Most pilots are shocked when they learn how little time they have when operating on 100 percent oxygen. The minimum pressure requirements typically result from the requirement to supply the cockpit crew with 15 minutes worth of oxygen at a cabin altitude of 8,000 feet. I believe one manufacturer adds an additional factor of 9 to 13 percent to account for leakage during the flight.
While certainly not intuitive, I was told by an engineer from a major oxygen equipment manufacturer, when operating on 100 percent oxygen, the rate of oxygen consumption will increase as the cabin altitude decreases. This is due to the design of the regulator. He went on to explain that if you dispatch with the MEL minimum oxygen level, and encountered the requirement to utilize 100 percent oxygen with the cabin altitude at sea level, you could find your supply depleted in as little as 10 minutes.
With a fully charged (1800 psi) 77 cubic ft cylinder installed and only a crew of two using 100 percent, you will have approximately 42 minutes at a cabin altitude of 8,000 feet. If you have a 115 cubic ft cylinder, your oxygen will last approximately 66 minutes.
The Kid
Last edited by Aztec Kid; 21st Dec 2007 at 04:30.
Reason: Format