Between the Terrain and the Anvil

When you hear about an accident in Taos, NM (7,095 feet MSL), you may reasonably assume that density altitude was a factor. While that’s often the case for accidents during warm weather at high altitudes, our subject today involves thunderstorms, which can happen anywhere.

On May 10th, 2013, the pilot of a Canadian-built Zenith 250 waited for a thunderstorm to pass before departing on Runway 04. Just before takeoff, the pilot obtained the automated weather observing system information, which reported wind from 120 degrees at 11 knots gusting to 22 knots. 

A quick spin of the E6B tells us that, at max gust, the crosswind component on the runway was almost 22 knots. Proceeding would be ill-advised for a low-hour pilot in a Skyhawk, but this pilot had over 1,100 hours in his logbook, 654 of which were in a Zenith 250. And, by the book, the wind was within the airplane's capabilities.

However, according to the pilot, the airplane performed an uncommanded descent shortly after takeoff when it encountered "turbulent crosswinds and downdrafts." The pilot turned the airplane but it still would not climb and continued to encounter downdrafts and turbulence as it approached rising terrain, obstacles, and the storm clouds. 

Long story short, the churning winds unceremoniously deposited the aircraft into a landfill near the airport. The pilot sustained serious injuries but lived to fly another day (the plane, not so much).

Let’s go back and look at one particular passage.

The airplane “...continued to encounter downdrafts and turbulence as it approached rising terrain, obstacles, and the storm clouds.”

• In the judgment of the pilot, the thunderstorm had traveled sufficiently beyond the airport to allow for a safe takeoff and departure.
• But, as the pilot struggled against the wake of the thunderstorm, he was concerned that he was getting too close to the storm clouds. 

You and I weren’t there to judge the scenario in person, but doesn’t that sound like the storm was still pretty close to the airplane?

Each time we embark on a flight, we make judgment calls regarding the weather. With increasing time in our logbooks, in type, and flying from the same airport, lax personal minimums can set in. 

In the absence of sober judgment, which can ebb away with every hour spent in uneventful flight, overconfidence or complacency often fills the void. We weren’t in the pilot’s head that day, but it’s a good bet that a lack of proficiency, impatience, and/or a lack of respect for convective weather were at play.

So, how would you have handled the situation?

The first thing to pop into my mind is the 20-mile rule. We’ve all heard discussions about staying at least 20 miles from thunderstorms, but they aren't a regulation and are not starkly defined. 

For example, have you ever defined from what part of the storm you should measure that 20 miles? The visible cloud? The red splotch on the radar?

The FAA specifies that, outside the storm cloud:

• Shear turbulence is encountered several thousand feet above and up to 20 miles laterally from a severe storm.
• Clear air turbulence may be encountered 20 or more miles from the anvil cloud edge.

The information is useful but doesn’t plainly offer advice.

If a storm is powerful enough to have a large anvil, it can extend far beyond the mesocyclone, or core, of the storm. The non-anvil components of a thunderstorm usually have a radius in the neighborhood of eight statute miles. 

If a thunderstorm has an anvil that extends seven miles past the edge of the core, you could assume the need to stay 20 miles away from the edge of the anvil, which extends seven miles from the storm’s core, which has a radius of eight miles. That’s 35 miles from the center of the storm.

In the end, it’s up to you to decide what to do with that information. For the accident outlined above, the NTSB identified the probable cause as “the pilot's improper decision to take off in gusting wind conditions, and the airplane's loss of climb performance following an encounter with turbulence and downdrafts.”

It doesn’t say anything about violating a rule, or even a rule-of-thumb, about remaining 20 miles from a thunderstorm. It just says the pilot should have known better.  

You may have noticed that this article hasn't offered specific advice either. Well, here you go. 

If you analyze the weather and feel that things will “probably” work out with storms in the area, chock your plane and go do something else. Heck, that advice goes for any unnecessary risk factor. 

Your knowledge, experience, and comfort with certain conditions and situations can be overshadowed by neglecting the periodic work and continual learning required to be a safe pilot. 

Quiz