I used the flight level change feature for all climbs and descents, and it does a great job, even in up- and downdrafts over the mountains. If vertical speed mode (which it and a lot of other autopilots have) is used, the airspeed will vary. But that is not the way we fly. In FLC mode, airspeed is held constant and the vertical speed varies with the ups and downs, as it should. The Columbia is one tough Utility Category airframe, with a positive limit load factor of 4.4 Gs and a maneuvering speed of 158 knots, so even in the bumps while descending I had the FLC on 158 and there it stayed as the vertical speed ranged from barely down to 1,500 feet per minute down. For more descent rate, the speed brakes can be used.