November 2005- If you operate your aircraft into large busy airports, this comment from the tower controller has almost become a cliché. “Caution wake turbulence,” your trusty government employee will say. “You are six miles behind a heavy Boeing 777. Wind calm, cleared to land.”
If you’ve been operating at large airports long you’ve heard this warning hundreds of times and nothing has happened to you. In a few instances your airplane went through a few seconds of rough air – nothing you couldn’t handle. Following a heavy 747 in past encounters only resulted in a little mild rocking and rolling.
Today is not going to be your day. You are VFR but it is pretty hazy and you are looking into the sun. You are five miles behind that aforementioned 777 and your speed relative to it is “compatible” (the same).
You feel a little bump and your artificial horizon quickly shows you to be in a ninety degree bank – then it tumbles. You are in the haze with the sun in your eyes which doesn’t give you a very good horizon for your recovery effort, you think you may be upside down and you are only five hundred feet above the ground. Like I said, this isn’t your day.
Denial: the first stage of grief
“This would never happen to me,” you chortle. “Why, I was in a bad wake turbulence roll one time at O’Hare and I forced my airplane upright with no problems at all and on instruments, too!”
Arguments like these have been given by pilots over the years who say they have flown through killer thunderstorms and came through okay. Please don’t try a story like that around experienced professional pilots. They will laugh behind your back and brand you for the geek you truly are.
No one who has survived penetrating a really severe thunderstorm or was lucky enough to survive a bad encounter with a wake brags about it. They know it was luck and not skill that saved their hides. A really rough nasty thunderstorm or the “perfect storm” of a wake encounter can kill the best of pilots. You should avoid both as strongly as you would an ABBA concert.
Wake Factoids
Roll Rate
If you think you can roll out of a wake turbulence encounter you may be right – then again, you may be very wrong. Your average, run-of-the-mill General Aviation Spam can or airline sub-sonic people mover is capable of a roll rate of approximately 40 to 65 degrees per second.
This figure leads to amazement among your passengers who look up to you like they would an aviation god. Your average, run-of-the-mill roll rate to be found in the bowels of a serious wing-tip vortice produced by a heavy aircraft is approximately 80 degrees per second, or twice the maximum roll rate of your aircraft.
An unfortunate encounter with this fact will not only have your passengers looking up to you like an aviation god – they will meet the real one very soon.
When you add the facts about your relative roll rates to the fact that you’ll be a little slow counteracting it because you’ll be very surprised, you can have a very unpleasant situation very quickly.
Most electrically driven attitude indicators won’t tumble through 360 degrees of roll but they will tumble at around 100 degrees of pitch. Suction driven attitude indicators like most of us still use will tumble at about 100 degrees of bank and 60 to 70 degrees of pitch.
Downwash
Not only will you find the exciting roll rate and rough air should you end up following a heavy into a wake turbulence situation, there is also downwash to contend with.
Wake turbulence moves downward and outward. This downward rate averages 500 fpm but can exceed 800 fpm. If you are flying your aircraft behind that big jet you may say “Hello, ground!” a few miles short of the runway threshold because the sink rate imposed on you will be more than your aircraft can outclimb at full, go-around power.
Parallel Runways
It is entirely possible that you might not even be following a heavy and still get the snot kicked out of you. Parallel runway operations on a day with stiff crosswinds might be an unseen problem. The vortices you encounter there won’t be quite as strong because before they get to you they have a minute or two to break up and the strong winds will help dissipate them as well.
He Ain’t Heavy – Right?
Just what is considered heavy? Anybody heavier than you! The FAA and ATC will tell you that a heavy is any aircraft that is capable of a gross weight of more than 300,000 pounds. It doesn’t matter if the aircraft actually weighs that much at the time. It just must be capable of that weight.
A few other aircraft, like the Boeing 757, will generate a warning for you if the controller is paying attention even though they don’t max out at 300,000. For some aerodynamic reason, following the 757 (even when I was in another 757) has led me into more wake turbulence problems than I care to admit.
Air traffic control will both warn you of the presence of a “heavy” in front of you on approach and will provide you with either five miles or two minutes of separation. When you are taking off at a busy airport they will usually go for the distance requirement because it takes less time than the two-minute time gap and with radar it is much easier for them to measure.
They usually release you for takeoff as the aircraft in front of you is about three or four miles out, assuming that it will take an extra mile or two for you to get airborne. You are very much within your rights as a pilot to refuse the takeoff clearance if you are uncomfortable at all about it.
You are also wise to request an off-course turn immediately after takeoff to avoid wake turbulence. It is unlikely that you’ll be able to outclimb the path of the heavy in front of you, meaning you’ll be in prime wake country if you continue to follow it. Don’t be afraid to ask for what you want and wait for it if necessary to make your flight safer.
What about Light Heavies?
A fully grossed-out DC-9 weighs in at over 105,000 pounds and a Boeing 727 can weigh as much as 184,000 pounds. That doesn’t seem very light to yo,u does it? Me neither.
Some of the worst wake turbulence I’ve ever encountered was behind a 727 on a calm day. Remember, in the eyes of ATC, “heavy” means over 300,000 pounds. To you, anybody heavier than your aircraft should be considered heavy.
“Avoidance” Isn’t the Name of a New Laxative
Have you noticed that all of the wake turbulence avoidance techniques blithely assume that you can see the other airplane? That is not the easiest thing to do when you are IFR or on a hazy day.
All of the charts and pictures in the AIM and other books show you how to visually avoid the wake of the biggies, but there are no hints on how to do it with limited visibility. Here are some hints:
Don’t Go Where the Big Airplanes Are
This sounds simplistic, doesn’t it? I know that the AOPA people demand the right to land at LAX behind a Lufthansa 747. That is perfectly okay with me and I’ll defend to the end their right to scare themselves to death.
Unless you have a real need to mix it up with the heavies, why do it? Secondary airports in big cities are more suited to us General Aviation types anyway. If you do need to go to airports frequented by big aircraft that is fine, but to go there on a regular basis just to prove you can is silly if you are serious about living to be as gray-haired as I am.
Use a Different Runway
ATC will probably offer this option to you, anyway—just to get you out of their hair.
Ask for More Separation
We’ve already discussed this but it bears repeating. You are not a wimp if you ask for more space – you are a wise and thoughtful pilot who will live another day.
Fly a “Dot High” and Land a Little Long
When flying on instruments, the FARs say that we must be on or above the electronic glide path if there is one provided for the runway. If you fly a dot above the glide path it will put you above the wake of the heavy who is probably coupled to the glide slope by at least his flight director and is flying right down the pipe.
A one-fourth scale deflection of the glide slope needle will only put you about five feet above the touchdown zone when you are over the threshold. Unless the weather is extremely bad you can live with being that high. Also, since you are likely at a big airport you have a lot of extra runway.
You’re In It – Now What?
Aggressive action is called for should you find your world tumbling out of control behind another aircraft. Here are a few techniques: Add power and fly it out of there. The airplane will naturally roll one way or the other. We have already established that it is very hard to fight this, so don’t.
Whichever way it rolls, try to stop it at about 30 degrees of bank (good luck) and use the bank to fly out of the wake at about a 45-degree angle. Go around.
There is no use trying to save this approach, especially if you are at low altitude. The vortice is still out there. Leave and come back later for another try. While fighting the rolling tendency, you should use your rudder as well as your ailerons. Use whatever it takes.
Practice Unusual Attitudes
Can you recover from an extreme angle of bank on or off of instruments without an artificial horizon or hazy outside references? There are some great courses out there taught by professionals that can teach you all about unusual attitudes and recoveries. The bonus is that they are really fun. You now have a safety excuse to go fly a Pitts or Citabria upside down.
Take Care of Business
Encountering wake turbulence is an emergency. Treat it as such. Do whatever you have to do to avoid it and if you accidentally enter one, get out of it.
Use your emergency authority as pilot in command if you have to. It is better to mess up the traffic pattern at O’Hare or LAX than to mess up their approach lights with wreckage.
Kevin Garrison’s aviation career began at age 15 as a lineboy in Lakeland, Fla. He came up through General Aviation and is currently a senior 767 captain. When not frightening passengers, Kevin plays tennis and lives on a horse farm in Kentucky, where he writes unsold humor projects and believes professional wrestling is real and all else is bogus.
