They both return false in NavMovementComponent.h. AFAIK the only difference between IsFalling() and IsFlying() is that IsFalling() is supposed to return true only when moving downwards in air while IsFlying() will return true when moving up or down in mid-air.
Instead you need to do a line trace to see if there is anything within a metre of the bottom of the vehicle. See the following. SQ_Go is the road sound producer which is on the bottom of the vehicle. It goes silent when the vehicle is in the air. The line trace does the check for this. It triggers TRUE if a line drawn 1m directly down from the vehicle hits anything. Hence by doing a NOT of the result you know if you are in the air.
You can drag from the vehicle movement component and use the GetWheels function to return an array of wheels that you can then plug into the IsInAir Function as shown below.