Turbulence

Aviation Topic of the Week
By Michael Oxner, December 7, 2003

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This week's topic:
Turbulence

With the advent of real world weather importation into Flight Simulator some time ago, many folks began to experience turbulence in the virtual skies for the first time. Most would set up ceilings and low visibility (once that became available) but they'd ignore setting turbulence. This week, we'll talk a little about turbulence, what can cause it, and how it's rated.

Definition
Types and Causes
    Mechanical Turbulence
    Convective Turbulence
    Clear Air Turbulence
Descriptive Ratings
ATC Handling of Turbulence

Definition

Turbulence is simply air that isn't smooth. The big things are what makes it unsmooth and the intensity. The AIP Canada, in the infamous MET section, has surprisingly little information on turbulence, except the reporting system detailed in MET 3.7. I will detail some of this information in another section below. There is also some information on Clear Air Turbulence in AIR 2.10.

Types and Causes

There are a few different types of turbulence that pilots will encounter in their travels. They are:

Mechanical Turbulence
Convective Turbulence
Clear Air Turbulence
Wake Turbulence

I won't go into Wake Turbulence this week, since that can be a topic of discussion all on its own. There will be plenty to say about the others this week.

Mechanical Turbulence

Anyone who has flown on a windy day can relate to Mechanical Turbulence. This type is caused when high winds blow over and around hills and get deflected into other currents of air, causing shifts in airflow. Similarly, buildings and trees, and any other form of obstruction can give rise to mechanical turbulence. In my own experience, winds over 15 knots almost always cause some amount of turbulence in the lower levels, and strong winds can cause nasty bumps upwards of 5,000 feet AGL without even the need of having significant mountain ranges nearby. When high winds are present, they are almost always accompanied by wind gusts, as well. Noted in weather sequences as "25G35", it means the "mean wind" is reported as 25 knots, with gusts up to 35 knots. In addition to turbulence causing up and down motions, the motions can be sudden side to side movements, as well as forward/aft. While the aircraft won't so much be jostled around by changes in forward speed, they can be very dangerous. A sudden loss of speed could lead to a stall, which is very dangerous, for example, when the aircraft is flaring for touchdown. High winds almost always result in abrupt fluctuations of airspeed on final, since mechanical turbulence is typically associated with the lower levels.

Convective Turbulence

This type of turbulence is normally associated with warm days over changing terrain. Some surfaces absorb the sun's energy and radiate it outward at a much higher rate than others. This gives rise (pardon the pun) to a light form of Convective Turbulence. As always, the sun heats the ground and the ground heats the air. We all know that hot air rises. Convective turbulence is born in situations like when hot pavement and grassy fields are in close proximity. The hot pavement heats the air above it at a higher rate than the rate at which the grassy field warms the air above it. As such, the air above the pavement will rise faster than that over the field. An aircraft flying from one area to the next will encounter vertical speed changes as he passes over these areas, and these changes in currents can be quite amazing in force. Air over water bodies, treed areas, rocky terrain, city scapes, and so on warms up at different rates. Add a gentle breeze in there to shift the upward currents around and you have for some bumps to fly through.

Taken to higher levels, hot summer days will often provide a more advanced, and dangerous, form of convective turbulence. Cumulus (CU) clouds are formed as warm, moist air rises and cools, thereby condensing. Again, we get into turbulent air as this process moves on. As it continues in the heat of a summer day, this effect can become monstrous. Towering Cumulus (TCU) clouds may form as the air rises higher and faster, leading to precipitation. Now you may have air rising in the cloud itself, sometimes with rain underneath the cloud creating an additional airflow in a downward direction, also causing turbulence under the cloud. More advanced yet, Cumulonimbus (CB) clouds may form from the rise of warm air if the airflow is fast and massive enough. These clouds can be quite tall, reaching into the upper flight levels where modern airliners fly. The tops of CBs are often sheared off by upper level winds, giving the anvil shape often seen. Now we're talking some real dangerous conditions for aircraft. Apart from the significant chances of icing in cloud, or hail forming and severely damaging the aircraft, the airflows inside TCUs can be so strong as to violently shift the aircraft around, even rising or descending faster than the aircraft is capable of. This is the true danger behind convective turbulence. There are many stories of aircraft entering TCUs in stable, controlled flight, only to exit at vastly higher or lower levels, in vastly different attitudes (sometimes being rolled inverted in the cloud in solid IFR weather) or being thrashed around inside the cloud to the point where heavy damage is sustained to wing spars and the fuselage. Airframe operating limits can be quickly exceeded, threatening the lives of all on board. Many people have been injured in aircraft cabins due to unsecured objects being thrown about, or even being caught unsuspecting while being out of their seats when turbulence hits.

I once tried to talk an IFR Cherokee out of attempting to punch through a line of thunderstorms based on SIGMET information and pilot reports (PIREPs) from other aircraft. Undaunted, the pilot continued on. Starting out a 5,000, he asked to climb to 7,000. Then, out of 7,700, asked for 9,000. 7,800 was as high as he got. Every time he keyed his mic I could hear the hail hitting his windows as the air around him sank faster than he could climb. The lowest altitude he reached was 1,600 feet over 1,200 foot hills known to have 300 foot cell phone antennas on them. He was IMC the whole time, so he had no realization of how close he really was to not making out of the line of cells alive. He finally exited the downdraft horizontally and was able to climb to a safe altitude. I don't think I was nearly as relieved as he was when he called clear of cloud and asked for a vector to the nearest airport.

Clear Air Turbulence

Just like what its name implies, this is turbulence associated with otherwise clear air. This is normally associated with upper wind shifts, such as those found near the jetstream. This high speed "river of air" has a core in which the highest wind speeds are found. The speeds are upwards of 200 knots in some cases. As an aircraft approaches the jetstream, the flow of air changes and strong eddies are encountered, typically just below and just above the core itself. This can be quite severe. The major difference between this type of turbulence and convective turbulence is that this one can't be seen. Every pilot knows there is rarely ever a good reason to fly into a thunderstorm, but they can be avoided either by use of weather radar or by looking out the window while the conditions are still good. CAT is just not visible, and as far as either a look out the window or a standard weather radar is concerned, things look smooth ahead. Jolts from severe CAT have significantly damaged aircraft frames, but have another danger. Once in cruise flight, the "fasten seatbelt" signs are often turned off. Passengers and crew will roam the cabin, serving meals, visiting the bathroom or other passengers (or, I suppose "visiting" other passengers in the bathroom), and suddenly with no warning, the aircraft can run into a bad spot of air. People have been severely injured, and even killed, from the jolts received by CAT. New systems are being developed to try to evaluate conditions ahead, but I'm not aware of any current system in use.

Descriptive Ratings

In order to aid pilots, a method of distributing pilot reported information has been put in place. This way, a PIlot REPort, or PIREP, can be made accessible to other pilots that are or will be operating in the area in which other pilots are or have been in. In order to provide advance warning of conditions to pilots who will operate in the area, there had to be a standardized way of reporting turbulence so that when a description is used in a report by one pilot, the next pilot hearing the report will know what is meant and can decide what to do with the information. Here is what the AIP Canada MET 3.7 has to say about turbulence reporting:

Intensity	Aircraft Reaction	Reaction Inside Aircraft
Light	Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch. roll, yaw). Report as "Light Turbulence". OR Turbulence that causes slight, rapid and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude. Report as "Light Chop".	Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered in walking.
Moderate	Turbulence that is similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. it usually causes variations in indicated airspeed. Report as "Moderate Turbulence". OR Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude. Report as "Moderate Chop".	Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.
Severe	Turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control. Report as "Severe Turbulence".	Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking impossible.

In addition to the reporting of the force of the turbulence, there are notes associated with the table above as published in the AIP.

NOTES:	1.	Occasional: Less than 1/3 of the time. Intermittent: 1/3 to 2/3. Continuous: More than 2/3.
	2.	Pilots should report location(s), time (UTC), intensity, whether in or near clouds, altitude, type of aircraft and, when applicable, duration of turbulence. Duration may be based on time between two locations or over a single location. All locations should be readily identifiable.
		Examples: Over REGINA 1232Z, moderate turbulence, in cloud FL310, B737 From 50 NM EAST of WINNIPEG to 30 NM WEST of Brandon, 1210 to 1250Z, occasional moderate chop, FL330, AIRBUS.
	3.	High level turbulence (normally above 15,000 feet ASL) not associated with cumuloform clouds, including thunderstorms, should be reported as CAT (Clear Air Turbulence) preceded by the appropriate intensity, or light or moderate chop

It's obvious as to why ATC would want information on locations, intensity, in or near clouds, and altitude. The time can be recorded at the time the report was taken, but should accompany a pilot report if the turbulence occurred earlier but is no longer occurring, or was experienced some time ago. The type of aircraft involved is important also, since moderate turbulence reported by a PA-31 Navajo might hardly be felt by a Boeing 747. On the other hand, if an Airbus 340 reports moderate turbulence, someone flying a Cessna 210 might do well to be very cautious about entering the area concerned.

ATC Handling of Turbulence

Normally, if pilots report turbulence to ATC, controllers will, workload permitting, pass this information along. If for no other reason, to attempt to ascertain pilot's intentions before they get to it and ask for a change in altitude or routing. Controllers know that pilots would rather fly in smooth air, and if it's practical for ATC to help out, generally speaking, it's done. The one exception to this is severe turbulence. This is a known threat to safety, and is handled very seriously by ATC. The simple mention of the word "severe" catches people's attention, pilots and controllers alike. Severe, as indicated in the table above, means that the aircraft may be temporarily out of control, and damage to the airframe can occur. ATC will relay PIREPs of severe turbulence as soon as practicable to other aircraft in the area, and to supervisors for further distribution to flight services personnel, and ultimately to Environment Canada personnel.

Additionally, ATC in Canada has direction to discontinue the use of vertical separation between aircraft if severe turbulence is reported in the area. Since the aircraft may experience large fluctuations in attitude and altitude, ATC must immediately move to provide another form of separation other than vertical, and this will typically, where available, mean resorting to radar separation for the airspace concerned. Any speed restrictions applied to aircraft should also, as soon as possible, be canceled. Even if they're not canceled, a smart controller would consider the fact that aircraft may have to significantly alter speed to weather the bumps. Most aircraft have a "rough air penetration" speed. This is the speed at which full deflection of control surfaces at a given weight will not cause structural damage to the airframe. This is usually lower than normal cruise speeds, and so aircraft encountering rough air will likely want to slow down, even if only a little, when encountering turbulence in the severe category.

ATC will take any reports of severe turbulence and will also want more information about the nature of the turbulence, the extent of the jolts felt, and whether there was any damage to the aircraft or injuries to persons on board. Since the pilots are likely to be quite busy trying to control the situation, these reports can be left until the aircraft has exited the area of severe. ATC will also want to know when the aircraft has passed the severe turbulence, both to ascertain the condition of the aircraft and the edge of the area of turbulence.

Well, that was a bumpy ride through the topic of turbulence. Did I miss anything? Have anything to say? Stories to tell about it? Send them to me at moxner@nbnet.nb.ca. I'd love to hear them. Feedback? I'll take that, too. Thanks for taking the time to read!