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Old Oct 20, 09, 5:46 pm   #1
 
Join Date: Oct 2008
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BA2156 19/10 Over Atlantic: Severe turbulence and emergency decent

'A British Airways Boeing 777-200, registration G-VIIR performing flight BA-2156 (dep. Oct 18th) from Antigua (Antigua and Barbuda) to London Gatwick ,EN (UK), was enroute at FL390 overhead the Atlantic about 8 minutes ahead of their next waypoint N46 W30, when the crew reported severe turbulence and requested to descent. Oceanic control was unable to clear the airplane due to conflicting traffic at FL380, FL370 and FL360. The crew then advised, that they were executing oceanic contingency procedures and performed an emergency descent to FL350, the crew reporting they had the conflicting traffic on their TCAS sceen. After the airplane had levelled at FL350, the flight was cleared to proceed on FL350 and reached London for a safe landing about 2:45 hours later about 15 minutes ahead of schedule.'

(from avherald.com)

I've flown this route before, anyone here on board to say what it was like?

Never been in 'severe turbulence'. Most people don't even know what 'severe turbulence' even is.

Must have been awful for the flight crew to make such a manoeuvre.

Last edited by hugolover; Oct 20, 09 at 5:53 pm.
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Old Oct 20, 09, 6:26 pm   #2
 
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Quote:
and performed an emergency descent to FL350
Perhaps the wrong choice of word in the report, but suffice to say, if they had the other aircraft on TCAS and were experiencing uncomfortable continuous rough ride, they chose a valid option. It is not really something I would expect to see reported in the media.

If you define discomfort as half the passengers barfing in continuous rough ride, as opposed to the shock one single, violent CAT-turbulence, I can imagine that they just had enough of the ride eventually. Atlantic weather systems are not pleasant by any means.
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Old Oct 20, 09, 6:29 pm   #3
 
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Quote:
Originally Posted by Lucifer UK View Post
Perhaps the wrong choice of word in the report, but suffice to say, if they had the other aircraft on TCAS and were experiencing uncomfortable continuous rough ride, they chose a valid option.
Well I'm just curious to hear what 'severe turbulence' is really like. Surprised there were no injuries among the cabin crew if they were walking about.
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Old Oct 20, 09, 8:45 pm   #4
 
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Originally Posted by hugolover View Post
Well I'm just curious to hear what 'severe turbulence' is really like. Surprised there were no injuries among the cabin crew if they were walking about.
I found an interesting link over here that describes quite a few experiences and information about detailed turbulence classifications:

I thought I knew severe turbulence....
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Old Oct 21, 09, 6:37 am   #5
 
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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

Last edited by Bellerophon; Oct 21, 09 at 6:52 am. Reason: Formatting
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Old Oct 21, 09, 8:27 am   #6
 
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Posts: 356
Thanks Bellerophon - really informative post!

Quote:
Originally Posted by Bellerophon View Post
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
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