Vd and Md on big jets
 

After viewing some company training material on a nasty jet upset/over speed I was wondering what the actual Vd and Md are on the big Boeings and what sort of tested margin is available.

For example, for a Vmo and MMO of 330/0.87 what would the Md be particularly? CS 25 just says Vc/Mc are limited to a max of 0.8 Vd/Md and a minimum Mach margin of 0.05 or 0.07 which suggests if you get an overspeed beyond 0.92 with the above Vmo you are rapidly becoming a test pilot?

if anyone knows or could point me towards the actual Vd/Md values established during flight test I’d be very grateful.

.Jwscud
Having spent around a half century in the industry, accumulating about 27,000 hours in PA18 thru to 777ER and instructed, examined and audited; I have never heard of Vc/Mc/Md, and Vd only in the context of a medical examination. What I have noticed though, with increasing frequency, is a focus on minutiae. For my money, if you don't know those values you are unlikely to be adversely affected.

Lighten up and concentrate on the relevant. Life will be far less stressful.

Maui

Jwscud

VD / MD is the Design Dive Speed
VC / MC is the Design Cruise Speed

Both used in flight testing for certification process (CS-25)

From the FAA Type Certificate Data Sheet of B767:

VD = 420 KCAS to 17,854 ft/.91M above 23,000 ft, linear variation between these points.VFC = 390 KCAS to 17,600 ft/382 KCAS at 23,000 ft/.87M above 26,000 ft, linear variation between these points.

From Airbus Safety First:

• How is VMO/MMO determined?

VMO/MMO is established with regards to the aircraft’s structural limits and it provides a margin to the design limit speed/Mach number VD/MD. VD/MD must be sufficiently above VMO/MMO to make it highly improbable that VD/MD will be inadvertently exceeded in commercial operations. Several certification criteria exist. As a result, on Airbus aircraft, MD is usually equal to MMO+ 0.07 and VD approximately equal to VMO + 35 kt.

The applicable VMO/MMO are indicated in each Aircraft Flight Manual. For example, VMO/MMO and VD/MD are given in the following table.[img]images/tab-1-Controlyourspeedincruise.jpg?r=0.6150412419197809[/img]

Sorry the table has not been copied properly. I will have to write down and post.

Maui,

Whilst your thrust on avoiding the minutae is well intentioned, here it is clearly wrong.

There have been several aircraft that have experienced upsets in the cruise recently, partly - I believe - due to the fear of Mmo exceedence by the PF. Usually these events have been caused by a sudden decreasing tailwind (or increasing headwind). The fear of watching the trend arrow heading quickly north to the red led the pilot to disconnect the ATHR in a panic that (IMHO) he was about to be reported through the FOQA system. All hell broke loose thereafter but that's the subject for another thread.

Airbus have been stressing recently - including at its most recent safety conference - that transient excursions beyond Mmo are OK as there is a huge margin to .... you guessed it Md (or Vd).

Had the pilots known of Vd or Md, they would been aware that the wings are not going to fall off at Mmo +1kt and their rather embarrassing upsets never would have occurred.

As Vilas said, Airbus FBW aircraft have a gap of about 35kts/M0.07 from Vmo/Mmo to Vd/Md respectively. I imagine that Boeing would be fairly similar, although flight testing procedures for non FBW certification vary slightly for the Dive.

Comp Stall. I am familiar with the scenario you discuss, and have had thrust levers slammed on backstops by PNF. As I explained to the miscreant there is about 30 knots above MMO before any maintenance action is triggered.
The fear of FOQA is the root cause of this syndrome, and the attitude negates the worth of the FOQA program. The key to resolution is more appropriate training and advice.
My advice: Be vigilant and sit on your hands: the system works well and rarely needs intervention.
Maui

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.

At those speeds the alleviated gust is decreased incrementally. Vb>Vc>Vd like that same for Mach limits