Continued VFR into IMC is when an aircraft operating under visual flight rules intentionally or unintentionally enters into instrument meteorological conditions. Flying an aircraft without visual reference to the ground can lead to a phenomenon known as spatial disorientation, which can cause the pilot to misperceive the angle, altitude, and speed at which they are traveling. This is considered a very serious safety hazard in general aviation. According to AOPA’s Nall Report, approximately 4% of general aviation accidents are weather related, yet these accidents account for more than 25% of all fatalities.[1]
If weather deteriorates during flight or the aircraft flies into clouds, a flight that started out under VFR may turn into a flight under IMC. This is known as VFR into IMC or Inadvertent Entry Into Instrument Meteorological Conditions (IIMC). IIMC is a dangerous situation that has resulted in many accidents,[2] as pilots may become subject to spatial disorientation without visual cues, leading to loss of control or controlled flight into terrain.[3] Statistics from the Federal Aviation Administration indicate that spatial disorientation is a factor in approximately 15% of general aviation accidents; of those, approximately 90% are fatal.[4] Other statistics indicate that 4% of general aviation accidents were attributable to weather; of those weather-related accidents, 50% resulted from VFR into IMC, and 72% of the VFR into IMC accidents were fatal.[5]
In the 180—Degree Turn Experiment conducted in 1954 by the University of Illinois, in order to establish a baseline for the effectiveness of instrument training, twenty non-instrument pilots flew from VFR into simulated IMC; after entry, all of them eventually reached a dangerous flight condition or attitude[a] over a period ranging from 20 to 480 seconds.[6]: 16 The average time to reach a dangerous condition was 178 seconds, echoed in the title of the "178 Seconds to Live" article distributed by the Federal Aviation Administration in 1993.[7] However critics of the FAA's article point out that the subjects were specifically chosen, as per the report, to (a) "have had no previous instrument experience under either simulated or actual conditions", and (b) "have had a minimum of experience in the Beechcraft Bonanza." In addition, "The Beechcraft Bonanza C-35 was selected for use in these case studies upon the basis of the preliminary flight testing which indicated that the technique would be most difficult to accomplish in the Bonanza." (These choices were not oversights on the part of the researchers, as the study's goal was to show the efficacy of instrument training and thus the scientific method requires establishing a a priori to compare against the post priori.)[3]
Nonetheless, the original 1954 study was simulating an aircraft the subjects had little to no experience with, was intentionally chosen to be the most difficult to succeed in, only provided a partial instrument panel, and the subjects themselves were chosen because of their lack of training and experience.[3][6] In addition, the "178 seconds" average time was extracted from the preliminary evaluation; after training for a standardized procedure to exit IMC, each student pilot was tested three times, and 59 of the 60 resulting simulated flights successfully resulted in a controlled descent out of the cloud deck without reaching a dangerous condition.[8] As a result, AOPA's and the FAA's 178 second claim is thought of by many as significantly mischaracterizing the reality of flight in 1954, and does not take into account modern planes, instruments, and training standards.[3]
Inadvertent entry into IMC is a situation where deteriorating weather prevents you from flying under visual meteorological conditions when you were planning to fly under VFR.
