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Welcome to summer! and thermal turbulence

From the Accident Review Committee

Your Accident Review Committee hopes your reintroduction to the flying season in spring went great!  Now we are in the heat of it, literally, with long days and lots of solar heating.  This makes it a good time to look more closely at the topic of turbulence in general - and thermal turbulence in particular - with the goal of reducing the risk factors of summer flying that contribute to accidents.

Preface: Causes of a Serious Accident

First, some preface remarks about accidents in general.  In serious accidents, we all want to know the reasons why it happened.  We want to know the root cause(s).  This yearning for an answer partly (or mostly) comes from the fear that it might happen to us.  We need to understand so we can convincingly tell ourselves that we are safe from the fate that occurred to our friends or fellow pilots. While this is a very useful and beneficial desire, it often results in “sound bite” answers to complex scenarios and risks: missing or minimizing the full list of ingredients that went into the recipe that resulted in the accident.

The importance of digging for that full list of ingredients is that in many (if not most!) serious incidents, it was the culmination of several factors that led to an unrecoverable situation for the pilot.  The elimination of perhaps just one factor would have left the pilot with the ability to successfully deal with the situation.  Distractions are great examples of seemingly minor factors that lead to more serious incidents.  Being distracted with operating a camera in flight has led to close calls or midair collisions.  Pilots adjusting their harnesses or zipping up shortly after launch has caused some exciting maneuvers or worse.  Most of the stories we know about pilots launching unhooked or with poorly pre-flighted gliders happened after getting distracted before launch. Take away the ingredient of distraction and the accident either doesn’t happen or the pilot is in a better position to deal with it.

Now there are absolutely solid principles, physics and factors involved that can be crisply articulated as having major contributions to the recipe of an accident: the root causes.  The point here is not to dismiss the primary role of the root causes, but rather to not miss the importance of the other contributing factors that may have had a non-linear, significant impact on either the pilot’s ability to deal with the big problem, or that may have even caused the big problem.

Assessing risks related to thermal turbulence

Visualizing “invisible” turbulence

Now, back to the topic of turbulence.  Let’s motivate this discussion with a quote that many have probably heard:  “If we could see the air we fly in, we wouldn’t.”  The point is that the air we fly in is very actively moving in many different directions and on different scales.  Large scale horizontal flows are mixed in with the smaller scale mechanical and thermal induced flows in all directions.  The analogy of water cascading over rocks in a fast flowing stream helps us visualize the invisible mechanical turbulence of the air we fly in.  We see smooth flows over round rocks in slowly moving water and chaotic tumbling flow behind sharp rock edges in fast flows. 

That visualization helps us understand how we easily soar in the nice ridge lift in front of a mountain ridge and why going behind the ridge in strong winds likely results in total loss of flight control of our wings no matter how good we are.  So never go behind the ridge right!?  Well, if we are flying in moderate winds and the ridge has a smooth round top, we might see pilots top landing back there.

The Answer is “It Depends”

This exception to the “Don’t land behind a ridge top” rule leads to a couple of important facts.  For many, if not most, of the flying situations we face, the answer is “It depends”, along with the more important corollary: “Just because you got away with it, doesn’t mean it was a good idea or low risk.”  For the ridge example, as the wind velocity increases and the sharp edges/obstacles on the ridge are more prominent, the chaotic turbulence that results behind the ridge also increases making controlling our wings in that area more and more difficult.  There is absolutely a region of the velocity/obstacle density spectrum above which our wings become totally uncontrollable (and no one could top land in those conditions); however, where that region occurs is highly dependent on the local factors of the ridge and the wind velocity/direction (so the answer to the question of “Is it safe to land behind that ridge top” is “It depends”). 

Between the no brainer conditions where anyone could top land and the uncontrollable conditions where nobody could, is an insidious slope of increasing difficulty/risk along which more talented pilots can pull off the top landing most of the time.  There are and will continue to be many things you see your fellow pilots do that may or may not be smart ideas depending on the conditions, the pilot’s experience level and wing type, etc.  Having a skeptical, somewhat pessimistic view of the merits of trying some new flying trick will serve you well in your personal risk management.

Thermal Turbulence and Time of Day / Time of Year

The characteristics of ridge lift and mechanical turbulence are also true for thermal turbulence, although it is tougher to “see” the increasing velocity/obstacle turbulence spectrum when it comes to thermals.  That said, as the vertical velocities of the air increase (common during mid-summer, dry, high lapse rate days) and the sharpness of the edges of the thermal increase (common on those punchy, small thermal, high pressure days) we can expect increasing thermal turbulence. This thermal turbulence may be severe enough to lose control of our wings for a moment or for good. 

This leads us to a useful metric for predicting potential thermal turbulence: time of day and time of year.  Early morning and early evening flights are the sweet spots for student training for good reasons.  In the morning the shining sun hasn’t heated things up enough for thermals to begin and in the early evening, the diminishing sunshine moderates the amount of heat energy in the thermals and they tend to be more benign.  As an example of the time of year effect, more experienced pilots can enjoy pleasant, mellow flights in the famous thermal producing Owens Valley in October, as opposed to the rippers in June/July.  When you are evaluating the amount of risk you want to expose yourself to from thermal turbulence, consider the time of day and time of year.

Altitude AGL in Risk Management

Now let’s shift to another strategy for risk management concerning thermal turbulence that has been a factor in several of our most tragic accidents over the years: altitude above ground level (AGL).  As mentioned earlier, turbulence in the invisible air we fly can make our aircraft permanently uncontrollable.  In that case, an emergency parachute may be the best - or only! - option for survival.  In order for an emergency parachute to be useful, however, we need sufficient altitude AGL to provide us the time to look, reach, pull, and throw our chute, followed by successful inflation.  While the answer of how much time/altitude is needed for a successful deployment depends on a variety circumstances, numbers in the 300-500 feet AGL range are reasonable and on the low end of the comfort zone. That said, do not hesitate to throw even if you think you are too low! Some reserves have been successfully deployed in 100 feet or less.

Tools for managing risk

“Safe Operating Envelope” (SOE)

The above discussions on the probability of thermal turbulence and the altitude AGL needed for successful emergency parachute deployment leads us to the useful concept of a “Safe Operating Envelope” (SOE).  Such an SOE can be depicted on a plot with the axes of “Probability of severe turbulence” vs “Altitude AGL” and with shaded areas ranging from red indicating high risk, to yellow for moderate risk, and to green for low risk (see Figure 1).

Safe Operating Envelope
Figure 1: Plot of Risk vs Probability of Turbulence and Altitude AGL

The purpose of the plot is to provide a picture that emphasizes what we already intuitively know: that flying close to the ground in likely situations of severe turbulence is a bad idea

The vertical axis (probability of severe turbulence) depicts how likely you might encounter air that would make your wing uncontrollable and require emergency chute deployment.  It starts at the origin with a very high probability of turbulence - equating to flying in situations such as sunny mid-day summer conditions of high pressure and high lapse rate, or flying in areas of known mechanical turbulence such as the lee side of a ridge in high winds.  You’d find a moderate chance of turbulence on days with less solar heating, a moderate lapse rate, etc.  A low chance of turbulence would be found early or late in the day, with light winds, and/or very little solar heating such as in the fall in many locations (the “Time of day, Time of year” idea).  While defining and identifying the probability of severe turbulence is an inexact science, we can find value in a general assessment of high, medium or low probability, which can be made considering the types of circumstances listed above.

The “Altitude AGL” axis of the plot is more straightforward to define, and easy to understand. The plot goes green at the point on the axis where a successful emergency chute deployment is likely.  That point is intended to be defined by pilot comfort, equipment, and training; however, as previously mentioned, it starts at the 300 to 500 foot range for most pilots.

Take Off and Landing

Now the astute in the audience will point out that every time we take off and land on a thermic day - hoping to soar - we are in the “High Risk” red zone on our SOE because we are starting at zero altitude AGL and have a reasonable probability of encountering turbulence.  This is true! That fact should be acknowledged and we should mitigate the associated risks as best we can.  On thermic days we should be hyper vigilant and stack as many mitigating factors on our side as possible during take off and landing.  Choose good cycles to launch in, fly quickly away from launch and gain altitude, don’t fly in known turbulence areas low, etc.  On landing a very useful strategy is to arrive high enough over our LZ that we can drift or climb in any thermals kicking off in the LZ and then land after they have passed.  Also consider that a very reasonable decision is to not fly on those very thermic days or at least not during the peak times of the solar heating.  The mountain will be there tomorrow!

Summary Thoughts

It is the hope of the ARC that this article stimulates further discussion between pilots and at club meetings on how we can mitigate the risks we encounter while flying. There were a lot of topics touched on in this article that deserve further discussion:

  • Distraction as a significant contributor to accident scenarios
  • Just because a pilot got away with it doesn’t mean it’s a good idea
  • Time of Day / Time of Year and thermal turbulence
  • Minimum altitude for effective chute deployment
  • Visualizing / predicting areas of turbulence at your local sites
  • Evaluating the current risk level along the “no brainer” to “nobody could/should do it” spectrum based on current conditions
  • Making the decision not to fly

While acknowledging that many of the ideas above might not be new, reviewing them and saying them a different way is always useful.  As any long time aviator will tell you, there are seldom new causes of accidents, just innovative ways to fall prey to old ones.


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