(h) Descending  When the ship is descending, air must be fed into the  system to maintain the differential pressure between the  inside of the envelope and the increasing atmospheric  pressure.  The scoops should be opened wide enough to per› mit adequate air flow.  The air scoops, ducts, and damper  valves can admit air into the ballonets at a sufficient rate  to maintain a gas pressure of 1.25 inches of water when the  ship is descending at a rate of 1200 feet per minute, at a  forward speed of 50 knots.  The air intake capacity naturally decreases as the  forward speed is reduced so that the maximum permissible  rate of descent is less than 1200 feet per minute when the  forward speed is less than  5.0  lrnots.  CAUTION:  1200 feet per minute is the maximum permissible  rate of descent.  The actual rate of descent  should be kept well below this value.  (i) Landing  At the time of landing it _usually becomes necessary  to throttle down the motors and the air scoop system can no  longer supply the required pressure to the ballonets.  An  auxiliary blower is provided for such conditions.  The blower is of sufficient capacity to maintain a  gas pressure of 0.5 inches of water at the gas manometer  -37-