Understanding and avoiding turbulence
By Karsten Shein
Comm-Inst, Climate Scientist
Wake vortices form downwind of the Cape Verde Islands in the tropical Atlantic in Apr 2004 as seen from NASA’s Terra satellite. The rough islands emerging from the relatively smooth sea disrupt the airflow around them, generating strong turbulent eddies in their wake.
In the early afternoon of Mar 5, 1966, Capt Bernard Dobson, pilot of BOAC Flight 911 (a Boeing 707) amended his flightplan from HND (Haneda, Tokyo, Japan) to HKG (Kai Tak, Hong Kong) to take advantage of some unusually clear skies and make a visual climb via Mount Fuji that would allow his 124 passengers a rare chance to view the iconic mountain from the air without its usual cloaking blanket of cloud.
Although the aircraft reached an altitude almost 5000 ft above the summit of the mountain, as it approached the 12,400-ft peak from the southeast witnesses on the ground saw it suddenly sprout white vapor trails, soon followed by debris.
In short order, the tail section detached and the aircraft entered a flat spin during which the starboard wing failed and the forward portion of the fuselage separated. The 707 crashed near the base of Mount Fuji and evidence, ranging from load testing on the recovered tail section to frames from a home movie camera found in the wreckage, all pointed to the aircraft having been subjected to a sudden and extreme load.
The white vapor trails were determined to have been fuel vaporizing in the cold air as the engines were ripped from their mounts. Indeed, a load meter on a US Navy rescue aircraft sent to locate the wreckage returned with readings ranging from –4 to +9G.
The likely culprit for this disaster was determined to be a strong mountain wave generated by Mount Fuji as a severe pressure gradient over the area generated winds aloft of 60–70 kts out of the northwest.
Post-accident analysis also indicated the presence of nearby lenticular and rotor clouds prior to the accident. Apparently, the turbulent forces generated by this mountain wave easily exceeded the 110% of gust load needed to cause structural failure of the airframe. Weather dangers come in all shapes and sizes, but few are more insidious than turbulence.
Rarely has severe turbulence been as deadly as that experienced near Mount Fuji that day in 1966, but turbulence has long been a leading cause of aircraft occupant injuries and occasional fatalities, and it continues to be a contributing factor in crashes ranging from single-engine cropdusters to the largest commercial aircraft.
Just about every pilot has experienced some form of turbulence. It is exceedingly common to encounter the unsettled air near the surface during a takeoff or landing. Solar energy heats the surface, which in turn heats the air above it. As this surface-layer air becomes buoyant and rises, we’re able to feel the force of the rising air buffet our aircraft from beneath.
Fotunately, such jostling is nothing more than a minor nuisance that dissipates quickly as soon as we clear the first layer of fair weather cumuli. But this sort of turbulence is generally not the type we need to be concerned with.
Turbulence is a function of windshear—that is, its one and only cause is the interaction between air moving one speed and direction and an adjacent flow moving at another speed and/or direction.
When a flow of air at one place is identical to the flow next to it, there is little resistance between the 2 flows and the boundary between them is considered laminar. Friction between the 2 flows, also known as eddy viscosity, only establishes when the flows are dissimilar.
When this occurs, the boundary between the flows becomes turbulent. What is occurring along a turbulent flow boundary is that, initially, the friction generates waves, similar to what happens when a fresh wind blows across a lake or ocean. If the flow differences are very small, only small waves—and light chop—will result.
But, if the flow difference is relatively large, bits of one flow are sheared off by the adjacent flow. The sheared bits spin off in eddies, similar to the small whirlpools you would see along the edges of the turbulent flow behind a boulder in a stream. It is these eddies that generate the turbulence you feel.
The more conflicting the 2 flows—whether it is a difference in speed, direction or both—the more significant these eddies, and the stronger the turbulence can become.
Mountain waves occur as strong winds are compressed over a mountain or range and establish waves downwind. The strongest turbulence is generally found beneath the first few wave crests, below the altitude of the mountain peak.
Although turbulence is a function of windshear, there are 2 basic ways in which turbulence can be generated—thermally or mechanically. Thermal turbulence is largely convective, ie, it is generated as air rises or descends due to heat-induced buoyancy.
The light chop we commonly experience on takeoff or landing in the summer is a primary example of thermal turbulence. Fortunately, this air normally rises at just a few feet per second and any eddies it generates will be small and weak. Although such turbulence can be widespread, its effects generally exist only in the few thousand feet nearest the surface.
As soon as we climb out of the surface layer—often marked by a layer of haze or the bases of the lowest cumulus layer—the air will smooth out almost immediately. Thunderstorms however, represent an altogether more violent version of thermal turbulence.
While the mechanism is the same as surface-layer chop, thunderstorms are the result of the atmosphere constraining the heated air near the surface until it has enough energy to break free and explode into the free atmosphere above. Now, instead of rising at a leisurely few feet per second, this air may rise at 100 fps or greater.
What’s more, cold air near the top of the storm may descend through the storm, or even descend around it at similar, if not even faster, rates. At such speeds, air moving vertically through an environment of relatively light horizontal flow is likely to generate a great deal of shear along the edges of the up and down drafts.
In general, the speed of the air in an eddy will be some fraction of the speed of the faster flow, so, given a 100-kt downdraft, the speed of air in the eddy may range anywhere from a few knots to 100—more likely closer to the 100-kt mark. What’s worse is that, because the eddy is spinning off from its source flow, there is no way of telling from which direction it may hit your aircraft.
In addition, even with a flow of 100 fps, the resulting eddies can spin hundreds of feet into the surrounding air. When an updraft and downdraft flow next to each other, the difference can easily exceed 250 fps, which can translate into a 200-kt-plus wind hitting your aircraft from any conceivable direction.