TM 1-1510-225-10 3A-116 a localizer frequency.  This advisory is inhibited by the weight on wheels strut switch. (6)   Aural Advisory “Minimum, Minimum” (Mode  6).    The  aural  advisory  “Minimum,  Minimum” indicates   that   the   aircraft   is   at   the   radio   altitude selected  by  the  crew  with  the  radio  altimeter  DH  set knob located, on the EFIS control panel.  This advisory is   canceled   when   valid   information   from   the   radio altimeter is lost, during climb, whenever the aircraft is above  1,000  feet  AGL,  or  whenever  the  aircraft  is  out of the operating altitude range of the radio altimeter. (7)   Aural “Altitude, Altitude” Advisory (Mode 7).        The    aural    advisory    “Altitude,    Altitude” indicates the approach to a pre-selected altitude as the aircraft  reaches  a  point  1,000  feet  from  the  selected altitude  or,  after  reaching  the  selected  altitude,  when the   aircraft   deviates   more   than   250   feet   from   the selected altitude. (8)   Aural   “Check   Trim,   Check   Trim,   Check Trim”   Advisory.      The   aural   advisory   “Check   Trim, Check  Trim,  Check  Trim”  indicates  that  the  autopilot has had a trim failure. (9)   Aural    “Autopilot”    Advisory.        The   aural advisory   “Autopilot”   indicates   that   the   autopilot   has disengaged. d.  Emergency  Procedures.     If  an  emergency or   malfunction   makes   it   necessary   to   disable   the GPAAS,   pull   the   GPAAS   POWER  circuit  breaker, located   on   the   right   sidewall   circuit   breaker   panel, Figure   2-6.      GPAAS   audio   may   be   turned   off   by pressing the VOICE OFF switch. 3A-34.  TRANSPONDER (APX-100). Refer to Chapter 3, Paragraph 3-5 for information on the APX-100 Transponder. 3A-35.  TRANSPONDER (MST 67A). a.    Description.    The  transponder  (MST  67A) responds  to  interrogation  to  allow  the  aircraft  to  be located  in  range  and  azimuth  by  a  ground  Air  Traffic Control   Radar   Beacon   System   radar   site.      Upon receiving    mode    A    or    mode    C    interrogation    the transponder  transmits  coded  returns  that  identify  the aircraft  by  code  number  and/or  reports  the  altitude  at which    the    aircraft    is    operating.        This    mode    S transponder  transmits  random  replies  called  squitter which include the unique mode S address assigned to the   transponder   at   installation.      The   ATC   mode   S ground  station  receives  this  mode  S  reply  and  can selectively   interrogate   one   specific   address.      The transponder     receives     on     a     frequency     of     1030 megahertz   and   transmits   coded   reply   pulses   on   a frequency   of   1090   megahertz.      The   range   of   the transponder  is  line  of  sight.    The  transponder  (MST 67A)  is  powered  through  a  3-ampere  circuit  breaker, placarded XPNDR NO. 2, located on the right sidewall circuit breaker panel, Figure 2-6. b.    Transponder Control Unit Operating Controls (MST 67A).  Refer to Figure 3A-69. (1)   Reply Indicator.   The reply indicator is an R  annunciator  located  in  the  upper  left   portion  of  the display.  The reply indicator illuminates to indicate that the transponder is replying to ground interrogation.  An interrogation    that    causes    the     R    annunciator    to illuminate will normally be processed with each sweep of   the   search   radar   that   will   normally   be   a   10   to 15 second interval. (2)   Ident Code/Flight Level Test Display. (3)   Photocell. The built-in photocell automatically controls display brightness. (4)   Ident Switch.   The ident switch, placarded IDT,  when  pressed  momentarily  then  released,  holds the ident reply for approximately 25 seconds to assure the   proper   reply   within   at   least   one   radar   sweep.   During    this    time,    the    IDT     annunciator     will     be illuminated   on   the   transponder   control   unit   display.   The ident function may also be activated from switches mounted    on    each    control    wheel,    placarded    ATC IDENT. (5)   Ident    Indicator.        The    ident    indicator, placarded IDT, illuminates when the ident function has been   activated   by   pressing   the   IDT  switch  on  the transponder  control  panel  or  one  of  the  control  wheel ident switches placarded ATC IDENT. (6)   Function   Switch.      The   function   switch, placarded  TST  /  SBY  /  ON  /  ALT  /  VFR,  is  used  to select the operating mode of the transponder. (a)   TST.    The  test  (TST)  position  of  the function   switch   provides   for   a   preflight   or   airborne check    of    transponder    operation.        This    check    is independent  of  any  ground  interrogation.    Setting  the function   switch   to   TST   should   cause   the   reply  (R) annunciator    to    flash    and    the    flight    level    (FL) annunciator to illuminate.  The transponder transmitter is inhibited during the test function.  After 3 seconds, the unit will revert to the altitude (ALT) reporting mode.