Air Pockets

Whoa!! We just hit an air pocket! I have heard that so many times. Or I’ll hear someone tell me about the time that they “dropped 1000 feet” during a flight.

First, let me assure you that there is no such thing as an air pocket. From what I hear from the people who ask about it or describe having hit one, it’s a ‘void’ of air or a place where there wasn’t any air and therefore, they felt a bump. If they did exist in the natural world, they (by that definition) would be vacuums in atmosphere. IF they were to exist at altitude, then they would also have to exist near the ground right? How many times have you been walking down the street, skiing, laying in a hammock or anything else and had a complete momentary loss of air because you were suddenly in a vacuum?

Next, “we dropped 1000 feet”. That statement is a highly inaccurate exaggeration of the sensation that someone had when their flight hit a bump. Let’s all agree that 1000 feet is 1/5 of a mile (a mile being 5280 feet) and that 60 miles/hour is equal to 1 mile/minute.
We can also agree that 3600 miles/hour is 1 mile/second (60x60=3600). If you were to ‘fall’ or ‘drop’ 1000 feet in a second, (which is the time frame I hear in most traveling stories) you would be traveling 1/5 of a mile in a second so therefore you would be going 1/5 of 3600 miles/hour for just a second. That’s an instantaneous drop at 720 mph (1/5 [or .2] x 3600 = 720). If you were to experience that, it would be like hitting a brick wall at over 700 mph and you’d die from the force, not to mention the plane would break apart.

So what’s really happening?

Air is a fluid just like water, only the molecules are farther apart (and of different composition). If you ever look at a river, you’ll notice that the water moves pretty smoothly when going slow and its rougher when moving fast. Also, there are little spots along the river called eddies. Eddies are where the river water swirls along a bank and comes back on itself creating a smooth very slowly rotating reverse current. This gives a good layman’s example of fluid dynamics where fluids will naturally go in different directions.

Take the above example and apply it to air. Rather than the shore, rocks, etc. creating ups and downs, temperature and pressure differentials cause the air to move all around. If the pressure is lower in one part of the country, the higher pressure area will move to equalize it, thus causing wind. The warm air will rise and cool air will fall (remember 7th grade science class? ;-) causing up and down drafts.

So now we go back in the plane and we go bump. What just happened was that we either hit an up or down draft or caught a change in the direction of the wind. This causes everyone to feel the plane shake a bit. How far did we drop? I can say from experience that while watching the altimeter during a really bumpy ride, we may gain or lose 5-20 feet on a normally rough ride. If we were to get in to severe turbulence (the kind where you think or though you were going to die for sure) we’d lose 50-200 feet at a time but not instantaneously. Severe turbulence is rarely encountered and when it is, it’s reported so no other planes fly through that area until it passes.

All in all, that bumpy ride you had last time you flew, or will have the time after next is really nothing to worry about, even though it is no fun.

Here’s a picture of Boeing testing the wing of the 777. All aircraft builders are required to do these tests on their planes to ensure that they meet safety standards.

you would be sitting here….and the wing tip would be out of view out of the window, and the plane is still going to fly.

Auto Landings

I did say in my last post that I'd discuss Auto Landings in another thread so here it is.

Most large modern jets have the ability to 'Auto Land'. A few of these planes are the : Boeing 747, 757, 767, 777, 787 (not yet in service) some 737s; the Airbus 318, 319, 320, 321, 330, 340 and 380. There are others planes that can auto land, but that list covers most of what you'd see as a passenger these days. Another thing to note is that not only does the plane have to be capable, the pilots and the airline have to be signed off to do auto landings.

Auto Landing by definition is the autopilot landing the airplane without pilot intervention. The reason this ability is necessary is to allow a landing in very low visibility at the destination airport, all the way down to zero visibility.

All normal landings are couducted by the pilot and are done visually even in low visibility. I landed in D.C. the other day with low overcast skys and didn't see the runway until 700 feet above the ground.

When the visibility drops below the minimum allowed altitude for that runway, which is usually 2000 feet or as determined by the FAA, a visual landing is no longer deemed safe. This means that we will conduct an auto landing.

How it works:

We first will get a weather report and if the visibility is reported near or below the minimum allowed for a normal visual or a low visibility visual landing, we'll set up an auto landing.

We have to ensure that everything that is required to fly the apporach and land is in working order. That includes, both autopilots, our GPS systems, our glass cockpit displays, radios, instrument landing system indications and a few more items.

If that is all working we proceed with a long briefing about what we are specifically going to do on the approach and landing and what we will do if something breaks or goes wrong. We then fly the apporach to landing. During the arrpoach, we have only our instruments to tell us where we are until we either see the runway lights or we feel the wheels touch down. Because of this, we are extra vigilant about our positiion and altitude. We will each monitor certian items and will 'call out' anything that has failed or doesn't look right.

At 100 feet above the ground, I will call "ALERT" and if everything is still where it should be, we continue and let the plane land itself. The only intervention by the pilot is pulling the thrust back to idle and then to reverse after touchdown, even the brakes are automatic.

Whew! We're down and slowed to about 60 mph on the runway. Now we turn off the auto pilot and drive or taxi the plane toward the gate to let everyone get on their way. If the visibility is really low, we have to follow lights in the ground matched to our charts to see where we are going. In reality, we may fly in, land and park at the gate never seeing any farther than 100 feet ahead.

Hopefully, this answers a few of those questions about what's really going on up front.