VD represents the design diving speed of an aircraft, which is the highest true airspeed the manufacturer tests the airframe against before structural failure or catastrophic flutter occurs. It is established during the engineering and flight test phases. Once VD is proven, engineers set the never-exceed speed at a lower margin, typically 10 to 20 percent slower, to keep operational pilots safe.
Heard on the job
Test engineering wants us to push the prototype right to VD on this next dive to validate the flutter models.
Insider tip
Line pilots will never see VD on their airspeed indicator, as the red line is drawn at the never-exceed speed to provide a structural safety buffer.