B777 Crew oxygen system

[MickG0105]

Originally Posted by Amadis of Gaul

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

The a/c is on the Malaysian registry

[MickG0105]

Originally Posted by spannersatcx

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]

Originally Posted by yotty

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]

Perhaps the official report mentioned above has some typographical errors and omissio

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]

Originally Posted by 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.

What were they investigating?

[MickG0105]

Originally Posted by Amadis of Gaul

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]

Originally Posted by MickG0105

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]

Originally Posted by Amadis of Gaul

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]

Originally Posted by MickG0105

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]

Originally Posted by Swedish Steve

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

Originally Posted by 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.

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]

Replenishment during A1 and A4 Checks

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?