Airbus A380

The Airbus A380 tag on the old Flying Engineer blog held a post about the dive speeds of the Airbus A320, with a nod to A380 flight testing. It was written in March 2012.

What the dive speed is

The dive speed is the absolute top speed an aircraft must never exceed. An aircraft usually has to be in a steep descent to reach it, because the engines cannot overcome drag in level flight. At that speed, strong vibration can threaten the structure.

Protection on Airbus fly by wire aircraft

In normal law, the flight envelope protections make it impossible to reach the dive speed. If the sidestick is held full forward and the airspeed passes VMO or MMO, the nose down authority fades smoothly to zero at about VMO plus 16 knots or MMO plus 0.04. That does not guarantee the speed will settle there. If the aircraft reaches MMO plus 0.04 or VMO plus 20 knots, a structural inspection is needed. Beyond MD (MMO plus 0.07) or VD (VMO plus 31 knots) on the A320 family, the structure can break up. The lower value must always be respected.

A320 speeds

• Dive speed MD/VD: Mach 0.89 and 381 knots.
• Maximum operating speed MMO/VMO: Mach 0.82 and 350 knots.
• Expedite descent speed set on the FCU, if available: Mach 0.8 and 340 knots.

The post included a chart of the speeds above VMO/MMO, the high speed protection range and what each means for the fly by wire system. It also linked a video of dive speed testing on the A380, whose MD is Mach 0.96, a speed the test crew do not enjoy reaching.