Monday, December 21, 2009

Wake Turbulence: Part 1 - What is it?

A while back (more specifically, a couple of months ago), a reader sent an inquiry about an airborne wake turbulence encounter that they experienced on a flight inbound to LAX:

Thanks for this great blog. A question for you: I was on an Alaska Airlines Flight from CYVR to LAX on Sunday. We came in from the coast and headed east with LAX to our south. We expected to turn south to line up for landing, but then we rolled/banked suddenly to the left (north) what felt like 45 degrees, eliciting gasps from everyone on board. The plane felt like it was out of control at that moment, but then it leveled off. A few seconds later, the same thing happened again. After we leveled off a second time, the pilot announced that it was “wake turbulence,” and immediately banked hard to the south, then seemed to come in for a much faster than usual landing. I fly a lot and have never experienced anything quite like this. I’m curious how common this is, how serious a situation it is, and whether “wake turbulence” is just one of those things, or a hazard that could have been avoided.
Boat Wake 2Boat Wake 1
(Photos borrowed from stock.xchng)

Airplanes produce wakes as they fly just as boats and ships create wakes as they move through the water. Unfortunately, the wakes created by aircraft are not normally visible, thus making them very hard to detect and avoid. The easiest way to visualize an aircraft's wake is the contrails that you can sometimes see as aircraft pass at altitude overhead:

Lufthansa 1,000 feet above us (from London to Boston) by Fly For Fun.
(Image borrowed from Gadling)

However, while the contrail makes the concept of an airplane's wake visible, the wake itself is actually something different. The contrail is generally caused by the hot engine exhaust condensing water vapor in the very cold air aloft. The wake is caused by the movement of the airplane through the air. The act of creating lift involves having air pressure below the wing greater than the air pressure above the wing. This is why airplane wings are, for the most part, not flat, nor are they symmetrical: the top of the wing is curved much more than the bottom of the wing:

Maximum wing thickness for a Typhoon(a) and a Mustang(b)
(Image borrowed from Air Warrior)

There are exceptions, naturally (aren't there always?): aerobatic stunt planes often do have symmetrical wings, while high speed military jets and balsa wood models have seemingly flat wings. In these cases, the angle of the wing to the airflow is used to make the air behave as if the wing were curved more on the top than the bottom. But I digress, as I so often do . . .

The difference in air pressures above and below the wing is what creates the lift that allows the airplane to fly. The wake is created by the air's natural tendency to even out the differing air pressures. Any area of high pressure is going to flood into an area of lower pressure, be it air or water or any other liquid or gas. When the pressures have become equalized, the flow will tend to relax.

As an airplane moves through the air, the higher pressure air below the wing swirls up into the lower pressure area created above the wing. This action is particularly prevalent at the wing tips where there's no wing to block the flow. This swirling air can best be pictured as a pair of horizontal tornados that extend behind the airplane from the wingtips:

As you can see in this graphic borrowed from an FAA training publication, the vortex coming off the left wing spins one direction, whereas the vortex off the right wing spins in the opposite direction.
On occasions when the humidity is high, the sudden change in air pressure can cause the water in the air to condense, illuminating these vortices:
File:Wingtip condensation.jpg
This normally occurs when the wing is creating a lot of lift at relatively low speed, such as approach to landing. In this photo, which I borrowed from Wikipedia, you can see the vortex created by the extended wing flap. We rarely see these at LAX because the ambient humidity is usually pretty low; my compatriots in more humid climes are doubtless more familiar with the sight. That said, here's a rainy day shot from a few months ago:
In this shot, you can see the vapor trails, as well as a foggy area just above the wing. This is condensation of the water vapor in the air, caused by the pressure drop above the wing. A wing that is not creating lift will not exhibit this localized vapor, as demonstrated by this shot of the same aircraft seen above, taken moments earlier - before it lifted off:
Nifty as they may be, none of these shots show the airplanes' wake; the fog above their wings is a result of the wings creating lift, and helps to illustrate the pressure differential that exists when the wings are creating lift. The vapor trails come closer to showing the wake, but the turbulence that we're discussing is another by-product of the wing creating lift, and tends to be invisible. However, in the following NASA video clips, smoke is used to show the effect of an airplane's wake:

Saturday, December 19, 2009

Home again

And we're back! I don't remember who it was that used to say that - maybe Johnny Carson - but in any case, I have returned (I do remember who said that, although I wasn't there at the time). This year's Christmas vacation was spent moving my household stuff from Memphis (where it's been in storage for about two and a half years), with a stop at the family farm in Texas. Over the past two weeks, I've been the captain of the biggest rental truck I could get:

The truck's as big as the house!

While driving across the country, I've had plenty of time to contemplate various options for a narrative about the whole thing, but I think the main points will suffice:
  • It rained. A lot. Loading day in Memphis had a 80-percent chance of rain when I left LA three days before and a 90-percent chance the day before. And rain it did: the entire day. When I arrived in Texas, where additional loading took place, it rained some more. For a week.
  • Hiring loaders was some of the best money I've ever spent. That truck went from empty to completely loaded in two hours. I still marvel at how well that went.
  • Driving one of these trucks across the country - or even across town, for that matter - will give you a much greater appreciation for the work done by professional truck drivers. It will also give you much more insight into how stupid many car drivers are around big trucks.
  • You think the truck rental costs a lot, wait til you have to buy the fuel!
After all the rain in Memphis and Houston, I returned to clear skies in the west. I heard that we had rain out here too while I was gone, and the mountains now have snow caps:

Downtown LA, as seen from the tower on a (rare) clear day.

And you know it's a clear day in LA when we can see the Hollywood sign from the tower!

I hope everyone has a safe and happy holiday season - Go Saints!

Sunday, November 29, 2009

Somethings new, something old

For my US readers, hope you had a good and safe Thanksgiving. For those of you who have not spent the last week feasting and shopping, you haven't missed much. Thanks to the on-going painting, plumbing, and deliveries at the house, I haven't got much for you this time, although I am working on assembling an upcoming series on wake turbulence. Updates during the month of December are going to be sparse, as I'll be off for the next couple of weeks and getting the move done (I hope).

This time around, I've got some more new LAX stuff:

AeroMexico and AeroMexico Connect have just moved from Terminal 6 to Terminal 2, making Terminal 2 now almost another international terminal. Hawaiian and Sun Country are the only domestic carriers using Terminal 2; all the rest are international: Air France, Virgin Atlantic, Air China, WestJet, Air Canada, KLM, Air New Zealand, LACSA, TACA, Volaris, and Avianca.

Speaking of Avianca, they've shifted their schedule around: instead of arriving late in the evening and departing that night, Avianca now arrives first thing in the morning and leaves a little before lunch time.

I've mentioned winglets appearing on more models of aircraft before, and here are the latest:

This Continental was the first B757-300 I've seen with winglets. Nearly everyone who operates the -200's has been retrofitting their fleets over the last several years. At LAX, that includes American, Continental, Delta/Northwest, and United. The only other operator of B753's at LAX is Delta/Northwest; I don't know yet if theirs are getting them too.

In addition to the B753's getting winglets, I've also mentioned that a number of carriers have started retrofitting their B767-300's with winglets. American was the first, and I think I've shown you Delta and LAN aircraft as well. Here are the latest additions:

Air New Zealand, who uses B763's to/from Rarotonga in the Cook Islands (the only carrier going directly between LAX and Rarotonga; flight time is around nine hours)

Hawaiian, who unlike most of the other carriers that offer service from LAX to the 50th state, only goes into Honolulu from LAX. American, Delta, and United also serve Kona, Lihue and Maui.

While researching for a previous entry, I ran across a press release announcing the latest Horizon special paint scheme - 'Comfortably Greener' - and the next day, it showed up at LAX.

And finally, something old - this Northwest B744 arrived from Japan last week on its last revenue flight in Northwest livery. After unloading, it left for the paint shop in Victorville. The next time we see it, it'll be wearing Delta colors.

Friday, November 20, 2009

Who's That? Part 16 - Rogue's Gallery revealed

As promised, this was a hodge-podge of special liveries and generics, along with whatever else I found laying around. When I was in New Orleans, they would call this "sweep the kitchen" or maybe even "sweep the swamp". For my simplicity, the carriers with multiple entries are grouped together.


Boise State Broncos
(love those colors!)

University of Oregon Ducks
(Although it sure looks like John Deere to me)

University of Washington Huskies

Bonus: Horizon also has a Dash 8 in Huskies colors.

US Airways

State of Arizona

State of Nevada

Arizona Cardinals

Philadelphia Eagles

Star Alliance

This was tricky because it isn't an airline, but rather an airline marketing alliance. I think I've mentioned the Star Alliance before; it was the world's first airline alliance, and I believe it is the largest of the three major airline alliances. There are about two dozen member airlines. An airline's membership in the Star Alliance is usually denoted by the star logo appearing on the aircraft, just aft of the cockpit windows. But there is also this Star Alliance livery which appears on a few aircraft, in this case another US Airways A319. As a controller, I don't care for it because it's very difficult to distinguish what airline the plane actually flies for: The operating airline's name/logo appears in relatively small print below the windows aft of the front door. This makes it difficult for the controllers and other pilots to figure out who the airplane is, especially at night. I've personally had situations where I've told another aircraft to follow or pass behind a United or US Air (among others) aircraft who unbeknown to me was in the Star Alliance livery, only to have the pilot not do what I instructed him to do because I had incorrectly described the airplane he was waiting for. Here are a few more Star Alliance-schemed aircraft:

You can see the Star Alliance logo on this Air Canada E190, between the cockpit window and front cabin door.

All Nippon B773

United B744

United B763

Unlike most other airlines, Singapore's Star Alliance livery retains the airline's logo on the tail, which makes identifying the airline much easier. I wish they were all this way.


Not to be outdone, the second-largest airline alliance, SkyTeam, has introduced its own special livery in commemoration of its tenth anniversary. Like Star Alliance, SkyTeam's member aircraft display the SkyTeam logo just aft of the cockpit, and the special-liveried aircraft have the parent airline's name/logo aft of the front door, below the window line. The tail I showed you was an Air France B773; here are couple more:

Delta (nee Northwest) B752

China Southern B772

This Northwest Airbus shows the usual placement of the SkyTeam logo on participating airlines' aircraft: Just aft of the cockpit window.


Okay, there really wasn't anything tricky about this one, I just recently started seeing them here again and got some pictures. Evergreen International is a cargo operator based in Oregon. We used to get Evergreen DC9's in the wee hours, but not for some time now.

Air Shuttle

Air Shuttle is the callsign for Mesa Airlines, who appears at LAX as a regional partner for US Airways. They fly mostly to/from Las Vegas and Phoenix using various models of CRJ like this CRJ2. Here's how they normally appear:


To commemorate 50 years of Alaska's statehood, Alaska Airlines held a contest for a special paint scheme to appear on a B734: The Spirit of Alaska Statehood was designed by a student from Sitka, Alaska. The design is different on each side of the aircraft, and I like the way the sled dog appears to be wearing shades:


The Vive Mexico campaign was launched earlier this year by Mexican President Calderon to reinvigorate the Mexican tourism industry.

I've been seeing this white-tailed Mexicana A320 for months now; apparently they've run out of logo decals. Interestingly, this airplane and the one with the Vive Mexico livery both carry French registrations - if you look closely, you can see that they're consecutive.


This was another sneaky one, as it isn't a regular airline. I knew that the armed services have been contracting out non-essential services (anybody remember Halliburton?), but until these guys showed up, I had no idea that the practice had extended to mid-air refueling service. Omega Air offers this specialized capability, which seems like really targeted marketing. As far as I know, the only users of mid-air refueling are the military forces around the world; a mid-flight top up is not available to the flying public at large. This is a B707 - an old and rarely-seen aircraft these days. Compare to the B747 that's tail-on to the camera.

Sun Country

Of all the sneaky shots, this one was the most underhanded: The tail of this B737 wears Aloha colors, but the fuselage carries the Sun Country name. Adding to the underhandedness is the fact that I actually took this shot a couple of years ago. This is how the plane (well, one of its fleet-mates actually) looks now: