TM  1-1520-240-10            2-3-9 This provides the FADEC system with back up electrical power in the event of a HMU integral alternator failure thus preventing loss of the PRI mode. Placing the B/U PWR switch to OFF will reduce operating time on the FADEC circuitry. The B/U PWR switch should always be ON during engine operation. d. OSPD 1, 2 Switch. The Over Speed test switch is a  three  position  switch  used  to  test  the  FADEC over- speed system. In the event of a NR overspeed of 114.8 percent, FADEC reduces fuel flow to a ground idle condi- tion. The system remains activated until the overspeed condition no longer exists, and will re-activate as soon as an overspeed re-occurs. The system contains provisions to inhibit overspeed trip command if the other engine has experienced a overspeed trip condition. To prevent inad- vertent operation during flight, this test is locked out if NR is greater than 81.3 percent. When performing an over- speed  test  with  the  engine  running  and  the  RRPM 79.0$1%, pressing the test switch to 1 or 2, lowers the overspeed trip threshold to 79.0$1% NR. At this time the system senses an overspeed and reduces the fuel flow. e. LOAD SHARE, PTIT/TRQ Switch. The primary FADEC system provides pilot selectable engine torque PTIT matching to govern the engines. Torque matching is normally the preferred option. The selected parameter is constantly compared between the two engines until the RRPM stabilizes at datum figure. The PTIT option may be used when one engine is running hot. N1 matching is engaged  automatically  if  the  selected  matching  mode fails. f. ENG START Switch. It is a three position switch, spring loaded to the center position, labeled 1 and 2. It is used to commence the start sequence on the respective engine. 2-3-33.  DECU Unit.  714A The two airframe mounted DECU’s, one for each engine, contain the primary and reversionary mode electronics. The DECUs are located on the left (sta 390) and right (sta 410) side of the aft cabin. Figure  2-3-6.  FADEC Panel  714A 2-3-34.  BIT.  714A The DECU contains a two-digit BIT display. When active, the display indicates the operating status of the FADEC system and power assurance test results. A complete list of the FADEC BIT fault codes are located at Table 2-3-1. The fault monitoring carried out by the DECU consists of: a. Power up tests. b. Fault tests designed to discover dormant faults. c. A set of repeated monitoring tests to detect faults occurring during normal operation. Fault information for the previous or current engine cycle can be seen on the DECU BIT display. The last engine cycle is reset on the first occurrence of start mode and not on  engine  shutdown.  During  engine  shutdown  (when ECL is at STOP or N1 is less than 10 percent) faults are not stored. Fault indications are stored in the DECU and are retained throughout the life of the control unit. How- ever, fault information prior to the previous cycle can only be accessed with specialized test equipment. During en- gine start the DECU BIT displays 88 for satisfactory test or if the test fails, a fault code. Faults are classified as either “HARD” or “SOFT”. In primary mode a hard fault will cause the FADEC to transfer to Reversionary , while in Reversionary a hard fault will cause the FADEC to “fail fixed” to a constant power condition. If a hard fault occurs in Primary after a hard fault exists in reversionary then the primary will fail fixed. In the event of a soft fault the FADEC will remain in the mode it was in prior to the fault but there may be some degradation or redundancy. All soft faults are less severe than a hard fault since the FADEC will not switch modes due to a soft fault. The activation of the BIT display is dependent upon the position of the ECL as follows: a. With the ECL at STOP, the fault information for the last engine cycle and current faults are displayed. b. When the ECL is positioned at GROUND only current faults are displayed. c. When the ECL is positioned at FLIGHT the dis- play will be turned off except as required for Power Assur- ance Test (PAT). 2-3-35.  Starting in Primary Mode.  714A   CAUTION The (P3) compressor pressure signal line going  to  the  DECU  contains  a  manually operated moisture drain valve. This valve shall  not  be  drained  while  the  engine  is running. NOTE Engine  may  not  start  if  REV  fail  caution  is illuminated. In primary mode, engine start is initiated with the ECL in the GND position. Select and hold the respective ENG START switch and allow the engine to accelerate to 10