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If there is a loss of/lack of air pressure at high altitude, without an explosive decompression (for instance as on Helios 522 or N47BA) are there any signs that pilots can look out for that they might not be getting enough oxygen?

Does an audible alarm exist on any commercial jets when cabin pressure is at a dangerous level?

It's my understanding that the brain is impaired, and reaction times are increased with oxygen deprivation. Are there any techniques to combat this, after noticing hypoxia onset? e.g. slowed breathing

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    $\begingroup$ In a Cessna Turbo 310 at 16,000 feet with two of us on oxygen, I decided to try a little experiment. I removed my mask and sat there for about 15 minutes. I felt fine, but then I tried a little exercise I had been told about. With pencil and paper I tried to do a couple of problems consisting of multiplying two 4-digit numbers. I found I couldn't do it, as I would forget the carry. Then I tried writing down the carry and was able to do it. After putting the oxygen back on, I checked both problems with a calculator. Both answers were wrong! $\endgroup$ – Terry Sep 17 '14 at 3:25
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Yes, all pressurized aircraft have cabin altitude (= altitude at which the same pressure as currently in the cabin normally occurs) indicator with alert when it exceeds safe value.

On Helios 522 that alarm did sound. The crew however mistook it for configuration alarm which only makes sense during take-off roll and thus considered it spurious. When the technician they called realized it was cabin altitude warning, the pilots were already too confused to carry out his orders.

The symptoms are already listed in the other answer. The problem is that the initial symptoms feel just like fatigue. One starts to feel sleepy and at that moment one might not realize it could be due to hypoxia as mental capacity is already reduced.

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A good amount of information can be found in the Operator's Guide to Human Factors in Aviation:

Hypoxia can be recognized from both objective (i.e., capable of being perceived by an observer) and subjective (i.e., perceived by the pilot only) symptoms. Objective signs include increased rate and depth of breathing, tachycardia, cyanosis (blue-colored lips and nails), mental confusion, anger, euphoria, poor judgment, loss of muscle coordination, slouching and loss of consciousness. Behavioral changes may be noted by the hypoxic individual, as well as by the observer. The subjective symptoms include breathlessness, apprehension, headache, dizziness, fatigue, nausea, hot and cold flashes, blurred vision, tunnel vision, tingling, and numbness.

The only way to reverse the effects of hypoxia is to provide the lungs with a greater partial pressure of oxygen. This can be done using either supplemental oxygen (mask), or descending to a lower altitude. Corrective action must be taken quickly, as any delay makes the effects of hypoxia worse.

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  • $\begingroup$ Thank you for the excellent reference. Especially the "Time of useful consciousness" table is quite interesting. $\endgroup$ – yankeekilo Sep 23 '14 at 11:49
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One option is to experience hypoxia in a controlled setting first, such that you know what altitude you might start feeling symptoms around and which symptoms are the ones you tend to express. The FAA offers a training course for this:

http://www.faa.gov/pilots/training/airman_education/aerospace_physiology/

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