Currently, within the helicopter community, there is much discussion of inadvertent or unintended flight into instrument meteorological conditions (IMC). I frequently hear speakers, instructors etc. use the phrase “you’re legal but …” when referring to the VFR requirements of Part 91 (14 CFR §91.155). This demonstrates a fundamental misunderstanding of the Part 91 rules. The cloud clearance and visibility requirements of Part 91 have nothing to do with keeping a pilot out of the clouds, rather, they are intended solely for aircraft separation. To separate VFR from IFR traffic and VFR from other VFR traffic using the “see and avoid” concept (§91.113 and AC 90-48D). For example, to operate VFR in Class E airspace (below 10,000 ft MSL) you must fly 500 ft below the clouds, 1000 ft above the clouds and 2000 ft laterally from the clouds. Why- separation between VFR and IFR aircraft? IFR aircraft typically descend at slower descent rates, hence the VFR pilot will be able to see and avoid the IFR traffic as it descends out of the clouds. But IFR traffic normally climbs at higher rates therefore, the 1000 ft requirement above the clouds and the closure rates with aircraft at the same altitude are potentially very fast, so we need 2000 ft to see and avoid. At altitudes above 10,000 ft MSL, traffic is flying at much faster airspeeds so the cloud clearance and visibility requirements are much greater. However, look at the requirements in Class G airspace below 1200 ft AGL. Here the VFR pilot is allowed to just stay “clear of clouds” and have only ½ mile visibility during the day. The reason is for all intents and purposes there is no IFR traffic that will come busting out of the clouds so it’s ok or “legal” to cuddle up right next to a cloud but not very safe. Another example is Class B airspace. In Class B the VFR pilot is again allowed to operate just clear of clouds. The reason being that the VFR pilot is not responsible for separation from IFR traffic using the see and avoid concept–ATC is. Everyone in Class B is operating on an ATC clearance, which is being used to separate traffic.
So pilots trying to determine safe personal weather minimums from the standpoint of avoiding an IIMC situation, especially in Class G airspace below 1200 ft AGL, should definitely not base their decision on Part 91 requirements. Instead, use local weather patterns, terrain and experience/comfort level.