If you ever needed proof of why it’s a good idea to keep your seat-belt attached when seated on a plane then this is it. Twelve people aboard a TAM A330 flight from Madrid to São Paulo, Brazil, were injured when the plane encountered severe turbulence off the northeast coast of Brazil early this morning. Any person or item not tied down was thrown against the ceiling as the plane suddenly descended. The crew diverted to nearby Fortaleza where the twelve people were taken to hospital.
In-flight turbulence is caused when an aircraft encounters an abrupt change in wind-direction and/or wind-speed as it flies through the air. This change in wind changes the speed of the airstream (airspeed) under and over the wings, which changes the lift they produce. A sudden loss in airspeed causes a sudden loss in lift and hence a sudden drop of the aircraft.
Convective clouds, such as the cumulonimbus of thunderstorms, are well-known sources of turbulence for aircraft as they contain strong up- and downdrafts. Pilots know to avoid flying through thunderstorms and on-board radar can easily show where to avoid them. Clear-Air Turbulence (CAT) – as the name implies – is turbulence encountered in clear air, i.e. away from these convective build-ups especially common in tropical regions and near the jet stream in temperate latitudes. CAT is very difficult to forecast accurately and therefore catches some flights out unawares. On-board radar can detect it ahead in some instances, which is why you may sometimes see the captain turn on the seat-belt sign and slow down slightly before you start feeling bumps, however, in many cases no warning is possible and accidents such as this one occur.
The Sigwx (Significant Weather) chart below is the actual chart the crew would have consulted before setting off on their flight on Sunday evening. These charts are available for every 6-hour period, and this one is valid for 0600Z (GMT) this morning. I have marked location of the turbulence with a red square. There are many strange symbols on these charts, which I won’t explain now, but the thing to note is the absence of any symbols near the incident location. There is an area of Isolated Cb (cumulonimbus) clouds to the north, with clouds extending up to Flight Level 450 (FL450, 45,000 ft). The sub-tropical jet stream (heavy black line with barbs and pennants) is well to the southeast and no CAT areas are nearby. This chart would suggest that turbulence was unlikely in the area where it actually occurred, and if anything they should have encountered it earlier on, further north in their route in that area of Cbs.
Of course we must await the outcome of any investigation there may be into this accident, but for the moment it serves to illustrate the importance of listening to the cabin crew briefing in which they recommend keeping your seat-belt attached at all times.
For further updates on the facts of this flight see AVherald.com.