Oxygen Endurance
 

Hi, I have a question about the oxygen system in our Boeing 747-400.
We questioned our system, because during the required short test we noticed that the pressure dropped 10 psi. This also happened during the next consecutive tests. Therefore we decided to don the masks (all still on the ground) and use it for 2 minutes at 100% (3 pilots) In these 2 minutes we lost 110 psi from the total of the 1500psi we normally carry on board (minimum required 700psi)
So, this means that when you fly low, and when using 100% oxygen (smoke etc.) you only have less than 30 minutes oxygen......
I and my colleagues thought that we would have more endurance.
Our manuals are all blank about this subjects....
What are your views and opinions about this????

There is a problem in the system in that airplane.

That is EXACTLY why you do the initial short test. You should see NO pressure drop.

Actually the nominal pressure in the oxygen system is 1800 psi.The low limit for despatch from base after a daily check is 1700 psi. It looks like you have a leak.

The lowest our table -the company put one in our QRH- goes is 8000ft; but there 1500psi should give you 5:50 manhours of 100% oxygen on the 744F. I suppose the flightdeck system is similar to the pax -400.

Depends on the flt and routing, LH, ULH over mountains or over the Atlantic, you can dispatch with 680 psi.

That is EXACTLY why you do the initial short test. You should see NO pressure drop.
According our FCOM a drop up to 100psi is acceptable....

Actually the nominal pressure in the oxygen system is 1800 psi.The low limit for despatch from base after a daily check is 1700 psi. It looks like you have a leak.
The lower limit for dispatch differs in our company per stretch. For example over the Himalaya we need to have 1650 psi, and from another station back to homebase 700psi is acceptable. On what do you base there is a leak? In case of a leak, the pressure should always drop, also when not using any masks...

The lowest our table -the company put one in our QRH- goes is 8000ft; but there 1500psi should give you 5:50 manhours of 100% oxygen on the 744F. I suppose the flightdeck system is similar to the pax -400.
No, the freighter has more bottles for cockpit and supernumary crew. The Pax version only has 1 cockpit bottle...
However i like to see your tables...

Depends on the flt and routing, LH, ULH over mountains or over the Atlantic, you can dispatch with 680 psi.
Like we do....

In the 737 the master oxygen valve is in the forward hold, so that the engineers can shut the O2 off when playing with the tank. When the valve is turned off, pressure remains in the lines leading to the cockpit - so the guage remains at bottle pressure, and there is no cockpit indication of the lack of O2 availability in this configuration, except an overly rapid drop in pressure during the O2 test.

reaction to the answers:
There is a problem in the system in that airplane.
That is EXACTLY why you do the initial short test. You should see NO pressure drop.

A drop of 100psi during the test is acceptable according our FCOM, so there shouldn't be the problem.

The lowest our table -the company put one in our QRH- goes is 8000ft; but there 1500psi should give you 5:50 manhours of 100% oxygen on the 744F. I suppose the flightdeck system is similar to the pax -400.

the system on the Freighter is different than the Pax version. the freighter has more bottles for fire surpression. However i like to see these tables...


I am still shocked to see that we only have 30 minutes of oxygen in the 744-400

Clearly understand the advantage of having hrs of oxygen in a -f to starve out a fire. 30 minutes should be plenty of time in a pax bird to decend (terrain providing) to a breathable altitude, too much more time and you would start killing off your payload.

A SINGLE 50-100 psi drop is acceptable in the short test, just because the gauge measurement might be on the edge of the 1500-1450 or 1500-1400 psi threshold. However, repeated 100 psi drops on multiple tests is NOT acceptable, at least in the freighter version.

Don't know about FAA regs but EU Ops requires 120 mins of oxygen for flight-deck crew.

Can you tell me where i can find this rule?
it is new to me!!!!
(we are talking about a pax version of the 747-400)

It is in EU OPS 1.770 Appendix 1.

PT6A

Oxygen Endurance
 

In the US, FAR 121.333 requires enough oxygen for each flight crewmember to have 2 hours' worth: 10 minutes' worth during a constant descent from the max certificated altitude for that aircraft to 10,000 feet, and then 110 minutes' worth at 10,000. It doesn't address some of the possible variations of flow rate (e.g., 100% vs. normal, normal vs. emergency), nor does it address different requirements for cargo carriers who utilize Class E compartments.

As far as flow rates, 100% at one atmosphere (i.e., sea level) is very, very high, but decreases as cabin altitude goes up. Emergency flow is fixed regardless of altitude (on most masks) and normal flow (again, on a "typical" mask) results in almost no oxygen being used at lower cabin altitudes, but will increase oxygen in the mix until you're breathing 100% oxygen -- even in Normal -- at higher altitudes, typically between 18,000' and 25,000'.

Another way of putting it....If you need 100% oxygen and you're not sure your supply will last as long as you need it, consider climbing the cabin altitude. Of course, if others are on the aircraft, this won't always be practical....

But then again, one of the top reasons for one to need oxygen in the first place is loss of cabin pressure.

It is not the TOP reason. It is one of the reasons.
It is as important as oxygen in case of smoke/fumes....

I agree, that's why I said one of the top reasons. The other one being the one you said.