Saturday, March 3, 2012

Rumble in the Jungle (Part 1) - Why Thunderstorms are Bad for Airplanes

Most flyers have experienced some sort of weather related delay. Sitting in the airport terminal for hours on end, your schedule is ruined, you complain and yell at the poor gate agent because you're now going to miss a wedding, a funeral, your kid's baseball game or your hot yoga class. All the airlines fault, right? "I'm never flying this airline again!" you say. Well, think that if you want but they have most likely just done you the biggest favor of your life. There are many things that you can blame the airlines for but delays due to severe weather is not one of them. For all the things humans have figured out how to control or circumvent, weather is not one of them. And, of all the little understood weather phenomenons that occur, the thunderstorm is probably one of the least understood. First, I can assure you that, no matter what you think, on your worst flight, unless you had the world's worst pilots, you have probably never flown directly thru a thunderstorm. It doesn't matter what you are flying in -  avoidance is key because, no matter what you are flying in, the laws of physics are not subject to repeal. Now, that's not to say you haven't possibly come close enough to a thunderstorm to get an autograph but, more than likely you haven't gotten "up close and personal" to see the ugliness that exists within. Let's delve into why.

To set the stage, three basic things need to exist for any run of the mill thunderstorm to form: moisture/water, instability in the atmosphere (read low pressure so things can "move around") and some sort of a lifting force (heat from the sun or a cold front moving thru to push things up into the air or some other force that can jam millions of tons of water 30,000+ feet into the air). That right there should give you the idea of the amount of energy contained within a thunderstorm. The best way I've heard it put is that if you compare a thunderstorm to a hydrogen bomb (you know, big mushroom cloud explosion that could evaporate an area the size of Rhode Island...sorry Rhode Islanders) a thunderstorm releases it's energy more slowly than a hydrogen bomb but has lots more energy than a hydrogen bomb. Impressive so far, right?

Let's look at all the nastiness that can occur in a thunderstorm.

First and foremost is turbulence and wind. If you refer to my post on turbulence, you'll see that there is some turbulence labeled as "severe" and "extreme". While these types can, in rare instances, occur outside of thunderstorms, you have a pretty good chance of finding them in a thunderstorm. Why there? Go back to those basic ingredients necessary for a thunderstorm. Instability means well.....instability! Air moving in many different directions unabated. Think that can create turbulence? You bet it can! Next - a lifting force. Some supercell thunderstorms like the ones that occurred last night can exhibit updrafts (air moving upward that develops a thunderstorm) in excess of 160mph! In aviation terms, that's about 12,000 feet per minute (fpm) or well in excess of the climb rates of commercial airliners....and even many fighter jets! Updrafts are bad enough. Now take that updraft and turn it into a downdraft with similar speeds. It doesn't take anything more than simple math to tell you what happens to an airplane capable of climbing at 5,000fpm that is stuck in a 12,000fpm downdraft (or what would be referred to as a "microburst"). The word "splat" comes to mind. Even a more "benign" storm or your run of the mill summer thunderstorm can have 5,000+fpm updrafts and downdrafts. Consider that your average Boeing 737 can climb roughly at a maximum of 6,000fpm lightly loaded at sea level/takeoff (that number goes down quickly as you get heavier and higher) and you can see why you don't mess with mother nature. Now, imagine all of this happening in close proximity to the ground and you get what is called "low level wind shear"(LLWS). There is a famous crash of a Delta Lockheed L-1011 that occurred in 1985 in TX that resulted in the development and subsequent mandating by the FAA of modern LLWS detection systems to ensure similar crashes never occur again. In addition to all of that, the stresses imposed on airplanes by those severe updrafts and downdrafts can greatly exceed the design limits of the airplane and the airplane can, well....break. Oddly enough, because of the power of updrafts and downdrafts, it has, over the years, rained some pretty crazy stuff. Fish and birds have gotten sucked up in an updraft and ultimately rained down and, yes, as the song goes, it has even rained a few men as some unfortunate skydivers have gotten caught up in thunderstorms. Here's one great story you have to read!
Would YOU fly thru this? Didn't think so.
An impressive supercell thunderstorm.

The second reason you don't tangle with thunderstorms is water. Wait, water!? You're probably thinking "you're telling me water can crash a plane!? Great! I'm never flying again!" Well, hold on a second and I'll explain. What happens to water when the temperature drops below 32 degrees Fahrenheit? It freezes right? Wrong! It sometimes freezes. 32 degrees is the magic number at which ice melts not at which water freezes. So now that we've thrown 5th grade science out the window, how does ice form then? Well, it forms when that below 32 degree water hits something solid that is below 32 degrees as well like airplane wing!  Now, lets join the rest of the world and use Celsius. Water freezes at 0 degrees Celsius (32F). Every 1,000 feet up you go, the temperature drops on average by about 2 degrees Celsius. So if you're on the ground on a nice 80 degree F (about 27C) summer day, the "freezing level" is at about 14,000 feet. While all commercial airliners have very robust and very capable ice protection systems, they can easily be overwhelmed by the enormous volume of water found in thunderstorms.  Trust me when I say this - ice does not do nice things to an airplane. Aside from that, what happens when you take that very large volume of water and stick it into one of those updrafts I talked about? Well, at some point the water does start to freeze together and get heavy enough that it starts to fall and you get hail. If you think hail is bad when it leaves little dents on the roof of your car sitting in your driveway, think of what happens when it hits an airplane doing 450 knots (or about 520mph). In extremely powerful thunderstorms, hail can actually shoot thousands of feet out of the TOP of the storm! The final thing I'll say on the water front is that there is so much water that there have been extremely isolated incidents where water in a thunderstorm combined with hail have overcome jet engines and have caused them to flame out. That's a lot of water!

The third but far less important reason you don't tangle with thunderstorms is lightning. Now, many airplanes have encounters with lightning regularly and they don't fall from the sky. In fact, one estimate says that every commercial airliner is hit on average once a year. Airplanes can and regularly do survive lightning strikes. Wires are shielded to protect avionics, fuel tanks are designed to withstand sparks created by lightning and the skin is typically made of aluminum which means it conducts the charge around the outside of the airplane. But, let's face it, there is a lot of energy contained in a bolt of lightning and if you can avoid it, you probably should. Just because the law of averages is stacked very comfortably in favor of airplanes surviving lightning strikes, why throw caution into the wind? And where does lightning come from? Well, all that water caught up in all that wind rubs together which causes the friction that creates the charged particles to form lightning. So, in all reality, you want to stay away from the stuff that creates the lightning even more than you want to stay away from the lightning itself. And, pilots tend to like happy passengers. There's a high "underwear change" factor when you have a plane that gets hit by lightning and, if you think thunder is loud on the ground. Try being 2 feet away from the lightning bolt that the thunder originates from.

By now, you're thinking "I AM NEVER FLYING AGAIN!" and, if you stopped reading at this point you'd probably be right in your thinking! But, planes do safely navigate around thunderstorms all the time. In my next post, I'll talk about how your pilots safely deal with thunderstorms.

Until then.....

Blue Skies,

1 comment:

  1. Jeff, this is amazingly educational. Thank you! You know, the most important reason we don't fly through thunderstorms is we want to keep our passengers safe.

    For all our us, we want to get moving as much as you do. But your safety is our most important concern. So when we stop the continuation of the flight because of weather- it's for your safety. Know that a delay could be the time you get to live to be late.