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I assume that even on an IFR flight, being in even marginal VMC can be helpful, and that pilots consciously or subconsciously use cues such as the horizon to assist them in their flight even though they are principally flying by instruments.

I was wondering if that makes a sudden transition to IMC a psychological challenge, particularly if unexpected. That must be quite a "task switch", scabbling for information, switching focus, and so on?

It seems like that might be much more of a personal challenge than when flying deep in IMC for some time (to a trained and qualified pilot/aircraft) when a routine of going from instrument to instrument is established along route.

Is this a major challenge or a minor or even unnoticeable one in the grand scheme of things?

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If you are an IFR pilot hand flying in VMC and suddenly your horizon reference disappears, or becomes untrustworthy (flying over sloping terrain in low visibility, or flying between cloud layers that are sloping), you do have to make a conscious mental transition from external scanning to internal scanning and it will increase the stress level slightly when you're new, but this declines as you get experienced doing it and it eventually becomes a non-event.

This is mainly because if you are flying IFR a lot, especially commercially, you will be doing this all the time on departures in instrument conditions, where you take-off visually, looking outside, and immediately enter cloud, often even before it's time to engage the autopilot (you wouldn't really want to to do a lot of IFR flying without one). So effectively you have to make that VMC to IMC transition on every departure in IMC weather that you hand fly while entering cloud and you get used to the look-out-to-look-in transition pretty fast.

So I would say the answer is yes to a new pilot, but quickly becomes no, once experienced a bit. That being said, "grey area" conditions, like I mentioned at the start, where you need to go on the gauges even though there is still a view outside, would be more of a challenge compared to just charging into cloud where the view goes from green to grey instantly and the distraction of a misleading outside view isn't there. That requires a bit more concentration and self discipline to ignore the view outside, and therefore a bit more experience to drop the stress level of the transition. But still, after doing it a number of times, it should be no big deal.

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For an inadvertent entry into IMC on a VFR flight by an instrument rated and current pilot flying an IFR certified aircraft, the initial transition into instrument flight would not be difficult - at least for a competent instrument pilot. They would immediately and instinctively transition into attitude instrument flight and could quickly gain control of the aircraft and maintain controlled flight.

The problem could come immediately thereafter as they are now operating an aircraft on instruments on a flight planned to be conducted under visual flight rules. These flights are far more liberal on route planning, terrain and obstacle clearance, traffic separation, etc. as they are totally dependent upon visual reference for these tasks. Often VFR flights are simply conducted by pilotage with only a cursory review of NOTAMS prior to departure. They’re flown along custom routes that may not make of use of any electronic navigation aids nor any consideration taken for signal coverage, reception altitudes, instrument procedures at the planned destination, etc. These factors can link together to form a accident chain very quickly. Fly into a valley with patchy clouds and there’s a GPS outage you didn’t note in the preflight, you’re screwed.

Likewise you are now flying blind, either in controlled or uncontrolled airspace with no means to see and avoid traffic nor do you have ATC radar services available to you. I’ve have the uncomfortable experience of watching a 737 pass over me at night in the clouds in KSAN Class B - all I saw of him was two luminous beams that were his landing lights glide over me. And that was on an IFR flight plan with ATC services. I’d hate to think of what that would have been like without it.

If this happens and you wander into Class B airspace, expect a 709 ride from the FAA. If this happens and you wander into the side of a mountain, you can expect a coffin.

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Your question is very, very complex. Well, not the question as such, but the things that have an effect to the situation you describe.

Anything that happens suddenly in an airplane has varying effects, depending on at least (but not limited to)

  • has the pilot received proper training for such situations
  • is the pilot proficient with regards to the situation (I don't mean in a regulatory sense)
  • what is the personality of the pilot like
  • what is the psychophysiological state of the pilot
  • in case of a multi-crew flight, how well is the crew coordinated

If any of the aforementioned is less than satisfactory, it is reasonable to assume something might hit the fan. And talking about reasonable, if something actually goes wrong, one way to describe the failure is to use the "Swiss Cheese Model", aka "the Reason Model" (not because it deals with reason or reasonable stuff, but because it was created by D. Orlandella and J. T. Reason).

Now, the IR rating itself, while of course is necessary for successfull flight in IMC, does not at all guarantee survival in poor visibility. More important is experience, and self discipline. You see, I'd say that marginal VFR is harder to fly than dead zero visibility IMC. In IMC there is usually very little to distract you, and since you have nothing to reference your situation with, you stick with the gauges the best you can. If there is some visibility, the risk is, that an inexperienced pilot, or one with poor airmanship discards the gauges and loses situational awareness.

Professional pilots that fly alot have probably the best abilities when it comes to situations like the one in question, if company culture is of the right sort (strictly safety oriented).

Flying by gauges is not that difficult if training and practise are in order. Perhaps the most difficult part is to recognize when to switch from visual to instruments. There absolutely should be no in-between!

To add up, I have to say that a pilot that finds oneself suddenly and unexpectedly in IMC is not a very good pilot. We all make mistakes, that's for sure, but if you are properly trained, have been practising rigorously, are physically and mentally fit and not the sort of person who either freezes or acts overly confident in surprizing situations, it would be weird if anything went horribly wrong.

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