Those boring announcements are there for a reason...

“All right, you can go ahead and let them up now.”

In pilot-speak, this is the captain telling me, the first officer, that he or she is satisfied that the flight is going to be smooth and the passengers should be free to move about the cabin. When I’m not flying the leg I get to make the announcement.

“*BING* Good morning, folks, from the flight deck. We’re currently at our cruising altitude of 36,000 feet. It looks like our ride should be nice and smooth today so I’m going to go ahead and turn off the ‘fasten seat belt’ sign. You’re free to move about the cabin but please keep your seatbelts on whenever you’re seated, just in case we run into any unexpected turbulence. Also if you could try and keep the aisles clear our cabin crew would really appreciate it. We’re showing an on time arrival, when we get a little closer I’ll be back to let you know what the weather looks like. Thank you for flying with us and have a great flight.”

Of course I don’t think 90% of the passengers actually listen to this announcement. I can’t blame them; before I was an airline pilot I didn’t either. But there’s a reason we make the announcements we do.

The most ignored direction we give our passengers is “please keep your seatbelts on whenever you’re seated.” I understand that they don’t want to wear the seatbelts any more than necessary. After all, if the flight has been smooth so far, and if we hit any turbulence, they’ll have time to get their seatbelts on, right?

I realized a few weeks ago that I didn’t really have a good way of explaining turbulence and what causes it to appear out of, literally, the clear blue sky. So here’s what I hope is a good explanation, concentrating on high-altitude turbulence that jets encounter.

While flying at high altitude you can encounter turbulence both within clouds and in clear air. The former is caused by convective currents.

Inside a cumulus cloud, air is cooling and sinking as well as warming and rising. You’ve seen this if you’ve watched a thunderstorm “build” (as warm, unstable air is lifted) and dissipate (as the warm air cools, it condenses, becomes saturated with water, and sinks again – this is when it’s raining). All these convective currents buffet the airplane as it flies through them. Luckily most jets fly above where most cumulus clouds thunderstorms form, with the exception of very strong thunderstorms. These can have vertical extends well above 45,000 feet, and these we fly around.

This kind of turbulence can be very dangerous, but since flying into thunderstorms is so risky, we use weather radar to go around all but the smallest cumulus clouds. Contrary to popular belief, we don’t TRY to make the ride rough. But sometimes, due to traffic or just the widespread nature of a storm system, we can’t go around it. However, since your flight crew knows that a cumulus cloud will hold at least some turbulence, we would never have the seat belt sign off if we anticipated flying through any.

The other common form of turbulence we encounter is called mountain wave turbulence. This is caused by air currents moving over the tops of mountains. On the leeward side of the mountain the air becomes very turbulent. I haven’t flown across any tall mountain ranges so I can’t offer much insight on mountain wave turbulence besides that.

The most critical form of turbulence with concern to passenger safety is Clear Air Turbulence, or CAT. This is turbulence that occurs without any sort of visual cue or warning, and is the reason we ask passengers to keep their seatbelts fastened whenever they’re seated.

CAT is caused as an aircraft moves between different bodies of air that are moving in different directions at different speeds. This occurs most often around the jet stream or frontal systems. I’ve flown with captains that were good enough at reading weather reports that they could tell by the “winds aloft” report where there would be some turbulence. Winds aloft is a weather report that lists the speed and direction of winds at different altitudes at certain locations – over commonly used navigation facilities and airports. The reporting starts at 3000 feet (with a few exceptions) and continue up to FL390 (39.000 feet).

By interpreting the change in wind speed and direction along the route, you can determine where you’re most likely to encounter CAT. If we use this technique to establish the likelihood of hitting CAT on the flight, we can anticipate the need to keep the fasten seat belt sign lit as well as ask the flight attendants to remain seated until we are sure the danger is past.

A quick search of the NTSB’s Aviation Accident Database revealed over 30 aircraft incidents involving CAT over the last 10 years. In the majority of the accidents the only injured people were the flight attendants, who are especially vulnerable to turbulence since they are not seating during the majority of the flight. Most of these incidents resulted in G loads to the airplane of less than +2Gs and -1Gs, which is about what you feel on a roller coaster. Imagine being on a roller coaster without your seatbelt on!

In several accident reports I’ve read, the flight crew had no reason to expect CAT. At most, weather was observed several miles ahead. In the vast majority of cases, no warning was given to the cabin. In a Northwest Airlines incident in 1972, five flight attendants and nine passengers were injured, two of them seriously, when the Boeing 747 entered “an area of unforecast and unexpected severe clear air turbulence when numerous occupants did not have their seatbelts fastened.” (NTSB report number: NTSB-AAR-72-27)

So please, when the flight crew asks you to keep your seatbelt on whenever you’re in your seat, do it! I guarantee your captain and first officer are doing the same.

(For more details on weather, pick up “Aviation Weather,” another great FAA publication (AC 00-06A).)