Vd — the 'design diving speed' — is required to be not less than 1.4 times the design cruise speed for a normal category aircraft. To receive certification, it must be demonstrated, possibly by analytical methods, that at Vd, the propeller, engine, engine mount, and airframe will be free from overspeeding, severe vibration, buffeting, control reversal and most importantly flutter and divergence. To provide some safety margin, Vne (the IAS which should never be exceeded in level flight, descent or other manoeuvre) is set at 90% of the lower of Vd or Vdf. Vdf is a diving speed which has been demonstrated by a test pilot without problem in test flights, and which must be lower than, or equal to, Vd.
Vne is always specified in the pilot's documentation as an indicated airspeed and usually marked on the ASI (the red line) but — unlike the performance airspeeds (also specified as indicated airspeeds and perhaps marked on the ASI) Vne is related to those structural characteristics and limitations associated with bending, twisting and flexibility which affect stability, control and even structural integrity. Limiting speeds are also associated with structural reaction to pilot induced loads and to gust induced loads. Limiting speeds could also be associated with other potential problems; for example, suction effects at particular speeds and attitudes might lead to canopy departure, or door or cowling security problems. Quite often Vne is limited by the critical flutter speed
VA
Recommended Max. Rough Air Manoeuring Velocity
design maneuvering speed (stalling speed at the maximum legal G-force, and hence the maximum speed at which abrupt, full deflection, control inputs will not cause the aircraft to exceed its G-force limit). Maneuvering speed is limited by aircraft structural characteristics.
בוקצור הVD זה מהירות הצלילה האפשרית בה בתנאים מתאימים הגלשן מסוגל לטוס בשלמות ,
לעומת ה VNE שזה המהירות המומלצת ע"י היצרן שאסור לעבור .
