PART I-DESCRIPTIVE

                 central off position, and can also be pressed in when in
                 the off position.  A warning system ON/OFF switch (108)
                 is also above the pressure gauge and a warning indicator
                 light is on the left forward console.
           (iv)  The ON-OFF valve must be turned on (fully forward)
                 before flight and left on for the duration of the flight.
                 The air inlet switch should be at NORMAL, but should
                 be moved forward to 100% OXYGEN if smoke or fuel
                 fumes enter the cockpit or symptoms of anoxia are
                 present.  The emergency switch should normally be
                 central but, in the event of accidental loss of cockpit
                 pressure, should be placed in either the left or right
                 positions.
           (v)   before flight the mask should be checked for fit by
                 pushing the emergency switch in fully in the central
                 position.  This will supply oxygen under pressure and
                 the mask can be adjusted until no leaks are apparent.
                 The firmer the switch is pressed, the greater the oxygen
                 pressure: when fully in, the pressure is five times as great
                 as with the switch in either of the side positions.
           (vi)  The warning system switch, when switched on before
                 flight, causes the indicator light to come on.  After two
                 minutes normal breathing the light will reduce from a
                 bright glow to a dim glow indication normal operation.
                 If the light glows brightly in flight it indicates no oxygen
                 flow or a steady flow, either of which indicate an
                 interruption of normal operation.

          37.    Cockpit air conditioning and pressurising

           (i)   The two position pressure control switch (68) is on the
                 left aft console and is labelled RAM and PRESS.  When
                 set to RAM cold ram air enters the cockpit.  This ram
                 air can be heated if required (see sub-para. (iii) below).
                 When set to PRESS air from the engine compressor is
                 delivered under pressure to the cockpit.  A pressure
                 selector switch (69) on the left aft console has two posi-
                 tions "2.75 p.s.i." and "5.00 p.s.i." and above 12,500 ft.
                 the cockpit is pressurised according to the setting of this
                 switch.  On the lower pressure setting a cockpit altitude
                 of 12,500 ft. is maintained until an actual altitude of
                 21,200 ft. is reached, and on the higher pressure setting
                 until an actual altitude of 31,000 ft. is reached.  Above
                 these altitudes the cockpit altitude will rise on a scale
                 so as to maintain a constant cockpit pressure 2.75 lb./sq.
                 in. or 5 lb./sq. in. greater than the outside air pressure,

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