View Full Version : Blue Line vs. V2
I'm about 5 hours into my multi commercial, and I was wondering what the difference between blue line, and V2 is. From what I understand, V2 is called "Single engine safety speed", but what does that exactly mean?
PS: Multi flying is THE BALLS!!!!!!!!!!!!
This might help: http://www.aerospaceweb.org/question/performance/q0088.shtml
I'll take a stab at this. I think than "blue line" speeds are safe single engine speeds for light twins. I believe that they evolved due to the misunderstanding of the criticality of VMCa, which got a lot of light twin drivers into fatal spin accidents when an engine failed. Blue line speeds are always higher than VMCa, and do not change, as they are based on max certified takeoff weight. Maintaing blue line after an engine failure will give you an airspeed higher than VMCa.
V2 is a "takeoff safety" speed calculated for transport category aircraft. It is usually 1.2 Vs, and will allow for a positive rate of climb, directional control, and required second segment climb obstacle clearance in the event of the failure of an engine after V1, which is decision speed. These speeds must be computed for each takeoff and are based on takeoff weight and other factors. They can vary quite a bit.
FB3 is on the mark. Because of the large variances in weight, V2 for a transport catagory aircraft is calculated for each flight. For example, the V2 for a 757-200 can be 125kts at 160,000lbs or up to 160kts at 260,000lbs. This is for a flap 15 deg. takeoff. We have data for 1, 5, 15, and 20 degree flap settings. You can see where a single blue line wouldn't work. Transport Catagory aircraft are certificated under a different part of the FAR's as well. Much more testing involved.
Just an additional "gee wiz"..... The trim setting calculated for a departure is for V2 at that particular CG. This helps guarantee a resonable control force in pitch if you happen to pop one on T.O..
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