May 1, 2004
By Bruce Landsberg
Bruce Landsberg, Air Safety Foundation executive director, has spent years studying Thor's handiwork.
It's thunderstorm season once again and throughout the magazine you will see plenty of references to why messing around in the big clouds is a bad idea (see " Wx Watch: Storm Season Insights," page 123). Searching AOPA Air Safety Foundation's database for accidents that occurred during the summer months will yield a number related to convective activity. Many of these accidents are fatal, but a number of pilots have just missed a storm and come back to tell stories about getting too close. In more than a few cases these pilots were staying what they thought was a safe distance away from storms when they got into trouble. A common thread is the speed at which conditions changed.
A Pitts was practicing aerobatics with thunderstorms in the area. A witness a mile from the accident site observed the airplane performing hammerhead stalls and rolls when a gust front came through and the wind increased from 5 mph to 30 mph within seconds. He then lost sight of the airplane as it descended behind a row of trees. The accident occurred within a minute after the gust front came through. Two weather reporting stations 14 and 18 miles from the accident were reporting heavy thunderstorms or very gusty winds. The pilot and passenger were seriously injured but survived. The NTSB called the accident the result of improper in-flight planning and decision making, and the pilot's failure to maintain control of the aircraft. The thunderstorm outflow was a contributing factor. It is possible that the pilot was unaware of the weather, but in today's environment it's easy to get a radar picture at the FBO or from a crew-lounge TV before launching on a local flight. It also doesn't hurt to look at the sky.
Sometimes the weather is well defined and the pilot tries to pick his way through. Almost every IFR pilot has been in a circumstance where there was at least some temptation to poke around. There's no harm in taking a look but the escape route must be planned. It's hard to know what you'll find and being conservative is really the only way to play, regardless of the many successes you've had in prior years. I keep reminding myself of that.
Here is a perfect description of a "sucker hole." The Ted Smith Aerostar pilot was aware of a line of thunderstorms near the departure airport and along the route of flight. Initially he paralleled the weather to the west for about 20 minutes looking for a hole to penetrate, and without success, turned back to the east. If the groundspeed was around 180 knots, that's 60 miles of bad road with no breach between storms. Climbing to 13,500 feet, the pilot noticed what seemed to be an opening to the south. He turned into it and was about 2 or 3 miles in when "the hole closed." The pilot reversed course and "accidentally penetrated a cell." What happened next was a classic thunderstorm encounter. The pilot stated he "lost control of the airplane, and was turned upside down...heading straight down toward the ground...traveling at a high rate of speed...the airspeed indicator was pegged." He was able to level the wings at 2,000 feet, reduce power, and raise the nose just before the airplane came to a "controlled crash landing" straight ahead in a sugarcane field. The Aerostar was destroyed and the pilot suffered serious injuries. There are seldom survivors in accidents like these. By definition, a sucker hole isn't one unless something bad happens — and you don't know that in advance.
A 118-hour private pilot on a VFR cross-country in a Cessna 172 diverted from his planned flight when he encountered a building squall line. It was a good decision, but as he was landing at the chosen alternate airport the situation deteriorated rapidly. The pilot reported a "normal touchdown" on Runway 17, and then an "extremely strong wind burst accompanied by rain" pushed the airplane onto the grass off the left side of the runway, where it struck a ditch and nosed over, coming to rest inverted. The runway was 6,400 feet long and 75 feet wide.
Twenty-three minutes after the accident the weather was reported as "wind from 340 degrees at 14 knots with gusts to 30 knots, visibility two and a half statute miles, with heavy thunderstorms and rain showers, and a wind shift with peak gusts at 44 knots." Prevailing winds were south but the gust front came from behind with no warning and nailed this pilot. Diverting a few minutes earlier might well have turned this into just a good hangar story.
The Beechcraft Duchess pilot was on an IFR flight plan but was circumnavigating a line of thunderstorms in visual meteorological conditions. This is a good technique, especially if there is no on-board weather avoidance gear. At 7,000 feet the airplane entered the clouds and light turbulence. The pilot reported that he disengaged the autopilot subsequent to entering instrument meteorological conditions and "was hand-flying for several minutes when we hit a couple of waves of heavier turbulence followed by a nearby lightning flash." Then the turbulence increased and the altitude deviated approximately 100 feet.
As the pilot attempted a minor control input to return to the assigned altitude, the airplane departed controlled flight. The pilot reported, "The airspeed indicator went full-scale deflection up, despite retarding throttles, and [the] VSI was full-scale deflection down. I felt that we were in a spiral/spin and I attempted to reverse this, but also felt largely unsuccessful in my efforts. We exited the clouds in a vertical nose-down orientation without rotation/ spin. An aggressive control input was made to arrest the descent and then a climb was started."
Control was regained after losing approximately 4,000 feet. The pilot reported that he wasn't sure how he recovered from the spin, but he was able to return to level flight and continue on to the destination at the assigned altitude. The landing was uneventful, but upon leaving the airplane he discovered "substantial wrinkled" damage to the airplane's wings. It was fortunate that the airplane did not shed any parts after the severe-turbulence encounter. Aerodynamically this sounds similar to the Aerostar pilot in the earlier example. However, with a slower aircraft there was more time to make a recovery, albeit with damage, before hitting the ground.
You'll remember that severe turbulence is defined in the Aeronautical Information Manual's rather antiseptic terms as the aircraft being "momentarily out of control." The flight controls of most airplanes are powerful. They can achieve any attitude almost in the blink of an eye, and to fly in conditions so strong that they are rendered momentarily useless should be avoided at all costs. Some survivors have described massive jolts to the airframe or a deluge of water if they actually penetrated the cell — frequently with leaks into the cabin. In some cases the flight instruments have been an unreadable blur. It may have been that the pilot was headed up when the panel was headed down, the instruments were reacting to the violent motion of the aircraft, or all of the above simultaneously. The noise is something to behold between the rain and possible loud protest from the passengers — this is not the place to be. It's a humbling and uncomfortable experience to surrender pilot-in-command authority to the elements. Most of us will enter into a Faustian bargain to survive the onslaught, and vow to be a whole lot more cautious next time.
Air carrier rules require a mandatory inspection after any severe-turbulence encounter before the airplane can be used again. It seems like a pretty good idea for Part 91 operations as well. Light-aircraft airframes can take some pretty high stresses, and they will actually go beyond what the pilot's operating handbook specifies in the limitations sections before breaking, unless of course some other fool has tested that particular airplane before you. In that case, the warranty is void. A thorough inspection after a severe-turbulence event is money well spent.
The airlines spend much more time operating near thunderstorms than most light GA pilots, but they have the equipment to make it a bit safer. On-board weather radar can provide a real-time tactical look ahead. Airline dispatchers make a point of learning all they can from their crews in a convective area and passing the information along to flights headed in or out. This is in addition to the tools that are available to GA pilots, such as some guidance from air traffic control and flight service. Airliners with high wing loads and heavy weights are much less likely to be tossed out of control than a typical GA aircraft, and yet they have plenty of convective-related problems as well.
Most airline thunderstorm "accidents" are really turbulence encounters where there is injury to passengers or flight attendants who weren't strapped in, and the narratives are similar. The flight crew generally knows about the weather, turns on the seat belt sign, and frequently advises the cabin crew to strap in as well. Typically, before everything and everybody can be tied down, the flight data recorder (and the airplane) will experience a minus 0.5-to-plus-2.4-G excursion with some altitude fluctuation, and someone is injured. The aircraft usually isn't damaged. In many cases it is determined that the flight crew was not adhering to the almost-universal admonishment to avoid severe storms by at least 20 miles. To be fair, though, there are times when all guidance is being followed and the atmosphere just creates a pothole.
When flying around convective weather it's worth noting how normal everything seems to be and how fast it can all deteriorate. Many other weather conditions gradually envelope the airplane, but Thor can deal a knockout punch instantly. This isn't to say storms just happen. There is usually plenty of visual and official warning. We just don't know exactly how bad it will be and in micro terms, exactly where or when. A few minutes or a few miles can make all the difference between an uneventful flight and headlines.
After reading this, you might decide that flying from March through October is inviting doom. It's not. Having spent much of my flying career in the Midwest and South, I've learned to believe the view out the windshield as opposed to any forecast; to ask lots of questions of ATC, flight service, and other pilots; and to build up real quality flight time avoiding the cauliflower clouds. Be flexible and plan an early departure before daytime heating gets the convection going. Several experienced pilot friends have described thunderstorms as "treacherous." This implies an anthropomorphic sense of evil or malice, which implies a living being. If that helps you to stay away, you can imbue them with any personality you wish.
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