Tim's interpretation of vertigo and subsequent assumption of steep spiral and/or inverted flight is spot on. I have instructed several dozen pilots leading to their instrument ratings, Instrument Flight Instructor ratings, and conducted several hundred instrument flight checks, and been able to induce vertigo in all but one pilot (I'll get him yet one of these days...! ).
It's amazing how much the mind wants to override what the instruments are telling you. When placed into a situation where you are suddenly in inadvertent IMC, you must simultaneously trust the instruments initially while also error-checking to ensure all the instruments are functioning properly, thus leading you to the right conclusion. In such a situation (inadvertent IMC, where the pilot is taken surprise) , it doesn't take much for a RATED pilot to get behind the airplane, never mind a pilot who hasn't received adequate instruction and developed the requisite experience to 1. identify the situation, 2. take initial corrective action, 3. evaluate the effectiveness of the corrective action while simultaneously error-checking the instrument indications to ensure all instruments are functioning/indicating properly. And getting behind the airplane can get deadly. Many flight instructors drill into our students and pilots seeking recurrent training that inadvertent encounters with IMC for untrained, unqualified, and/or non-current instrument pilots result in a sobering outcome: 178 seconds to live*.
Hope this helps,
Don
Don Desfosse, ATP, CFII, FAA Safety Team Lead Representative (FAA Boston FSDO)
* "An AOPA-funded study conducted in 1954 by Jesse W. Stonecipher, then chief flight instructor at the University of Illinois, Urbana, indicated that the average life expectancy of a non-instrument rated pilot in IMC was 178 seconds from the onset to loss of control due to spatial disorientation. Of the 20 subjects tested, 19 entered graveyard spirals and the 20th stalled his aircraft. None of them lasted over eight minutes and all had the same skills training. There’s no reason to believe those figures have changed in the past half century."
— Illusions: Spatial Disorientation and Loss of Control by Dick McKinney. Business & Commercial Aviation magazine, March 2008 pg. 63.