Despite the fact that the Pratt & Whitney Canada PT6C-67A engines are installed on the first BA609 prototype tiltrotor, Bell is letting the development schedule of that aircraft slip, with its previously planned first flight moving from last month to sometime this summer. Assembly of a further pair of prototypes has also slowed dramatically. According to sources, the delays are less due to troubles with the V-22 Osprey military tiltrotor than with continued softness in the commercial vertical lift market.