Following on from this question and the answer mentioning Big Sky Theory: When was the last collision in IMC outside controlled airspace? I'm interested in finding empirical evidence for Big Sky Theory in class-G and IMC.

Most collisions seem to occur in VMC (VFR or IFR) or in controlled airspace although it's difficult to get the exact details from the descriptions online. There are some reports on Pprune, but many incidents have one-line write-ups that don't mention the meteorological conditions of the airspace classification.

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    $\begingroup$ Around the world or in a specific country? $\endgroup$
    – Ben
    Jul 11, 2023 at 9:55
  • $\begingroup$ @Ben Anywhere. I'm interested in empirical evidence for Big Sky Theory in class-G and IMC. Good point, I'll edit the question. $\endgroup$ Jul 11, 2023 at 10:00
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    $\begingroup$ This sort of accident happening rarely might mean that the activity is pretty safe (i.e. big sky theory works), or it might mean that the activity, while not particular safe, is so rare that the accident rate is low despite the risks. Most places somebody would need to fly IMC for any great length of time tend to be controlled airspace, and the airspace that the airplanes that fly the most (time, miles) is Class A because that's where the efficiency is to be found. "Big Sky Theory" is only tested when 2 aircraft are somewhat close to start with, and that seems maybe rare in IMC + Class G. $\endgroup$
    – Ralph J
    Jul 12, 2023 at 0:39

1 Answer 1


In February 2020, there was a mid-air collision involving two IFR aircraft on training flights, in class G airspace in Australia. It occurred near a small airfield north of Melbourne. You can read the investigation report here. Essentially, while ATC did provide traffic advice to each aircraft, the pilots did not recognise the risk and so did not coordinate over the CTAF.

  • $\begingroup$ +1 From that report the collision occurred at 4,100ft and the conditions were broken with a base of about 3,200 ft amsl - I think that's within the ICAO definition of IMC - and the collision occurred in class G. $\endgroup$ Jul 14, 2023 at 10:46
  • $\begingroup$ @Dave, note that "IMC" simply means the pilot is unable to fly at their current altitude solely by visual reference outside the aircraft. The ceilings can be 10,000' above the ground (so "VFR" at the reporting station) and an aircraft flying at 11,000' above the ground could be IMC within that cloud layer. $\endgroup$
    – randomhead
    Dec 19, 2023 at 16:36

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