop sync, the propeller rpm is coarsely  matched by the pilot and the system is engaged. The prop  sync adjusts the rpm of the “slave” engine to precisely  match the rpm of the “master” engine and then maintains  that relationship. Checking crossfeed operation on the ground with a quick  repositioning of the fuel selectors does nothing more than  ensure freedom of motion of the handle. To actually check  crossfeed operation, a complete, functional crossfeed system  check should be accomplished. To do this, each engine should  be operated from its crossfeed position during the run-up. The  engines should be checked individually and allowed to run at  moderate power (1,500 rpm minimum) for at least 1 minute  to ensure that fuel flow can be established from the crossfeed  source. Upon completion of the check, each engine should be  operated for at least 1 minute at moderate power from the main  (takeoff) fuel tanks to reconfirm fuel flow prior to takeoff. This suggested check is not required prior to every flight.  Crossfeed lines are ideal places for water and debris to  accumulate unless they are used from time to time and  drained using their external drains during preflight. Crossfeed  is ordinarily not used for completing single-engine flights  when an alternate airport is readily at hand, and it is never  used during takeoff or landings. The prop sync should be disengaged when the pilot selects a  new propeller rpm and then re-engaged after the new rpm is  set. The prop sync should always be off for takeoff, landing,  and single-engine operation. The AFM/POH should be  consulted for system description and limitations. A variation on the propeller synchronizer is the propeller  synchrophaser. Prop synchrophase acts much like a  synchronizer to precisely match rpm, but the synchrophaser  goes one step further. It not only matches rpm but actually  compares and adjusts the positions of the individual blades  of the propellers in their arcs. There can be significant  propeller noise and vibration reductions with a propeller  synchrophaser. From the pilot’s perspective, operation of  a propeller synchronizer and a propeller synchrophaser are  very similar. A synchrophaser is also commonly referred to as  prop sync, although that is not entirely correct nomenclature  from a technical standpoint. Combustion heaters are common on multiengine airplanes.  A combustion heater is best described as a small furnace that  burns gasoline to produce heated air for occupant comfort  and windshield defogging. Most are thermostatically operated  and have a separate hour meter to record time in service  for maintenance purposes. Automatic over temperature  protection is provided by a thermal switch mounted on the  unit that cannot be accessed in flight. This requires the pilot  or mechanic to actually visually inspect the unit for possible  heat damage in order to reset the switch. As a pilot aid to manually synchronizing the propellers, some  twins have a small gauge mounted in or by the tachometer(s)  with a propeller symbol on a disk that spins. The pilot  manually fine tunes the engine rpm so as to stop disk rotation,  thereby synchronizing the propellers. This is a useful backup  to synchronizing engine rpm using the audible propeller  beat. This gauge is also found installed with most propeller  synchronizer and synchrophase systems. Some synchrophase  systems use a knob for the pilot to control the phase angle. When finished with the combustion heater, a cool-down  period is required. Most heaters require that outside air be  permitted to circulate through the unit for at least 15 seconds  in flight or that the ventilation fan can be operated for at least  2 minutes on the ground. Failure to provide an adequate cool  down usually trips the thermal switch and renders the heater  inoperative until the switch is reset. Flight director/autopilot (FD/AP) systems are common on the  better-equipped multiengine airplanes. The system integrates  pitch, roll, heading, altitude, and radio navigation signals in  a computer. The outputs, called computed commands, are  displayed on a flight command indicator (FCI). The FCI  replaces the conventional attitude indicator on the instrument  panel. The FCI is occasionally referred to as a flight director  indicator (FDI) or as an attitude director indicator (ADI).  Fuel crossfeed systems are also unique to multiengine  airplanes. Using crossfeed, an engine can draw fuel from a  fuel tank located in the opposite wing. On most multiengine airplanes, operation in the crossfeed  mode is an emergency procedure used to extend airplane  range and endurance in OEI flight. There are a few models  that permit crossfeed as a normal, fuel balancing technique  in normal operation, but these are not common. The AFM/ Flight Director/Autopilot Fuel Crossfeed Combustion Heater Propeller Synchronization 12-6