A320 Dive Speeds: VD and MD

This page collects what The Flying Engineer wrote about VD and MD, the dive speeds of the Airbus A320. It follows the post "Diving into the A320: Dive Speeds", first published on 18 March 2012.

What the dive speed is

The dive speed is the absolute maximum speed for an aircraft. It must never be exceeded. Reaching it normally needs a dive, a steep descent, because the engines cannot overcome the drag in level flight at that speed. At the dive speed the aircraft vibrates heavily, and its structure is at risk. VD is the dive speed in knots, and MD is the dive speed as a Mach number.

A320 speeds

For the A320, the lower of the speed and the Mach number must always be respected.

• Dive speeds, VD and MD: 381 knots and Mach 0.89
• Maximum operating speeds, VMO and MMO: 350 knots and Mach 0.82
• Expedite descent speed, as set on the FCU: 340 knots and Mach 0.80

Why a fly by wire Airbus does not normally reach VD or MD

On Airbus fly by wire aircraft the flight envelope protections in normal law make it very hard to reach the dive speed. Suppose the sidestick is held fully forward and the speed passes VMO or MMO. The nose down authority then reduces smoothly to zero at about VMO plus 16 knots, or MMO plus 0.04. This is the high speed protection range. It does not promise that the speed will settle at that value.

What speeds above VMO and MMO mean

• If the aircraft reaches or passes MMO plus 0.04, or VMO plus 20 knots, a structural inspection is needed.
• Beyond MD, which is MMO plus 0.07, or VD, which is VMO plus 31 knots, the structure can break up. This applies to the A320 family.

How serious it is

The original post showed a Flight Crew Bulletin on speeds above VMO and MMO, and a drawing of how each speed matters to the flight control system. It also linked a test flight video of the Airbus A380 going to its MD, which is Mach 0.96. The test crew were said to dread taking the aircraft that far.