Each warning light on a 737 needs four wires to it, so it's not so simple, but as you say it can be done. Really the failure in this case was to do with training. The crew should have known that an intermittent horn in flight meant excessive cabin altitude, yet for some reason they decided it was to do with the takeoff configuration warning system (which only works on ground). The multiple use of one aural warning for two things is not unusual in Boeings. All Boeings of the 1960s and 70s era (and that includes the NG) use intermittent and continous horn warnings in exactly the same way as the 737.Having multiple aural warnings is not always a good idea either. Several different warnings going off at once will only cause confusion, The MD80 is an example of an aircraft with a lot of different voice warnings some of which are hard to understand and distinguish and not always obvious how to clear or cancel.

When you must route a wire, 2 or four, are the same, an hour or 2 more in man power.I agree that error was caused by human factors regarding the training, however, more the plane is Murpy proof, more it is human error free.A voice alerting "Warning, pressurization" or "cabin altitude" is better than re-using an existent sound well known as it sounds all days on ground. ;)Training can be improved, aircraft can be improved, both is better ;)

When you must route a wire, 2 or four, are the same, an hour or 2 more in man power.I agree that error was caused by human factors regarding the training, however, more the plane is Murpy proof, more it is human error free.A voice alerting "Warning, pressurization" or "cabin altitude" is better than re-using an existent sound well known as it sounds all days on ground. ;)Training can be improved, aircraft can be improved, both is better ;)

If all you had to do was just grab two or four strands of wire and route them however you wanted then yes it would be easy. However it needs to be designed, approved, certified. Its possible that more connectors would need to be added to the plane. Its a lot more than a couple hours. As far as im concerned any pilot who hears a horn at 10,000 and assumes its the takeoff config should not be flying. Shows a complete lack of knowelge. To assume a warning horn is faulty is quite the risk to take.

If all you had to do was just grab two or four strands of wire and route them however you wanted then yes it would be easy. However it needs to be designed, approved, certified. Its possible that more connectors would need to be added to the plane. Its a lot more than a couple hours. As far as im concerned any pilot who hears a horn at 10,000 and assumes its the takeoff config should not be flying. Shows a complete lack of knowelge. To assume a warning horn is faulty is quite the risk to take.

2 hours are the difference between 2 wires and 4.In a previous post I talked about projects and other aspects that take its count in a work like it. Doing that is pretty simple, the altitude switch is just below the control cabin, near the aural warning module, the aural warning module probably has free pins in the connector, if not, a splice can be used to route new wires to the lamp/s, a connection is then needed to the master dim and test module, but it is all possible with existent equipments.Also, a mod. kit could include a new connector (male and female) to replace a full one with a bigger one, by re-routing all wires (existing and newers) to the new connector.It is all possible and can be done with a day or 2 of hangar.

When you must route a wire, 2 or four, are the same, an hour or 2 more in man power.I agree that error was caused by human factors regarding the training, however, more the plane is Murpy proof, more it is human error free.A voice alerting "Warning, pressurization" or "cabin altitude" is better than re-using an existent sound well known as it sounds all days on ground. ;)Training can be improved, aircraft can be improved, both is better ;)

Yes, but they can't change the aural warnings. It would have to be done on all 737s because of the common type rating and would be a major piece of work. Every manual would have to change too. All 737 pilots would have to be retrained to identify the new aural warnings. All because one unfortunate crew had forgotten what the cabin altitude warning horn sounded like.The intermittent warning is unique in the air, it means excessive cabin altitude. There should be no confusion. Adding a light helps confirm the cause, but it really isn't necessary. The same design was used on the 707, 720, 727 and 747.

Yes, but they can't change the aural warnings. It would have to be done on all 737s because of the common type rating and would be a major piece of work. Every manual would have to change too. All 737 pilots would have to be retrained to identify the new aural warnings. All because one unfortunate crew had forgotten what the cabin altitude warning horn sounded like.The intermittent warning is unique in the air, it means excessive cabin altitude. There should be no confusion. Adding a light helps confirm the cause, but it really isn't necessary. The same design was used on the 707, 720, 727 and 747.

This topic itself demonstrates that a light could "enlight" a not good trained crew (or already suffering of hypoxia) much more than the sound.The modification could be also not mandatory, so it should be a company option.Adding the light without removing the aural warning will help in replacing only one page on the manual.I found that all manuals can contain 20000 WARNINGS or CAUTIONS about the usage of some items, but, you will find always an human that will skip a step, or a warning.I cannot think that both pilots were never been trained on the simulator for a depressurization, they probably forgot what the sound was, partially for the beginning of Hypoxia, but a light or a thing that is only for it like the voice, an EICAS message, cannot be misunderstood.We can talk about human factors....We can talk about the tunintair ATR crashed in the sea for a wrong fuel quantity gauge.Neither the technician, the store, the captain or the first officer recognized that there was a visible difference (fuel maximum values) between atr42 and 72 instruments, noone found discrepancies in the refueling quantities neither in the feel of the aircraft weight at take off...Pilots probably found the "error" when the second engine shut down.

Tim,As John said crew masks don't drop down automatically. Why didn't they do anything when the passengers masks dopped I hear you ask?Correct me if I'm wrong but the pressurisation switch was set to manual. Therefore unless the crew were manually controlling the a/c pressurisation, then the aircraft will not pressurise. So therefore it will not depressurise.The depressurisation you are referring to where the masks will drop down and people are in crontrol is rapid decompression caused by some type of fault. Usually a structural failure. Up to this time passengers are conscious and fully functioning.

When the plane is on the ground the cabin pressure is equal to the outside air pressure. As the plane climbs the pressure in the cabin will decrease (either due to small leaks or due to the outflow valve being open), so the cabin will in fact depressurise, even if it was not actively pressurised by the flight crew.The cabin oxygen masks are triggered by cabin altitude (i.e. air pressure inside the cabin). Off the top of my head, if the cabin altitude exceeds 14,000' the masks automatically deploy. It doesn't matter if the cabin altitude reaches 14,000' within seconds (rapid or explosive decompression) or only after a period of several tens of minutes (this accident).I know that when KLM takes delivery of a new plane they take it up to above that altitude with the pressurisation off to make sure that the system works (presumably other airlines do too). Inside the cockpit however there are no automatic masks. I'm also not sure if the crew recieves any warning about the cabin masks deploying, but it's possible their thinking was already detoriated due to the hypoxia by then.Regardless, finding out who was to blame for an accident does exactly nothing useful apart from giving judgemental types a false sense of security(* I wouldn't make such a stupid mistake, so it could never happen to me *). It's far more useful to figure out how to prevent such an accident from occuring again. Demanding that all pilots (and for that matter technicians, designers, ATCOs, etc) become superhuman beings that never make mistakes is not a viable solution.

There is an amber light ( PASS OXY ON) next to the Passenger Oxygen Switch, located on the aft overhead panel.If I remember correctly, this will not trigger a Master Caution or an aural warning, since the aural warning comes way before the automatic deployment of the masks. The cabin altitude horn will sound at 10,000 ft. cabin altitude, the masks will automatically deploy at 14,000 ft.Fun Fact: the Passenger Oxygen Switch is commonly referred too as the "OH F@CK-button".

When OXY ON is triggered (and all masks in cabin deployes) the Master caution and Overhead will turn on ;).But, in a slow decompression, the part of the flight between the horn and the mask deploy is too much and pilots are probably already "unserviceable".I forgot about the cutout button, so, the wires from the altitude switch goes to the pressure controller before going to the aural warning module, and this is a thing that could be used to simplify a modification. It should be sufficient to do a new pressurization panel with a light (that also triggers the master caution)alerting the crew about pressurization. All can be done internally on the panels.The alternative is the existent new light system on the main panels that added TAKE OFF CONFIG and CABIN ALTITUDE lights, but it adds more work.

One thing puzzles me about the NGX on this subject. I have made the same error twice on the NGX where I did a "no packs" take off with engine bleeds closed, both packs set to off and APU off. Unfortunately, after take off I forgot to deal with the packs or bleeds. Serious I know. However, the puzzle is that on both occasions it wasn't until I reached about FL310 that the klaxon sounded. Anyone know why this is?Iain Smith

Edited January 28, 201214 yr by iainsmith

If you have the pressurization controller in AUTO mode, it will attempt to control the rate of climb by closing the outflow valve completely. The outflow valve of the Helios 737 was IIRC almost, if not fully open.

One thing puzzles me about the NGX on this subject. I have made the same error twice on the NGX where I did a "no packs" take off with engine bleeds closed, both packs set to off and APU off. Unfortunately, after take off I forgot to deal with the packs or bleeds. Serious I know. However, the puzzle is that on both occasions it wasn't until I reached about FL310 that the klaxon sounded. Anyone know why this is?Iain Smith

As Fabo said in your case the outflow valve in auto mode closes to let the pressure to remain inside.In the helios accident the packs were running, this situation provides positive pressure and fresh conditioned air in the cabin. The outflow valve in the helios was left in a position where the air pumped by the packs was a bit less than the air going out from the various holes in the cabin (outflow valves, drains, vents) this causes the pilots to not recognize the depressurization, oxigen will decrease slowly and the effects will be latent.I also think that when they heard the aural warning, they probably checked the bleeds and packs as they are the most probable cause.

Thanks Peter & Andrea. Something else I've learned!Iain Smith

No, as the aircraft descends with about 1500fpm they do not have enough time left.

The F-16 pilot is quite sure the guy was making gestures with his hands passing through 8000 ft.Boeing never made use of the tech they got off MD when they bought them out. On MD jets from the MD-8 0 onwards it says "CABIN ALTITUDE!", so there is no mistaking the problem.Best regards,Robin.

Not quite true.... The newer 737's do indicate the difference between take off config and cabin altitude.I couldn't remember whether the pass OXY masks triggered the master caution or not, thanks for reminding me Andrea ;-)

We all know electromagnetic impulses along with some chemicals are what constitute our memory, thinking and what transmits commands to the rest of the body via the nerves. So with less of them, we start not beeing able to remember what we've learned, take much more time to take decisions and move slower. Neurologists estimate that we forget about 75%, maybe more, of what we learned if prolongedly exposing the brain to low oxygenation situations.

Re: brain function/memory, I think it's only 'electromagnetic' if you've had the cybernetic implants done.Nerves work via action potentials, which are 'electric'. Memory itself is poorly understood, maybe changes in molecular stucture of molecules, but no one really understands this well.Regardless, human brain performance suffers with hypoxia. I've been at 18,000 feet quite a few times. With a few days acclimatisation, it's not too bad. With more acute onset, you might not think so clearly.This is an interesting accident, like all accidents, in terms of lessons learned. I'd agree with the suggestions that the warning could have been more specific - a warning for a situation where your brain function is likely to be impaired (pressurisation issues) should be unmistakable and not involve troubleshooting/logic etc. Suboptimal design here.Given that, I suspect most pilots would have figured this out even when a little obtunded and would have reached for the crew O2 in time...