This is quite interesting...
The pilot of the single-engine, non-pressurized airplane in cruise flight at 25,000 feet above mean sea level requested and was issued a descent clearance to 12,000 feet. The pilot acknowledged the clearance, but the airplane did not descend. Air traffic control (ATC) noted that the pilot sounded "in distress and out of breath." The pilot was issued the clearance multiple times, but the airplane never descended. The last radio transmission received from the airplane was the pilot's labored breathing. Approximately 1 hour later, the airplane crossed directly over the destination airport at 25,000 feet, and maintained its on-course heading. National Guard aircraft scrambled to intercept the airplane were unable to gain the pilot's attention. The intercepting pilots observed an "unresponsive individual who appeared to be unconscious." The airplane continued in cruise flight at 25,000 feet for another hour after passing the destination airport before it slowed, departed controlled flight, and descended into terrain. All major components of the airplane were accounted for at the accident site. Examination of non-volatile memory from the accident airplane revealed that the onboard oxygen system had 29 percent of its total oxygen capacity remaining when the accident occurred. The airplane was equipped with a factory-installed oxygen system that the pilot had augmented by installing a supplemental pulse-demand oxygen system several months prior to the accident. The manufacturers of both systems explicitly advised against the use of non-original components with their respective systems. The pilot routinely used masks from the airplane's original oxygen system with components from the supplemental system he installed, and even noted the occurrence of a previous encounter with hypoxia in his pilot logbook as a result of this practice.The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
The pilot's improper modification of the certified, on-board oxygen system, which resulted in incapacitation due to hypoxia, and the airplane's subsequent uncontrolled descent into terrain
The NTSB is of course correct in that a fitted o2 system must be operated in accordance with its FAA approved flight manual supplement, and one thus cannot legally fit any equipment (such as a demand regulator) to the low pressure outlet, but that is a completely irrelevant finding because the same pilot could have been flying the same plane with a
portable o2 system with a demand reg....
Funnily enough I now fly with 1 or 2 48 cu. ft. carbon/kevlar cylinders and the Precise Oxygen
mechanical demand regulators. These mechanical regulators are
nothing like as good as the MH O2D2 electronic ones, but they don't need electrical power, and the lightweight cylinders have sidestepped the difficult refilling issues in Europe.
The MH regulator fails
shut when the power fails; a little fact which the man on their exhibition stand at EDNY denied but it is true. I have had some reliability problems with the Precise regs (not least the company almost having apparently gone bust recently; their premises were found to be locked up for weeks, with no responses to any comms) but at least they have a much simpler operating mode.
There is nothing actually wrong with fitting any demand reg onto the outlet of a fitted o2 system, so long as the pressure is within the specified range for the demand reg, and you are aware of the failure modes (which are no different to a portable system, of course).
What the mostly useless NTSB report doesn't say is whether anything was wrong with the equipment.
The pilot was apparently using the original Cirrus bag-reservoir masks, with the O2D2 reg. He was a mug, because the bag will stop the reg from sensing the inhalation, and it won't work properly. But then he would have been hypoxic on all previous such flights as well.
However, the O2D2 reg gives you various audio warnings if it doesn't see breathing so maybe it did see just enough breathing to dispense just a little gas... or maybe it was faulty?