Wake Turbulence Again? Maybe not.

March 4, 2009 by Bruce Landsberg

Last week’s crash of a Turkish Airlines 737-800 landing in the Netherlands has everyone speculating on what might have happened. Wake turbulence from a Boeing 757 that is rumored to be the preceding aircraft, is one of the theories. The weather doesn’t appear to have been an issue with one possible caveat. Light winds can cause wake to hang for a long time and turn a no-brainer approach into a real head banger.

Based on sketchy reports at this time, only two minutes elapsed between aircraft when witnesses said the 737 came ” ‘straight down.” Reliable witness reports are often an oxymoron so, as usual, we’ll be waiting to see what the real deal is. The 757 is known to have a particularly strong wake and while it technically isn’t classified as a heavy, maybe it should be treated as such. ATC does allow extra spacing after these aircraft but perhaps it wasn’t enough, even for a big aircraft like the 737.

We discussed wake dangers as recently as November Rolled involving a Learjet in Mexico and if that were to be the cause again here, take it to heart that below and behind is not a good place to be. With the recent spate of air carrier crashes (USAir in the Hudson with a total power loss, Continental in Buffalo possibly due to icing and now a possible wake encounter, it’s worth remembering that the basics of flight apply across the board – doesn’t matter what you’re flying.

Late breaking news – now it appears that a misreading altimeter caused the autothrottles to think it was time to flare and land. They reduced power to idle and by the time the crew caught it, the aircraft was out of airspeed. I’ll reiterate the last sentence – it’s worth remembering that the basics of flight apply across the board – doesn’t matter what you’re flying. Also shows the danger of speculation.

Bruce Landsberg
Senior Safety Advisor, Air Safety Institute

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8 Responses to “Wake Turbulence Again? Maybe not.”

  1. Alex Kovnat Says:

    I checked Avweb (www.avweb.com) and they provided a link to a press release by the Dutch Safety Board, which is investigating the Turkish Airlines tragedy.

    The cause was not wake turbulence but rather, the radio altimeter system sent misleading information to the plane’s autothrottle system to retard engine thrust at too great a height above the ground. Here’s the deal: At 1950 feet (I’m not sure whether they meant above sea level or above ground level), the radio altimeter (a form of radar which indicates aircraft height above ground; used mainly at 2500 feet or less to assist landings) falsely indicated the height was only 8 feet.

    The autopilot system then retarded engine thrust according to this false information, resulting in the aircraft losing airspeed. From what the Dutch Safety Board says in their press announcement, the flight crew recognized what was happening and applied power, but it was too late to avert the tragedy.

    What’s strange is this: There are two radio altimeters, but only one of them failed. Why wasn’t the plane equipped to recognize when one of the radio altimeters has suddenly gone haywire and disregard the false indication?

    For years, the FAA has required that flight crews observe the concept of “sterile cockpit”, whereby conversation between pilot and copilot shall be strictly limited to professional business at hand. For more information, see FAR/AIM and look up Federal Air Regulations, Part 135, specifically 135.100, Flight crewmember duties.

    If the crew had been strictly on the ball as called for in the letter and spirit of 135.100 (or the equivalent in Part 121), perhaps they would have recognized immediately that the system had gone haywire and acted while there was enough time.

    An unfortuntate characteristic of jet engines is, they do not respond instantly to demands for more thrust and furthermore even if they did, an aircraft flying too slowly cannot accelerate instantaneously. That’s why stabilized approaches are so important when flying jets.

    The lessons we can learn from the Turkish Airlines tragedy are as follows:

    1. Avionics suites must be designed to recognize when components have gone haywire.

    2. The system must be designed so the captain or first officer can dismiss the autopilot immediately and fly the plane manually.

    3. Observe sterile cockpit below 10,000 feet and of course, BE ON THE BALL!

  2. Phil Stang Says:

    It is always very interesting what can be learned from other people’s mistakes. A similar type of accident involved a GIV several years ago with auto-throttles playing a major role. In this case the throttles were inadvertently selected on or left on during landing. This did not permit ground spoilers and T/R’s to deploy properly resulting in a departure from the runway and total destruction of the aircraft. After this accident we adopted an SOP for the crew to verify the auto-throttles are disconnected by 50 ft prior to landing.

    For aircraft such as this 737 and the GV auto-throttles retard to idle automatically prior to landing. I am not sure an SOP change would help in this case. The crew must be ready to react swiftly when operating close to the ground. The failure of the throttles is no different than encountering a wind shear or or any other unexpected thing that could happen at that last moment.

  3. David Piccone Says:

    It is too early to tell exactly what caused this crash; mechanical, aeronautical, physiological or a combination of all three events. I am not typed in this aircraft so it may have different procedures for autolanding than the one that I am currently flying. However, when anyone is doing an autolanding in any aircraft, the PF (pilot flying) and PM (pilot monitoring) have got to be ready to either go around or disconnect failed systems and manually fly the aircraft in the event of an excedence of autoland approach parameters, ie. anticipate a go-around. This is not a cheap shot at the crew but, a comment that there is more to this crash than just a mechanical problem and anomalies do occur during real time (IMC and VMC) autolands. Just a question, was the crew current in performing autolands and had the aircraft been autolanded or mechanically tested IAW the airlines operating specifications?

  4. Leonard Nash Says:

    In several of the articles/blogs I have read on several web sites, I haven’t read much mention of the ‘new breed’ of pilot coming up through the ranks.

    I learned and taught “seat of the pants” attitude flying by reference to the actual horizon as seen out the windscreen in the late 80′s and early 90′s. I am currently an ATP with two type ratings flying mid-sized newer corporate jet aircraft with gadgets I couldn’t have dreamed I’d now have.

    This ‘new breed pilot’ today started learning to fly on their home computer and other simulator based ‘training’ and video games. I am fearful we will see many more “pilot error” accidents moving forward due to a shear lack of true pilot experience.

    When I came up through the ranks, you couldn’t dream of getting into a jet aircraft until you had several thousands of flight hours flying everything in the GA arsenal. Then, when you did actually get there (turbo props and/or jets), you flew with a senior captain at least “around the calendar” here in the Northeast.

    Today, commuters have kids in jets with much less than one thousand hours total time. Sometimes much less than that through these ab-initio fast track trainings offered by some foreign airlines just to fill vacant front seats cheaply. Airlines are unwilling to pay for experience and the flying public will pay the price airlines are unwilling to pay.

    Newer pilots are more and more dependent upon and rely greatly (too much in my opinion) on the ever increasing automation and better avionics in the cockpit, which I believe sometimes attempts to replace real world experience.

    When these items fail, for whatever reason, the pilot must be there to correct accordingly and immediately.

    These new “training methods” are no replacement for experience and wisdom that comes from “coming up through the ranks” and several thousand hours of actual piloting experience BEFORE you get your hands on the controls of a jet
    or commuter aircraft carrying dozens or hundreds of the flying public!

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  6. Sophie Says:

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  8. Doctor Németh J. Says:

    Permission request to use the picture on your website. Wake turbulence: blog.aopa.org.leading edge.
    Purpose: use in an educational and informative folder on ” Fear of Flight”.
    In case of approval, reference will be made to your website.
    Many thanks
    Doctor Németh, Belgium, 3600 Genk

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