Twin Proficiency: Confident “Go” Decision, (thunderstorm turbulence) Don’t Drive on the Greens Part II

Twin Proficiency: Confident “Go” Decision, (thunderstorm turbulence) Don’t Drive on the Greens Part II

Twin Proficiency: Confident “Go” Decision, (thunderstorm turbulence) Don’t Drive on the Greens Part II




I spoke with a pilot at Oshkosh who had very recently survived a harrowing encounter with thunderstorm turbulence as he was flying IFR between cells. The pilot was working with Center to find a path between the growing cells. ATC suggested a deviation based on its real-time weather radar display; the pilot’s onboard NEXRAD upload agreed with the controller’s description of the cells’ locations. So, the pilot confidently accepted the new course.

With no warning, in the clouds but outside the areas of heavy precipitation on both ATC radar and the NEXRAD uplink, the airplane suddenly hit strong turbulence. It was thrown into a steep bank that disengaged the autopilot and caused the airplane to enter a sprial. The pilot recovered after entering visual conditions beneath the cloud deck, at roughly 1,300 feet above the ground. He thinks he may have momentarily blacked out from the G-load of pulling out of the dive.

ATC granted the pilot’s request to land at the nearest airport reporting VMC, which was uneventful. Shaken up, his brief visual inspection of the airplane revealed no apparent damage. Later, the pilot and his mechanic inspected the airplane more closely. The pilot had not looked under the fuselage, but the mechanic noticed the fuselage skin below the wing’s carry-through spars was wrinkled. Removing the cabin interior, he found wrinkling in the carry-through itself. Further, the wing attach fittings appeared to be bent. The pilot and his wife were extremely lucky to have survived. The airplane was totaled.

Radar and the life cycle of a thunderstorm

One of the challenges of making an informed, confident “go” decision when thunderstorms are developing is precisely how to plan your route to avoid the threat. It’s easy to look at the greens and yellows and reds on a radar screen or uplink display and think: “As long as I stay out of the yellows and reds I’m okay.” After all, green often indicates very light precipitation. Certainly a piston twin or light turbine can handle that.

The pilot who lost control and miraculously recovered asked me how he encountered such significant turbulence while flying outside the yellow and red areas of his iPad’s NEXRAD map, and those Center told him were painted on their screen. I told him this is because radar doesn’t indicate the earliest, but still dangerous, phase of a thunderstorm’s development: the Updraft Stage. I took the experienced pilot back to his knowledge test preparation and asked him to name the three stages of a thunderstorm’s development. He recalled them as the Cumulus or Updraft stage; the Mature phase; and the Downdraft or Dissipating stage. I then asked him–and this is a key point–what defines the beginning of the mature stage. The answer: it’s when precipitation begins to fall from the cloud.

Then I queried: What’s the greatest hazard of flying into a thunderstorm? He correctly said “turbulence.” I have recently been asked “How much rain can an airplane fly through?” The answer is “it depends”–some engines may have air filters or inlets that don’t stand up to rain. Some canard surfaces are known to have adverse aerodynamic reactions to rainflow, and high-efficiency laminar airfoils may lose some of their lift-generation capability in the rain. But, for most of us, the beating of rain against the airplane makes noise but does not alone create a hazard. What is dangerous is the wind shear effect of a boiling, fluid atmosphere.

The critical concept: Radar doesn’t detect turbulence, it displays the precipitation that sometimes (but not always) coincides with areas of strong turbulence. It’s turbulence, not precipitation, that is the hazard of flying near a thunderstorm. And, by definition, the Updraft stage of thunderstorm development is invisible to radar, despite the potentially damaging wind shear it contains–as soon as the storm appears on radar it is no longer in the Updraft stage.

Further, Center air traffic control radars are optimized for aircraft detection, not weather returns. Often, they cannot detect areas of light precipitation that may be the first indication of a mature-stage thunderstorm. The ability to paint weather on ATC scopes varies widely from one ATC facility to another. And, the rate at which individual thunderstorms can build is so great that what appears to be a clear path can close in before an airplane can traverse the space between the cells. That may include your escape path, too.

Life cycle of Cloud formation and how it affects turbulence

Twenty miles from where?

The old advice about avoiding thunderstorm activity–to remain 20 miles from the edges of any storm cell’s radar return, and to stay in visual conditions if at all possible while doing so–is just as valid now as it was in the days before weather data uplinks and moving map displays. The true purpose of these technoloiges is not to help us penetrate areas of precipitation returns; it’s to make it easier for us to stay 20 miles or more away.

There are additional factors of Base Reflectivity vs. Composite radars (the former reads only what is happening at the base of the clouds, while the latter depicts precipitation in the cloud in cross-section, providing advance warning of heavy precipitation), and the time between radar observation and transmission through NEXRAD uplinks (the latency period, which isn’t as bad in areas of active radar returns as has been reported). Even the colors used to identify specific levels of precipitation activity are very different from one weather radar data provider to another.

Dr. David Strahle developed an operating logic in the 1960s that pioneered digital transmission of weather radar information, eventually into the cockpit in flight. Dave is known as “the father of datalink radar.” A very active IFR pilot, he is an expert on weather radar interpretation and regularly consults with Government and private authorities and air crash investigators. His profession is as a radiologist…who better to analyze radar cross-section returns with a fresh, new outlook than someone whose industry-leading career is interpreting cross-section imagery of cancer patients?

Dr. Strahle is the extremely-rare authority who not only knows his stuff, but is also makes it extremely easy to understand the very advanced topics as a speaker and lecturer. I’ve heard him speak on inflight radar uplinks four times, and would do so again in a minute. I highly encourage you to watch for him on the speaking circuit and attend his presentations. In his Oshkosh 2015 talk, he emphasized: It’s generally safe to fly through areas of light precipitation (“green” returns on most radar plots), if there is no moderate or greater precipitation associated with those clouds. However, Dr. Strahle tells us, if there is any moderate precipitation in the radar plot (generally yellow), you need to remain at least 10 miles away from even the light (green) returns that surround the heavier precipitation. If there is heavy (often, but not always, orange or red) or extreme (darker red, white or other) precipitation, remain at least 20 miles away from even the light (green) returns.

Why is this? Research shows that individual thunderstorm cells will “share” or “exchange” energy, creating massive areas of instability and turbulence between them that may be invisible to radar…and even to the eye. If there is any precipitaiton at all in an area of storms with moderate or greater precipitation returns, anywhere in the cluster, researchers tell us there is the chance of extreme turbulence hazard. Dr. Strahle warns that if thunderstorm complex has enough energy to create yellow or red radar returns, it has enough potential to create turbulence anywhere within or near the cloud.

Avoid the Turbulence areas. Even the Green

So, what exactly should you stay 20 miles away from? If there’s moderate, heavy or extreme precipiation in the cell at all (yellows, reds or worse), it’s not safe to be anywhere in the precipitation footprint of that cell. Remaining 20 miles clear of that thunderstorm means staying 20 miles or more away from the outside edges of even the lightest, green radar returns.

Knowing what you know now, it’s a lot easier to make an informed go/no-go decision for flying when storms threaten. You know you’re not pushing your luck if you plan your flight to remain well away from anything that shows up on radar, to avoid the Updraft stage of individual storm cells and the potential of encountering significant turbulence as cells share and exchange energy between them. If you’re enroute and can’t stay well away from all the radar returns surrounding heavy precipitation, deviate right away. If you can avoid driving on the greens, as it were, you can make a confident “go” decision.

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1 Comment

  • Jim K October 5, 2015 at 11:10 am

    Thomas, thanks for this article. For years I’ve heard about staying 20 miles away from cells but I have violated that rule slightly a few times with little or no consequence and other times I have been well outside the 20 mile footprint and been tossed around quite a bit. I really didn’t understand until now that key to this rule is to determine if there is moderate to heavy precip WITHIN the cell footprint. I now realize that those cells where I entered the green area and didn’t get a rough ride were cells that had little or no yellow and red.

    This is much clearer now, my thanks.

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