Diving into the A320: Dive Speeds

One lesser known fact about the Airbus A320 is its dive speed: what it is, why it matters, and what happens if an aircraft gets close to it.

What is the dive speed?

The dive speed is the absolute maximum speed of the aircraft. It must never be flown above it. To reach it, the aircraft normally has to enter a dive, a steep descent, because the engines cannot make enough thrust to beat the drag in level flight.

At the dive speed, strong vibrations can build up in the structure. They put the structural integrity of the aircraft at stake. That is why the dive speed is tested only by test pilots during certification, and never flown in service.

Why an A320 should never get there

On the Airbus fly-by-wire aircraft, the flight envelope protections in normal law stop the aircraft from reaching the dive speed. If the pilot holds the sidestick fully forward and the speed goes past VMO or MMO, the nose-down authority of the sidestick smoothly reduces to zero at about VMO + 16 kt or MMO + 0.04.

That does not guarantee the speed will stop there. The rules after that are clear:

• If MMO + 0.04 or VMO + 20 kt is reached or passed, a structural inspection is needed after the flight.
• Beyond MD (MMO + 0.07) or VD (VMO + 31 kt) on the A320 family, the structure can break up.

The A320 numbers

Here are the speeds for the A320, as Mach number and knots. The lower of the two always applies:

• Dive speeds (MD / VD): Mach 0.89 / 381 kt
• Maximum operating speeds (MMO / VMO): Mach 0.82 / 350 kt
• Expedite descent (on the FCU, where fitted): Mach 0.80 / 340 kt

How seriously test pilots take it

To see how serious VD and MD are, look at the flight tests of the Airbus A380. Its MD is Mach 0.96, and the test crew openly say they dread taking the aircraft that far. They fly it once, carefully, to prove the aircraft can survive it, so that line pilots never have to.

For more on how Airbus aircraft are built and flown, see our aircraft manufacturers guide.