One generally accepted definition of
Severe Turbulence is as follows:
Severe Turbulence causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control.
Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking impossible.
It may additionally be categorized as Occasional, Intermittent or Continuous.
- Occasional......Less than 1/3 of the time.
- Intermittent...1/3 to 2/3.
- Continuous.....More than 2/3.
Without knowing any details of this event, it strikes me as unlikely that an
Emergency Descent was actually performed.
An Emergency Descent is a high rate-of-descent manoeuvre, generally only performed when the cabin is losing, or has lost, pressurisation. It is performed in order to get the aircraft down out of the higher altitudes (where the ambient air pressure is so low it will not sustain life) and down to altitudes where the air is breathable, as soon as possible.
My assumption would be that, because ATC were unable to provide the desired descent clearance (due to other aircraft in the vicinity at lower, intervening, altitudes) the Captain declared an
Emergency and used that authority to descend, at his own risk, to a lower altitude.
There are published contingency procedures on the
North
Atlantic
Track system in place to allow one to do this, with a minimum of risk, when necessary.
Briefly, this involves turning left or right off your published NAT (the next nearest NAT will be 60 nm away) until you are 15 nm displaced, turning to parallel your original track, and only then descending to your desired altitude.
All other aircraft in the vicinity would have been advised by the crew as to what they intended doing, on a VHF air-to-air frequency that is monitored by crews over the Atlantic, and the crew would have been keeping a very close watch on their
Traffic
Collision
Avoidance
System screens for any possible conflict.
I would wager that the descent itself, once the aircraft had been correctly positioned away from its NAT, was in fact a measured and unhurried descent (the opposite of an emergency descent) in order to avoid any possibility of triggering a warning on TCAS.
An unusual procedure certainly, but by no means unheard of. I've done it twice in a career, once following an engine failure, once due to cold fuel.
Best Regards
Bellerophon