Thunderstorms and airplanes and pilots: they don’t go together

Are there fewer thunderstorm-related private airplane crashes now than there were before Nexrad was beamed into almost every cockpit?

The answer is yes, no and maybe. The reason for the vacillation is the simple fact that we have little or no information on exposure. Given what we do have, and given the raw number of this type of accident then and now, I think it is pretty safe to say there has been no significant change in the relative rate of thunderstorm-related accidents.

In 1982, when writing about thunderstorm accidents, I noted that there always seems to be a spike around the 4th of July. There were apparently two in close proximity to the 4th in 2017, 35 years later. To me, these were particularly sad because one involved fathers and sons flying together and the other one grandparents and grandchildren flying together. The airplanes involved were a 1979 Cessna 421 and a 1966 Piper Aztec. From news reports, it appears that the airframe failed in flight in both cases. More about airplanes and thunderstorms later.

Garmin 696
That weather picture is great, but it’s not real time.

When we first got Nexrad in the cockpit, there was much talk about the fact that the picture seen on the screen is a bit behind the true picture. To begin, it was said that this could be as much as 20 minutes. That number was later pared down to as short as four minutes. Whatever, a lag is a lag and this can be important when dealing with something as dynamic as cumulonimbus clouds.

I was motoring along at 17,000 feet one summer afternoon, headed home to Maryland from Sporty’s in Ohio. The building cumulus were there, well below, but I would be coming up over the mountains shortly, where cumulus development is usually enhanced.

I saw an ambitious cu ahead and a peek through my sight level suggested it would soon grow through my altitude. There was nothing on Nexrad. I thought about going higher but decided to stay put and skirt by the cloud.

As I flew closer, the top of the cloud was well above 17,000 and it appeared to be growing rapidly.

The cloud top was probably well above 20,000 feet when I passed by and there was a smudge of green shown on Nexrad. Close visual examination suggested that it was an angry cloud and certainly no place to fly.

I was several minutes past the cloud when yellow started showing and within a few more minutes there was some red. That cloud was clearly blossoming into a cumulonimbus.

The important things learned were that the lag in the Nexrad display this day was maybe ten minutes and that a cloud can be mean enough to beat the heck out of you long before it shows on the display.

I thought about this when reading NTSB’s probable cause of a recent thunderstorm-related accident: Contributing to this accident were the pilot’s continued flight into forecast adverse weather conditions, and his reliance on weather technologies with known limitations and processing delays.

The NTSB looked at the information available to the pilot on his weather display and at the actual weather in the area and said that it was likely that the pilot flew into a developing rain shower and updraft and that it would have been difficult for the pilot’s equipment to pick up this new rain shower development. What the pilot apparently saw on the screen was heavy rain and thunderstorms to the north and west of the accident flight track and not much where he was flying.

When conditions are ripe for thunderstorm development, the flying conditions can go from good to bad quite quickly.

One of the basics of using radar information is to gauge the strength of a storm and the potential for turbulence based on the gradient, or the distance involved in going from no precip to heavy precip. If the picture turns red or magenta or whatever is used for the highest rainfall rate, in a short distance, it is likely a strong, mean storm.

The rain itself is not the problem. What the picture just described suggests is that the distance between the warm moist air rushing in to construct the storm and the air cascading downward with the rain is short and the wind shear turbulence in that area will be severe.

Think about rapids in a river: The water is moving over rocks that are stationary yet there is a lot of disturbance. If the rocks were moving in the opposite direction, as is the case with the in/out air in a storm, the disturbance would be greatly magnified. So, even if shooting rapids in a canoe is your excitement, doing double or more than that in a light airplane might be more excitement than you can stand.

thunderstorm cloud
Picking your way through a storm can work, but it’s a crap shoot.

I have heard pilots say they were going to pick their way through an area of thunderstorms and while there is no doubt that many are successful, it is still a crap shoot. It can be a maelstrom in there and, especially if your weather information is from Nexrad, the best practice is to avoid areas of thunderstorms. Close work is never a good idea but if it has to be done, airborne weather radar is the best tool.

Everybody has heard tales of thunderstorm penetrations and this used to be openly discussed in airline and especially military flying.

I worked for a while as a Link Instructor at a USAF contract school, teaching instrument flying to aviation cadets and student officers. The year was 1954 and Air Force Manual 51-37 (Restricted) was the bible. Read what it had to say about thunderstorms:

Sometime in the course of a pilot’s career he will be required to fly through a thunderstorm because of the importance of a military mission, or when there is no other possible route of flight. It is essential, therefore, that a pilot understand and employ the proper techniques for safe flight through a thunderstorm. On the first fight through a thunderstorm the pilot does not usually find any path that seems desirable. If he keeps cool, however, he will undoubtedly discover that his fear (and that is what it is) does not result from the actual predicament of the moment. He is afraid of imaginary dangers that lie ahead: things unseen, things that could happen. Passage through some thunderstorms is rough, the rain is heavy, lightning brilliant, turbulence is usually moderate to severe, and the hazards of ice and hail can be expected. These conditions, plus the possibility of structural failure within the aircraft, resulting from overcontrolling by the pilot, demand constant attention. Proper technique and procedure, however, will minimize or eliminate the dangers.

Sound like fun so far? The manual goes on to outline the recommended procedures best employed in an actual thunderstorm penetration. This was prepared in 1951 and was based on the experience of all the flying that was done in World War Two, flying that was done by young low-time pilots who had to fly with the mindset that if a thunderstorm got in the way of an important mission, fly through it. Do it right and you might come through unscathed. (I got one Amazon hit on AFM 51-37 so if you want a look at this publication you might give that a try. Or, eBay might yield one. I have always treated my copy, issued to me by the USAF, as a rare treasure.)

So how did they tell those kids to behave in a thunderstorm? I’ll share a couple of quotes.

The least amount of severe turbulence is found at or below 6,000 feet above the average surrounding terrain.

This was taken as gospel for years and many pilots think it is still true. However, the only jet airliner (a BAC 1-11) to suffer a structural failure around thunderstorm activity was lost in an attempted low-altitude squall line penetration. Lower wasn’t better that day.

The roll cloud, which is undeserving of the reputation for containing severe turbulence, is found near the 6,000 foot level, and at that altitude the turbulence is not severe. Severe turbulence can be found at any altitude if the right conditions exist; however, at the lower altitudes, gusts occur less frequently.

They really did believe that and I knew more than one contract flight instructor who flew a T-6G into a summer thunderstorm to try to prove that was true.

Thunderstorm cross section
Where is the severe turbulence? Everywhere.

There has been a great lot of research done on the subject since 1951 and we have had the benefit of learning from thousands of thunderstorm-related accidents in all types of airplanes. One thing that has proven to be true over and over is that old manual’s observation that severe turbulence can be found at any altitude.

One thing I have always thought about when studying thunderstorm crashes is the USAF admonition to keep cool. That is still a key these many years later. In some accounts of thunderstorm accidents it seems like the pilot either panicked, or, simply gave up.

When considering light, under 6,000 pounds MTOW, airplanes and thunderstorms the first thing to understand is that while one of these airplanes flown by a skilled pilot might make it through a garden-variety storm, the strong stuff is mostly out of the question.

Before I go into the gust tolerance of our airplanes, consider the superlative downburst that has become lore. If I recall correctly, it occurred near Andrews AFB (close to Washington, DC) and the number was 6,000 feet per minute. The corresponding updrafts would likely not have been quite as strong but such a condition would make the design gust of 30 or 50 fps (1,800 to 3,000 fpm) for Part 23 airplanes seem a tad puny. If you wonder about the 30 or 50, the first number is for original certifications before the early 1970s and the second for airplanes developed after that time.

The majority of the airplanes in the fleet today are covered by the lower design gust strength but a change in calculating the onset of the gust meant that the airplane designed to 50 fps is probably not much stronger that the one designed for 30 fps because the lower number was based on an instantaneous onset where the higher number considered a slightly more gradual onset.

The good news is that the incidence of an airframe failing because of a vertical gust encounter in or around a thunderstorm is relatively rare, especially if the airspeed is anywhere near Va, maneuvering speed, where the airplane will stall as the limit load factor is reached.

There is a lot of logic involved when considering the strength of the airplane in relation to turbulence. For example, the airplane will do best when the span loading is pretty equal, or, tanks full and weight brought up to maximum with cabin load. Burn up most of the fuel and there is more weight in the cabin and less in the wings so there’s more bending load on the wings when you pop a vertical gust. That is why maneuvering speed is lower at lighter weights.

Some airplanes have a maximum zero fuel weight. That means what it says, all weight above that value has to be in fuel, and that weight is established to keep wing bending loads under control in turbulence.

The bigger question in light airplanes is controllability. Larger airplanes have higher wing loading so they just naturally do better in convective turbulence. I rode through a thunderstorm on the cockpit jump seat in a Boeing 727 and while it certainly was not a good ride it wasn’t the wild ride that it would have been in a small airplane with lighter wing loading.

Thunderstorm buildup
It may not be on the radar yet, but you don’t want to fly through it.

I have always thought that the size of the airplane has as much to do with the response to turbulence as does wing loading. When the wind shear turbulence created by the in/out air interaction of a thunderstorm is visualized, there is a lot of rolling and tumbling in there. A smaller airplane might, in its entirety, be within one of those rolls and tumbles momentarily which could mean the available control authority would not be adequate to overcome the rolling and/or pitching moment. That too would pass but it might well leave the airplane in an unusual attitude. The slower the airplane, too, the longer it would be in that bad spot.

If you have ever watched cockpit video of a jet fighter penetrating a thunderstorm, it looks like a wham-bam-boom affair that lasts only seconds. At slower speeds it would last for a lot longer with the only good news being that the area of greatest vertical wind shear in a storm comes as the heaviest precipitation is encountered and can’t last too long.

The critical thing is to maintain control. When you think back to the pilots that AFM 51-37 was addressing, they had all been exposed to aerobatics so they were accustomed to using whatever control was necessary and they had the ability to return the airplane to wings-level flight from any attitude.

Many private pilots don’t even move the controls through the complete range before takeoff and there are a lot of landing accidents, especially in crosswind conditions, where the control authority to solve the problem was there, but the pilot didn’t use it and lost control. Most pilots never see a level of turbulence where full use of the controls is required to maintain control and keep the wings level but it can happen.

In thunderstorm accidents, the sequence usually starts with a loss of roll control. That is why keeping the wings as level as possible is primary. If roll control is lost, the natural tendency of the airplane is to enter a spiral dive where the airspeed increases rapidly and is soon outside the envelope. Then the airplane is in a condition where either turbulence or use of the controls can cause a structural failure.

In accident reconstruction, they go to great lengths to determine the sequence of the failures when the airframe lets go. That might be useful for a lot of things but it does nothing for the people who were in the airplane because the outcome is the same whether the wings or the horizontal tail is the point of the first failure. (Vertical tail failures have come first but that is quite rare.)

May reading about it be your closest encounter to all this.

In closing, I offer one more quote from Air Force Manual 51-37. It was put there for a reason and, given the average age of the private airplane fleet, the words are a bit like rolling a hand grenade under the old bunk but perhaps they are food for thought.

An airplane that is over ten years old cannot be expected to withstand the same stresses that a newer one can.

I do wonder if the USAF has told the pilots who are flying their grandfather’s B-52s and KC-135s about that.

18 Comments

  • Why is there ANY Nexrad delay? Everything in the chain -radar propagation and return, radio transmission and computer processing, is speed-of-light stuff.

    • I’ve read that there is a conversion delay from the sweep of the radar to a digital signal, then uplinking it to satellites, then Nexrad filtering it to the cockpit. I’ve read (perhaps here) that that link can average 10-20 minutes…it’s not the “transmission” time, but converting the radar information that causes the delay. I’ve certainly witnessed that delay as Dick Collins asserts here – you’ve already seen the CB before Nexrad starts displaying anything….

  • Counting on one weather depiction resource can get a pilot in a lot of trouble. Richard, unsaid was that you were looking outside the cockpit with your eyeballs also and not counting on NEXRAD alone. Radar, listening to what others say about asking for vectors and ATC are all part of the plan. Unfortunately Flight Watch has gone away.

    There is no single silver bullet to ward off weather advoidance risks.

  • Thank you! I always read your columns/articles with great interest as you have an uncanny knack of conveying your years of experience, with over-the-backyard-fence wisdom (“lower wasn’t better that day.”) Every IFR pilot, if they are honest, on at least one flight, has looked at the Nexrad display, compared it to the windshield, and “hoped” the display was telling it like it is, just before penetrating the clouds. I sometimes felt that tightening the seatbelt was akin to tightening one’s sphincter, you were preparing for the unknown, but “hoped” it would be unremarkable. I don’t think electronics/downlinks has really added any noteworthy changes to increase our “educated guessing” in entering the “wisps” adjacent to a CB. Hopefully that will change….until then, thanks for stirring in your facts with years of wisdom…LAR

    PS – we exchanged hellos once in Denver Airport, and I still consider it a great treat to have shaken Dick Collin’s hand…and yes, I do miss my Malibu..!

  • I flew a 1977 T-210N in business for a number of years with Dick Collins as my vicarious mentor. We had poor man’s weather avoidance on board the 210 – a Stormscope but no radar. After 1500 hours in the 210 we upgraded to a TBM 700 that had both radar and a new Stormscope. I insisteded on the installation of the new Stormscope in the TBM because my experience in the 210 had proven to me that by avoiding lightening – that one can avoid destructive wind shear.

    Since that time and several thousand additional hours of flight time, the simple paradigm of [lightening=wind shear] has never let me down when flying in the area of thunderstorms while avoiding convective activity.

    So, many years later, I continue to remain befuddled about all of the angst expressed over thunderstorm avoidance using radar/NEXTRAD when a much simpler and profound solution based upon simple physics has been available to aviators for over 40 years now.

    • Jim,
      Thanks for the great advice and sharing your valuable experience.
      What I hear you saying is , ” weather avoidance “.
      The Art of determining how to avoid hazardous weather, is life saving practice.
      Detect and avoid, if you penetrate the unknown you might not make it …

      Cheers !!!

  • We must also remember that NEXRAD is not an aviation product. It provides meteorologists with more accurate precipitation information for the general public.

    In that instance, the storms are travelling to the people. Transmission delay is not a problem because people can see it coming and take precautions/ cover.

    As pilots, we travel to the storm much faster than they move. We, in essence, are over-running the data. The access to nexrad data is invaluable but we must consider the limitations of the original intended use.

    Additionally, NEXRAD radar is designed to report surface precipitation, (Is it going to rain on my golf game?), not precipitation aloft. There is some correlation with airborne precipitation, in recognition of additional uses of the product, but that correlation takes time.

    Your eyes, when you can see the buildups, and airborne radar used with appropriate clearances from returns when you can’t see them remain the best tools for T’Storm avoidance.

    • Excellent points Charles !!!
      Many believe the ” salesman”, technology will take care of them. Not knowing the basics of technology and the basics of meteorology has proven to be fatal.
      Common sense and respect for the energy packed in a CB ready to be released.
      After the energy is released,the air is smooth and a pleasure to fly through.
      Thanks !!!

  • I was shocked to hear when Scott Crossfield met his maker in his strut braced 210 in an area of bad storms. He had an early version of portable satellite weather and I believe the unit was brand new to him. If it didn’t work for whatever reason, I’m sure Mr. Crossfield would have resorted to his “normal” way of flying without radar to avoid the bad stuff. As a guy who would have read Air Force Manual 51-37 and flown many aircraft at the extremes of performance limits, I have a difficult time thinking he would let a storm get the best of him and a pretty sturdy airframe. I can only think that he confidently entered an area of storms that showed up as benign on his new radar display and was caught unprepared. Just my 2 cents.

    • ” During the flight, he encountered a Level 6 thunderstorm.

      Scott Crossfield requested to deviate from his planned course to avoid the severe turbulence. Atlanta Center authorized his request and he began to turn. Approximately 30 seconds later, at 11:10 a.m., radar contact was lost near Ludville, Georgia. The last indication was that the Cessna was descending through 5,500 feet.”

  • I read your article as always with great interest. Sadly enough the BAC 1-11 was not the only jetliner accident due to thunderstorm activity. I can still remember the horror when I heard of the crash of a Fokker F-28 that lost the right wing in a thunderstorm here in the Netherlands:
    http://aviation-safety.net/database/record.php?id=19811006-0
    It shows that even in a country with normally rather “moderate” thunderstorms bad things can happen.

  • Wow. This scared me enough to re-think this entire subject and vow to be even more afraid of convective Wx than I already am!

  • We are all told to avoid thunderstorms at all costs, but any pilot flying significant IMC will one day encounter a Thunderstorm. I have encountered three. One by being vectored into a big one by ATC who kept insisting that I go North toward the storm as it was “50 miles from me”, when I wanted to turn West. He flew me right into it, even while I keep requesting a left turn before I got to the storm. It was 50 miles North of me before I took off, and then verified by ATC at 50 miles. He kept me going North despite my continued insistence that I really needed to turn West on course. Really bad weather day and I was the only one on frequency? I did turn carefully West in the Storm, very unhappy with ATC. The rain was so violent that it took most of the paint and primer off the leading edges. Exeedlingly rough, a waterfall of rain, and lightning flashes. It was so rough my eyes were bouncing in my eye sockets and the needles were bouncing so hard I could barely read them.
    The second was with a new Stormscope that I pushed too hard and close to and suddenly found myself surrounded, IE, in it! I have encountered the same sudden surrounding in a friends plane on the ground. He had a new Stormscope, and we could see the storm coming. So we got in his tied down plane and watched it coming toward us. When it still seemed 5-10 miles from us it suddenly engulfed us. This is unnerving unless you understand to stay further from it than it seems.
    The third time was when the weather was closing in around Galveston, and I thought I could beat it out. I didn’t. Turned out to be the most destructive storm and tornados in local history. Significant tornado damage in the millions of dollars.
    So, in addition to telling pilots not to go there, we need some information about what to do when it happens.
    It will be a lot rougher in there than you think it could possibly be, but the plane can survive it, if you do it correctly. That is, don’t get a wing low, don’t worry about maintaining altitude, slow down, and as a last resort, lower the landing gear. Do not lower the flaps, as that significantly lowers the G loading the plane can handle.
    Fly the Plane.
    Keep the wings level.
    Keep the pitch level. Do not try to hold altitude, because your speed will significantly vary if you do. Just ride the updrafts and downdrafts, and keep everything level.
    Don’t turn. The two major airline failures were due to the pilots trying to turn out of the Thunderstorm. Turning decreases the structural G load the plane can handle. Technically turning increases the G load.
    Slow Down. Slow down generally to about half the speed between stall speed and maneuvering speed. I like to be at about 60% above stall instead of half.
    The transition between an updraft and a downdraft will be even rougher, and close to out of control for seconds. Keep it level and slow. Lower the gear. You may lose a gear door, but that’s better than the alternative.
    Go straight through. Most Thunderstorms are not much larger than five miles in diameter, and you will soon be through it.
    Lowering the gear stabilizes the plane and makes it easier to keep it level.
    The rain will be unbelievable and make you wonder if the windshield will hold up. It usually will. The DC8 as I remember did have their windshield broken, and the rain drowned both engines. The plane did not break in that extreme turbulence, but they hit a telephone pole with a wingtip after landing after congratulating themselves on making it down. Very sad ending.
    The lightning will be even louder, and much brighter. As Earnest Gann said, lower your seat, turn on your inside lights to the brightest, and ignore the flashes.
    If in the unlikely event that ATC asks why your altitude has changed, explain (shortly) why. If he persists, ask for a block altitude. The chances that any other plane will be in there with you are pretty slim. (g)
    For some reason two of my encounters included getting in a massive smooth updraft. It was suddenly smoother, quieter, and very rapid. I did not worry about my altitude.
    With a giant “Bang” that sounded almost as if the plane was breaking up, the updraft ended, it got very much rougher, and then I entered a downdraft. Being above 6000′ makes you feel happier that you won’t pancake into the ground.
    I have been told that the roughest altitude is around 18,000′, but prefer not to experiment further.
    I think most Thunderstorms, if not all, can be handled if you keep the plane slow, and level in both roll and pitch.
    Slow down, keep the wings level, keep the pitch level, and hang on.
    The windshield will not burst. I was concerned the first time that it would, and I wondered how I would die? Would I be beaten to death by the rain, or drowned first?
    I have used both a Stormscope and on board Radar, and the Stormscope seems to be preferable, but three times may be enough for a lifetime of encounters, but not enough to draw any absolute conclusions that a Stormscope will keep you safer. I lean that way.
    The beginning stages of a storm have little rain and associated lightning, so even with the best of intentions and equipment, if you fly IFR much, you will encounter a Thunderstorm. I once flew by a massive rain storm, but before the lightning had started, and therefore not showing on a Stormscope. It was unusually smooth even though I was very close.
    You will survive if you don’t panic, and don’t lose control of the plane. Get a wing too far down will speed you up and cause a break-up, or even a break-up after falling out of the cloud and pulling back too hard on the yoke.
    I think Scott Crossfield of all people, may have lost control because of the suddenness of the encounter. The plane, even though strut-brased, then came apart in an ensuing overspeed.
    I hope my few experiences will be helpful to someone in the future.

  • Early in my career (late 60s), I was literally “spit” out of the top of a building cu coming out of Tallahassee IFR enroute to Opa Locka (Miami) in my Beech Travelair (no radar). Told Jax Center I could not hold my assigned altitude. Also, several years later, had to make the big 180 enroute to Nassau in my Twin Beech 18 — both radial engines went quiet as they were starved for air and started windmilling. Both came back to life, only after turning around. I thought I was going through a “soft spot” on my radar in a squall line across the Gulfstream. Nothing soft about it.

  • Another great article Richard. I like the Nexrad for the precision and ability to make flight plan changes to widely avoid the bad stuff. I have over the years noted a good correlation with the stormscope and severe turbulence. I’ve stumbled into a couple storms and made it out. In the end, fate is the hunter.

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