🪂 Flying in turbulence
🚨 Turbulence creates two problems for aviators: loss of control and stress on the aircraft. Strong turbulence can nose you over or lift a wing wildly or even stall you by rapidly changing your angle of attack. When this happens close to the ground it is disconcerting at best. Such turbulence can also produce gust loads that can break or fold certain aircraft. To combat the first problem we need to speed up for quicker control. To combat the second problem we need to slow down so as to reduce the suddenness of the gusts. It is obvious we need a compromise here. ⚙️ The tried and true rule in most aviation circles is to fly 1.5 times your stall speed in turbulence to help prevent an inadvertent stall and avoid overloading your wings. Mechanical turbulence exists close to the terrain in its most virulent form. You can avoid it by remaining above it (several hundred feet up), waiting until the wind abates or landing in flat open terrain. Close to the ground mechanical turbulence has a preferred eddy orientation with the axis perpendicular to the wind as shown in the illustration due to the rolling action caused by the ground. This is especially true in the lower several feet. Within 60 feet (20 m) of the surface the eddies become oriented in all directions with random energy. What this means is that a landing into the wind brings you face to face with turbulence that tends to cause pitch changes, unless it meets one wing only. Good control speed is in order during a turbulent landing. ☀️ Thermal turbulence can be anywhere from the ground up to the thermal-based clouds. However, it will be worse near inversion layers and in high winds. Sometimes these winds are only at certain levels or in specific areas so can be avoided. Despite the normal presence of higher winds aloft, thermal turbulence is often less severe the higher we climb for the thermal gets more organized and broadens out. In any case, the best way to avoid thermal turbulence is to wait until solar heating tapers off. ✂️ Shear turbulence generally can be escaped by descending below the shearing layer. If you are powering up and encounter shear, simply power back down. On the other hand, if you are powerless and descend into shear all you can do is hold on and drop below it. Shear rarely extends to the ground. 🚁 Avoiding turbulence from other aircraft is of utmost importance, especially if the aircraft is larger than you. The details are described in flying manuals, but the main idea is to avoid the downwind side of the other aircraft's path for several minutes. Helicopters create a terrific amount of turbulence that lingers below their flight path. Avoid this deadly air by heading the other way.
🌬️ Lee – flyable or not?
The road to heaven is paved with good intentions; and one of these is to not fly in lee. However, if we don’t find any lift on the windward side, maybe we’ll get lucky over on the leeward side? Here is a passage from paraglider pilot Burkhard Martens (author of the book Thermal Flying).
🛑 Stable Day (Extremely Dangerous)
To do this in relative safety we must understand that lee isn’t just lee. Illustration 1 depicts a lee situation on a stable day. It could be an autumn day in the north Alps, with high pressure and a strong inversion. The temperature hardly decreases with altitude, in this example only about 1 degree per 1000m, or a temperature gradient (see chapter 9) of 0,1°C/100m. If the wind is on the face of the mountain it may still be soarable, and the airmass being pushed up over the mountain will still cool down dry-adiabatically, i.e. with 1 degree/ 100m (3°/1000ft). Once the air mass reaches the top, parts of it will be a 9°C colder than the surrounding air, and very much heavier. On the lee side of the mountain the cold, dense air rushes violently back down, causing extreme turbulence on its way. Flying here is not an option, even for pro’s.
🌪️ Illustrating the Turbulence
To illustrate the violence of such a leeside air movement we only need to consider that thermals begin to rise by temperature differences of as little as 2°C – a thermal stemming from a temperature difference of 9°C would be very extreme indeed, probably showing climb rates well beyond 20m/s.
✅ Unstable Day (More Feasible)
Illustration 2 The same mountain, now surrounded by extremely unstable air where the temperature decreases dramatically with altitude. Again, air is being pushed up over the mountain by the wind and getting chilled dry-adiabatically – but this time the temperature decrease just matches that of the surrounding air since the surrounding air is 11 degrees at ridge level, and the rising airmass has been cooled down to 10 degrees on its way up. It will still sink back down on the lee side, but with the low difference in temperature the movement will be much more benign. Anyone flying into this lee still needs to fly actively, but it is fully feasible and survivable, as opposed to the previous situation.
🌬️ The Wind Factor
Both illustrations assume weak winds and are consciously drawn more extreme than reality would normally be, but the example serves the purpose of explaining the differences between lee flying on stable and unstable days. The examples dealt with the turbulence caused by the downrush of cold, dense air due to pressure differences. The turbulence caused by the wind flowing over the obstacle adds to the complexity of the picture but again we can assume that it is proportional to the wind strength.
💡 HINT: I consider leeside flying in moderate winds and unstable conditions to be fully doable by experienced pilots, but I personally still seek to avoid it. If I see that leeside flying is inevitable on a cross country flight, I try to balance the risks with the possible rewards. Is the wind really not too strong? Is the airmass unstable? If I can say yes to these two, and there are emergency landings available I may decide to do it – but the landings are important because if I don’t find anything I’ll be on the ground soon due to the increased descent in the lee.
— Burkhard Martens
✅ SUMMARY
🔄 Turbulence is with us on an intermittent basis at least until the sun burns out. We have to live with it, fly through it and avoid its most severe forms. We can do the latter by understanding how the various types of turbulence are created and what signs indicate their presence. The use of flowing water as a model and a little imagination help us visualize where the dragons lurk and where the flying is comfortable. Sport aviators should use skill and judgement to finesse themselves through the air rather than plow through the rough spots.
😎 Soaring pilots choose to fly in thermals and a certain amount of wind which naturally introduce them to turbulence. Non-soaring pilots often pick and choose their conditions to minimize turbulence, but even so they occasionally run into textured air with a capital T. To feel at home in the air all aviators need to taste a bit of the rough stuff and swallow it with a smile. We'll leave the white-knuckle gnarly rides to the race car drivers, but accept a certain amount of bounce as being part of the aerial territory.
