V1/V2/VR and density alt
 

Morning all

Background info.
On the 767 T/O chart has two regions high and low pertaining to density alt and flat rated thrust. The v1/v2 are higher by a few knots, and very rarely throughout the range, v2 is one knot out.

Question.
Why are the high and low speeds different , why is V2 almost unaffected.

I wont attempt to suggest an answer because I don't want to direct the answers in any one direction and although anonymous its embarrassing.

tar

V1 and Vr are highly Runway oriented figures, that is, Accelerate-Stop and Accelerate-Go must be contained within the available Runway Length/s. These are highly Ground Speed dependant, which, after wind correction, are TAS dependent.

As you would be well aware, CAS and Density Height yield the actual TAS, thus, correction to CAS must be made. (Actually CAS and Density Height yields Density Airspeed (DAS), which, at typical Takeoff altitudes is extremely close to TAS).

As V2 is an airborne speed, and NOT runway dependent, the only correction required is for conversion to EAS, which at higher Takeoff altitudes and V2 speeds is only a minor adjustment, 1 knot or so as you say.

Caveat - V2 must be reached by the screen height, thus there is a SMALL degree of Runway dependency, but this small correction is typically accommodated for in the Vr adjustment.

Regards,

Old Smokey