TM  1-1520-240-10            2-3-7 bustion camber and the other is at the aft end to ensure complete drainage. 2-3-29.  FADEC Description. 714A   Each engine is controlled by its own Full Authority Digital  Electronic  control  System  (FADEC)  which  pro- vides the following features: a. Automatic start scheduling. b. 1 and 2 engine load sharing. c. Power turbine speed governing. d. Transient  load  anticipation  (using  rotor  speed and collective pitch rates). e. Transient torque smoothing (using N2 rates). f. Contingency power capability to meet aircraft de- mand. g. Acceleration and deceleration control. h. Engine temperature limiting throughout the oper- ating range. i. Surge avoidance. j. Compressor bleed band scheduling.. k. Fuel flow limiting. l. Engine fail detection. m.   Power assurance test. n. Engine history/fault recording. o. Engine-to-engine     communication     (via     data bus). p. Automatic switchover to reversionary backup in the event of a FADEC primary system failure. The  FADEC  provides  automatic  engine  start,  simulta- neously  sequencing  ignition,  start  fuel,  and  stabilized operation at idle. A data link between 1 and 2 engine FADEC systems transmits signals to achieve load sha- ring. It also provides control of N1 speed and NR (N2) output shaft speed to maintain the rotor system at a near constant RRPM throughout all flight power demand con- ditions. FADEC provides smooth acceleration and over- temperature protection when ECLs (both together) are moved  from  GROUND  to  FLIGHT.  Overtemperature protection is provided (through the DECU temperature limiting function) by control system thermocouple inter- face  at  the  power  turbine  inlet.  The  control  system compares PTIT temperature signals with reference limits to  calculate  and  provide  appropriate  N1  acceleration. During starts, an absolute 816_C limit is set and if ex- ceeded an engine out indication and shutdown will occur. If compressor performance deteriorates for any reason, surge detection automatically allows recovery from com- pressor instability while protecting the engine from dam- age due to overtemperature. The FADEC system consists of: q. The  Digital  Electronic  Control  Unit  (DECU)  in- cludes a primary mode and a reversionary section for backup (fig. 2-3-7). r. The Hydromechanical Metering Assembly (HMA),  includes  Hydromechanical  Fuel  Metering  Unit (HMU) and fuel pump unit for all fuel metering to support both primary and reversionary fuel metering, a self-con- tained alternator for powering the FADEC electronics, a primary  and  revisionary  compressor  bleed  air  control, and redundant speed sensing. s. ENG COND panel (fig. 2-3-5). t. FADEC control panel (fig. 2-3-6). u. RPM  INC/DEC  (Beep)  switch  THRUST  CONT Lever. (1) On  714A   engine installations, engine beep switches are only active when in reversionary mode. (2) Each switch is labeled NO. 1 or NO. 2 which is used to adjust RRPM when in reversionary mode. (3) Operation   of   the   beep   switches   on   the 714A   in the reversionary mode are the same as for the 712   except that each switch operates respective en- gine independently. If only one engine is in reversionary mode, the RRPM will not change, as it is governed by the engine in primary mode. 2-3-30.  Reversionary System.  714A NOTE Aircrew  should  be  alert  to  the  possibility  of abrupt NR and engine power changes when operating the FADEC in single or dual engine REV mode (s). The  reversionary  (backup  mode)  automatically  takes control of the engine if the primary mode fails or if se- lected by the operator via the FADEC panel, REV switch. When  an  engine  is  operating  in  reversionary  mode, FADEC  provides  engine  and  rotor  control  through  N1 speed    governor,    beep    control,    and    thrust    pitch compensator. When both engines are in reversionary mode, RRPM will require   more   pilot   attention   since   proportional   rotor speed governor will not hold speed as accurately as the primary systems. With large collective changes, the rotor speed  can  change  up  to  $3  percent  from  a  nominal setting. The reversionary system provides the following control functions: a. Automatic start sequencing including over tem- perature protection, but not start abort. b. Pilot controlled start fuel enrichment/derichment, if required, through ECL modulation. c. Ground idle set 55 "5 percent with ECL at GND.