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Aexeron

A question regarding Pressurization

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I have recently experienced a problem with the 737NGX pressurization system, entirely my fault: The Pressurization Mode Selector (ironically, PMS) on the Digital Cabin Pressure Control System (DCPCS) was set to Manual. On my initial overhead scan, I completely missed the "MANUAL" light (perhaps because it was green), and continued on. About 3500 meters up, I encountered what sounded like the takeoff configuration aural warning, obviously sparking some confusion. About 5000 up, and the Master Caution appeared, along with "AIR COND" on the Flight Director's recall. Initially, I thought something was wrong with just that -- the air conditioning. I tried tweaking a few knobs, but nothing seemed to work. Then, I looked up at the upper overhead and saw "PASS OXY ON." Uh oh. It clicked then, I had a pressurization error! I had not even thought of it due to the similarities between the Takeoff Configuration aural warning and the Pressurization aural warnings.

 

Since then, I have done a bit more research on the pressurization system in 737s, and noticed a very similar incident to mine: Helios 522. Following that incident, Boeing added a "CABIN ALTITUDE" warning on panels P1 and P3 of the 737, as shown:

 

cabinaltitudelights.jpg

 

My question is: Why doesn't the PMDG 737NGX have these lights? Shown below is the current configuration of panel P3:

74777095.png

 

 

Is there the possibility of these lights being added to clarify an already bad situation?

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The real aircraft NGX is modelled after did not have this mod, thus the NGX does not have this mod. AFAIK there are no plans to add it, at this point.

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Hi there,

first Helios 522 was a 737-300, not a NG. Then in order not to confuse the pilots too much, aural Warnings are used for different situations, i.e. the "Takeoff" Warning. The sound only means Takeoff warning when you are about to take off. It also sounds for example with flaps 15 or more and gear not extended. You as a (typerated) pilot should what the bell means at what time. Under heavy workload you wouldnt be able (or at least the most of us) top distinguish between different rhythm to identify a problem. This is why same some is used in different Phases of flight. But: This does not explain, why they (Boeing) took the light away.. it helps for clarification but again one more light that flashes and might draw your attention from something more important... cheers

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The TOCW horn going off in mid flight is clearly something unusual (because one has already taken-off!) Which is why it is usually reserved for cabin height being above 10,000ft. It is usually a very loud attention getting warning. The cabin altitude can be a result of a slow leakage or sudden depressurisation. In either case stick the masks on. Depending on a/c type oxygen is released into the cabin ring main manually or automatically which is conditional on cabin altitude. Unless the a/c is equiped with chemical cannisters (which are a one time only use and last a few minutes).

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The TOCW horn going off in mid flight is clearly something unusual (because one has already taken-off!) Which is why it is usually reserved for cabin height being above 10,000ft. It is usually a very loud attention getting warning. The cabin altitude can be a result of a slow leakage or sudden depressurisation. In either case stick the masks on. Depending on a/c type oxygen is released into the cabin ring main manually or automatically which is conditional on cabin altitude. Unless the a/c is equiped with chemical cannisters (which are a one time only use and last a few minutes).

 

In the case of Helios 522, the cockpit oxygen masks did not automatically deploy, so the pilots had no indication of pressurization problems. Given that the takeoff config aural warning and the pressurization aural warning is the same tone, a pilot would think that the takeoff config warning was on the fritz, mainly because pressurization warnings are very far and few between. This is supported by contact between Helios 522 and Helios engineering department -- "Where is the takeoff config circuit breaker?"

 

Boeing added these lights to allow pilots to differentiate between two very similar warning tones. It shouldn't be too hard to implement on the NGX in future service packs.

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I never understood why Boeing never decided to simply make the CA>10K warning an aural alert similar to the GPWS warnings. Right now if you are flying towards a mountain, you don't hear a loud ding, you hear "WHOOP WHOOP - PULL UP!" or "TERRAIN TERRAIN - PULL UP!" telling you just what's wrong. Similarly, if you're cabin altitude climbs above 10,000 feet, it seems to me like Boeing should have added an aural alert similar to "WHOOP WHOOP - CABIN ALTITUDE!" - The fact that they didn't seems to have led a lot of room for confusion amongst crews, in my opinion.

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So many easy ways to save lives, so many bucks to change the aircraft certificates, training, etc.

 

The answer is $$$$$$.

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In the case of Helios 522, the cockpit oxygen masks did not automatically deploy,

 

They never deploy, the pilots have to manually pull them out of their little storage box and the masks themselves are much different from the passenger masks.

 

http://www.b737.org.uk/emergency_equipment.htm

 

 

Given that the takeoff config aural warning and the pressurization aural warning is the same tone, a pilot would think that the takeoff config warning was on the fritz, mainly because pressurization warnings are very far and few between. This is supported by contact between Helios 522 and Helios engineering department -- "Where is the takeoff config circuit breaker?"

 

That and hypoxia also added to the confusion. According to the Greek investigation the FO had prior issues with not following checklists so I can see how he missed the auto switch during his preflight panel flow.

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That and hypoxia also added to the confusion. According to the Greek investigation the FO had prior issues with not following checklists so I can see how he missed the auto switch during his preflight panel flow.

 

Confusion could be averted by having clearer warnings. Hence why they added the lights to differentiate the two warning types. Pressurization warnings, being so important, should be very clear and understandable.

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They never deploy, the pilots have to manually pull them out of their little storage box and the masks themselves are much different from the passenger masks.

 

http://www.b737.org....y_equipment.htm

 

 

That and hypoxia also added to the confusion. According to the Greek investigation the FO had prior issues with not following checklists so I can see how he missed the auto switch during his preflight panel flow.

 

 

Perhaps not enough training is/was given. The TOCW horn means such "only" on the ground during the take-off roll or a high speed taxi. If it sounds in the air i.e. once airborne! it isn't the TOCW horn any more its the cabin pressure warning. Obviously to hear the horn at FL350 and "not" during take-off means that the take-off was probably ok or at least 7 out of 10! Also it depends on the type of warning given. Is it a gentle reminder as is the stall warning of the A330 ("Stall-Stall, sotto voce!). Or is it as in the VC10 a very loud intermittent horn (in the real sense!) that most definately gets your attention.

 

As for hypoxia this is why it is imperative that pilots don their masks as soon as the warning is given. Regardless of whether the cabin leak is slow or sudden.

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I never understood why Boeing never decided to simply make the CA>10K warning an aural alert similar to the GPWS warnings. Right now if you are flying towards a mountain, you don't hear a loud ding, you hear "WHOOP WHOOP - PULL UP!" or "TERRAIN TERRAIN - PULL UP!" telling you just what's wrong. Similarly, if you're cabin altitude climbs above 10,000 feet, it seems to me like Boeing should have added an aural alert similar to "WHOOP WHOOP - CABIN ALTITUDE!" - The fact that they didn't seems to have led a lot of room for confusion amongst crews, in my opinion.

 

Because in-built warnings are an essential part of the airplane and as they were built in the sixties, there was not technology to make them speak out the condition. EGPWS is meanwhile a new technology that was glued on to the 737 and is not an integral part of the airframe itself.

 

The issue is then similar to why EICAS was not introduced in the NG - crew training, crew rating and type certification.

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the cockpit oxygen masks did not automatically deploy, so the pilots had no indication of pressurization problems.

 

Hi Aexeron,

 

There are no such thing in the cockpit. The oxygen mask are quick donning type.

 

Regards,

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Because in-built warnings are an essential part of the airplane and as they were built in the sixties, there was not technology to make them speak out the condition. EGPWS is meanwhile a new technology that was glued on to the 737 and is not an integral part of the airframe itself.

 

The issue is then similar to why EICAS was not introduced in the NG - crew training, crew rating and type certification.

 

While true, I always figured they could just foot the bill and integrate more modern updates to the airplane, including but not limited to switching the audio hardware and software and .wav file associated with the horn, and replacing it with newer hard/software and a "WHOOP WHOOP - CABIN ALTITUDE."

 

While I understand it's not nearly as easy as that, the point is that as time progresses, aircraft evolve to meet the demands of the present. The 737 NG is extremely new, and considerations such as new aural alerts and better crew advisories should have been given more thought by Boeing - and perhaps they'll consider it with the 737 NEO.

 

Or maybe they won't.

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It's not that Boeing doesn't want to do it. The problem is airlines won't pay extra for it.

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I still remember as I once forgot to turn the PACK's back on after takeoff. At about 5.000ft. I heard the "takeoff warning".

Out of instinct I kicked out the autopilot, and maintained the altitude manually, after that I started to figure out what was wrong. Since I knew what the "takeoff warning" sound sounded like, I payed attention to the flaps and the gear - but there was nothing. After a quick scan of the overhead panel I realized that my cabin altitude was pretty high and that both PACK's were still off. I guess this happens because of the lack of a co-pilot in FS. :P

 

What I wanted to say is that, hearing the takeoff warning sound during your climb and not knowing that the same sound is used for cabin altitude warnings, causes a lot of confusion and the time for figuring out what it is is too long. As previously said above, an aural "CABIN ALTITUDE" warning would solve the issue within a couple of seconds.

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What I wanted to say is that, hearing the takeoff warning sound during your climb and not knowing that the same sound is used for cabin altitude warnings, causes a lot of confusion and the time for figuring out what it is is too long. As previously said above, an aural "CABIN ALTITUDE" warning would solve the issue within a couple of seconds.

 

And that's exactly the reason why a type rating doesn't only take 20 minutes.

A good trained pilot, who probably would have read the manuals more than once, should know what this horn means in this stage of the flight.

It is the same with the EGPWS. You also don't just fly straight into a cloud in a valley until you hear the "PULL UP" sound. Of course it would be "easier" as you don't have to know where you are and you won't have to use charts, but do you think this is gonna work?

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It's not that Boeing doesn't want to do it. The problem is airlines won't pay extra for it.

Ding ding ding.

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I agree. Since this particular situation I know what that warning sound is for and can indicate during the mentioned stage of flight.

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Is it a gentle reminder as is the stall warning of the A330 ("Stall-Stall, sotto voce!).

 

The Airbus alarm is not a gentile reminder, I've got to experience a stall test in an A330 cockpit on the ground and that warning is very loud and to the point.

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The MD-11 when in the same situation will tell you with words "Cabin Altitude" so you know what is wrong, in addition to the Boieng style warnings.

Why this wasn't put on the 737 we will never know, but the Betty in the MD-11 sure works out better than the Boeing alerts of dings and dongs and other weird noises.

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When voices or aurals are played with the same speaker they need a priority, with the 737 aural warning you don't need a priority as TCAS, EGPWS are played with the speakers, the aural ding, dongs, and bells are played by an independent system. Much easy, much cheaper, and with a trained crew it is easy to understand the warning.

Howevwer the light could help, and is one of the things that they added.

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The light is a direct result from Helios 522 since the 737 never had a visual warning before (as far as I know) and on other aircraft you will get a EICAS or ECAM warning. On the A320 the pressurization page will pop up in addition to the ECAM warning so it differs from aircraft to aircraft.

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Aaaahhhhhhh! I remember when gauges were gauges! :-)

The cabin height gauge was deliberately positioned to be in the way of PO's left knee!

 

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