Here’s what the factory told me….

In the early 1980s Hughes proposed some optional product improvements for the 369F (mostly for external load operations). Hughes called the helicopter with these performance improvements the 369F+ (marketing designation). The FAA determined that these changes were “significant” and therefore required a model designation change to the 369FF. The production line was rolling and 20+ 369Fs were delivered to various customers around the world and our customer support personnel were tasked to make the necessary mods (see note 12 on the H3WE TCDS) and attach an additional data plate designating the helicopter as a 369FF. The goal was to convert all 369F to 369FF. Unfortunately a couple conversions were not documented or never done therefore we were forced to include the 369F in most service documents hence forward.

Basically Hughes increased the 5 minute shaft horsepower limit from 375 to 425. The corresponding 5 minute torque limited was increased from 327 ft-lb to 371 ft-lb. (this required a new torque gage with correct markings and a placard 50 KIAS limit).

Hughes also required a new drive shaft interconnect and Kamatics couplings.