In an F-16, at what altitude is supplemental oxygen considered essential life support?

Question

Combat aircraft like the F-16 employs hypobaric pressurisation with supplemental oxygen from a regulator.

However, at what point does the manual call for the pilot to start using the regulator, what altitude?

As a bonus:

What is the maximum altitude one can sustain flight with only hypobaric pressurization under normal operation? Can the pilot fly at say, 20 thousand feet, without ever needing to care about the regulator?

My own reasoning:

It is possible to ascend Mount Everest (approximately 30 thousand feet) without oxygen. Obviously not everyone can do this, but, for a limited time, it seems plausible that the sheer altitude wouldn't be fatal. However, I imagine the manual calls for the regulator a lot earlier than when it becomes dangerous to not have it, for practical and safety reasons. I just have no idea when that point would be, perhaps the manual always states to have the regulator on, that would be the safest option.

Bonus part:

Again, I don't know.

The hypobaric pressurization alone would alleviate somewhat the need for supplemental oxygen, so perhaps it is possible to survive, although not comfortably so, beyond 30 thousand feet for multiple hours, thanks to the pressurization.

Answer 1

The cabin of the F-16 airplane is pressurized via bleed air from the engine, which provides bleed air for this, environmental control and anti-G systems. See the schematics below.

The pressurization schedule is automatically controlled according to the chart shown below.

Onboard oxygen for breathing is provided by a liquid oxygen system controlled by an oxygen regulator. See schematic below.

This is a diluter demand oxygen regulator which also provides pressure demand modes for cabin altitudes above 30,000 ft as well as for anti G enhancement. As the oxygen mask is typically worn during all phases of operation, the regulator would be in use throughout this.

As for oxygen requirements, the military has similar ones to civil aviation. Pilots need to be using oxygen at cabin altitudes above 14,000 ft at all times in order to remain fully alert with all cognitive functions intact for flight ops and combat.

And, in regards to your comments about climbing to the top of Mount Everest, no, human beings cannot survive up there for extended periods of time without supplemental oxygen. People who have scaled Everest without using oxygen have

1. Been in exquisite physical condition prior to making the ascent
2. have acclimated themselves to higher altitudes at the base camps and
3. expedite the ascent to the summit and return below 26,000 ft.

Answer 2

Pressurization to a particular cabin altitude creates a breathing environment identical to actually being at that altitude. For example if you are cruising in an airliner at 35,000 feet with a cabin altitude of 7,000 feet this is no different than being on top of a 7,000 foot mountain, other than the difference in temperature and humidity.

As shown in the chart in one of the other answers the F-16 pressurization system can operate with a pressure differential of 5.0 psi. This means that at an altitude of 23,000 feet you would have a cabin altitude of 8,000 feet, which is the maximum cabin altitude allowed in airline operation because of the wide ranges in medical conditions of airline passengers. At 26,500 feet you would have a cabin altitude of 10,000 feet, which is generally considered a safe unpressurized altitude for pilots. At 34,000 feet you would have a cabin altitude of 14,000 feet, which is the maximum allowed for general aviation pilots. And yes someone who is an experienced mountain climber who is aware of their physical limits could probably go higher, they could for example fly at 44,000 feet which would have a cabin altitude of 18,000 feet.

However this doesn’t mean that it’s safe for fighter pilots to doff their oxygen masks during flight, for multiple reasons. High g-force maneuvers make it harder to breath, the positive pressure that is provided by oxygen masks can help with this. Oxygen masks are also needed when ejecting, and there may not always be time to put the mask back on in time. Also a cabin leak in a fighter jet will possibly lead to a much more rapid depressurization than would typically occur in a much larger aircraft cabin. Again there might not be enough time to put on the oxygen mask or descend to a lower altitude prior to losing consciousness.

The reason that airliners can cruise at 40,000 feet or higher and still maintain an 8,000 foot cabin altitude is because they have a higher pressure differential limit on the airframe, typically between 8 and 9 psi. Concorde notably had a maximum differential of 10.7 psi, which allowed it to maintain a cabin altitude of 6,000 feet at its service ceiling of 60,000 feet.