TM 1-1510-218-10 2-77 inadvertent switching to the  HI  position during system shutdown. NOTE Erratic operation of the magnetic compass may  occur  while  windshield  heat  is  being used. 2-68.  PRESSURIZATION SYSTEM. a.   Description.   A mixture of bleed air from the engines, and ambient air, is available for pressurization to the cabin at a rate of approximately 10 to 17 pounds per minute.  Approximately 75% N₁ is required   for   normal   pressurization.       Approximately 85%   N₁   C   D   is   required   when   operating   with   one engine.   The flow control unit of   each engine controls the  bleed  air  from  the  engine  to  make  it  usable  for pressurization, by mixing ambient air with the bleed air depending upon aircraft altitude and ambient temperature.   On take-off,  excessive  pressure  bumps are  prevented  by  landing  gear safety switch actuated solenoids incorporated in the flow control units.  These solenoids,  through  a  time  delay,  stage  the  input  of ambient     air     flow     by     allowing     ambient     air     flow introduction through the left flow control unit first, then 4 seconds later,  air  flow  through  the  right  flow  control unit. b.  Pressure   Differential    C    D    T1   .            The pressure   vessel   is   designed   for   a   normal   working pressure  differential  of  6.0  psi,  which  will  provide  a cabin   pressure   altitude   of   3870   feet   at   an   aircraft altitude of 20,000 feet, and a nominal cabin altitude of 9840 feet at an aircraft altitude of 31,000 feet. c.   Pressure  Differential   T2  .      The  pressure vessel   is   designed   for   a   normal   working   pressure differential    of    6.5 psi,    which    will    provide    a    cabin pressure altitude of  8,000 feet at an aircraft altitude of 29,700  feet,  and  a  cabin  altitude  of  10,000  feet  at  an aircraft   altitude   of   34,000   feet.      At   an   altitude   of 35,000 feet the aircraft will have a 10,400-foot cabin. d.  Cabin Altitude and Rate-of-Climb Controller  C D1  .   A control panel is installed on the copilot's  side  of  the  instrument  panel  for  operation  of the system.   A knob, placarded  INCR  RATE, controls the   rate   of   change   of   pressurization.       A   control, placarded   CABIN   CONTROLLER,   is   used   to   set desired    cabin    altitude.        The    selected    altitude    is displayed  on  a  mechanically  coupled  dial  above  the control,   placarded   CABIN   ALT   /   FT.      Mechanically coupled   to   the   cabin   altitude   dial   is   a   second   dial, placarded   ACFT   X   1000.      This   dial   indicates   the maximum altitude the aircraft may be flown to maintain the desired cabin altitude without exceeding the design pressure   differential.      A   switch,   placarded    CABIN PRESS DUMP / PRESS / TEST, is provided to control pressurization.      The   switch   is   spring   loaded   to   the PRESS   position.      In the   DUMP  position,  the  safety valve     will     be     opened     and     the     cabin     will     be depressurized  to  the  aircraft  altitude.    In  the   PRESS position,   cabin   altitude   is   controlled   by   the   CABIN CONTROLLER   control.      In   the   TEST  position,  the landing   gear   safety   switch   is   bypassed   to   enable testing  of  the  pressurization  system  on  the  ground.   Operating instructions are contained in Chapter 8. e.   Cabin Altitude And Rate-of-Climb Controller D2 T1 .   A control panel is installed on the pedestal    for    operation    of    the    system.        A   knob, placarded  RATE  MIN  /  MAX,  controls  pressurization rate  of  change.    A control, placarded   CABIN  ALT is used   to   set   desired   cabin   altitude.      The   selected altitude  is  displayed  on  a  mechanically  coupled  dial above  the  control,  placarded    CABIN  ALT  1000  FT.   Mechanically  coupled  to  the  cabin  altitude  dial  is  a second  dial,  placarded   ACFT   ALT   1000.     This  dial indicates  the  maximum  altitude  the  aircraft  may  be flown  to  maintain  the  desired  cabin  altitude  without exceeding  the  design  pressure  differential.    A  switch, placarded  CABIN PRESS DUMP / PRESS / TEST, is provided   to   control   pressurization.       The   switch   is spring  loaded  to  the   PRESS  position.    In the   DUMP position, the safety valve will be opened and the cabin will  be  depressurized  to  the  aircraft  altitude.    In  the PRESS  position,  cabin  altitude  is  controlled  by  the CABIN ALT control.  In the  TEST position, the landing gear safety switch is bypassed to enable testing of the pressurization   system   on   the   ground.       Operating instructions are contained in Chapter 8. f.    Pressurization    Controller    T2    .             The pressurization    controller,    located    on    the    pedestal extension,  provides  a  display  of  the  selected  altitude; an  altitude  selector,  and  a  rate  control  selector.    The cabin  and  aircraft  altitude  display  is  a  mechanically coupled  dial.     The  outer  scale,  (CABIN  ALT)  of  the display, indicates the selected cabin altitude; the inner scale (ACFT ALT) indicates the corresponding altitude at   which   the   maximum   differential   pressure   would occur.   The  indicated  value  on  each  scale is read per placard   ALT-FT  X  I000.      The  rate  control  selector, placarded   RATE   INC,   regulates   the   rate   at   which cabin  pressure  ascends  or  descends  to  the  selected altitude.  The rate change selected may be from 200 to 2000 feet per minute. g.  Cabin  Rate-Of-Climb Indicator   C  D1  .    An indicator, placarded  CABIN  CLIMB, is installed in the instrument panel, Figure 2-17, above the cabin altitude and     rate-of-climb    controller.          It    is    calibrated    in thousands-of-feet per-minute change in cabin altitude.