An apparently “lesser known” fact about the Airbus A320 is the dive speed, its significance, and the associated consequences.

The dive speed is the absolute maximum speed above which the aircraft must not fly. Typically, to achieve this speed, the aircraft must enter a dive (steep descent), as the engines cannot produce sufficient thrust to overcome aerodynamic drag in level flight. At the dive speed, excessive aircraft vibrations develop which put the aircraft structural integrity at stake.

On the Airbus fly by wire aircraft, it is not possible to reach the dive speed, due to the flight envelope protections available in normal law. If the sidestick is maintained full forward, and the airspeed crosses VMO/MMO, the pitch nose-down authority smoothly reduces to zero at approximately VMO +16 / MMO + 0.04. This however, does not guarantee the airspeed stabilizing at this speed.

If MMO + 0.04 / VMO + 20kts is reached or exceeded, then a structural inspection is necessary. Beyond MD (= MMO+0.07) / VD (= VMO + 31kts) (A320 family), structural disintegration can occur.

Here are the speeds for the A320, in Mach number and Kts. The lesser value must always be respected, at all times:

Dive Speeds:

MD/VD = M0.89/381kts

Maximum Operating Speeds:

MMO/VMO = M0.82/350kts.

Expedite Descent (as on FCU, if available)

M0.8/340kts

The graphical representation of speeds above VMO/MMO, on the left (made by The Flying Engineer), gives you a clearer picture of the speeds, their significance for the FBW system, and the consequences.

To understand the seriousness of the VD/MD, take a look at the video below, which involves the VD/MD testing of the Airbus A380. The MD for the A380 is Mach 0.96, and the test crew dread taking the airplane that far.