After retarding the throttles for an en route descent, what speed should you maintain until reaching 310 KCAS?

Maintaining a speed of 0.74 MACH during an en route descent after retarding the throttles is appropriate because it allows the aircraft to descend safely while still being within the operational envelope, maximizing fuel efficiency and maintaining control. The C-17 is designed to operate effectively in a range of speeds, and 0.74 MACH provides a good balance between efficiency and safety during descent.

As the aircraft descends, it is critical to manage airspeed within specified parameters to ensure the structural integrity of the aircraft and adhere to air traffic control requirements. Transitioning to 310 KCAS can be done smoothly from 0.74 MACH, which typically aligns with the descent profile and operational limitations of the C-17.

In contrast, higher speeds, such as 0.80 MACH or options like 360 KCAS, may exceed the safe operational limits during descent, leading to unnecessary fuel consumption and potential violation of speed restrictions. Lower speeds like 270 KCAS may not adequately support the descent profile or efficiency needed during the phase, potentially resulting in a less effective or too slow descent. Therefore, the selected speed of 0.74 MACH is the most suitable choice for the scenario presented.