Keyboard and Rudder: A blog on the Art of Flying

Wednesday, October 28, 2015

Weather pictures speak a thousand words, Part 2: Weather hazards

Part 1 had lots of pictures of air masses demonstrating how much the atmosphere flows like water. This time, let's see what happens when the weather misbehaves.

Icing

The problem with ice isn't, as many pilots think, that it adds a lot of weight. In reality, it doesn't weigh all that much compared to the enormous performance penalty it creates by changing the shape and/or smoothness of the wing surface. After all, how efficient would your wing be if it were made from rock candy?

That nub on the windshield wiper is put there specifically to ice up. That way you can easily see if you're picking up ice, since it's right in your field of view--unlike the wings.

Thunderstorms

Some days, you have no choice but to dodge towering cumulus clouds:

At least there was a gap between those. Here's something you don't want to see sitting right over your initial approach fix:

Sure, thunderstorms are pretty when you're sitting on the ground:

They're not so pretty when you're trying to get from one airport to another, however. This is a classic frontal line, where a cold front plows through, lifting the air ahead of it and creating a long line of thunderbumpers:

And this (like the first thunderstorm picture above) one is a classic air mass or "pop-up" thunderstorm. It's easy to identify because it's all by itself:

In What do beer and thunderstorms have in common? I wrote about what helps a pocket of convection build into a monster like this. If there isn't enough energy to create something like the big guy above, you might end up with a failed thunderstorm that never happened, like this:

You can tell from the wispiness of the cloud that it tried to get going but fizzled out. (Not surprisingly, as I took that picture at 7:12 a.m. before the heat of the day added enough fuel to the atmospheric fire. There were thunderstorms there later that afternoon.) However, if there is enough energy to get the convection ball rolling, you end up with something like the next four pictures, each taken two minutes apart:

See how fast that grew? In less than eight minutes, it went from a little puff to a decently-growing towering cumulus. If I'd have had a chance to take more, you probably would have seen it keep growing even more. It's almost like a bomb going off, which is what this pocket of convection looked like:

That's why in the post Why there is no reason to fly through a thunderstorm in peacetime I compared the average thunderstorm's power and an atomic bomb's power.

Ever wonder what rain would look like if you could see it from the side instead of having it fall directly on your head? On the ground, you can't do that, but from the air you see it all the time. It's the misty stuff in these pictures:

And here you can see some virga, which is rain that evaporates before making it to the ground:

Here I am getting rained on while above the clouds! Why? Because there's another layer above me:

But as the day goes on and the sun begins to set, the ground begins to cool and more energy isn't being fed into the convection system anymore. Things begin to calm down and die out:

Even when it's cruddy on the ground, it can be beautiful above. Check out next week's post for how pretty it can be once you blast through the gray on the ground.

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The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a Master-level participant in the FAA's WINGS program and a former FAASafety Team representative. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

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Weather pictures speak a thousand words, Part 1: The air flowing like water

During the private pilot ground school I teach at the local college, we spend two full weeks on weather. Some people's eyes glaze over at the technical discussion of things like heat exchange processes, fronts, air masses, standard pseudoadiabatic lapse rates, and the other things that go into the atmospheric dynamo that creates the weather.

Personally, I love weather, which will probably come as no surprise to those of you who have seen all of the posts I've written on it throughout the last several years. One of the things that I enjoy most about flying is that you're not just talking about some abstract concept: you're actually up in the atmosphere dealing with weather on every single flight. That, after all, is why we spend so much time on it in class!

This time, instead of the typical post analyzing a certain weather phenomenon, I'm just going to show pictures of the processes in action. Lots and lots of pretty pictures. In fact, so many pictures I need to split this one into more than one post!

Air Masses

Weather comes from air moving. It's that simple. Everything else is details. The air starts moving because the equator heats up more than the poles because the sun hits the equator more directly. After that, the warm air tries to flow toward the colder poles. It doesn't make it there due to things like the coriolis effect, but its attempt it what sets the weather process in motion.

I said the air tries to flow from the equator to the poles. That's because one of the other important things to keep in mind is that air behaves like a liquid.

Yes, this is a picture of actual water. However, you'll notice that you'll see the same sort of behavior in many of the pictures to come.

Imagine dropping a pebble into a very gently flowing stream. It's easy to visualize what would happen: you would get ripples that would follow the current.

Now imagine that instead of dropping a pebble downward, you threw it up into the sky. That wouldn't do much, but if you heat a parcel of air (like, for example, by having a big smokestack with a lot of hot air rising out of it), you can do almost the same thing, as you can see in the next two pictures.

Now imagine if you had the same stream, but on the stream bed were some ridges. As the stream flows along, the water at the bottom gets pushed upward when it hits the ridges. This would cause some small waves or bulges on the surface. In Life on the Mississippi, Mark Twain recounts how he learned the art of reading the Mississippi River. One of the things an experienced riverboat pilot could do was to be able to read what's under the surface just by seeing what the surface looked like.

Air does the same thing and has the same telltale signs. I spend much of my flying time going across and along the Blue Ridge Mountains. I have numerous pictures of "gravity waves", which is the technical term for ripples in the air that happen when air hits something like the ridges of the Blue Ridge.

First, here's what the mountains themselves look like:

Now here's what happens when air flows across those ridges:

If the cloud remains connected between waves, the gravity wave "humps" are still there on top:

Here's an excellent cap cloud:

If the conditions are just right, the waves can actually crest! These are Kelvin-Helmholz waves, and can be seen where two fluids meet at different velocities. They can even be seen clearly and beautifully in the atmospheric bands of Jupiter, but on Earth they're easy to spot along muddy riverbanks where the slower water near the bank encounters the faster current in mid-river, or when a breeze blows over the ocean, or (like what's happening in this picture) a faster layer of air rides over a slower one:

Continuing with the stream metaphor: what would happen if the stream hit an obstacle that it was too shallow to go over? It would back up and be dammed, right? Or if you live near water, you've probably seen a breakwall, which is a man-made obstacle placed in front of waves to cut down on erosion.

If the conditions are right (meaning stable air and lots of moisture in that air), the air will do the same thing water would do: it will hit the obstacle and either try to crash over it or get backed up behind it. The next picture is what happens when it flows over it. Recall that as the air rises, it cools. Once it cools to its dewpoint, it dumps its moisture and creates a nice cloud that traces the ridgeline almost perfectly:

The next picture was taken outside the airport entrance at State College, PA, early in the morning. I was standing there for about 20 minutes watching the veeeeerrrrrryyyy slooooowww process of the air hitting the mountain like water hitting a breakwall and slowly splashing up and over it. And by "slowly" I mean about half an hour for one "wave"!

The next two pictures show the same thing happening a couple of months later:

Here is the air getting dammed when it hits the ridge. The sharply defined line is it stopping when it hits the ridge and getting backed up like water in a reservoir. If you look closely you can see the mountains it is hitting:

To cap off part 1, here's some very stable air trapping a cloud in between two ridges, with the skyline of New York City in the distance:

In part 2, there are more pretty pictures. This time, it's about weather hazards: icing and thunderstorms. If you've ever wondered what rain looks like from above, head over there now!

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Wednesday, October 21, 2015

Why are some pilots more likely to declare an emergency than others?

Not long ago, I got into a debate over at the Flying Friends Facebook group on when one should declare an emergency. This just happened to be a couple of days before the sick passenger emergency in the last post. The original post was on a vacuum failure in IMC. I said I would declare an emergency. Many others said they wouldn't. One poster even ridiculed another who said he would.

My position was seconded by a retired 30,000+ hour airline pilot and given another nod by a current 10,000+ hour airline pilot. So my small-by-comparison 3,000+ hours were vindicated. However, as the discussion lengthened, a couple of things really stuck out at me:

1. The lower the number of hours, the less likely one would be to declare an emergency. This is exactly the opposite of what one would expect. After all, it's just logical that the more experienced pilots would consider fewer things emergencies. We get hammered in simulator sessions every six months with serious emergencies, so we have practiced things repeatedly that every pilot hopes they will never see. We have had more than a taste of things gone wrong. One would think that after those experiences, we would shake off something "minor" like a bad attitude indicator.

2. Everyone who was a professional pilot agreed with me. Some general aviation pilots agreed, but everyone who disagreed was a general aviation pilot.

These trends really surprised me. I've spent a lot of time thinking about them and trying to understand where they came from. These trends, I think, reveal a clear distinction between the mindset of the professional versus those who fly by choice rather than check. Here are three sources for that different mindset that I think are likely.

1

I know there is, in some segment of the GA community, a disdain for airline pilots. We are looked down on by this set as mere "bus drivers". However, bus drivers drive thousands and thousands of miles a year on all sorts of roads no matter whether it's snowing or raining or any of a myriad of other sub-par conditions that keep Joe Driver's sports car in the garage. By the same token, we flying "bus drivers" fly in conditions from 36-knot crosswinds to dodging summertime air mass thunderstorms to shooting an ILS approach to a snow-covered runway where the visibility is 1/2 mile because the snow is still coming down. While Joe Pilot (who thinks airline pilots have it so easy) is sitting at his computer reading Air Fact's "Go or No Go" column and saying "No Go" to marginal VFR, we're in the air flying in the soup.

In other words, we've seen a lot. One commenter disagreed extremely harshly with me that a vacuum failure in IMC was an emergency. Ironically, I have more time just in actual instrument conditions alone than all of the hours he has in his logbook put together!

The last two paragraphs are not meant to puff myself up. Mark Twain is often erroneously given credit for saying, "When I was a boy of 14, my father was so ignorant I could hardly stand to have the old man around. But when I got to be 21, I was astonished at how much the old man had learned in seven years." When I had 300 hours, I was a hotshot CFI who knew it all. Now that I have 3000 hours, I know I don't know it all. However, those paragraphs are driving at this point:

With experience comes not just the skill required to meet danger if it arrives, but the ability to recognize it when has arrived.

In other words, the old adage (which Wikiquote attributes to the astronaut Frank Borman, although he was almost definitely just passing it down, as it has been around a lot longer than 2008) that "A superior pilot uses his superior judgment to avoid situations which require the use of his superior skill." When you combine that with the other old saw, "Good judgment comes from experience. Experience comes from bad judgment," you get the sentence above.

So some of the reluctance to declare an emergency in this situation may simply be due to not having enough experience to recognize the danger that a dead vacuum pump poses to the safe outcome of a flight. Why get others involved over something as "trivial" as that? After all, isn't that why we practice partial panel flying as part of getting an instrument rating in the first place?

Anything that puts the safe outcome of a flight in doubt can be considered an emergency. Personally, I've spent enough hours slogging along in the soup that I know how quickly things can change and how insidious vertigo can be. That's why I would say right away that if my attitude indicator flopped over, I'd be declaring an emergency immediately.

2

Pilots are, in general, "can do" people. After all, just to get a private pilot certificate requires studying for and passing a written exam, the discipline to stick with 40+ hours of flying lessons even through the inevitable ups and downs, more studying for an oral exam, and all the time spent polishing everything up for the flight portion of the checkride.

The process tends to weed out those without the determination to set a goal and follow it all the way through to completion. That's why being in the top 2% of IQ qualifies you as a "genius" and gets you into Mensa, but successfully attaining a pilot certificate is something that less than .2% of the population has ever done. If you look it one way, it's 10x harder to become a pilot than it is to become a genius! (Yes, I am being facetious here.)

That determination and self-reliance is a good thing in most life situations, but it can also lead to pilots being unable to admit when they need help. They think that they should be able to handle anything on their own, so they are unwilling to declare an emergency in situations where they probably should.

Asking for help and/or declaring an emergency is not a sign of weakness. Being stubborn is not a sign of strength: it is a sign of weakness disguised as strength. There is no room in the cockpit for weakness at a critical moment, so check your ego and your stubbornness and if you need help, ask for it.

3

I've saved what I believe to be the most likely for the biggest number of pilots for last. There are probably many pilots who are reluctant to declare an emergency simply because they're reluctant to talk to ATC at all. They may realize something really bad is going on, and they may be ~~humble~~ wise enough to consider declaring an emergency, but they're too intimidated by the process of talking to someone "in control", or they may be unfamiliar with all the resources available to them if they'll just ask.

It may seem totally unbelievable that someone would be too afraid to ask for help, but I've seen the fear of "the system" in too many pilots to think it won't happen. I once gave a flight review to someone who had been flying for over two decades. As part of it, I asked him to plan a cross-country flight to a particular airport. He did a good job planning it, except he had planned a detour that added several dozen miles to the flight just so he could avoid transiting some Class B airspace along the way. And this is just one story of many like that.

It may be reluctance or it may simply be a general unawareness of what help a pilot can get by declaring an emergency; in either case, the outcome is the same: when you need help, it's not there, but it's waiting for you!

One of the reasons that I think that this is a big reason why airline pilots don't have a problem declaring an emergency when necessary whereas many GA pilots do is that airline pilots spend all day, every day working in the system. We talk to controllers hundreds of times a day. We're in the system so much we often have the next frequency tuned into the standby slot 30 miles before we're even switched over because we have them memorized. In fact, I know of more than one airline pilot who is in the system so much that they file IFR even if it's "clear and a million" because they're afraid to fly without ATC now!

This day-in, day-out familiarity with the system makes it second nature to us to declare an emergency if we need to. We know that controllers are humans, and we know they're an important member of the team. We know that like any good team-mate, they'll help us out if we need them to. In fact, in response to one commenter who said partial panel was no big deal, it was the 30,000 hour guy who said, "I'm glad that you're such a good pilot, but I need all the help I can get!"

Does this mean you should declare emergencies for everything? Of course not. In my 700 flights at the airline, we've only declared two emergencies. That means 99.72% of the time we don't (and there have been plenty of non-emergency things that have gone wrong on the other 698). In my 1900 flights total, I've only had three emergencies--one of which was a total engine failure. That means 99.95% of the time, I didn't declare anything. Here's hoping your rate is 100%, but if you need the help, don't let other people tell you that you shouldn't ask!

-

If you ever get into a situation where you do declare an emergency, here is a short list of the things you should do immediately:

1. Fly the airplane
2. Perform your memory items (you did memorize the bold printed items in your plane's POH, right?)
3. Fly the airplane
4. Run your checklist(s)
5. Fly the airplane

Remember: when you declare an emergency, it is still your airplane! ATC will do everything they can to help, but that doesn't mean they can fly the aircraft for you. It is still your responsibility to Get Things Done. Look at your emergency as a nail. If you ask, ATC can give you a hammer, but they can't use it for you: it's up to you to drive that nail in.

I'm going to get more into the responsibility that comes with being a pilot in the next post. See you next Wednesday!

Wednesday, October 14, 2015

What is an emergency?

In the previous post, I told the story of a recent emergency we had on a flight. It was easy to declare an emergency in this case because there was a big red warning light flashing on the glareshield saying that we may be on fire. However, most incidents that may occur in flight are not that cut and dried. So what exactly constitutes an emergency?

There is a saying in aviation that goes, "Some pilots will declare an emergency because of a failed magneto check. Others, upon having a wing fall off in flight, will merely request a lower altitude." This is one way of saying that there is no exact way to determine what constitutes an emergency.

The truth of this was borne out the first time we declared an emergency. Yes, the first time. The one I wrote about last post wasn't the first time I'd been in the cockpit when an emergency was declared. However, the first time was much more toward the first end ("mag check") of the spectrum.

The first time, we were heading to Charleston, WV. We were about 10 minutes out when the flight attendant called us to say that there was a passenger who was very pale and was having trouble breathing. We asked her to give the passenger oxygen and report back in five minutes. We had already started our descent, and in that part of the country there was no nearer airport, so we were definitely going to continue to the destination. We were also the only airplane in the entire airspace, so priority was not going to be a problem.

I called our operations staff for the normal "in range" call, which is something we do 10-20 minutes out to let them know we're getting close so they can start getting prepared for our arrival. If we have any special needs when we get there, like a passenger who will need a wheelchair, an unaccompanied minor, etc., we let them know as part of the call. This time, I told them we'd need an ambulance to meet the plane and briefly explained why.

Since "Charlie West" isn't a very big airport, the taxi time after landing is short, so we'd be at the gate in no time. For these reasons, I personally wouldn't declare an emergency. Everything would already be waiting for us and there is no other air traffic we'd need priority over. All we need to do is concentrate on the flight and land.

However, the Captain did declare an emergency. Is this the wrong decision? Not at all. Would not declaring be the wrong decision? No. I look at it as overkill, but no harm done except for a bunch of extra paperwork. The outcome would have been the same in either case under these particular circumstances.

(In case you're curious, I don't know what caused the passenger's problem. Once we landed and got to the gate, he was taken out of the airplane on a stretcher to the ambulance that was there waiting for him. However, he was up and walking under his own power within about 15 minutes of receiving treatment from the paramedics.)

What if we were in the same circumstances but going to Newark instead of Charleston? In that case, I wouldn't hesitate to declare an emergency. Why?

Because in Newark, there are a ton of other planes flying around, and sequencing all of us into the 20-mile conga line that New York Approach's controller/magicians keep churning along would take several extra precious minutes. In addition, once on the ground, the taxi time can be substantial. Declaring an emergency in this case could be the difference between life and death, as it would cut many, many minutes off the time to the gate. In this case, not declaring an emergency would be foolish.

One of our other pilots was in this scenario once, and on landing tower cleared them straight to the gate. This may not sound like much if you're used to a smaller airport where the taxi time is relatively short and you go directly from tower to ground and you're at the ramp in no time. In big airports, there's also a ramp frequency you use near the gate, which adds even more time. You also may have to give way to one or more other planes on the way in, since at Newark it's not usual to have at least a couple dozen airplanes moving on the taxiways at the same time. Being cleared straight to the gate means everyone else is getting pushed out of your way, and is a big deal when making your way around an airport as complex as this:

Even by large airport standards, Newark is a maze.

So not only is it sometimes hard to tell whether something may not be enough to be elevated to "emergency" status, sometimes the same thing definitely IS an emergency and under other circumstances may simply be an urgent condition.

In short:

Q: What is an emergency?
A: Anything that you believe reduces the certainty of the safe outcome of your flight.

Note the words "anything" and "you believe". When you're the pilot, an emergency is what you say it is. Not everyone will agree, but you're the one whose posterior is up in the air, not firmly attached to a chair on the ground.

This may be more useful to you than the FAA's definition in the generally-excellent Pilot's Handbook of Aeronautical Knowledge, which unhelpfully just says that an emergency is "a distress or urgent condition." (Thanks for nothing, PHAK!) However, that's not all there is to the answer.

In the next post, I'll go into some of the psychology involved in making the emergency declaration decision and examine a surprising difference between what GA pilots and airline pilots consider an emergency.

See you next Wednesday!

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Wednesday, September 30, 2015

Emergency!

It's a beautiful day for flying. The winds are light, the sky is clear, and the temperature is perfect. It's the sort of day where you'd almost feel guilty if you didn't take advantage of the opportunity, since the beginning of fall means days like this are numbered.

It's just a short hop from Wilkes-Barre/Scranton over to Newark. Not even half an hour in the air at 270 knots, and less than an hour from gate to gate. Nonetheless, those few dozen minutes take you through some magnificent scenery, from the Pocono Mountains and the "tricky triangle" of the Pocono Raceway, over the Delaware Water Gap that runs through the Worthington State Forest and marks the border between Pennsylvania and New Jersey, over thick woods dotted with lakes carved out by glaciers 20,000 years ago, to farmland and then cities packed more and more densely before the trip ends just on the other side of the Hudson from the magnificent skyline. What could go wrong?

Well, just like any flight, nothing will probably go wrong, but something always can. And in this case, something did.

We're about 10 miles northwest of the Boonton Reservoir. It's easy to spot from the air, as it has a small island in the middle of it; in fact, it's so easy to spot it's used as a visual checkpoint:

Boonton Reservoir is the lake marked with a flag at the upper left. Newark is the big airport near the bottom right.

How do I know this? Because I'm keeping the general picture in my mind of where we are as the flight progresses. That's nothing unusual; in fact, that's exactly what you're supposed to be doing!

We've already started our descent and begun the pre-landing preparations. Everything is going just as it has on almost all of the hundreds and hundreds of flights I've done before. And then a blinking red light comes...

SMOKE

We have dozens of yellow caution lights for all sorts of minor issues, from the parking brake being set to hydraulic systems to any of the multitude of power sources. These cause a yellow flashing light to come on. We have only a handful of red ones: the ones that are REALLY serious. This was one of the red ones.

This one's job was to tell us that there might be a fire in the cargo compartment. What are we going to do about it?

The same thing you should do every minute of every flight: fly the airplane.

On this particular leg, I was the Pilot Not Flying and the Captain was the Pilot Flying. He did exactly what needed to be done: he flew the airplane while we dealt with the problem. What does "dealing with the problem" mean? The same thing it should mean for you, and the same thing you should have been trained for (and we are trained for every six months):

Don't just do something--sit there!

This is an old aviation adage that has a world of wisdom wrapped up in it. It doesn't mean do nothing, it means do the right thing in the right way at the right time. This is an easy 3-step process:

1. Fly the airplane
2. Keep calm
3. Run your procedures

From the sectional excerpt above, you can see that we were close to both Essex County and Morristown Municipal. Both of them were suitable for landing. We continued past them and on to Newark. Why?

Because we were busy doing those three steps. The Captain was flying the airplane, we were both calm, and I was running the checklist for a smoke warning annunciator. To make a sudden, drastic change in destination would require a whole new plan of action; something probably not best done while in the middle of trying to determine if the back of the aircraft is on fire. One thing at a time: the right thing in the right way at the right time.

While I'm just beginning the checklist, New York Approach spits out in their rapid-fire way a descent, a heading, and a frequency change. Since I was busy managing the checklist, I simply repeated the frequency, ignored the rest, and checked in with the new controller in a calm voice,

"New York, [Flight Number] declaring an emergency. We've had a smoke warning light come on. No further assistance required at this time."

Basically, in a few short sentences, I told ATC--who are a vital part of the team in this--that we have a problem, that it might be a serious one, we're coming to Newark and we need to do it in the most direct way, and we're busy dealing with it so keep the chatter and instructions to a minimum while we handle it.

And that's exactly what they did and we did. They gave us a simple heading that pointed us straight to the airport, gave us an altitude that would set us up on a downwind leg for the runway they would have cleared and waiting for us, and asked very few questions except for the standard souls on board and fuel remaining ones.

In the meantime, I was running through the checklist, informing the flight attendant of the situation and asking her to see whether there actually was smoke coming from the cargo compartment or not, getting the final before landing items done, and so on. After all, this sort of thing has happened to me many times before--except all the other times were in the simulator.

After a couple of minutes, the flight attendant reported that there was no sign of anything unusual. It was likely a false alarm, but we continued to handle the situation exactly the same. Fly the airplane, keep calm, and carry on.

After landing, a half dozen fire trucks were waiting for us on the taxiway we'd be likely to turn off. Notice in my initial call-up above that I never asked for them. I didn't need to: ATC is highly trained too and smart enough to know that where there's smoke, there's fire. All I had to do was tell them our situation and let them do the job of getting the resources supplied.

As we taxied off the runway, I thanked ATC for their help and switched to the frequency the fire trucks use. They made a pass around the plane and saw nothing usual either. They scanned the cargo compartment with their thermal cameras and verified that there was no fire. It was just a bad sensor after all.

Any time a big red flashing light goes off in the cockpit, it's easy to declare an emergency. But what about when things are seemingly more minor? In the next post, I'll get into a bit more about what constitutes an "emergency". See you next Wednesday!

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The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a FAASafety Team representative and Master-level participant in the FAA's WINGS program. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

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Wednesday, September 16, 2015

How to crush something without touching it

What if I told you I can crush a water bottle without touching it? You probably wouldn't believe me, would you? Of course you wouldn't. How can something get crushed without having something doing the crushing?

But I can. I have proof:

Crushed it!

I did not touch this water bottle for at least 30 minutes before I took this picture, and when I put it in that pouch, it was a normal-looking, quite round bottle of water.

So what happened? And how did I crush it without touching it? Well, to be technically correct, I personally didn't crush it; I just set up the conditions for something else to do the actual crushing.)

The thing that did the actual squishing was atmospheric pressure: that invisible ocean of atmosphere that surrounds us. You can't see it, you can't hold it in your hand, and you don't even notice that it is pushing down on you with the weight of an entire bowling ball on every square inch of your body. That thing.

You don't usually notice it at all, since our bodies basically push out against it with the same 14.7 pounds per square inch (psi) of pressure that it pushes in on us with. We're usually in equilibrium with it, so we take no notice of it. Kind of like in-laws who live on the other side of the country: they're there, but we don't notice them.

If the bottle was perfectly fine half an hour ago, why isn't it fine in this picture? Because of something we did in the time in between: we descended from our cruising altitude and landed.

When we were up at 24,000 feet, the pressure inside the cabin was roughly the same as it would be if we were only at 8,000 feet. Think of climbing a few thousand feet up a mountain outside Denver: that's about how thin the air inside the cabin would have been.

By the time we landed, the pressure inside the cabin was a mere 313 feet above sea level, which is the elevation of Washington-Dulles International Airport. So in the meantime, the effective change in air pressure was the same as climbing 7,700 feet down a mountain. There's a lot more air down here. That means a lot more air to push on the bottle.

At 8,000 feet, the atmosphere was pushing on the bottle with a weight of 10.9 pounds per square inch. Because of that, when I drank the last of the water and put the cap back on, the air pressure inside the bottle was 10.9 psi, the bottle was perfectly round, and everyone was happy.

However, by the time we landed, the air pressure was up to 14.6 psi. More air = more pressure on the bottle. However, since I didn't touch the bottle, the air pressure inside it was still only 10.9 psi. Since there was more pressure outside than inside, the bottle collapsed. By opening the cap and letting air in, the bottle would "inflate" back to its round shape, since the pressure inside the bottle and outside would both be 14.6 psi.

If things get crushed as you descend, why wouldn't the bottle start bulging as we're climbing up? After all, the pressure inside the bottle would be 14.7 psi if it were filled at sea level, and back to 10.9 at 8,000 feet. It should do the opposite in the climb as it did in the descent, right?

Right. It does just that, actually. However, the bottle tends to be filled with water and not air on the way up, making less room for the more compressible air. Also, the bottle stretching outward is much less outwardly visible than its collapsing inward. Nonetheless, it is bulging. It's a lot easier to see in the seal on this Pringles can, which was bulging in cruise just before I opened it:

That seal is just bursting with flavor at a cabin altitude of about 6,000 feet.

This expansion and contraction cycle is why your ears pop twice during a flight. On the way up, the air on the inside of your eardrum is trying to push it outward. On the way back down, the air on the outside of it pushes it inward.

Imagine if we could hook up some gears and a meter to the outside of the water bottle. The gears could drive some sort of indicator that shows us how much air pressure is around us based on how much the bottle bulges or gets crushed. That would be a neat way to indirectly measure how high we are.

Don't rush to the patent office just yet, as someone already came up with this idea a long time ago. (Long before airplanes, even. Mark Twain mentions a device using a similar concept in his book A Tramp Abroad, which was written way back in 1880, and the idea wasn't new even then.) There already is something in the airplane that uses this expansion and contraction of trapped air inside a container and puts it to good use. A very vital thing, actually. It's called the altimeter.

Figure 7-2 from the Pilot's Handbook of Aeronautical Knowledge. You can download the entire book for free from the FAA's website. Your tax dollars at work.

That thing labeled "Aneroid wafers" that looks like a stack of discs or an accordion's bellows operates on the same general principle as the water bottle. As the wafers bulge outward, they're connected to some gears that are connected to the dial on the altimeter's face and make the hands indicate the altitude winding upward. As they shrink, the hands start going in the opposite direction. If the aneroid wafers are not bulging or shrinking, you're in level flight.

In the altimeter's case, it's connected to an air pressure source outside the airplane (the static port) so it isn't measuring the air pressure inside the cabin like the water bottle was, but the principle is exactly the same.

When you're learning to fly, everything in aviation seems to run off of two sources of energy: money and magic. While it has always been the case that flying isn't cheap, I think it's an excellent investment because of the magic it brings along with it. Your instruments, however, don't run on magic. They actually run on simple principles of physics that are harnessed in a way that sometimes looks like magic. And it can turn you into a magician who is able to crush things just by looking at them.

See you next Wednesday!

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The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a FAASafety Team representative and Master-level participant in the FAA's WINGS program. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

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Wednesday, September 9, 2015

What you can't see can hurt you: Avoiding wake turbulence

I recently came across a very nice picture of wingtip vortices. (Unfortunately, it was passed to me without any information on who took it or where. If you know, please let me know so I can give them the credit they deserve.)

With the exception of some NASA tests with equipment designed to help make them visible, these are usually invisible. In the picture below, you get a nice view of what these vortices do to the air, and where wake turbulence comes from. (Throughout this post, for simplicity's sake I'll refer to wingtip vortices and wake turbulence interchangeably.)

Notice the airport in the background.

These are behind every airplane that is creating lift, from a 747 to a 172. In fact, in your training you may have done a nice steep turn, and toward the end of it felt a bump seemingly out of nowhere. That was you doing such a good circle that you hit your own wake turbulence.

If they do this to a thin cloud layer, imagine what they'd do to you if your airplane was in that path. Well, the FAA has made sure you have a scary idea of what they could do, as they point out on pages 13-15 and 16:

"The vortices from larger aircraft pose problems to encountering aircraft. The wake of these aircraft can impose rolling moments exceeding the roll-control authority of the encountering aircraft. Also, the turbulence generated within the vortices can damage aircraft components and equipment if encountered at close range."

This is a very dry, boring way of saying that flying into them can flip your butt over and/or rip your wings off. As exciting as falling head over heels for wingtip vortices might sound, their behavior is fortunately very predictable, which means there's an easy way to avoid them.

Since they're part of the air mass, they move along with it. They also tend to sink. Since we usually try to land into the wind, this means they're coming toward you but sinking at the same time. Solution: just stay above the glidepath of the plane ahead that caused them and you'll avoid them.

I use this quite often when I'm flying the Dash-8. Although at 43,000 pounds it's not a particularly small airplane, it's a wee speck compared to the 767s, 747s, and A340s I've landed behind at Newark. In these cases, I intentionally fly half a dot high on the glideslope.

Notice I said half a dot high. This is on a two-dot glideslope indicator. If you have a five-dot indicator, this is less than two dots. The figure below shows a glideslope indicator that's as high as you need.

Don't use more than this; you don't need to stay extremely high to stay above wake turbulence. It's tempting to do so if you think along the lines of "better safe than sorry", but staying way too high will make the final stage of the approach unnecessarily difficult. If you're flying visually or you don't have a glideslope receiver, use the PAPI lights to help you out. All you need for them is 3 white and 1 red. Again, no more.

At 5 miles out, this is only about 100 feet high. As you get closer to the runway, that gets smaller and smaller. By a mile out (further out than you'd be turning base if you were flying a traffic pattern), this is well under 50 feet. The top part of the following diagram shows you what I mean:

Well, that's great if you're coming in to land, but what if you're taking off behind a monster?

In general, the bigger the wingtip vortices, the bigger the plane. This is actually good news, because the bigger the plane, the more runway it tends to use on takeoff. This means that you can probably lift off well ahead of them. However, that's not the full solution, as the chances are very, very good that they can outclimb you by a lot once their wheels do break the ground.

Now what? Remember that wingtip vortices have predictable behavior, and they move along with the air mass. That means if there's a crosswind, they will slide along the side of the runway. Once you're reached a safe altitude, turn a bit in order to keep them on one side of you. If the crosswind is from the right, the vortices will be moving from right to left over the runway. Just turn a bit to the right to keep them on your left and you'll remain clear. If there's a left crosswind, turn to the left.

Most towered fields where GA planes mix in with the big ones understand what it takes to keep the shiny side up under these circumstances. Not only will they not have a problem with you sidestepping, they will expect you to do it on your own. When they say, "Cessna 1234 cleared for takeoff runway 27, caution wake turbulence departing Boeing 737", they are alerting you of the possibility of encountering wingtip vortices AND giving you implicit permission to maneuver within reason to avoid them.

Although they may make for a pretty picture, wingtip vortices can make for an ugly encounter. By keeping this post's tools in your bag, you can look forward to years of smooth flying!

See you next Wednesday!

Like Larry the Flying Guy on Facebook:

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The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a FAASafety Team representative and Master-level participant in the FAA's WINGS program. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

It takes hours of work to bring each Keyboard & Rudder post to you. If you've found it useful, please consider making an easy one-time or recurring donation via PayPal in any amount you choose.

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