Apologies for the typo in the thread title...

 

On a flight out of KSFO to KORD, The departure out of San Francisco went off without a hitch. While monitoring the flight profile and JUST after making the cabin announcement in FS2Crew about reaching cruise altitude, the master caution light came on! There were no warning horns, and a very quick glance of the panels didn't indicate anything out of the ordinary (I quickly checked engine monitoring and hydraulic screens first) before cancelling the light (no indication was lit on the minidisplay either)

 

I then checked the failures page and it indicated "Oxygen masks deployed"

 

Cabin pressure and temp both indicated normal though... I ended up doing a very rapid descent to below 10,000 and ended up making a successful emergency landing at an airport whose runway was JUST long enough and ended up almost rolling off the end of it :(

 

My question is: when masks are deployed, should emergency action to descend below 10,000 supersede what the instruments in the cockpit are reading? It all turned out ok but the descent from FL 370 to 10,000 would have been a really scary ride in the real jet :)

 

My first failure in a long while... Glad it was "relatively" benign...

Did you forget to set the cruise altitude to 37000 in the pressurisation panel on the overhead?

Did you forget to set the cruise altitude to 37000 in the pressurisation panel on the overhead?

 

That was actually one of the things I checked right away, since I had that problem once before :) But then I got a warning horn... Both Cruise and destination altitudes were set correctly, and from the original message, pressure and temp indications were all normal

 

I would guess that deployment of the masks is modelled as a "Failure" and in real life might actually trigger due to a fault in circuitry or sensors?

That was actually one of the things I checked right away, since I had that problem once before :) But then I got a warning horn... Both Cruise and destination altitudes were set correctly...

 

Were you in AUTO/ALTN modes and not MANUAL?

Were you in AUTO/ALTN modes and not MANUAL?

 

Yes :) Pressure selector dial was set to Auto as per the preflight check... I use FS2Crew and that is a visual check step of the procedure. I always run thru the entire preflight from Cold and Dark...

Did you forget to set the cruise altitude to 37000 in the pressurisation panel on the overhead?

I see this come up a lot. Failing to set the cruise altitude in the pressurization panel will not cause a cabinaltitude warning or the masks to deploy. As long as you have the packs running it will keep you pressurized below 10,000 foot cavbin altitude.

Were the PACKS on/auto? Engine bleeds on?

I once forgot to turn them on and got an altitude warning during my climb, but you obviously did not have any altitude warnings, right?

Mask deployement is always something that happens after the altitude horn has sounded, the only way to override and deploy the oxygen masks before it is to do it m anually from the switch.

So, If you didn't the cabin altitude reached 14000fts.

Altitude selection in the panel will not cause this.

Also cabin temperature will not cause this. It could be a freezer or an oven, but the masks deploys at 14000fts cabin altitude or when switched on.

The causes for cabin altitude to climb are commonly found in packs or bleed configuration, a bit less from pressurization on manual.

Passenger masks uses chemical oxygen generators to produce it, when the chemical reaction starts it cannot be stopped, To initiate the reaction the mask needs to be pulled by at least one passenger for each generator (a generator for each row left and one on each row right),

Now, assuming we have 100 people onboard seeing that mask deploys, I think that at least 5 of them will pull the mask also if there is no emergency.

In that case, you are wasting the 12mins. of oxygen that will be no more avaiable. This means that after that, if there is an emergency those people will die if an emergency descend must be done for a fast depressurization.

I think (but it must be checked on the procedures) that if mask deploys for an error and someone use them the plane must begin a descent to a safe altitude. I don't think it must be an emergency descend but it must be done pretty soon.

It is always better to err on the side of safety. Remember that if you are flying at a very high altitude you may have only a matter of seconds to react and put on your own mask and perform an emergency descent. As a sim pilot you don't have the luxury of a PO. Therefore in such an emergency you don't in reality have time to go through all the checks before hypoxia sets in. You're pretty much a gonner after that!

Whilst dependant on the actual a/c the best emergency descent procedure is: Don mask, close throttles, speed brakes out, wing over in direction of escape routes, don't let your speed exceed VMo Vno. As you descend into thicker air your airspeed will rapidly increase so watch that and don't pull back too much on the stick. The disadvatage of an a/c equiped with chemical oxygen generators is that you have around 15 minutes of oxygen only. Ok it only takes you 2 minutes to descend to 15,000ft from a high cruise altitude, but you have to land almost immediately. Not so easy if you are in a high mountainous region. For you as a pilot it is mandatory to be on oxygen above a cabin height of 10,000ft. A/c equiped with liquid oxygen have the advantage of being able to continue their flight after having made a descent to at or below FL150.

Thank you all for the great information! I agree it was better to err on the side if caution ( though masks deployed, there were no alarms, gauge indications, or settings that were out of place ) I THINK the mask deployment itself was a failure of some kind of the simulated system, and not a triggered event due to cabin altitude climb... I'm just guessing on that though but Its something I suppose only the NGX programmers can tell us :)

 

In the end though, as in real life, it's better to have a bunch of screaming passengers standing around the tiny terminal of a podunk airport in the styx, complaining that its too hot and the one coke machine in the terminal is broken, then a bunch of lifeless one's, motionless in the a/c awaiting the plane to run out of fuel and plow nose first into a residential area...

 

Thanks all again for the great info!

 

 

 

First disclaimer: I am not a licensed pilot rated on the 737NG.

Second disclaimer: Neither am I a casual simmer without much of a clue about how stuff works in real commercial aviation.

 

Several points I have to add to the discussion:

-There is a strong propensity in simulator pilots to go diving for ground as soon as any smallest indication of altitude problem is observed. Doing this is very often not required and, in the end, a bad judgement call (such as seems in this case).

 

Now vololiberista is right about donning your mask to protect yourself from hypoxia, but doing an emergency descent for the simple problem of, say, single pressure controller or something going tits up, is a clear overreaction, same as doing that for an oxygen masks indication (possibly even faulty one) - since you confirmed the air pressure being normal, you should have stayed on altitude and clarified, thereby saving possible injury in the cabin, high level of discomfort on passengers, and non-normal level of stress on airframe, in some areas even risk of collision with terrain, or another traffic.

 

Assuming you would have stayed on altitude, confirmed correct pressurisation operation, and breathable air mix, you should go about to find out why has the lamp gone on, without apparent reason. Could have been an electrical gremlin of some sort, or maybe some mechanical damage, intentional or unintentional act in the cabin, maybe something as simple as someone smacking his head in the overhead compartment just in the very unfortunate way that the masks descended.

 

There was a good point brought up, that once the chemical generator was activated, those masks are done for and can not be counted on to work in a real emergency - but that, in itself, definitely is no reason to race down to 10K ft, but only to see what can be done. Say this kind of thing happened, and one row of masks fell down, and someone activated the generator - maybe you can just move the pax to another row and go on to your destination. Maybe you can continue on on a breathable altitude, or altitude where oxygen masks for pax would not be mandated, maybe you can land somewhere, block the row off, and then continue?

 

Even if you decide that you want to descend, you can clear it with ATC and just descend in normal matter, not needing to do it in full emergency style.

 

Also note that procedures might differ airline by airline, and while one airline will allow you to continue with just reseating the pax, another one might want you to land regardless.

Fabo, it is partially correct what you say.

It is correct that, if light is on, but other things are normal, you don't need to do an emergency descent.

However, if the masks are dropped (this may be confirmed by the cabin crew) they will ALL drop if the light is correctly on, as the light is only for the opening actuation, it will not see how many will open.

Now, the crew can confirm that the masks are dropped, and without a wiring fault we can imagine that they are down.

The crew will now inform the passengers about the problem and will probably tell them to not pull the masks (the exact opposite on what they say in the demo).

Crew can spend minutes to check if a mask is used or not, spending more minutes to move passengers to another (if present) free seat.

In those minutes the aircraft is always exposed to the same danger of decompression.

The cabin crew is also equipped with oxy bottles, but, how they can make sure that a guy will not pull the mask after the check? how they can assure that there is no generator working?

An emergency descend is not required as there is no emergency, but, I'm pretty sure that for safety is better to descend to a lower altitude.

As soon as the aircraft lands, ground personnell will check the generators, stow back the masks, if one or more oxy gen were used it is possible to replace them or simply declare the relative row/s inop.

First disclaimer: I am not a licensed pilot rated on the 737NG.

Second disclaimer: Neither am I a casual simmer without much of a clue about how stuff works in real commercial aviation.

 

Several points I have to add to the discussion:

-There is a strong propensity in simulator pilots to go diving for ground as soon as any smallest indication of altitude problem is observed. Doing this is very often not required and, in the end, a bad judgement call (such as seems in this case).

 

Now vololiberista is right about donning your mask to protect yourself from hypoxia, but doing an emergency descent for the simple problem of, say, single pressure controller or something going tits up, is a clear overreaction, same as doing that for an oxygen masks indication (possibly even faulty one) - since you confirmed the air pressure being normal, you should have stayed on altitude and clarified, thereby saving possible injury in the cabin, high level of discomfort on passengers, and non-normal level of stress on airframe, in some areas even risk of collision with terrain, or another traffic.

 

Assuming you would have stayed on altitude, confirmed correct pressurisation operation, and breathable air mix, you should go about to find out why has the lamp gone on, without apparent reason. Could have been an electrical gremlin of some sort, or maybe some mechanical damage, intentional or unintentional act in the cabin, maybe something as simple as someone smacking his head in the overhead compartment just in the very unfortunate way that the masks descended.

 

There was a good point brought up, that once the chemical generator was activated, those masks are done for and can not be counted on to work in a real emergency - but that, in itself, definitely is no reason to race down to 10K ft, but only to see what can be done. Say this kind of thing happened, and one row of masks fell down, and someone activated the generator - maybe you can just move the pax to another row and go on to your destination. Maybe you can continue on on a breathable altitude, or altitude where oxygen masks for pax would not be mandated, maybe you can land somewhere, block the row off, and then continue?

 

Even if you decide that you want to descend, you can clear it with ATC and just descend in normal matter, not needing to do it in full emergency style.

 

Also note that procedures might differ airline by airline, and while one airline will allow you to continue with just reseating the pax, another one might want you to land regardless.

 

Let's imagine you are at FL380 all is well and has been for some hours. Suddenly you get a warning that oxygen masks have been deployed. You have at most 30 seconds of rapidly deteriorating consciousness. Less if you are smoker or have even a minor lung infection. You simply do not have the time to go through all your checks to discover if it is a false warning. If you want to spend time doing your checks then you are putting the lives of your passengers at risk. You have to have a cast iron certainty that the alarm is false. How can you be sure that that "certainty" is in itself due to the onset of hypoxia?

In real life the oxygen system is reliable and so the best option is always to get the a/c down to a breathable atmosphere and then sort the problem out. If something does then turn out to be faulty your company can't complain seeing as they sent you on your journey with a technical no go item!

Also an emergency descent is an "emergency" therefore you don't "clear" it with ATC. You tell them afterwards. The P.O. assuming he is PNF can make the Mayday call immediately but it is not good practice as his role is to monitor the instruments and the flight profile. During a real emergency descent things happen very quickly. You may have pax and cabin crew wandering about as you initiate the descent which is why you "wing over" until the nose is almost vertical. This avoids negative G which in this situation you really do not want!

Passengers get very frightened by an emergency descent manoeuvre. It comes out of the blue and nobody says anything until the a/c has been recovered.

Situational awareness and being prepared to expect the unexpected are essential components of airmanship. Practise the drills in the company sim and remember that complacency, over confidence, sky god itis, are not to be taken into the flight deck!!

However, if the masks are dropped (this may be confirmed by the cabin crew) they will ALL drop if the light is correctly on, as the light is only for the opening actuation, it will not see how many will open.

 

Indeed, unlike you, I am not this familiar with the 737. Thanks for the input.

Apparently if all the masks went down, all should be considered in use, and, using layman sense, a descent has to be planned so that at the latest in 12 minutes after the masks dropped (or other, if usage time is different), the aircraft should be at breathable air altitude (or, if regulations allow, on altitude where operating passenger oxygen system is not required - I can not quote, but I am vaguely aware that at least some regulators allow flights of pressurised airliners without emergency oxygen system, at levels higher than usable breathable air level, but low enough that there would be no medical damage to passengers by the time aircraft could descend in the event of depressurisation)

 

As soon as the aircraft lands, ground personnell will check the generators, stow back the masks, if one or more oxy gen were used it is possible to replace them or simply declare the relative row/s inop.

 

Indeed.

 

You simply do not have the time to go through all your checks to discover if it is a false warning. If you want to spend time doing your checks then you are putting the lives of your passengers at risk. You have to have a cast iron certainty that the alarm is false. How can you be sure that that "certainty" is in itself due to the onset of hypoxia?

 

Missing altitude horn would be your first clue. Of course, you should be putting on your own mask, thereby alleviating the risk of hypoxia, or the effects thereof.

 

I guess that if you have your mask on, confirm that there is no altitude horn, that cabin pressure gauge reads normal pressure and differential, cabin vertical speed is normal, packs, pressurization controller and outflow valve operate normally, and there is no discomfort in your ears (where even normal cabin climb and descend can create some discomfort, let alone depressurization severe enough to be dangerous), you can consider yourself quite sure that there is no pressurisation problem.

The altitude horn "could" be faulty so could any warning lights. In an a/c equiped with liquid oxygen an active flow indicator must be acted upon even without the usual tale tale cabin/cockpit mist.

This is the oxygen panel from D Maltby's VC10.