Continued VFR into IMC

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 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]

Inadvertent entry into Instrument Meteorological Conditions

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, twenty student pilots flew from VFR into simulated IMC; after entry, all of them eventually reached a dangerous flight condition or attitude[lower-alpha 1] 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, the original 1954 study was noted for simulating an aircraft the subjects had little to no experience with, and only providing a partial instrument panel.[3] 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]

Examples of accidents involving continued VFR into IMC

See also

Notes
  1. The authors defined the dangerous flight condition or attitude as one of the following four situations:[6]: 8 
    1. A stall, either normal or accelerated
    2. A bank exceeding 45°
    3. An excessive speed (more than normal fast cruise)
    4. Obvious or prolonged loss of altitude or directional orientation

References
  1. Ison, David (February 6, 2016). "Understanding VFR Into IMC Accidents". AOPA. Retrieved 11 February 2021.
  2. "Training Fact Sheet – Inadvertent Entry Into Instrument Meteorological Conditions (IIMC)" (PDF). 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.{{cite web}}: CS1 maint: url-status (link)
  3. Rowland, David (February 5, 2017). "Surviving VFR into IMC". AV Web. Retrieved 26 February 2021.
  4. LeCompte, Tom (September 2008). "The Disorient Express". Air & Space. Retrieved 26 February 2021.
  5. Ison, David (February 6, 2016). "Understanding VFR Into IMC Accidents". Plane & Pilot. Retrieved 26 February 2021.
  6. Aulls Bryan, Leslie; Stonecipher, Jesse W.; Aron, Karl (1954). 180-degree turn experiment. University of Illinois. ASIN B0007EXGMI. LCCN a54009717. OCLC 4736008. OL 207786M.
  7. "178 Seconds to Live" (PDF). Federal Aviation Administration. Retrieved 26 February 2021.
  8. Dubois, William E. (February 4, 2016). "The lost lessons of '178 seconds to live'". P&E: Proficiency [blog]. Aircraft Owners & Pilots Association. Retrieved 26 February 2021.
  9. Durfee, James R.; Gurney, Chan; Denny, Harmar D.; Minetti, G. Joseph; Hector, Louis J. (September 23, 1959). Aircraft Accident Report (PDF) (Report). Civil Aeronautics Board. Archived from the original (PDF) on February 26, 2009. Retrieved February 4, 2009.
  10. "Rapid Descent Into Terrain, Island Express Helicopters Inc., Sikorsky S-76B, N72EX, Calabasas, California, January 26, 2020" (PDF). National Transportation Safety Board. 2021-02-09. NTSB/AAR-21/01. Retrieved 2021-02-26.
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