B777 Crew oxygen system

[MickG0105]

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.

[FullWings]

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...

[MickG0105]

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.

[yotty]

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.

[vapilot2004]

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.

[MickG0105]

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.

[vapilot2004]

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.

[yotty]

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

[MickG0105]

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.

[NSEU]

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

[TURIN]

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.

[MickG0105]

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.

[Amadis of Gaul]

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?

[NSEU]

The mask check is 5 seconds at 100%,

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.

[MickG0105]

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

[MickG0105]

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?
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.

[NSEU]

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.

[MickG0105]

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.

[Amadis of Gaul]

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.

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

[spannersatcx]

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

[MickG0105]

Interesting...what's the country of registry?
The a/c is on the Malaysian registry

[MickG0105]

As stated in your other thread, not unusual, what is unusual is allowing it to get that low in the first place!
Yep, there's something amiss. The a/c had an A1 check 12 days prior and an extended stayover check 14 days prior. It's hard to believe that the O2 wasn't topped up on either of those occasions.

[NSEU]

Assuming the report was official, then I would assume they knew what they were talking about.

Having said that, it can be quite difficult to judge how much oxygen is being used, especially if the requirement to top up oxygen to 1850 is required every time an aircraft returns to main base (which might only be 2 or 4 sectors). Engineers on the line certainly wouldn't know if there was high oxygen usage unless he/she worked 7 days a week and there were only a couple of aircraft in the fleet. Really, only large leaks are ever going to draw the attention of a line maintenance engineer (e.g. ones which generate low oxygen messages in flight after only one sector). Topping up the oxygen so frequently also makes it difficult for the people looking for abnormal trends in Technical Log book reports (Maintenance Control). I'd be surprised if anyone had a good idea of what's normal or not. Bottle temperature variations also have to be considered. If a pilot insisted that a bottle to be topped up to 1850psi in mid-winter in London (because his basic checklist said it must be this), you may see over 2000psi on an aircraft baking in the sun on the ground on the Gold Coast (in mid-summer). Hopefully the engineers could persuade the pilot that the 1850 value is based on 21C (70F) bottle temperature.

Did the report mention charter operations?

(EDIT: just read your latest messages. Questions to be asked: What are the top up requirements for Malaysian? Where was the A1 check carried out and by whom? The carrier or a third-party?

[MickG0105]

NSEU,
The report is official and I tend to assume they know what they are talking about up to the point I read something that looks like they don't know what they are talking about.
There were no charter ops and MAS do their own maintenance so the A1 was performed by MAS at KL (as was the overnight).
The top up is recorded as follows:
S/N 4918752 –Night Stop. Crew oxygen system pressure reads 1120 psi (EICAS).
Action taken. Crew oxygen system replenished to 1800 psi – EICAS. AMM 12-15-08 refers.
Quiet bizarrely the report states "The minimum pressure for despatch as per the MAS Minimum Equipment
List (MEL) is 310 psi at 35°C for 2-man crew and with a 2 cylinder configuration (as installed on MAS B777 fleet)."
Now, that simply beggars belief!

[yotty]

MickG0105 Those figures do seem a bit on the low side, the figures from "a large UK airline operating 777s" are 900 psi for 2 crew, 1300 psi for 3 crew and 1600 psi for 4 crew. Min dispatch from UK 1400 psi or 1600 psi with 4 crew.

[yotty]

NSEU I suppose it really depends on your definition of line and main base. Our aircraft return to a main base every day, but we work them on the line, so only occasionally do they go down to the base for a hangar input. We handle around 10 a/c per day. If a particular aircraft has significantly more oxygen servicing it becomes "common knowledge" among the engineers. Also if defect coding has been carried out correctly it's easy to search on our particular computer system for any oxygen related entries making it easy to identify a trend. So in a way we do have a good idea whats normal or not.

[NSEU]

If a particular aircraft has significantly more oxygen servicing it becomes "common knowledge" among the engineers.

Perhaps I'm trying to compare apples with oranges. Different fleet, different rules, different routes. If one of our aircraft flew away from main base and did a 4 sector flight, it would be unlikely that the same engineers would see the aircraft the next day. Also, it wouldn't raise any eyebrows if the aircraft required another O2 service on the return to main base. Thanks to the lack of dilligence by some (outsourced) engineers who didn't reopen the bottles after servicing them, our flight crews were subsequently required to suck on the O2 during their preflights until the levels made a noticeable drop . If you do this for 4 sectors, the O2 is, of course, going to drop 10 psi or so. If you only top up the O2 to just over 1850, the aircraft will most likely need a service when it returns. Because of safety concerns, you can't fill up the bottles quickly, and because of time constraints, you may only have the time to fill up the bottles to just over 1850. The requirement for engineers to check the O2 levels everytime an aircraft returns to main base (which may also be on the line as you say) may have been removed since I left my old airline, but I know old habits die hard. Dedicated engineers will still check and service the system because they know the aircraft will be delayed if there is a last minute call to a departing aircraft... especially if some of the sectors involve routing over the Himalayas.

Perhaps the official report mentioned above has some typographical errors and omissions. Wouldn't 310psi generate some kind of EICAS message for crew O2?

[MickG0105]

The report is the Factual Information Report released by a Malaysian Ministry of Transport ICAO Annex 13 Safety Investigation Team, so I'd like to think it would be typo and omission free.
Regarding the EICAS CREW OXYGEN LOW warning, I can't find a reference to how low the pressure needs to be to generate that warning.

[Khaosai]

I think it's a figure of 400 or 500 PSI to generate the Crew Oxygen Low advisory message.

[Swedish Steve]

Regarding the EICAS CREW OXYGEN LOW warning, I can't find a reference to how low the pressure needs to be to generate that warning.
This EICAS is low pressure in the flight deck, i.e. the supply valve is closed.
It is not low bottle pressure.

[sleeve of wizard]

Swedish Steve that is not correct.
Indications
Oxygen Shutoff Valve/Indicator
The gauge on the cylinder shows cylinder pressure. A pressure transducer also measures the pressure. The information goes to the AIMS. The pressure shows on the status display. An advisory message shows if the bottle pressure goes below 500 psi.

[Amadis of Gaul]

The report is the Factual Information Report released by a Malaysian Ministry of Transport ICAO Annex 13 Safety Investigation Team, so I'd like to think it would be typo and omission free.


What were they investigating?

[MickG0105]

What were they investigating?
The disappearance of a MAS B777 on 8 March 2014.

[BuzzBox]

Having flown the 777 for many years, I agree with spannersatcx. A drop of 730psi over 50 days and 75 flights is "not unusual, what is unusual is allowing it to get that low in the first place!"

[Amadis of Gaul]

The disappearance of a MAS B777 on 8 March 2014.

I guess I'm not following you. What has that to do with the airplane you're working on?

[MickG0105]

I guess I'm not following you. What has that to do with the airplane you're working on?
I'm not working on an aircraft. I have been developing a hypothesis regarding the loss of MH370; that there was a problem with the crew oxygen system that led to
a. a fire, that disabled the transponder, forced the crew to abandon the cockpit (but not before selecting a track back towards Kota Bahru and descending to FL340 using HDG and ALT Hold) and subsequently destroyed much of the cockpit before,
b. burning a hole through the fuselage and decompressing the airplane.
Events may well have overtaken me, it appears a piece of wreckage showing signs of fire may have been recovered from MH370.

[Amadis of Gaul]

I'm not working on an aircraft. I have been developing a hypothesis regarding the loss of MH370; that there was a problem with the crew oxygen system that led to
a. a fire, that disabled the transponder, forced the crew to abandon the cockpit (but not before selecting a track back towards Kota Bahru and descending to FL340 using HDG and ALT Hold) and subsequently destroyed much of the cockpit before,
b. burning a hole through the fuselage and decompressing the airplane.
Events may well have overtaken me, it appears a piece of wreckage showing signs of fire may have been recovered from MH370.

Talk about thinking outside the box...

[NSEU]

This EICAS is low pressure in the flight deck, i.e. the supply valve is closed.
It is not low bottle pressure.

Swedish Steve that is not correct.
Indications
Oxygen Shutoff Valve/Indicator
The gauge on the cylinder shows cylinder pressure. A pressure transducer also measures the pressure. The information goes to the AIMS. The pressure shows on the status display. An advisory message shows if the bottle pressure goes below 500 psi.

Depends on your viewpoint. The pressure transducer shows pressure in the line between the bottle/s and the cockpit. If the bottle shutoff valve is closed, then it won't read the bottle pressure. At ground/air transition, the line bleed valve is opened and the pressure in the line is bled for 25 seconds (to atmosphere). If the bottle shutoff valves are closed, the pressure in the line will drop, triggering the EICAS message.

Seems like overkill, adding this kind of complexity and still have the crew check the supply during preflights. Then there is the possibility of valve leaks and an increase servicing (= more system disturbance, especially if there is no remote fill port)...

I'm not going to get into the fire debate... This has already been beaten to death in the hundreds of forum pages in the original and spinoff message threads relating to the MAS incident.

[MickG0105]

Following on from previous enquiries;


Can anyone tell me if checking and replenishing crew oxygen is covered in either A1 or A4 checks for the B777?

Does anyone have any relevant documentation that they might be able to share or direct me to?