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To clarify, I'm an aviation enthusiast, not a pilot. I have a question for ATR pilots (or anyone with knowledge of the ATR systems).

While travelling on an ATR-72, about 15 mins after takeoff (and presumably at cruising altitude) I felt a very rapid deceleration - to the point I felt pressure on the seatbelt. At the same moment, I felt a sudden change in the air pressure. Checking my altimeter-measuring watch, I saw a reading of 760mb - well bellow the normal ~1000. No masks dropped, however. Several people in the cabin became upset and worried. I was sitting on the second row and, although I can't be certain behind the closed door, I'm pretty sure I heard a 'whoop whoop' alert from the cockpit.

The journey continued without diversion or any emergency landing (frustratingly, no crew announcement either). I did notice that the air pressure remained bellow 800mb for the rest of the ~30min flight, beginning to rise only during descent.

I was wondering if anyone has a clue what might have happened. My best guess is some sort of engine trouble leading to bleed air problems, causing the plane to fly at a lower altitude (unpressurized) for the rest of the flight. But then again, is it normal procedure not to divert/return? I should mention that we had departed a large metropolitan airport headed for a regional one, so a return to the largest, better-equipped airport made more sense, I would think.

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    $\begingroup$ Your quotations of pressure don't make sense. 1000mb equates to an altitude of about 370 feet assuming a "standard day". A typical pressurised cabin will be about 760mb, which you quote as "well below the normal pressure". Below 800mb is what I would expect on a normal flight. Please also edit in the airline and flight number so that we can check the expected flight profile. In my experience, the deceleration from climb to cruise is very noticeable in all turbo props and what you describe all sounds "normal". $\endgroup$ – Simon Jan 3 '16 at 9:53
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    $\begingroup$ Finally, "whoop whoop" is only ever heard close to the ground (it's part of the GPWS). $\endgroup$ – Simon Jan 3 '16 at 9:56
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    $\begingroup$ Airplane cabins typically pressurize as high as 8000' cabin altitude or a pressure around 760 mb. That observation is 100% normal. Masks won't drop until sometime around 600 mb. $\endgroup$ – casey Jan 3 '16 at 10:28
  • $\begingroup$ Thank you for the input. The incident I described happened a very long time ago, so there are no expected flight profile data available. I was unaware of the pressurization data you mention. I referred to ~1000mb as normal (and <800mb as abnormal) because of my experience/observation of other flights, where that same altimeter-measuring watch would indicate ~1000mb (sometimes as low as 960mb) throughout the flight. What, in fact, prompted me to check my watch during the incident was an intense pain in my ears (which in fact lasted for a couple of days). $\endgroup$ – Digital Dracula Jan 3 '16 at 10:40
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    $\begingroup$ Your watch has a problem. 960-1000mb is not normal and would indicate a serious problem. Since you have multiple observations, your watch must be wrong. The pain in your ear is the only other unusual thing and was most likely caused by congestion in your sinuses preventing you from equalising the pressure naturally. $\endgroup$ – Simon Jan 3 '16 at 11:15