As stated above, the reason the QRH permits you to maintain your cruise altitude is because:
- As the cabin pressure differential is maintained by the outflow valve, not the pack, the system is capable of operating at maximum differential on one air conditioning system, albeit with reduced airflow.
- You have planned your flight at that cruise altitude, and even though you will have allowed fuel for depressurisation, it is probably safer to keep to the original plan than to complicate the rest of the flight with a forced decent, and
- The statistical chance of suffering a separate, independant failure during the remainder of the flight is sufficiently remote.
The reason the MEL requires a higher safety margin is because you no longer require two independant failures of the airconditioning system for a depressurisation, but only a single failure. Under those circumstances a higher margin of safety is required to allow a flight
to depart.
Why 25,000? 25,000 is simply a "line in the sand", like the 10,000' limit (14,000 in the USA) for pilots to use oxygen. At 10,000
most people will be able to sustain consciousness indefinitely, with blood oxygen saturation areound 90%. However, while oxygen pressure decreases a bit less than linearly with altitude, the ability of the hemoglobin to hold oxygen follows a much different curve, called the oxygen dissociation curve.
There is a big change for the worse in the hemoglobin's ability to combine with oxygen that occurs in the low twenties.
At 25,000 feet the oxygen saturation is only 55%. The partial pressure of oxygen in alveolar air at 25,000 feet is 14% of 281.8 mm Hg or 39.5 mm Hg -- slightly less than that normally found in venous blood returning from the tissues. Above 25,000 feet oxygen will diffuse
from your blood into the air. Any time over three minutes or so at this altitude, and brain cells are dying!
Altitudes above 25,000 feet are know by mountain climbers as "The Death Zone,". Given that climbers who work in these altitudes spend weeks at high levels getting acclimatised, allowing their bodies to increase the amount of heamoglobin in their blood, and still experience deaths, this isn't just a "cute term".
Keeping the aircraft below 25,000 feet is simply an added safety measure for your passengers, given the increased likelyhood of suffering a depressurisation while flying on one pack.
Image referred to from the article:
When Humans Fly High: What Pilots Should Know About High-Altitude Physiology, Hypoxia, and Rapid Decompression at
Avweb.