Flight Crew Air Transport Pilot (ATP)
Physiological - Other
Aircraft Equipment Problem Critical
Flight Crew Took Evasive Action
Aircraft
The First Officer was the pilot flying on this leg and and just prior to reaching the LAR VOR we both noticed symptoms of hypoxia. The First Officer noticed the cabin altimeter was showing somewhere between 12 and 13 thousand feet; maybe a little higher.  There was no associated CAB ALT HI annunciator.  I checked the  annunciators with a press to test and the CAB ALT HI tested normally.  After a few moments of disbelief we elected to don our oxygen masks and requested a lower altitude; which was granted.  I ran the Cabin  Decompression Emergency Checklist.  I elected not to deploy the cabin oxygen masks since we were descending and cabin altitude was now indicated below 12;000 feet.  Crew and ATC communications were  difficult due to the oxygen masks.  Once we reached 10;000 we removed our masks and landed normally.  We did not declare an emergency since we were given a descent.  It should be noted that the left environmental system was MEL'ed under 21-14 and the cabin was quite cold which we assumed was a result of the MEL. The CAB ALT HI is supposed to be our safety net and it did not illuminate.  Had this been working we would have discovered this earlier in the flight.  I don't feel that we could have done anything differently as  a crew.
During cruise flight at FL190 near Laramie VOR I noticed the onset of mild hypoxia symptoms that reminded me of my prior general aviation flying experience at altitudes between 10;000 feet and 14;500 feet without supplemental oxygen.  No red annunciators (to include CAB ALT HI) were illuminated.  I glanced at the cabin altimeter and noticed that the cabin altitude was indicating somewhere between 12;000 feet and 13;000 feet.  I was somewhat in disbelief due to the lack of CAB ALT HI being illuminated; so I asked the Captain to verify that I was reading the cabin altitude gauge correctly.  Upon confirmation of the high cabin altitude; we donned oxygen masks; and requested a lower altitude from ATC which was granted.  Communication was immediately established using the oxygen mask microphones; and our communication with ATC was not compromised.  However; there was significant background noise due to breathing as we left the hot interphone switch on for most of the event.  During descent the Captain ran the emergency checklist for cabin decompression.  The Captain checked all annunciators using the 'press to test' feature and they tested normally.  There were no signs of a rapid decompression or explosive decompression.  No emergency was declared since we were given an immediate descent when we requested it.  At the lower altitude the cabin had descended somewhat to below 12;000 feet.  Because of this and the initial cabin altitude noted at below 13;000 feet; we elected not to deploy cabin oxygen masks.  After descending further; with cabin altitude below 10;000 feet; we removed our oxygen masks and continued normally.  On this particular flight and the leg prior to it (DEN to SHR) the left environmental system was MEL'ed (21-14); placarded; and we reviewed the MEL and associated procedure prior to each departure.  We noted a cold cabin temperature on both legs of the flight and had ensured that the cabin temperature was set to maximum during each flight.  We attributed the low temperature to the inoperative left environmental system and outside air temperature below standard.  The cabin pressurization was checked during the climb checklist on both flights and no anomalies in pressurization during initial climb were noted.  I also observed the Captain test the annunciators prior to each flight and I did not observe any malfunctioning annunciators.The CAB ALT HI annunciator should warn of cabin altitudes in excess of 10;000 feet.  We believe this part of the aircraft's systems failed.  Without this operational our only warnings of high cabin altitude are instrument scan and physiological symptoms.  I believe my prior experience in general aviation flight at altitudes above 10;000 feet and my prior attendance of an FAA high-altitude physiology course (including altitude chamber training to recognize my personal symptoms of hypoxia) helped to identify the issue quickly.  Despite the maximum altitude of the B1900 being restricted to FL250 I believe this training would be useful for other flight crews in our aircraft and would like to request that the company subsidize this training for flight crew members who have not yet experienced it.
BE1900 flight crew reports detecting symptoms of hypoxia while cruising at FL190.  The cabin altitude is noted between 12- and 13;000 feet and oxygen masks are doned and a descent is initiated.  There was no associated CAB ALT HI annunciation although the light tested normally.