April 26, 2011

Thunderstorms are Beautiful - From a Distance

Every year I receive a reminder from my employer that thunderstorm season is on its way.  Frankly, I don't need to be reminded, as thunderstorms are generally hard to miss, but someone making a lot more money than me thinks I need to be reminded.  Spring brings with it an increase in turbulence related injuries for our cabin crews, concerns over lightning strikes for our ground workers and a plethora of added concerns for our pilots and dispatchers...but it's high altitude transitions across lines of thunderstorms like the one described below that I dread the most.


It was the first week in April this year before my first encounter with a line of severe thunderstorms.  The day started in Dallas with an early morning flight to Las Vegas where I was scheduled to sit for about an hour then turn around and fly back to where I had started.  The weather was absolutely gorgeous in both cities, although the Dallas area was forecasting a possibility of storms in the afternoon...not particularly unusual for a Spring day in Texas.  The flight to Las Vegas was uneventful with clear skies and a smooth ride all the way to touchdown.

The flight back to Dallas was equally uneventful until the descent, where we encountered a small, but building line of storms.  I wouldn't even classify this line of rain showers as a thunderstorm at this point, just rain clouds with some vertical development.  Ft. Worth Center, the Air Traffic Control facility handling our flight, allowed us to delay our descent which allowed us to fly over the top of the relatively short storm.  We were forced to make a few small deviations late in the descent but never encountered any turbulence. 

After a short time on the ground in Dallas, we continued on to Birmingham, Alabama for a long layover in preparation for an afternoon flight back to Dallas the next day.  Over night, the small area of rain showers that we easily topped on our way into Dallas the day before had grown quite mature, stretching from Louisiana to Pennsylvania, and was literally on our doorstep by the time we were scheduled to depart.  The picture below shows what the storm looked like when we left Birmingham.  The green line depicts our track through the weather. 


The next picture is a closer look at our track.  Take a look and see if you think you would have taken the same path before you continue reading.


We took off from Birmingham, headed west and were granted an unrestricted climb to a requested cruise altitude of 34,000 feet.  We experience light to moderate turbulence during the first few minutes of our climb and asked the Flight Attendants to remain seated until we were above the rough air.  The departure controller instructed us to climb on a westerly heading directly toward the approaching weather, but after a quick glance at the weather radar we could see that this was not going to work.

Atlanta Center was working several flights through the area that were having some success with a path through a small break in the storm just to the west of Birmingham.  If you look at the map above, you can probably see the hole he wanted us to fly through.  The problem was that the aircraft making their way through this hole were already at their cruise altitude and above much of the weather you see on the map.  We, on the other hand, would not be able to fly through the same hole without first making it to 34,000.

As we climbed, it quickly became clear that we needed another plan, so we requested a left turn to parallel the storm until we had gained enough altitude to make our way through another hole.  As we reached our planned cruise altitude of 34,000 feet, a break in the storm became apparent and we made our move.  We widened out to the left initially then made a gradual turn to the west and picked our way through the hole. 

Hole? You say you don't see a hole? As bad as it looks in the pictures above, we were able to find a safe path around the worst of the storm and managed a pretty smooth ride.  One thing you must remember is that the weather maps you see online or on the evening news are top-down views of the storms and do not depict the height of a thunderstorm.  Air Traffic Controllers see this same 2D view of weather and will often warn pilots that they are about to fly through heavy rain showers that they are actually thousands of feet above.  A thunderstorm is a three dimensional beast and the 2D nature of most maps just doesn't paint the full picture.

I took the picture below just after we made our turn to the west.  On the right side of the screen you see "WX+T" over "+0.5".  This indicates that my NAV screen was set to display weather radar and turbulence information and that the radar beam was directed .5 degrees nose up...basically level.



It's at about this point that I usually get a tingling sensation in my toes that's my body's way of telling me to pay attention. I had already checked the aircraft performance charts and found that we were capable of climbing another 2,000 feet if needed...but I had no intention of climbing.  That extra 2,000 feet was the extra performance that I needed to feel comfortable as we crossed the storm ahead.  The thunderstorms were "embedded" which meant the storm cells you see on the radar screen above were hidden from view by cloud cover.  As we entered the clouds, we would have to engage the wing and engine anti-ice systems to keep the engines, wings and tail clear of ice.

The anti-ice systems on most jet aircraft use air from the engines to heat the leading edges of the engines, wing and tail and the air used to keep these areas clear of ice is a direct draw on the engine.  Any power draw on the engine will affect the aircraft's ability to maintain altitude.  As an example, an MD80 that weighs 133,700 pounds is capable of maintaining 34,000 feet with the engine and wing anti-ice systems turned off.  With those systems turned on, the aircraft would have to weigh less than 130,300 pounds to maintain the same altitude...a 3,400 pound reduction in the lift capability of the aircraft.  For these reasons, I wanted to stay at 34,000.

One nice thing about the average line of thunderstorms is that they're usually only about 10 to 20 miles wide.  So that uncomfortable feeling I get from flying around a thunderstorm so close to the maximum capability of the aircraft only lasts for a few minutes...then it's gone.  It also brings to mind an old saying.  "There are old pilots and there are bold pilots, but there are no old, bold pilots."  I'm not ashamed of the uncomfortable feeling I get when operating near the edge of the envelope.  I intend to be an old pilot...not a bold one.

We encountered light to moderate turbulence and at one point as we flew through a significant downdraft, the auto throttles advanced the engines to full power in an attempt to maintain the speed I had selected on the flight guidance panel.  Since we had elected to stay at a lower altitude, there was ample power to maintain our speed and altitude.  With the engines sitting steady at max cruise power, I was pleased with the decision not to climb to a higher altitude.

The ride through weather such as this always seems to be the worst just before you break out the back side of the storm and into clear air.  As we broke out into blue sky with my concern and tingly toes in the rear view mirror, I took a moment to look back and appreciate the storm we would leave behind.  Thunderstorms are beautiful...from a distance.

6 comments:

  1. Brad, that was a great example of the differing utilities of NEXRAD-type info and wx radar info. Indeed if I'd seen the picture you included on the Garmnin 396 in my 182, there's no way I'd have attempted the track you chose. NEXRAD is a weather avoidance tool, radar is a weather penetration tool.

    NEXRAD gives you a composite picture, showing essentially the worst-case return from any altitude at each lat-lon coordinate. Look at this recent post of mine for a minor example of the reason you have to supplement NEXRAD with input from the Mark I eyeball.

    Finally, "embedded" is a dirty word for us FLIB-drivers. I'll leave that stuff to pro's like you.

    Thanks for sharing your insights; it helps make all of us safer.

    Regards,

    Frank

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  2. Wonderful input Frank. Thanks so much for your comment. "NEXRAD is a weather avoidance tool, radar is a weather penetration tool" I often wish we had a NEXRAD display available in our cockpits. A combination of the two would make for a very well informed decision.

    Thanks again.

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  3. We had a ton of AA diversions to OKC just the other day.Glad to here you all were able to make it back home. Good thing Frontier flies to DEN. We were out of there just as the first round of diversions from DFW came in. We have seen a lot of new aircraft out here with the storm system out east.

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  4. Terrific post, and the point about the two-dimensional vs. three-dimensional images of the storm line is an important one.

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  5. Wow, we passengers and airplane/aircraft lovers really have no idea about the pressures (and tingly toes) our pilots face.

    It's about time this one showed some appreciation for what you all do for us.

    I'm going to create a link to this blog from my website at www.Airplane-and-Aircraft.com/airplane-pictures

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  6. Terrific post the point of two dimensional vs three is good.

    Education

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