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.
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.
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.
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.
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.