AIM Text
Flight planning into a CTA may be accomplished prior to flight. Use the predicted coldest temperature for plus or minus 1 hour of the estimated time of arrival and compare against the CTA published temperature. If the predicted temperature is at or below CTA temperature, calculate an altitude correction using TBL 7-3-1. This correction may be used at the CTA if the actual arrival temperature is the same as the temperature used to calculate the altitude correction during preflight planning.
Source: FAA Aeronautical Information Manual · current edition · paragraph 7-3-2.
Research Notes
AIM 7-3-2 covers Vortex Generation — the physics of how wake vortices are produced.
The vortex-generation principle: Lift is produced by pressure differential between upper and lower wing surfaces. At the wing tip, high-pressure air flows around the tip to the lower pressure upper surface, creating a vortex. The vortex is continuously generated as long as the wing is producing lift.
Maximum vortex strength conditions: Heavy, slow, clean configuration. A 747 at landing weight, minimum speed, gear up and flaps retracted produces the most powerful wake.
Minimum vortex strength: A light, fast, dirty aircraft (heavy aircraft with full flaps and spoilers deployed) produces less concentrated wake — the high-drag configuration dissipates energy into turbulence rather than concentrating it into vortices.