Vd and Md on big jets

[Owain Glyndwr]

These are the CS25 requirements which define Vd/Md

(b) Design dive speed, VD. VD must be selected so that VC/MC is not greater than 0·8 VD/MD, or so that the minimum speed margin between VC/MC and VD/MD is the greater of the following values:
(1)
(i) For aeroplanes not equipped with a high speed protection function:
From an initial condition of stabilised flight at VC/MC, the aeroplane is upset, flown for 20 seconds along a flight path 7·5º below the initial path, and then pulled up at a load factor of 1·5 g (0·5 g acceleration increment). The speed increase occurring in this manoeuvre may be calculated if reliable or conservative aerodynamic data issued. Power as specified in CS 25.175 (b)(1)(iv) is assumed until the pullup is initiated, at which time power reduction and the use of pilot controlled drag devices may be assumed;
(ii) For aeroplanes equipped with a high speed protection function: In lieu of subparagraph (b)(1)(i), the speed increase above VC/MC resulting from the greater of the following manoeuvres must be established:
(A) From an initial condition of stabilised flight at VC/MC, the aeroplane is upset so as to take up a new flight path 7.5° below the initial path. Control application, up to full authority, is made to try and maintain this new flight path. Twenty seconds after achieving the new flight path, manual recovery is made at a load factor of 1.5 g (0.5 g acceleration increment), or such greater load factor that is automatically applied by the system with the pilot’s pitch control neutral. The speed increase occurring in this manoeuvre may be calculated if reliable or conservative aerodynamic data is used. Power as specified in CS 25.175(b)(1)(iv) is assumed until recovery is made,
(B) From a speed below VC/MC, with power to maintain stabilised level flight at this speed, the aeroplane is upset so as to accelerate through VC/MC at a flight path 15° below the initial path (or at the steepest nose down attitude that the system will permit with full control authority if less than 15°). Pilot controls may be in neutral position after reaching VC/MC and before recovery is initiated. Recovery may be initiated 3 seconds after operation of high speed, attitude, or other alerting system by application of a load factor of 1.5 g (0.5 g acceleration increment), or such greater load factor that is automatically applied by the system with the pilot’s pitch control neutral. Power may be reduced simultaneously. All other means of decelerating the aeroplane, the use of which is authorised up to the highest speed reached in the manoeuvre, may be
used. The interval between successive pilot actions must not be less than 1 second (See AMC 25.335(b)(1)(ii)).

(2) The minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts, and penetration of jet streams and cold fronts) and for instrument errors and airframe production variations. These factors may be considered on a probability basis. The margin at altitude where MC is limited by compressibility effects must not be less than 0.07M unless a lower margin is determined using a rational analysis that includes the effects of any automatic systems. In any case, the margin may not be reduced to less than 0.05M. (See AMC 25.335(b)(2)) the use of pilot controlled drag devices may be assumed.

Typically, at cruise altitudes, the 7.5 deg dive for 20 secs results in an altitude loss of around 2000 ft and a Mach increase around 0.06M.

As Sidestick says, there is no way you are going to get to Md in level flight at cruise conditions because the drag rise kicks in.