unsafe (adj)  DANGEROUS  IN THESE CONDITIONS, IT IS DANGEROUS TO REMOVE THE CARTRIDGES FROM THE FIRE EXTINGUISHER BOTTLE.  In these conditions, it is unsafe to remove cartridges from the fire extinguisher bottle.
 UNSATISFACTORILY (adv)  In an unsatisfactory "manner"  IF THE ELEVATORS OPERATE UNSATISFACTORILY, MOVE THE CONTROL COLUMN.  
 UNSATISFACTORY (adj)  Not satisfactory  IF THE CONDITION OF THE FILTERS IS UNSATISFACTORY, REPLACE THEM.  
 unscrew (v)  REMOVE, LOOSEN, TURN  REMOVE THE SPACERS.  Unscrew the spacers.
       LOOSEN THE CAPTIVE BOLTS.  Unscrew the captive bolts.
       TURN THE END FITTING TO ALIGN THE RIGGING HOLES.  Unscrew the end fitting to align the rigging holes.
 UNSERVICEABLE (adj)  Not "fit" for its "purpose"  REPLACE THE UNSERVICEABLE BUSHES.  
unstable (adj)  STABLE  MAKE SURE THAT THE AIRCRAFT IS STABLE.  Avoid unstable conditions of aircraft.
 UNTIL (con)  "Up to" the time that  MOVE THE LEVER UNTIL THE END OF THE LINK TOUCHES THE LOCK BAR.  
 UNTIL (pre)  "Up to" the time of  KEEP THE FIRE BOTTLE ARMED UNTIL THE END OF THE TEST.