Fig. VI  Air Scoops  (b) Valve Settings  (See Buaer Manual 12-304)  The pressure in the forward and aft ballonets is limit› ed by the forward and aft automatic air release valves. These  two valves are set to operate at different pressures for  reasons that will become apparent further on.  The valve of the forward ballonet is set to start open› ing when the air pressure in the forward ballonet, as read  at the car manometer, reaches 1.50 inches of water.  The  valve of the aft ballonet is set to begin to open at an air  pressure of 2.00 inches of water.  When the ballonets are partly inflated there is a  difference of pressure bet• Neen the air in the ballonets and  the gas in the envelope.  This difference depends upon the  -25-