October 26, 2012

Upside Down in an Airliner?

Even if you don't have a clear understanding of the instrumentation, the following picture should send shivers down your spine...


This old fashion "six-pack" instrument panel...quickly becoming a rare sight...paints a dire picture.  The most alarming instrument of course is the one in the middle...the "attitude indicator" or Primary Flight Display as we call it.  This jet is in a 120 degree left bank accompanied by a 10 degree nose up attitude.  At 5,240 feet above sea level descending at 1,500 feet per minute with an indicated airspeed of 200 knots, the pilot doesn't have much room for recovery.  What you can't see in the picture is that the wing is clean (flaps and slats retracted) and the minimum maneuvering speed is 235 knots.  Alarming to say the least.

You might be wondering two things.  First...how on earth did the pilot (me) let the aircraft get into this predicament in the first place and second...how could he (again...it's me) be so derelict in his duties to stop and take a picture instead of reacting to a clearly life threatening situation.  The answer to both is that this is a simulated event.  I recently attended recurrent training, a four day refresher course that my airline puts me through every 9 months.  Day one and two consisted of ground school classes on various subjects.  Day three consisted of a two hour simulator brief followed by four hours in the sim practicing all types of approaches and emergency situations.  Day four consisted of a LOFT (Line Oriented Flight Training) during which we flew from point A to point B in real time, encountering a number of unusual situations and mechanical abnormalities along the way. 

After the LOFT, my training partner and I received two hours of "Advanced Maneuvers Training" where we were subjected to the situation pictured above.  The picture could depict one of any number of scenarios, but the most likely, especially at such a low altitude, is an encounter with wake turbulence.

An airplane makes a wake through the air much like a boat makes a wake in the water.  Except that instead of a one dimensional spread on the surface of the water, the wake behind an airplane spreads out and sinks.  The wake rolls off the wingtips, creating little tornadoes in its path, and is most severe behind large aircraft at low airspeeds.  To visualize this, picture dragging your hand through the surface of the water in a pool.  If you pull your hand through the water quickly, like a speed boat skimming across the surface of the water, the result is a relatively small wake.  Now drag your hand through the water a little slower and allow your hand to sink deeper into the water.  The result is a much larger wake.  A large airplane flying at approach speeds (relatively slow) creates the largest wake.


There are ample opportunities to encounter wake turbulence as aircraft criss-cross the skies, but there are really only three scenarios when it becomes a common threat.  The most unlikely encounter is during cruise flight.  Large commercial aircraft typically fly roads in the sky called Jet Airways.  It is possible that one airplane could fly the exact path of another, especially with advances in navigation technologies like GPS that literally put aircraft within inches of an airway centerline.  But GPS navigation has also resulted in the ability to "cut the corner" and fly direct between points, thus decreasing the chance of encountering wake turbulence while at altitude.  Also, since wake turbulence tends to sink over time, it is highly unlikely to encounter wake turbulence even when flying at the minimum required distance behind another airplane.

A more common opportunity to encounter this type of turbulence is on an approach.  As I mentioned before, an airplane creates the largest wake when it is traveling at slower airspeeds as when approaching for landing.  As the picture below suggests, there is a pre-determined glide path that a pilot is expected to fly when approaching an airport.  And again, since wake turbulence sinks, as along as each pilot follows the prescribed "glide slope," an encounter with the wake from a preceding airplane is unlikely.

ILS 25L, Los Angeles, California
However, pilots don't always fly the glide slope.  The pilot ahead on an approach may have started down late or could have been held up high by an approach controller.  Either way, if the airplane ahead is high on the glide slope, then its wake could easily sink into your flight path.  To complicate matters, it is often difficult to know if the guy ahead is high or low, so a pilot must always be prepared for such an encounter.

The last and, in my experience, most common opportunity for a wake turbulence event is during the initial climb after takeoff.  Every flight instructor tells new students to make note of the lift-off and touchdown points of arriving and departing aircraft.  Wingtip vortices begin when an airplane lifts off the runway and end when it touches down.  If you can lift off before the preceding departure and climb above its flight path, you will avoid wake turbulence.  You will also avoid wake turbulence if you fly above the flight path of an arriving aircraft and land beyond its touchdown point.

The MD82 I fly is an old design that takes significantly more runway to takeoff than newer aircraft of similar size like the Boeing 737 and Airbus A320.   Even larger aircraft like the Boeing 757 often use less runway than my old MD82.  The result is that I often find myself in the danger zone after lift-off and during the initial climb.  In this scenario, the best course of action is distance and the passage of time.  When taking off behind the heaviest of aircraft, five to six miles in-trail and/or 2 minutes, is usually enough time for the wake from a preceding departure to sink and dissipate to a point that it is no longer dangerous.   


What you see above is the most probable cause of the situation I found myself in.  Air rolls off the wingtips of just about any airplane and forms a spiral flow of air like a small tornado.  The "vortex" formed by a significantly heavy aircraft is large enough to completely envelope an MD82.  When encountered, this little tornado will roll the airplane on its longitudinal axes and could easily flip the aircraft upside down.  I went from level flight to what you saw in the top picture in two seconds...and as I mentioned before, the proximity to the ground combined with my speed left little room to recover.

Speed and altitude are your friend!

The proper recovery technique involves a combination of aileron and rudder inputs in the opposite direction of the roll in addition to pitch and power management.  It is also important to note that most any airplane is structurally designed to withstand full authority rudder input.  But it is not designed to accept a full reversal...that is, to push one rudder to the floor, then immediately reverse inputs and push the other rudder to the floor.  In the event pictured above, I pushed on the right rudder and commanded almost full aileron input to the right to counteract the roll caused by a wake turbulence encounter.  I pushed the nose down slightly below the horizon in order to control speed and increased power on the engines.  If managed correctly, you should be able to safely exit the wake with minimal altitude loss.  In this event, I lost less than 1,000 feet before recovering to straight and level flight.

We refer to this as "Unusual Attitude" training.  Wake turbulence is just one example of an event that could induce an unusual attitude, but the training is invaluable and is something we practice frequently during our regular training cycle.  As is the case with many aspects of aviation, avoidance is the best policy, but rest assured, your pilot has been well trained.




8 comments:

  1. Brad, at what point in the upset do you just continue the roll in the departure direction, and recover level flight after a 360 degree excursion? (After all, the galley is already a mess.)

    Regards,

    Frank

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    Replies
    1. Excellent question Frank. It has certainly occurred to me that going all the way around could happen. To be honest, I've had nightmares on the subject. I'll cross that bridge when I come to it...but oh my heavens, I hope I never come to that bridge!

      I've encountered some wicked wake turbulence in my career, but I've never had more than a 30 degree uncommanded bank. There isn't anything in any manual I've ever seen that gives any guidance to the possibility of going all the way around. To my knowledge, it has never happened in a transport category aircraft. That said, my grandfather got into the wake of a C-5 in a V-tailed Bonanza years ago. He never flew again.

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  2. Hi Brad,
    In this particular case, you say the minimum maneuvering speed is 235 knots. How does one determine this speed in upset attitudes? Is it laid down in the manuals? Is this speed different for different upset attitudes and aircraft configurations? Then how does one arrive at it in the thick of handling an emergency?
    Pranesh

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    Replies
    1. Pranesh,

      We set min maneuver speed in two situations...at cruise and for the approach. At cruise, the min speed is figured for a clean wing (flaps and slats retracted) and changes for weight, altitude and temperature. We were set up for an approach in the top picture in the post. The three top bugs were set for minimum speeds for different flap/slat settings. The top bug, 235 knots, is the min speed with flaps/slats retracted. The next bug, roughly 185 knots, is the min speed with the slats extended. The next bug, set to about 157 knots, is the min speed with flaps and slats extended.

      I hesitate to complicate the matter, but in this situation you could bank up to 30 degrees with a clean wing if above 235 knots. You could slow to 185 knots with a clean wing, but between 185 and 235 you would be restricted to a 15 degree bank. The same applies to the other bugs.

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    2. Hi Brad,
      Thanks for the reply. The article is nice, made easier to understand by the pictures and diagrams you have added.
      PS: The new look of your blog is nice. It is easy to read the black text against a white background.
      Pranesh

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  3. Nice article, thanks for the information.

    ReplyDelete
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