WHILE DRIVING ON STEEP DECLINE ON ICY PACKED SNOW UNPACKED MOUNTAIN ROAD IN 4X4 LOW IN MANUAL 1ST GEAR WITH HILL DESCENT ASSIST ENGAGED, THE TRUCK STALLED AND WOULD NOT RESTART. (APPARENTLY ONE CANNOT RESTART THE F150 EVEN IN NEUTRAL IF THY TRUCK IS MOVING. HAVE TO COME TO COMPLETE STOP AND SHIFT BACK TO PARK TO BE ABLE TO RESTART ENGINE. VERY DANGEROUS DESIGN! IF TRUCK STALLS WHILE MOVING THE DRIVER BECOMES A SITTING PASSENGER UNTIL CAN BRING TRUCK TO STOP TO RESTART IT.) THE TRUCK THEN STARTED SLIDING OUT OF CONTROL ON THE ICY PACKED SNOW COVERED STEEP MOUNTAIN GRAVEL ROAD BECAUSE I COULD NOT USE THE STALLED ENGINE TO SLOW THE TRUCK DOWN. ENDED UP SLIDING OFF THE DROP OFF EDGE OF THE ROAD AND THE TRUCK ALMOST ROLLED OVER. VERY CLOSE TO DEADLY SITUATION. AFTER TOW TRUCK GOT THE F150 BACK ON THE ROAD I WAS ABLE TO DRIVE THE F150 SAFELY DOWN REST OF ICY ROAD WITHOUT HILL DESCENT ASSIST.