Tags
0.89, 381kts, A320, A380, Airbus, Airbus A380, Authority, Dive Speed, Expedite Descent, Flight Test, High, HSP, Mach, MD, MMO, Overspeed, Proection, Sidestick, SPeed, Structural Damage, VD, VD/MD, VMO, Warning
An apparently “lesser known” fact about the Airbus A320 is the dive speed, its significance, and the associated consequences.
A Flight Crew Bulletin detailing the dive speeds and other speeds above VMO/MMO. (Click to enlarge)
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:
Graphical representation of the speeds, their significance & consequences. HSP is High Speed protection range.
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.
The Flying Engineer 
Great post. Just to add my two cents, on the A320 structural disintegration due to over-stressing needn’t necessarily occur right above VD/MD (especially at non-enveloping g’s); the concern rather is about buffeting or flutter above the design dive speed (the buffet on the A380 video is pretty scary!).
One of the easiest ways to over-stress the airframe, oddly, isn’t at such high speeds; rather it’s at the VC/MC knee point at a pull-up of greater than the certified 2.5g with heavy payload and not much fuel in the wings to relieve the upward bending.
For that, you’d have to be unlucky enough to be close to your MZFW, with the aircraft in alternate law, and facing the need to pull-up that hard, though – thank goodness that’s hardly likely!
This is a great post. And interesting. In all the years of flying jets… now the A330. I have never heard of the “Dive” speed before. Is this A320 specific, or company driven? Perhaps my company just didn’t want us to know. So we’re faced with MMO/VMO. Would you know if the A330 has a dive speed and what it is?
Thank you!
PS… I figured out the problem. This is really Karlene. I guess I am Happy54. lol
Hello Karlene,
The Dive speeds are determined by the airframe manufacturer.
I sadly do not have access to the A330 FCBs. However,on page 12 of FAA TYPE CERTIFICATE DATA SHEET NO. A46NM (concerning the A330), the following speeds have been mentioned for all variants of the -200 & -300:
Airspeed Limits (Indicated Airspeed, IAS, unless otherwise stated):
• Maximum Operating Limit Speed/Mach, VMO/MMO 330 KIAS / .86
• Design Diving Speed, VD 365 KIAS / .93
• Design Maneuvering Speed, VA Refer to AFM Performance Section
Hope this answers your question!
Vd is carried out on all certificated a/c during the development program and FAA require a Vd demonstration for all externally mounted protrusions (ie) antennae, radomes, etc.on modified aircraft.
The principle behind Vd is as follows:
The maximum speed the pilot is expected to fly is at Vc. Vd is then calculated by assuming a 15 degree dive, and it’s there to provide extra margins over the maximum allowed speeds, to account for tailwinds, upsets due to gust, etc.
In other words, it’s not an important speed as far as flying is concerned; the importance mainly lies for structural designers, and the guys doing the flutter/buffet calculations.
As Barry Hubbard says, Vd needs to be demonstrated in flight test, as it is a part of the aircraft’s certificated operational regime. No conspiracy there!