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CajunRon

PMDG 737 NGX Pressurization Warning - Helios Airways Flight 522

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Posted (edited)

I came across an interesting accident re-enactment on the FlightChannel on YouTube.  This was the crash of Helios Airways Flight 522.   The highly experienced pilots misinterpreted the cabin pressurization alarm with the takeoff configuration alarm which sounded identical.   This was on a 737-300 I think.   The alarm went off at 10000 ft and they couldn't understand why the takeoff configuration alarm was sounding while in the air when it is only suppose to sound on the ground.   Amazingly even after calling maintenance, they never identified that problem was with the pressurization system.  It had been place on manual mode and left there by an engineer checking a door seal problem and they failed three times in the preflight and after take off checklist to identify this.  Everyone perished on that flight. (One cabin steward remained conscious but didn't know how to fly the aircraft and so died in the crash when it ran out of fuel)

What is interesting is that in my own experience with the PMDG 737-800 on some rare occasions I have gotten that alarm myself but didn't know what it was (I've never taken cabin pressurization seriously while sitting in my desk chair at 350 ft ASL while simming).   Amazingly, other than the audible alarm there is NOTHING that I could find that identified the problem.   On my last encounter I did finally put two and two together and finally diagnosed and found that the bleed air was off (have no idea how it got in that position).   I think I corrected the problem at 18000 ft so I and my virtual crew and passengers survived.

But more interesting is that, according the the accident re-enactment, in March of 2011 the FAA issued an air worthiness directive requiring all 737s of the 100 to 500 class to have two additional cockpit warning lights, presumably (though it was not clear) to differentiate between a takeoff config and a cabin pressurization alarm.    Curiously, the PMDG 737 NGX doesn't model this additional warning light (I frantically searched for one when I got the alarm).    Assuming that PMDG modeled the 737-800 correctly and that there is no required additional warning light in that model, it raises the question why would the FAA confine its directive to only the 100 to 500 class and not the 600 to 900 class which by my personal experience suffers from the same deficiency?

Here's the link to the YouTube video I think you'll find it really interesting. 

https://www.youtube.com/watch?v=Go3K0UUt2Us

Edited by CajunRon

Ronnie Pertuit

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Posted (edited)

A friend of mine flies 737-800s and all of their fleet have had a red warning light  "cabin pressure" retrofitted adjacent the brake pressure gauge. I'm not sure it was a thing when PMDG modelled their NGX 8 years ago in 2011 (Helios accident was 2005 though). And I'm not aware if their fitment to NGs was mandatory or something that operator opted to do

Edited by ckyliu

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On 6/6/2019 at 11:05 PM, CajunRon said:

Amazingly, other than the audible alarm there is NOTHING that I could find that identified the problem.

Except for the cabin altimeter and differential pressure gauge on the overhead panel, and the pressurization mode indicator lights, and the switch itself.

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Joe Diamond

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Because when I'm hypoxic and the takeoff configuration warning is sounding at FL200+ the first thing I think to check in the absence of any other warnings is the cabin altimeter of course


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6 hours ago, ckyliu said:

Because when I'm hypoxic and the takeoff configuration warning is sounding at FL200+ the first thing I think to check in the absence of any other warnings is the cabin altimeter of course

word not allowed design. But put the mask on if you hear the horn and you're not taking off or landing.

We have verbal alerts that tell us if we're on a runway, going too slow or forget to set the altimeter. But they couldn't put a verbal alert in the box. And forget an EICAS.


Matt Cee

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They could have put an EICAS in the NG. That would have made identifying problems easier and resolved the aural alert conflict. But Boeing was under pressure to keep things as common as possible across all 737 versions. 


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Posted (edited)

That's where many of the 737s problems stem from, trying to maintain commonality with a 50 year old design. I've called it reheated leftovers for a long time. The 757 cockpit is over 35 years old and yet in many regards more ergonomic, logical and sophisticated than that in a brand new 737 MAX. I think the MAX was a rehash too far they were pushed in to by competitors, and recent events would support that theory.

Edited by ckyliu

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I think nearly all real 737 pilots agree with you!

 

Fundamentally, you need to get below the surface a bit. The 737 pre-dates the concept of the Databus, 1553 or whatever. Components were wired to their respective sides in autopilot, instruments and warning lights. Overhead sections shared common wiring harnesses so it was relatively easy to route a voltage to a common warning light on the MWP. 

Later aircraft benefitted from General Dynamic's work on the YF-16 and have this mythical databus. So do modern cars. Think of it like this. On an older car, you operate your electric windows. Your car has a live circuit, switched on by a relay via the ignition switch. This circuit goes to a fuse, then on to the power side of the window switch. When you operate it, it sends power to the motor. Job done.

The 737, all of them, are like this.

Now, on a modern car, the power goes to a UCH, body computer. This powers up from the ignition signal, interacting with the security system to get information from the key before powering up the car. 

Operate the window switch sends a coded signal to the UCH, which in turn operates the window. It monitors load, limit switching etc.

This is the 757 and all following designs.

In the Classic, they fitted CFM engines. Then they fitted the systems to take signals from instruments and produce symbology for the CRT displays. It's again additional wiring going down each side, but with minimal crossing over.

The NG introduced more of this. With a full CDS that can get info from either side as the Flight Control Computers can send information across. There is more interaction on each side, but not full clarity. We have switches on the overhead to send signals to either side if needed.

This is how an old aeroplane evolves. It explains why the AOA signals each FCC receive tend to stay on-side as well. The FCC is a complex name for the Flight DIrector autopilot but as it's the computer part, this is where the algorithms come from for speed trim, mach trim and now MCAS. There really isn't anything else on board that clever!

To have changed the MAX would have meant starting from scratch. There is a sh1tload of wiring in the 737 flight deck. Think about the cost and re-certification requirements on that one and you can see why they just add bits like they do.

 

 

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Mark Jason Harris.

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Posted (edited)
On 6/12/2019 at 1:53 PM, JoeDiamond said:

Except for the cabin altimeter and differential pressure gauge on the overhead panel, and the pressurization mode indicator lights, and the switch itself.

Never mind

Edited by CajunRon

Ronnie Pertuit

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Posted (edited)
21 hours ago, MarkJHarris said:

To have changed the MAX would have meant starting from scratch. There is a sh1tload of wiring in the 737 flight deck. Think about the cost and re-certification requirements on that one and you can see why they just add bits like they do.

 

Indeed, but Boeing could have done it as the investment would have more than paid for itself in terms of cheaper production and lower maintenance costs for operators. And of course it’s exactly what they did do with the 747. Imagine if Boeing had decided to keep the 747-400 as an analogue aircraft for commonality with the Classic.

Never mind the MAX, Boeing should have bitten the bullet and modernised the systems in the NG. Actually there already are a lot of digital systems on the 737, just not very visible in the flightdeck.

Edited by kevinh
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3 hours ago, kevinh said:

Indeed, but Boeing could have done it as the investment would have more than paid for itself in terms of cheaper production and lower maintenance costs for operators. And of course it’s exactly what they did do with the 747. Imagine if Boeing had decided to keep the 747-400 as an analogue aircraft for commonality with the Classic.

Never mind the MAX, Boeing should have bitten the bullet and modernised the systems in the NG. Actually there already are a lot of digital systems on the 737, just not very visible in the flightdeck.

The 747 is a different kettle of fish IMHO. For starters it was a three crew aircraft. The whole flightdeck layout and function was changing, the engines, systems and everything was updated at once. There is remarkably little change with the Max verses the NG. The MWP is still the same as the -100!

 


Mark Jason Harris.

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B737NG Pilot. Ex Q400, BAe146, ATP and Flying Instructor in the dim and distant past! Now renewed my SEP to fly a friend's  C182RG 

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7 hours ago, MarkJHarris said:

The 747 is a different kettle of fish IMHO. For starters it was a three crew aircraft. The whole flightdeck layout and function was changing, the engines, systems and everything was updated at once. There is remarkably little change with the Max verses the NG. The MWP is still the same as the -100!

 

Yes, but the point is it could have been done on the 737 too. With a production run like the NG and MAX have enjoyed the cost of redesign could easily have been absorbed. However as I understand it Boeing was under heavy pressure from a few big customers to maintain a common type rating between the Jurassic, the Classic and the NG. Hence the minimal changes you mention.


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Boeing should have just updated the 757 and put that marvelous bird back into production. Southwest and Rynair could just keep buying NGs if they're too cheap to upgrade crew training. In my unlearned opinion.


Vic green

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Posted (edited)
On 6/24/2019 at 2:48 PM, MarkJHarris said:

To have changed the MAX would have meant starting from scratch. There is a sh1tload of wiring in the 737 flight deck. Think about the cost and re-certification requirements on that one and you can see why they just add bits like they do.

That's why Boeing is able to offer significant discounts on the 737 models and still maintain profitability. Cash-Cow 😉

Edited by MartinAOA

Martin Rojo

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