The regulatory definitions of “ground” visibility and “flight” visibility are significantly different. Flight visibility is determined by the pilot from the cockpit. On the other hand, ground visibility is officially reported by the United States National Weather Service (or an accredited observer) at an airport. It is a nuance that has important implications for pilots.
It is flight visibility that controls compliance with the VFR weather minimums, whether it is in controlled airspace (Classes B, C, D, and E), or in uncontrolled airspace (Class G)—that is, except for operations at the limited number of airports that report ground visibility (see “Pilot Counsel: VFR Weather Minimums,” AOPA Pilot November and December 2005). For airports that do report ground visibility, FAR 91.155(d) states that “except as provided in 91.157 [special VFR weather minimums] …no person may take off or land an aircraft, or enter the traffic pattern of an airport, under VFR” unless ground visibility at the airport is at least three statute miles.
A recent FAA Chief Counsel interpretation (2012-28) spells out some of the important implications of the two different definitions. It describes a scenario where a pilot on a VFR flight plan arrives at an airport without an operating control tower, in Class E airspace to the surface. The pilot picks up the AWOS information for that airport that reports a ceiling of 100 feet and visibility as one-fourth of a mile with calm winds. The pilot flies over the airport and determines that there is a fog bank over one end of the runway, which obscures the last 1,000 feet, but the other 9,000 feet are clear. The pilot then reports the flight visibility to ATC as unlimited and determines that he can remain clear of clouds and requests a special VFR clearance to land on the clear end of the runway. May the pilot land legally, VFR or special VFR?
No. The interpretation says that the pilot would be in violation if he lands at the airport. It says that in order for a pilot to land an aircraft or enter the traffic pattern of an airport, ground visibility at the airport must be at least three statute miles. The AWOS reported a ceiling 100 feet and visibility of one-fourth of a mile. That report precludes a legal landing. The report does not meet the visibility requirements as outlined in FAR 91.155(d) quoted above.
The interpretation also says that ATC may not grant a special VFR clearance because ground visibility must be at least one statute mile as required by FAR 91.157. The pilot’s report of flight visibility cannot supersede the AWOS report of ground visibility. The answer is the same for a Part 135 operation that also is bound by the Part 91 weather minimums and also may have additional VFR visibility requirements under Part 135.
There is some danger that the interpretation could be over read, restricting perfectly legal operations. What could be confusing as charted, and not addressed in the interpretation, is that there are airports underlying controlled airspace that do not report ground visibility. In other words, these airports are in Class G uncontrolled airspace. For example, the Class E controlled airspace does not begin at the surface of the airport, but in fact leaves a cushion of at least 700 feet of uncontrolled airspace above the airport. It takes special care to note the charting legend to make the analysis. The applicable VFR weather minimums in the underlying Class G uncontrolled airspace are very different. For one thing, the visibility minimums that apply are flight visibility minimums (determined from the cockpit). Then, if the flight operates at or below 1,200 feet above the surface, the minimums are daytime, one mile flight visibility and clear of clouds and nighttime, flight visibility of three statute miles and cloud clearance of 500 feet below, 1,000 feet above, and 2,000 feet horizontally. There are even lower nighttime minimums that apply in an airport traffic pattern within one-half mile of the runway. Then, an airplane (or powered parachute or weight-shift-control aircraft) may operate during night hours with as little as one mile flight visibility and clear of clouds. A helicopter may be operated clear of clouds if operated at a speed that allows the pilot adequate opportunity to see and avoid other traffic and obstructions.
The take-away from the interpretation is to remind us that the regulatory definitions of “ground” visibility and “flight” visibility are different—and the difference could have important operational implications