Sunday, October 29, 2006

Welcome to the new blog. I have changed to this service because I can actually send text messages via my cell phone to the blog. This will be very handy when I'm traveling on the road. *subject to change, of course!*

So I went for a bike ride today, despite 25mph winds. Why do they always seem to be a headwind...?

Wind is a pretty big deal these days in the northwest, but it's always a big deal to pilots. In a previous post I talked about a frontal passage. According to, a FRONTAL PASSAGE is the passage of a front over a specific point on the surface. It is reflected by the change in dew point and temperature, the shift in wind direction, and the change in atmospheric pressure. Accompanying a passage may be precipitation and clouds. May be referred to as "fropa."

This is what often causes TURBULENCE, the irregular and instantaneous motions of air which is made up of a number of small of eddies that travel in the general air current. Atmospheric turbulence is caused by random fluctuations in the wind flow. It can be caused by thermal or convective currents, differences in terrain and wind speed, along a frontal zone, or variation in temperature and pressure.

What is of a concern to pilots and their passengers is Clear-Air Turbulence (often abbreviated CAT and sometimes colloquially referred to as "air pockets"), the erratic movement of air masses in the absence of any visual cues (such as clouds). Clear-Air Turbulence is caused when bodies of air moving at widely different speeds meet; at high altitudes (7,000-12,000 metres/23,000-39,000 feet) this is frequently encountered around jet streams or sometimes near mountain ranges. Clear-Air Turbulence is impossible to detect either with the naked eye or with radar, meaning that it is difficult to avoid. However, it can be remotely detected with instruments that can measure turbulence with optical techniques, such as scintillometers.

This kind of turbulence creates a hazard for air navigation. The rapid changes in the speed and direction of the air mass cause the lift created by an aircraft's wings to vary quickly and unpredictably, making for a rough flight. From:

I get asked about turbulence quite a bit, and I try to reassure the jumpy by telling them they have probably not even come close to how bumpy the air could be to still have the turbulence considered "moderate." There is still severe and extreme to go...

As pilots, we are asked to give PIREPS (PIlot REPorts) to ATC (Air Traffic Control). According to the Airman's Information Manual:

PIREPs Relating to Turbulence
a. When encountering turbulence, pilots are urgently requested to report such conditions to ATC as soon as practicable. PIREPs relating to turbulence should state:
1. Aircraft location.
2. Time of occurrence in UTC.
3. Turbulence intensity.
4. Whether the turbulence occurred in or near clouds.
5. Aircraft altitude or flight level.
6. Type of aircraft.
7. Duration of turbulence.

EXAMPLE-1. Over Omaha, 1232Z, moderate turbulence in clouds at Flight Level three one zero, Boeing 707.2. From five zero miles south of Albuquerque to three zero miles north of Phoenix, 1250Z, occasional moderate chop at Flight Level three three zero, DC8.

b. Duration and classification of intensity should be made using TBL 7-1-9.

TBL 7-1-9
Turbulence Reporting Criteria Table
Intensity, Aircraft Reaction, Reaction Inside Aircraft, Reporting Term-Definition

Light: Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, yaw). Report as Light Turbulence.
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.

Occasional-Less than 1/3 of the time.

Intermittent-1/3 to 2/3.

Continuous-More than 2/3.

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.
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.

1. Pilots should report location(s), time (UTC), intensity, whether in or near clouds, altitude, type of aircraft and, when applicable, duration of turbulence.

2. Duration may be based on time between two locations or over a single location. All locations should be readily identifiable.

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 are impossible.

a. Over Omaha. 1232Z, Moderate Turbulence, in cloud, Flight Level 310, B707.

Extreme Turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage. Report as Extreme Turbulence.

b. From 50 miles south of Albuquerque to 30 miles north of Phoenix, 1210Z to 1250Z, occasional Moderate Chop, Flight Level 330, DC8.

High level turbulence (normally above 15,000 feet ASL) not associated with cumuliform cloudiness, including thunderstorms, should be reported as CAT (clear air turbulence) preceded by the appropriate intensity, or light or moderate chop.

Sorry, this one got a little long. I learn when I do this, I hope you do, too!

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