VFR flight operates under the idea of "see and avoid." Otherwise known as looking outside the cockpit and not hitting anything or anyone else. However, clouds and low visibility hinders the ability to see and avoid traffic and terrain. Therefore, every pilot operating under VFR is expected to maintain a minimum flight visibility and distance from clouds at all times.
The actual distance and flight visibility a pilot must maintain changes based on the airspace being flown in. These distances and visibilities are listed under 14 CFR 91.155 and are commonly referred to as basic VFR Weather Minimums.
Class B airspace is one of, if not the most, busiest airspace category.
Class B airspace encompasses many of the largest airports in America and serves thousands of aircraft every day.
In Class B airspace, all aircraft are essentially treated as IFR traffic even if being flown under VFR. ATC will provide spacing and traffic advisories removing much of the pilot's burden to see and avoid; however, it is still the pilot's responsibility to watch for other traffic at all times.
Since ATC provides these services to VFR pilots, the VFR weather minimums for Class B is the least restrictive of all airspace.
VFR pilots must maintain a minimum of 3sm forward flight visibility and remain clear of clouds at all times. Clear of clouds means that a pilot can get as close to a cloud as they wish. The pilot simply can not enter the cloud.
Class C and D airspaces offer VFR pilots some air traffic control services. Pilots are expected to maintain radio contact with ATC at all times; however, ATC services are reduced compared to Class B ATC services.
This reduction in services offered to VFR pilots requires the pilot to maintain more vigilance for other traffic.
To allow for a greater chance to identify and react to other traffic, the FAA limits how close a pilot can get to a cloud in both the vertical and horizontal directions.
In Class C and D airspace, a pilot is expected to maintain a distance of 1,000ft above, 500ft below, or 2,000ft to the side of any cloud. The forward flight visibility remains the same as Class B airspace at 3sm.
Class E airspace is considered controlled airspace. However, aircraft are not required to communicate or coordinate with air traffic control while operating in Class E airspace. This reduces ATC service and places the role of traffic and collision avoidance solely on the pilot's ability to see and avoid other traffic.
Class E airspace can potentially extend from the surface to 18,000ft MSL. This wide altitude range presents a danger due to aircraft traveling at higher airspeeds as altitude increases.
As a general rule, an aircraft's true airspeed will increase by about 2% for every 1,000ft of additional altitude. Aircraft operating at altitudes near 18,000ft MSL can therefore experience as much as a 36% increase in their true airspeed.
As true airspeed increases, the time a pilot has to react to a potential traffic conflict decreases.
To alleviate these dangers, the FAA categories Class E airspace VFR weather minimums into two categories:
At and above 10,000ft MSL, a pilot is expected to have a minimum forward fight of 5sm. By having a visibility of 5sm, a pilot will have a greater chance of spotting aircraft and maneuvering to avoid a collision.
The minimum clearance from clouds vertically becomes 1,000ft above and below. VFR aircraft also must maintain a horizontal distance of 1sm from clouds while operating in Class E at and above 10,000ft.
Below 10,000ft, the increase in TAS is reduced due to the lower altitudes.
Therefore, the minimum forward flight visibility can be reduced from 5sm to 3sm.
The minimum cloud clearance in Class E airspace below 10,000ft also reduces to only 1,000ft above, 500ft below, and 2,000ft horizontally.
Class G airspace is considered to be uncontrolled airspace. ATC has no authority or responsibility to control aircraft operating in Class G airspace. Class G airspace can also extend from the surface up to 14,500ft.
Since Class G airspace is both uncontrolled and has a wide range of altitudes, the FAA categorizes the required VFR weather minimums by both altitude and time of day.
This categorization debatably makes Class G airspace the most challenging to memorize or apply properly.
At all times of day, when operating in Class G airspace above 1,200ft AGL and above 10,000ft MSL, a pilot is expected only to operate if a 5sm forward flight visibility is maintained and a minimum cloud clearance of 1,000ft above, 1,000ft below, and 1sm horizontally is maintained.
When operating above 1,200ft AGL but below 10,000ft MSL, the VFR weather minimums change based on the time of day.
During the day, pilots are expected to maintain forward flight visibility of at least 1sm. At night this minimum increases to 3sm.
The required cloud clearance is the same for both day and night. A pilot must maintain a vertical distance of 1,000ft above, 500ft below, and 2,000ft horizontally in Class G airspace at these altitudes.
When operating at and below 1,200ft AGL in Class G airspace, a pilot must remain clear of clouds during the day and have a minimum forward flight visibility of 1sm.
At night in Class G airspace at and below 1,200ft AGL, a pilot must have forward flight visibility of 3sm, as well as a vertical distance of 1,000ft above, 500ft, below, and 2,000ft horizontally from clouds at all time.
While it should go without saying, all these minimum VFR weather minimums are just that—a minimum.
A Pilot's personal minimums should always be in mind, and no pilot should try to push their luck with the weather. Class G airspace allows a pilot to fly in less than ideal conditions legally. In some instances, what is legal may not be safe.