Happydays, VMCA (Minimum control speed airbourne) does change dependant on, most importantly, altitude and temperature. Bear in mind that two of the most essential conditions used to determine VMCA are FULL thrust on the remaining engine(s) and FULL rudder deflection to control the asymetric condition.
To keep it simple, an engine is producing less thrust when firewalled at 35000' than it is at 5000' (due to, amongst other things, a reduction in air density) resulting in a decrease in the minimum control speed. So, whilst full thrust is applied in both instances, external conditions dictate that the actual value of the thrust being produced is less at altitude. In effect, the rudder is more effective and is able to control the aircraft to a lower speed.
The conditions used to determine VMCA are set, but the nature of the conditions change with variations in things such as altitude, temperature, density etc.
Hope this helps.
GD.
Last edited by Go_Detent; 18th January 2007 at 17:40.