AF 447 Thread No. 8

[Ian W]

ITCZ Turbulence is not like 'academic' turbulence

From DozyWB

Lyman,

This is the definition of "Moderate" turbulence, but the bolding is mine:

Quote:
Turbulence that is similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed.
or
Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude.
From : Tutorial: wind shear and turbulence

Quote:
The meteorological categories for wind gusts in general (as measured with an anemometer) are:
Category 1: weak — ≥ 5 m/s to <10 m/s
Category 2: moderate — ≥ 10 m/s to <15 m/s
Category 3: strong — ≥ 15 m/s to <25 m/s
Category 4: severe — ≥ 25 m/s
The meteorological categorisation restated for vertical gust measurement might be:
Weak — ≥ 16 fps to <25 fps
Moderate to strong — ≥ 25 fps to <50 fps
Strong to severe — ≥ 50 fps to <80 fps
Extreme — ≥ 80 fps (or 66 fps [20 m/s] might be used)
Note that Moderate doesn't even come close to the equivalent of 100kts (50fps = 29.6kts).

Outside of the numbers, I sat in the sim - I saw it happen and I *heard* it happen. I experienced an infinitesimal fraction of the concern and fear those pilots felt as the aircraft ran away from me. I stand by the assertion that the climb, warning and deviation from assigned altitude were predominantly caused by deliberate action by the PF.

I can't convince you - of this I'm sure. But I've gone to further lengths than I ever have before - assisted by people to whom I am in eternal debt - to prove that I'm not talking out of my ar*e here, and I'm as certain as it's possible to be that the aircraft did nothing to make things harder for them.

Dozy,
I know this is late - but you really should have looked at turbulence as it occurs in the Inter-Tropical Convergence Zone. These are not weak mid-west thunderstorms - but a LOT more powerful:

"ITCZ thunderstorms are slightly different from midlatitude storms. The differences are primarily due to the different atmospheric conditions in these 2 locations. The strong thunderstorms we endeavor to avoid in the midlatitudes are often due to 2 distinct air masses lying on top of each other.

In these situations, warm, humid air gets trapped beneath a middle to upper-level cold and dry pool of air. A small temperature inversion caps the surface air until it builds up enough energy to burst through the cap and rise explosively.

It is the excessive speed of the updraft and corresponding down drafts of cold, dry air that give these storms their immense danger to aviation.

Vertical air movement can easily exceed 50 kts and has been measured in excess of 100 kts. Furthermore, the boundary between a strong updraft and a strong downdraft means a violent shear zone with air flowing through it at a combined speed of potentially well over 100 kts."

Wx Brief

Remember you are in moderate turbulence - until you hit extreme turbulence. If it was easy to see all the time it would always be avoided.