In Air Battles (namely air RB), Setting another players aircraft on fire should automatically count as severe damage.
While not as severe as control loss, Fires cause such significant damage to an aircraft that it severely hinders that enemies ability to continue.
Engine fires are pretty self explanatory. Setting them on fire means A) Reduced acceleration B)Reduced top speed C) Zero acceleration (if single engine or reliant on ALL engines to maintain flight such as the SM.79s) D) Loss of control (Aircraft dependent if multi-engine) E)Reduced manoeuvrability
Drop tank fires severely reduce an aircraft’s time in battle
Internal fuel fires reduce the enemy’s time in flight (to potentially zero instantly), cause airframe wide damage and can result in a kill by the fire alone.
In conclusion, setting a plane or helicopter on fire causes more than enough damage by its mere existence to warrant it resulting in a severe damage