B777 Crew oxygen system
 

A question for anyone familiar with the servicing of the cockpit crew oxygen system on the B777.
Would a drop in pressure of some 730 psi (from fully charged at 1850 psi to 1120 psi) over about a 50 day period raise any concerns? Over the 50 day period the aircraft was used on 75 flights and accumulated some 500 hours.

The procedure in our FCOM is to test the oxygen on emergency for 5s and make sure the pressure doesn’t drop by more than 50psi, to ensure it’s actually tuned on. Multiply this procedure by 75 and by 2/3/4 pilots and that’s going to account for a lot of the loss...

Thank you for that, FW.

It might be worth bearing in mind that it's possible that unrecorded servicing may have taken place, in which case there might be an underlying problem.

In addition to SOP mask tests, the airplane itself bleeds a bit of oxygen for about 30 seconds upon engine start. This test is enabled when there is an air to ground mode change, indicating there was a flight cycle. The purpose is to confirm available O2 pressure and volume downstream of the shutoff valves.

Yotty, vapilot2004, thank you.
I'm struck by the fact that nearly 40% of the capacity of the system (over 10 hours worth of breathable oxygen at 35,000 feet) was bled off in what appears to be a relatively short period.

MickG, Indeed. Considering the size of the tanks on the triple 7, that seems like a large loss, even if we consider SOP mask tests and aircraft self-testing cycles over 50 days.

Having charged the crew oxygen today, it appears to have been a single bottle no more than 4 feet long!! :eek:

Mmm ... the set-up I'm referring to has two cylinders located in the left side of the main equipment centre. Each cylinder holds 3150 litres of oxygen at 1850 psi.

Some 777's have two bottles, some have one.

In addition it is a maintenance requirement of several operators I have come across for O2 mask tests to be carried out at every Transit Inspection.

As Fullwings says, that's a lot of Oxygen.

Thanks TURIN. The mask check is 5 seconds at 100%, then there's 15 seconds bleed on first engine start to confirm pressure. By my calculations that should account for no more than 200 psi over 75 flights.

In 500 hours I imagine there would have been quite a few trips to the lav with the remaining pilot donning the mask. Did you account for that?

Company policy? I recall our airline had a policy of making sure the oxygen level actually dropped during the test (747-400). It certainly kept our engineers in a job, topping up crew O2. You may find that crews don't time the 5 seconds, so tests may run longer.

Was the last pressure check taken after the tank temperatures had stabilised on the ground. Some tanks are close to the fuselage skin, so skin temperatures can affect the readings (circa 100 psi). Note that some Airbus types have temperature compensation.

Anyway, if in doubt.... The plumbing from the 777 MEC to the cockpit is probably easier to access than many aircraft, so leak checks are probably not too onerous.

Mmm ... Excellent point! No, I haven't. Thanks for that, I shall run those numbers.

Thank you for that tip but the aircraft in question has quick donning masks and never operated above FL450 so there was never any requirement for use in flight.

Mick, are you asking this question from the point of view of Maintenance Control with only one aircraft in your fleet?

Is the aircraft currently flying? You say that 1850 is the normal pressure, but the aircraft has gone for 50 days without servicing back to this pressure? Do your regulations allow the bottle/s to be topped up in-situ? Or do the bottles have to be removed? I'm sure your local major airline would be willing to do a service for you and check for leaks in typical spots (bottles, regulator, masks). My former airline didn't have any 777's, but we routinely topped up the bottles for other operators (some didn't let the O2 levels drop to this level, though). We had adaptors for left and right hand threaded connections on refill points and we also did bottle top-ups in-situ.

NSEU, I'm asking the question because I am looking at an investigation report that says "the decay in pressure from the nominal value of 1850 psi [to 1120 psi over 75 flights] was not unusual.". However, it strikes me as highly unusual. By my calculations, pre-flight mask testing and bleed on engine start up should have accounted for no more than a 200-250 psi drop in pressure over 75 flights, nowhere near the 730 psi drop recorded.
I don't have access to the maintenance records so I was hoping someone familiar with the B777 crew oxy system, a driver or maintainer, could say yea or nay, not unusual or unusual.

Interesting...what's the country of registry?

As stated in your other thread, not unusual, what is unusual is allowing it to get that low in the first place!