Singapore Airlines Boeing 777-300ER
Photo credit: N509FZ( https://tinyurl.com/5hnmz5vp)
62 Seconds of Deadly Severe Turbulence
Last week, one passenger died and about 30 passengers were injured when a Singapore Airlines Boeing 777 encountered severe turbulence over the Irrawaddy Delta, Myanmar.
The Singapore Ministry of Transport TSIB just released a preliminary a report that the rapid changes in G (Gravity) forces over a 4.6-second duration likely caused the injuries to crew and passengers. The incident was attributed to the swift transition between climb and descent and not the actual change in altitude.
Positive G forces pinned sitting passengers in their seats, while negative G forces sent unrestrained passengers hurtling upwards, causing unbelted passengers to become airborne. The aircraft experienced a transition from negative 1.5G to positive 1.5G within 4.6 seconds, causing airborne occupants to fall back down.
The rates of climb and descent were not excessive, being only half of the typical rates during normal take-off and descent. These changes occurred over 62 seconds, possibly consistent with the updrafts and downdrafts of thunderstorm (cumulonimbus) clouds.
For passengers who consistently wear seat belts, even when the seat belt signs are off, the risk of injury during turbulence is minimal. Keeping your belt on whenever seated is the best way to stay physically safe. Have you ever heard of a pilot being hurt in turbulence? They are never injured because they always wear their seat belts.
My sympathies go out to the injured cabin crew, who are most vulnerable due to their duties. When the potential for turbulence arises, the commander should immediately alert the cabin crew to cease their services and secure themselves with seat belts.
Encounters with turbulence powerful enough to injure people and damage aircraft interiors are frightening but generally uncommon. Turbulent clouds are visible on weather radar, and pilots avoid them at all costs. Even clear air turbulence, which is not visible, can be tracked by analysing shear rates on computer flight plans, used alongside with weather charts provided to pilots before departure.
A useful guide for pilots is the forecast shear rate values indicated under the “S” or “SR” column in the Computer Flight Plan. Company policies set tolerance levels, usually from 0 to 20. If the value is 0, no turbulence is expected. Values from 01 to 04 indicate light turbulence, 05 to 09 indicate moderate turbulence, and values of 10 and above suggest severe turbulence.
I once experienced a shear rate value of 16 on a flight over India, but the severe turbulence did not materialize. Nonetheless, I prepared my crew for a turbulent encounter because it is always better to be safe than sorry. Most times, the forecasts are accurate, ensuring we can take necessary precautions.
Disclaimer: This article is for general information only.
View preliminary report of the Singapore Ministry of Transport TSIB Report here https://www.mot.gov.sg/news/details/transport-safety-investigation-bureau-preliminary-investigation-findings-of-incident-involving-sq321