I've heard that some pilots have this difficulty to determine the correct use of both features. So, which is the difference between using the vertical speed, instead using the flight level change during a descent? When, and how to use it?
FLC mode maintains airspeed during a climb or descent, while VS mode maintains a specific vertical speed.
Often air traffic controllers will request that you "maintain 250 knots in the descent" or something to that effect, which is much easier to achieve when using Flight Level Change. As another answer points out, by maintaining airspeed, climbs are made safer as the autopilot will not attempt to maintain a rate of climb regardless of whether or not the airspeed is safe.
FLC mode is typically only available in advanced autopilots, and as far as I know they need to connect to an Air Data Computer to get the necessary information.
How to use FLC would depend on the equipment available to you:
- aircraft with auto throttle will maintain speed and an appropriate rate of descent automatically.
- for aircraft without auto throttle you'd set your target altitude, enable FLC, adjust your target speed if needed, and set your throttle for climb or descent to maintain the desired altitude change rate.
I'm referencing the Garmin GFC700 as it's the only autopilot I've used with FLC mode, but I imagine the same steps and rules would apply.
Vertical speed should never be used when climbing because you could stall if insufficient power is added for the requested climb rate. Using FLC when climbing guarantees you maintain the best airspeed (best rate or best angle, depending on the situation)
On the other hand, when descending I always use VS mode because it gets you down to the proper altitude in time (if you've calculated the required VS before starting your descent). It also prevents physical discomfort for you and your passengers. If you use FLC during the descent you have to be very careful with power in order not to exceed safe airspeeds (in addition to the discomfort caused by excessive descent rates)
Not an expert opinion but just a personal opinion...
They are different modes that control aircraft pitch in different ways, and as a pilot you select what you want based on your current requirements. Note that the modes even behave differently depending on the aircraft / autopilot, so referring to the actual aircraft/autopilot documentation is essential.
Many autopilots don't even have a Flight Level Change mode, but instead have an Indicated Airspeed (IAS) mode.
Autopilot modes like these are typically "dumb" and just do what they are told, without taking anything else into consideration.
Regardless of which mode that is used, remember that it is the pilot responsibility to monitor the aircraft and to make sure that it is doing what they want it to do!
Indicated Airspeed / Flight Level Change
- This mode usually adjusts the pitch of the aircraft in order to maintain a specific airspeed. In simple systems, the only inputs that are used are airspeed and aircraft pitch.
- You usually control the vertical speed of the aircraft by making power adjustments:
- By reducing power, your vertical speed will decrease.
- By increasing power, your vertical speed will increase.
- In some systems your actual altitude is compared to the selected target altitude and a minimum climb or descent is maintained even if the airspeed can't be maintained.
- In some systems the aircraft altitude is used to change the behavior (for instance, in one aircraft that I have flown, if you are below 8,000 ft MSL and hit FLC, it will descend at 1,000 FPM but if you are above 8,000 feet it specifies a 250 KT IAS hold).
- In some systems, the pitch is adjusted to maintain a specific climb/descent schedule which automatically changes based on altitude. For instance, a descent schedule might be Mach 0.80 until 29,000 ft., 350 KIAS until 10,000 ft. and 250 KIAS below 10,000 ft.
- Another aircraft that I have flown in the past even had two climb schedules. You could toggle between the "normal" climb and the "high speed" climb by hitting the FLC button more than once.
- A lot of autopilots (especially older ones) tend to oscillate around the target airspeed and constantly pitch up and down while trying to fly a specific airspeed. For this reason (keeping in mind that passenger comfort could be an issue) many people avoid speed modes, especially at higher altitudes.
- This mode usually adjusts the pitch of the aircraft in order to maintain a specific vertical speed. Again, it is usually a "dumb" mode and doesn't look at anything but vertical speed and aircraft pitch.
In "full up" FMC/VNAV/Autothrottle airplanes, Level Change tends to be an "all or nothing" option for power: full climb power to go up, and flight idle to come down (with pitch holding the commanded speed in both cases). With Vertical Speed, the pitch holds your climb rate constant, and the throttles vary to hold your airspeed. With a sufficiently large V/S value commanded, you reach either full climb thrust or flight idle, and then the throttle can't affect airspeed any more -- so you can lose airspeed with a steep climb commanded, or gain it with a steep descent.
And then at some point, you look up & wonder why we're so fast/slow, and realize that V/S has done you in, and you go back to flying the airplane. :)
One reason that I personally prefer V/S over level change in certain circumstances is that the latter tends to be fairly aggressive about maintaining speed, so in gusty conditions the autopilot (in level change) will end up pushing & pulling quite a bit to hold the speed +/- a knot or two. I think that tends to be an uncomfortable ride, so I'll go to V/S and let the speed fluctuate (the autothrottles can chase it, or I deselect them & just leave a stable power setting) while the pitch attitude stays constant.
At some point, you go back to a speed mode (i.e. once you level off), but for the stable pitch instead of constant push/pull/push/pull, V/S has its advantages. Just don't ignore it long enough to let your speed do undesirable things to you!