According to the Pilot/Controller Glossary, instrument meteorological conditions (IMC) exist when visual meteorological conditions (VMC) minima are not met. We all learned these minima in our training, such as 3sm visibility, 500′ below clouds, 1,000′ above clouds, and 2,000′ horizontal from clouds in Class E airspace below 10,000’ msl.
What you may not know is that there is another definition of IMC that is found in an FAA interpretation letter from 1984 referred to as the “Moonless Night” letter. The letter is about logging actual IMC time but it also has safety implications – and lessons – for the all-too-common “VFR into IMC” accident scenario.
That letter says that actual instrument flight conditions exist “when some outside conditions make it necessary for the pilot to use the aircraft instruments in order to maintain adequate control over the aircraft. Typically, these conditions involve adverse weather conditions…Actual instrument conditions may occur in the case you described, a moonless night over the ocean with no discernible horizon, if use of the instruments is necessary to maintain adequate control over the aircraft.”
Members Alan Connor and Jay Patrikar were recently on an animal rescue flight where, despite being in forecast, reported, and actual VMC conditions (no clouds and ground visibility of least 8 miles), the horizon was completely obscured by haze or smoke. It would have been very easy for a VFR-only pilot to have difficulty in these conditions, especially without an autopilot.
This situation reminded Alan of his instrument check ride where his DPE told him, “If you can’t see the horizon, you are in IMC.”
The lesson, even on what appears to be an easy VFR day, is to always pay attention to the actual conditions and take the appropriate action based on your experience and training.
This article has more information about the “Moonless Night” letter: https://www.ifr-magazine.com/technique/actual-conditions/
