In short: Rod Machado explains that today’s VFR cloud clearances and airspace boundaries grew out of decades of rule changes driven by faster airplanes. Larger cloud distances at higher altitudes, the old Continental Control Area and the 18,000-foot base of positive control all trace back to aircraft performance and navigation limits of their era.
Video: “The History of Our National Airspace System for Pilots” by Rod Machado, from his YouTube channel.
What you’ll learn
- How 1930s contact flight rules handled cloud clearance
- Why you need more distance above a cloud than below it
- What the Continental Control Area was and how its base moved from 24,000 to 14,500 feet
- Why the larger cloud clearances now apply at and above 10,000 feet MSL
- Why the FAA chose 18,000 feet for positive control instead of 24,000
- Rod’s practical view of regulatory cloud distances
From contact flight to modern cloud clearances
In this excerpt from his private pilot eLearning ground school, Rod walks through how the rules evolved, with plenty of jokes along the way. A 1938 Bureau of Air Commerce rule book required pilots on contact flight, something like today’s VFR, to stay 300 feet above or below clouds, or 500 feet if there was precipitation, plus 2,000 feet horizontally. By 1945 the precipitation clause was gone, leaving a consistent 500-foot vertical distance.
In the mid-1950s the rules changed to 500 feet below, 1,000 feet above and 2,000 feet horizontally. Rod’s explanation for the extra room above: airplanes, especially the new jets, typically climb much faster than they descend, so a larger buffer above the cloud gives you a better chance to see and be seen.
Faster airplanes, bigger buffers
With no airspeed limits at the time, a VFR pilot 2,000 feet from a cloud at high altitude could meet a fast airplane emerging from it with almost no warning. In 1957 the Continental Control Area was created at and above 24,000 feet MSL, requiring five miles of visibility and 1,000 feet vertical and one mile horizontal cloud separation. In 1960 its base dropped to 14,500 feet, a figure Rod says was originally tied to Mount Whitney. By the mid-1960s those larger values applied at and above 10,000 feet MSL, with the familiar 500 below, 1,000 above and 2,000 horizontal, plus three miles visibility, in controlled airspace below that.
Rod notes that even with the 250-knot limit below 10,000 feet, an airplane exiting a cloud near that altitude is moving close to 300 knots true. His advice is to treat regulatory distances as legal minimums and ask yourself how far you would really like to be from a cloud.
Why 18,000 feet
When positive control expanded in 1961, the NBAA proposed starting it at 24,000 feet. The FAA rejected that largely because of VOR frequency protection: low-altitude airway VORs were spaced to avoid interference only below 18,000 feet. The agency also judged that most small airplanes didn’t use higher altitudes. Rod suggests that, in today’s GPS era, raising the base to 24,000 feet seems worth considering.
Go deeper with Rod Machado
This lesson is an excerpt from Rod’s private pilot ground school, where airspace, weather minimums and the reasons behind them are covered in depth. Learn more in Rod Machado’s 40-Hour Private Pilot eLearning Ground School.
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