To inform Pace and others, there have long been accepted descriptors of turbulence. When the level of turbulence is reported in a PIREP, it is important to consider the type of aircraft that encountered said turbulence. Altitude also gives a clue at what airspeed the turbulence was encountered. If the aircraft was on approach, the airspeed was most likely below 250 kts. CAT is usually encountered at cruise, so for a jet airliner, the airspeed was probably in excess of 400 kts.
Remember that CAT usually occurs at the intersection of the Jet Stream core, the tropopause and the borders of a warm and cold air mass. An aberrant eddy of air is thought to mix up the Jet Stream core with the resultant turbulence. Because CAT it is transient, i.e. dynamic, it is nearly impossible to track and predict. Way back in the 1960's, Northwest Orient Airlines researched CAT, with their airliners equipped with special sensors. A CAT reporting program was a result of the study. Conclusion(s): At cruise, CAT was almost always encountered just before a sudden and rapid increase in OAT (Outside Air Temperature). It was nearly impossible to anticipate. Subsequently, NWA installed an audible and visual warning system to alert pilots of impending CAT based on a rapid increase of OAT at cruise. The CAT in-flight warning system never caught on for several reasons.
LEVELS OF TURBULENCE: Aircraft Reaction and What Occupants feel:
Level 1, Light Turbulence that momentarily causes slight, erratic changes in altitude and/ or attitude (pitch, roll, yaw) 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.
Level 2, Moderate This will cause changes in accelerometer readings of around 0.5 – 1.0g at the aircraft’s centre of gravity. Changes in altitude and/ or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.
Level 3, Severe This will cause changes in the accelerometer reading of greater than 1g at aircraft’s centre of gravity. Large, abrupt changes in altitude and/ or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food services and walking are impossible.
Level 4, Extreme Turbulence in which the aircraft is violently tossed about and is practically impossible to control.
Last edited by evansb; 2nd Sep 2016 at 06:47.