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Cabin air pressure modelled?

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Hi,Watched a program on Discovery the other day that was about Helios Flight 522 that tragically crashed because the cabin pressure switch was in it's 'Manual' position from start instead of the normal 'Auto' position which left the a/c depressurized from start. Lots of alarms and warning messages should have been triggered by this during the climb phase but they found that maybe there was a problem with the a/c in that these different warning messages failed which in turn left the pilots unknown of the problem until it was too late and they passed out. Very tragic!Anyway I though I should try to simulate this in the PMDG 738 and I guess I should be lucky I was in the comfort of my own home and not in the actual a/c because no alarms were triggered in my case either...Maybe it was just me doing some mistake since I'm not quite sure on how these systems work or isn't this modelled in the PMDG 738?Also noticed when looking at the PMDG 738 in spot view that the valve at the rear of the a/c that will open if you manually want to decompressure the a/c doesn't seem to move regardless of moving the switch on the overhead panel but again...maybe it's just me not knowing exactly how this works. To me it looked open all the time?Anyway it would be interesting with some comments.Best,

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Can't remember if the valve position (external view) is supposed to change, but I didn't see it move today during testing. However the cabin altitude (cockpit indication) rises and the differential pressure decreases if you manually decompress the cabin.I'm pretty sure that the alarm isn't modelled in the PMDG 737NG.CheersQ>

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Odd to see this thread as we looked at this very incident during my own real world recurrent emergency procedures training today. Some of the facts I found interesting were:1. A/c fully serviceable.2. The crew thought or might have had a TO warning configuration warning.3. The capt was out of his seat and pulled a CB to stop the noise...which stopped or prevented the Cabin alt warning horn sounding.4. The Capt was German and the F/O Greek and communication was an issue.5. The last communication receieved was passing thru 22,000ft.6. The aircraft had been in for maintenance and the controller was in manual.Scary stuff.CheersSteve

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According to the documentary on Discovery I watched the pilots did report a problem with the air conditioning system shortly after start but that only led to a decision that the problem should be checked at next stop...and the a/c had had it's air conditioning system serviced/repaired more than once before this incident...What I find a bit odd is how come the pilots wasn't warned by their own physical symphtoms like the numb feeling in the lips etc and for that reason chose to land the a/c and have the problem checked instead of keep climbing but maybe they were already too disoriented to act in a logical way...very scary!!

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And therein lies the truly scary thing about hypoxia, by the time you realise something's seriously wrong there's often not much you can do much about it.

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"6. The aircraft had been in for maintenance and the controller was in manual."I would be interested to know pilots' thoughts on this.Should the pilots expect their controls to be in the right places after maintenance?Cheers.Q>

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I would guess in the end it's always the pilots responsibility to know everything is in it's normal order and in this case there is also a green light illuminated on the overhead panel when the switch is in it's 'Manual' setting...you could think the pilots should have seen this during the preflight checklists and reacted to it but it's always easy to say afterwards...

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>"6. The aircraft had been in for maintenance and the>controller was in manual.">>I would be interested to know pilots' thoughts on this.>>Should the pilots expect their controls to be in the right>places after maintenance?>>Cheers.>Q> Speaking from experience both flying and dispatching in the real world the answer is: NO!The absolute worst time to fly an aircraft is right after an annual or 100 hour check. I've found loose bolts, things missing and, once, a torque wrench in the engine cowling...FAA rules: Pilot is responsible for his aircrafts airworthyness.Dave Hart

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Exactly, Mark. USAF and FAA used to offer barometric chamber orientation for private pilots.. don't know if they still do. I lost pressure in a C414 at FL220... the first indication was my ears mildly popped. It took at look at the cabin pressure guage to figure out that pressure was gone. There was NO OTHER immediate symptom. This emphasizes how insidious the problem really is. I told ATC what happened and immediately started descent, which in the piston twin is not like a falling jet so it took a couple of minutes to get below 16000 where the hypoxia symptoms seems to back-off a little (didn't don mask) and of course down at 12000 all was better. Very interesting experience.

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You are quite correct Dave in that the pilot is ultimately responsible. However, as this case shows, if the aircraft had arrived ex maintenance with the control in the proper position they would all still be alive today. Airline safety is like a series of swiss cheese pieces placed alongside one another. The object is to have the cheese pieces organised so that the holes dont line up. In otherwords each piece of cheese represents one opportunity to catch an error. In this case the first piece of cheese was maintenance and the second the pilots. Both bits of cheese were aligned so the there was a hole straight thru.What I find surprising is that the aircraft never pressurised and the noise from the plug type doors would have been very loud. Masks would have dropped from the ceiling yet no one in the cabin called to find out what was going on and why the aircraft was still climbing. CheersSteve

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" However, as this case shows, if the aircraft had arrived ex maintenance with the control in the proper position they would all still be alive today."My 2c's worth on this issue...Looking at the Maintenance Manuals for, say, the 737NG, it does specify that the aircraft be returned to its usual condition (after pressurization system checks).Some airlines call for maintenance departure checks to be carried out prior to the pilots' arrival after extended transit stops (as well as after major maintenance), but not all. Unfortunately, the engineers doing the checks may not be qualified in all trade categories, so may not immediately recognize that a switch is in the wrong position. Having all category engineers available for these checks is probably not economically viable. Pilots, on the other hand, are qualified in all systems.Pilots _expecting_ the switches to be in the wrong position may actually be in a better position to identify switches left in the wrong position.I've noticed that some pilots take great delight in pointing out that a switch is not in the correct position (even if the engineer has to walk a half kilometer to the aircraft to move the switch to it's correct position, which, in some cases could have been moved by the pilot without him/her leaving his/her seat). More likely than not, the engineer who is called to the aircraft wasn't responsible for the switch being in the wrong position anyway... It may even have been a pilot who put it in the wrong position on a previous flight (a pilot may misidentify a switch)... or it may have been accidentally bumped during flight :( Also, engineers are not always in a position to "change the system", whereas an official report from a pilot would go through the correct channels and generate some kind of maintenance alert for all to see.Of course, you could always blame bad/ancient airplane design... That is... annunciator/warning lights not being visible in certain lighting conditions, switch positions not clear, lack of EICAS systems to identify the cause of generic aural warnings, etc.Cheers.Q>

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Q being an ex maintenance engineer myself I am in the unique position of being able to see both 'sides' point of view. I am in no way trying to apportion blame to any side and your comments regarding pilots expecting to find switches in the wrong position being in a better position to identify switches left in the wrong position is valid and the way by which I operate. However it is a different story when one looks at probabilities in the context of airline safety. Let me explain. 1. A/c leaves maint with switch incorrect. Pilot finds and corrects.2. A/C leaves maint with switch incorrect. Pilot misses it.3. A/C leaves maint with switch correct. Pilot chks switch.4. A/C leaves maint with switch correct. Pilot misses switch.As you can see from above had the A/C left maintenance with the switch in the correct position the disaster could not have occurred ...period, whether the pilot checked the switch or not. This is the first line of defence and is the reason why it is so important that A/C be returned to service in a correctly configured way. Ah but the pilot should have checked I hear you say. Yep he should have. But what if he THOUGHT he had? Impossible I hear you say. Consider this. What if just before reaching that point (and it may be the last item on that panel) he was interupted....fuel sheet signing, more blankets, return of logbook...anything, after all there are a myriad of things going on pre-departure. After this interuption he returns to his checks. He thinks now where was I.......oh yeah I had finished that and was about to move onto the next panel....switch missed. It doesn't help that most switches on panels are the same shape and in close proximity. How many times have you read a checklist and been interupted. When you return to it can you remember if the item you are holding at was acknowleged or still to be called? These are all human factors and are the single biggest cause of accidents. This is why it is so important to ensure systems are in place to minimise the chances of human error due to these natural human factors. Having an aircraft returned to service with switches in the correct position is one small but very important step in minimising these chances of error. As is proven in this case had this simple procedure been done the probabilty of an error would have been eliminated and them all being alive today.I cannot see how it could be loooked at in any other way....or do we just repeat this scenario a few years down the track?CheersSteve

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"But what if he THOUGHT he had? Impossible I hear you say. Consider this. What if just before reaching that point (and it may be the last item on that panel) he was interupted....fuel sheet signing, more blankets, return of logbook...anything, after all there are a myriad of things going on pre-departure. After this interuption he returns to his checks. He thinks now where was I.......oh yeah I had finished that and was about to move onto the next panel....switch missed. "And it's not possible for engineers to do this also? ;)Engineers have checklists just like pilots, but interruptions happen to everyone. Also, in a Line Maintenance environment, engineers have to juggle multiple aircraft, whereas the pilot only has one aircraft to contend with. Engineers are interrupted by the same people who interrupt pilots (cabin crew, refuellers, load controllers, pax handling,...)BTW, does your airline have the engineers set up the cockpit in a standard config? (and how many errors have you picked up even with this system operating?)Sure, we've all had to watch the movie with the guy poking the rod through the swiss cheese blocks, but has there really been any less accidents as a result? Seems that these safety courses were only designed to lessen the liability of management. Safety costs money... and airlines are not willing to spend that money where it's most needed (lol... some airlines now have cargo handlers doing the walkarounds).BTW, this message thread (diversion) was not intended as a finger pointing exercise, just a "what is expected of us?" discussion ;)Cheers.Q>

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>>Sure, we've all had to watch the movie with the guy poking the>rod through the swiss cheese blocks, but has there really been>any less accidents as a result? Seems that these safety>courses were only designed to lessen the liability of>management. Safety costs money... and airlines are not willing>to spend that money where it's most needed (lol... some>airlines now have cargo handlers doing the walkarounds).I hear you completely. Most of them are just box ticking exercises and designed to reduce insurance premiums. However despite all that rubbish, in the Helios case, there is no escaping the fact that if the panel had been reconfigured to a line state after maintenance it wouldn't have happened and if the pilot had performed his checks properly it wouldn't have happened. It certainly isn't a cure all but why settle for one layer of protection when you can have two? Even more so with gargo handlers doing walkarounds......whoever the first person was that came up with that idea should be put up against a wall and shot. Most western airline operators seem to be laxing into a state of 'safety complacency' and the above does little to disprove that.In the industry 'Safety first' has been replaced with 'safety at reasonable cost' with a dutch auction amongst airlines as to the term reasonable.CheersSteve

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As a maintenance pup, I quickly got tired of dealing with systems problems that the pilots found during their pre-flight walk arounds. Seemed to happen all the time. So I finally got the Flight Ops book and copied off the sections that describe

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And I believe that:1. An aural warning sounded passing 10000 feet and again at 14000.2. Passing 14000 feet the cabin oxygen masks deployed.Plenty of clues. A case of all the random holes lining up.NeilYPAD

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A question about the oxygen masks, I know they will drop down in front of the PAX in the cabin but how does it work in the cockpit for the pilots - is it automated there as well?

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No automation... The pilots have to be able to don their masks at any time. Also, it might be difficult to install the mask boxes/containers above their heads (not much room).Cheers.Q>

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Excellent post Sam. Nothing to add or ask. I'm stunned at the level of competence (or lack of, to be exact) you describe the majority of MXs to possess.However, like already pointed out. All layers of safety are still layers of safety, whether or not the responsibility of that safety is in the hand of the pilot or the mechanic. And since it's been decided a long ago, that the pilot is responsible for his cockpit and the way it's been set up, then there really is only one way to see this: Mechanics should be taught to assist/minimize problems as much as humanly possible, but the PILOT should claim full responsibility of how things were when they pulled the chocks. This way you get the extra layer of safety, but nothing is still compromised in a way that incompetent people would be given responsibility over things they have no understanding.Tero

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Hey, B737-Classic, big green oxygen bottle in the cargo hold, 1500psi constant supply to four oxygen masks in the cockpit - one for each pilot, one for each observer.

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