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Space is infinite in its complexity. It affords us majestic vistas of beauty—an open blue canvas for our imagination that often pocks up with smidges of greyish-white clouds. Space is also infinite in its potential. From trolling our imagination into creating giant rockets that carry us, humans, into space, the final frontier. And tampering our mental reserves with turbulent mischief.
And there in lurks the adventure. Within the expansiveness of space there are “known, unknowns and known knowns,” to quote Rumsfeld. The known knowns are easier to define as in flying into the clouds in IFR but to understand the known unknowns, well they have a way to defy our vision and catch us by surprise.
Turbulence is unpredictable. It is at its basic level of understanding, an irregular motion of air that causes eddies and vertical motion. By “Eddies” we mean the circular, chaotic motion of air, churned by the mechanics of wind, obstructions, and heat. All three axis of an aircraft are subject to the vagaries of turbulence. From light turbulence which causes a minor change in the lateral, longitudinal and vertical axis through moderate turbulence where the aircraft movement is rapid but remains in the control of the pilot and then extreme turbulence where the control of the aircraft is lost.
Turbulence at its core is caused by two forces: a mechanical force and a vertical thermal force. The mechanical force is via the wind velocity and direction. Wind velocity of greater than 20 knots with higher gusts, especially flowing over rough terrain, can lead to an unwelcome flight. Even stable airmass of constant 20 knot winds as it traverses over uneven terrain will cause eddies or burbles, or “potholes in the sky” that will “involuntarily” move the aircraft in its various axes. The changing force and the varying direction due to terrain and thermal convection creates the dynamics of unpredictable wind vector that causes turbulence.
Note that with the heat of a summer’s day, as the beach goers enjoy the sun, water and sand, the pilot faces the uncomfortable bumps that turn the ice cream into a milk shake in his stomach. The eddies mixing with the vertical motion of the heat generating lift of the heated air makes for discomfort. The up motion of the lift vector from the surface temperatures, and the perpendicular wind vectors pushing the heated air along, make for a dynamism in space.
Mountainous terrain has its own share of drama for the pilot who is cruising over flat terrain and can see the terrain rising in front of him. On the East Coast, the Appalachians, and in the West, the Rockies, pose an interesting threat. The former can produce some interesting chatter in the aircraft all the way up to 10,000 feet.
Flying to Oklahoma from New Jersey in my G36 Bonanza, I once encountered a mountain wave where the indicated airspeed changed from 92 knots to 164 knots without a single bump. The mountain waves occur due to winds blowing perpendicular to the mountain ridge at greater than 25 knots in a stable atmosphere creating updrafts and downdrafts over the mountainous ridge.
Imagine the thrill of 164 knots followed slowly and progressively to 92 knots where the heart starts to miss a beat as the angle of attack indicator goes from full green to blue and yellow. If one were to add a high K index and blow some lift into the atmosphere, that combination can be a rough transit especially within the clouds. On the western part of our country where the mountains are nearing 15,000 feet at some points, there are additional things to worry about.
The eddies created by those outcropping landmasses are gigantic, causing rotor clouds to form on the leeward side of the mountain. These rotor clouds usually form between 15,000 and 20,000 feet and can take an aircraft down if one curiously or inadvertently enters it. Similar mechanics are at play in clear air turbulence. Only you cannot see but if you get into it. You will certainly feel the impact and taste your stomach acid if not a whole lot more.
The thermal turbulence occurs over large land masses across the country. Flying over the barren desert land of Northern Texas, Southern New Mexico, and Arizona in the heat of a summer’s day can be quite the challenge that will force your passengers to take a commercial flight the next time around. Been there and some have done that too. The summer sun heats the land which forces the air parcels to rise. The vegetation slows down the thermal activity a bit, hence the air parcels are uneven, rising here and falling there. This up and down motion of air creates eddies and mess with the various axes of an aircraft.
Suffice it is to say that flying at a higher altitude in the summer is a preferred mode of operation. Add the wind to the mix and it takes on a whole new meaning to the term discomfort over a larger parcel of space. Such convective currents displace air rapidly and thus the uneven heating and the gusty winds can be a real challenge for pilots. In the summer, one can see a haze layer between the warm surface air and the clear westerly flow of cooler air. It is best to fly above the haze layer to avoid turbulence. If there is a small scattered cumulous cloud layer, then better to fly above that layer where the air is stable, and comfort guaranteed.
I once flew with a newly minted Private pilot in his Piper Archer. Upon takeoff, the turbulence was minimal, so he had a wide grin of seeing the aircraft perform under his command. As we got higher to 4,500 feet, the turbulence got unnerving for him and he said, “you take over!” I did, slowed it down to maneuvering speed and put the gear down. The aircraft settled down bumping along gingerly to our destination. I reminded him about the nature of Va or maneuvering speed that guarantees a stall rather than potential damage to the aircraft. Or in severe cases, a breakup of the aircraft in the air. Perhaps a lesson learned.
One can see the fluid dynamics of pouring cold milk in a cup of hot coffee. The milk churns and churns until it finds equilibrium. The equilibrium in a confined space is easy to see, but in the infinite space where several weather phenomena are competing and contemplating their actions at once, equilibrium is not often the result.
Cold weather fronts that push the warm front over and above them as they slide closer to the earth’s surface underneath, due to the denser colder air, create the same turbulence by lifting the warmer air ahead atop the colder denser air slipping below. This effect is not too dissimilar to eddies forming around the hills and the mountainous terrain.
If you fly long enough you will encounter some or most of these nature’s phenomena hurled at you. One such one was over front range airport in Colorado where I was happily watching the pilot flying his Debonaire on an ILS approach in VFR conditions. While he was wearing view limiting device, I had my eyes wide open. I could see a sudden shaft of rain in front by about seven miles. The clouds decided to empty their burden.
As we got crossed the final approach fix, the rain shaft grew. I looked at the airspeed and it was jumping +/-5-10 knots. Ah, a wind shear and a lot worse was going to mess with our landing. I asked the pilot to declare a missed approach and request vectors back for another ILS. He was alarmed but true to his flight objective, he requested the vectors which were granted.
As we turned back, the wind shear hit. The airspeed decreased and the downdraft caused him to pitch up to maintain altitude and I helped push the throttle in to maintain airspeed as our groundspeed built up from 120 knots to over 150knots in short order. His heart must have skipped a few beats as he took his Foggles off in a hurry to see what was going on. But there was nothing to see in the 180-degree direction. Everything was behind us. As soon as we reached the IAF, I checked the ATIS once again and the airport reported winds gusting to 34 knots in wind swept rain. He took his Foggles off again and looked at me with the “deer in headlight” look. What? We deviated to another airport and waited for the cell to move or dissipate, which it did, and I took the chance to debrief the pilot of what had just happened. It was a good lesson for both of us.
Here’s an example of moderate to severe turbulence in a Bonanza while in the clouds climbing through 7,000 feet.
Turbulence is common but never to be taken lightly. It can make milkshakes in your stomach, it can hurl unsecured objects around in the aircraft, create deviations in airspeed, altitude and pitch, yaw and bank axis and even cast you and your aircraft as a discarded toy or in extreme circumstance break it apart. So be prepared and done take it lightly.
Learn to understand nature and its vagaries. Weather is not only “flying in the clouds,” it is a everything that happens in space.
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You have a wonderful way to describe a weather phenomenon that we all experienced but unable to see – well said !!
Good article and insights one turbulence.
You say that you cannot predict it, but I could argue that you can… to a point, but perhaps not the intensity. There are a LOT of turbulence indicators and conditions that cause it, and if there’s none, you’ll probably get a smooth ride.
I hate the bumps and will do whatever I can to avoid it. Often it can be avoided with altitude, but sometimes it pays to alter the route. I’ve flown my jet as low as FL180 to avoid it and my Bonanza as high as FL180 to avoid it and have had pretty good luck in my 27000+ hours. Only hit severe twice, both in rapidly moving fronts.
Keep up the good work.
Some years earlier, a Japanese photographer visited Nepal. This was his second visit. First was, when he was nearing thirty years of age. Now he was plus sixty. He owned a photographic school in Osaka. His main task was to photograph mountains, when the sunlight first hits the top of the peaks.
Flight was scheduled for me for a few months. I was to operate out of a grass strip in Lamidanda, east of Kathmandu. There were no runway lights and no electricity. A person would stand at the other end of the runway, swinging a lantern. He was my beacon that keep me straight down the runway. ATC people did not get up at night to comfort us. Aircraft was a Cessna Caravan 208.
It was the month of November. We used to fly out of the airstrip at 2 am. As we reached up towards higher flight levels of more than 250, turbulence was expected as usual. Strong winds (jet-stream) are common in the Himalayan regions during cold climate. At FL 290, my ground speed would be around fifty knots or so when flying from east to west. As I banked my aircraft towards east to start the photo-shoot run, my groundspeed would increase to 240 knots. So, you can imagine mountain waves. I did not see any wave clouds. But I could clearly see some kind of vapour curling on the east slope of the mountain side. I must avoid the top of that mountain or avoid the east slope. To correct wind drift, I had to bank sixty degrees. That was exciting.
One day I was at FL 280 flying near Mount Makalu 27,766 feet. Unfortunately, that day I drifted towards the east of the peak. I saw my precious airspeed drop. I shouted out, “we are going down…turbulence. Grab something and hold. Cessna just nose-dived and went to roll. I had no control. Nature took over. When the Cessna recovered, nose was pointing east towards Mt. Kanchenjunga and not towards the vertical wall of Makalu. I lost around three thousand feet or so. We diverted and headed back to our base in Lamidanda. Our Japanese friend was hurt quite badly. Because of his photo actions, he was not strapped so as to be quite movable. This was a frightening flight. We then flew him to Kathmandu for medical services. Stress tests were carried on the aircraft.
Photographer was medically fit after ten days and the schedules resumed. I flew was then well prepared.
Good article on turbulence, and loved the included video. Looking at those wave clouds as he was taking off, and watching as he climbed into them thinking “this is going to get rough pretty soon…” I’m still wondering how you managed to put the wheels down in a Piper Archer :-)