Airplanes

[entropy]

Picture perfect castle nut installation however.

[GJM]

Picture perfect castle nut installation however.

It makes you wonder what else on these newer aircraft Boeing missed. I assume the continued airworthiness programs airlines use are predicated on starting with an airworthy aircraft.

It will be interesting to learn whether the pressurization messages were related or unrelated to this failure.

For those familiar with the pressurization system on this aircraft, I am assuming at the low altitude it wasn't near max diff, and if this happened in the high 30's at max diff, it could have been far worse to the airframe, leaving aside the passenger's oxygen needs.

[entropy]

It makes you wonder what else on these newer aircraft Boeing missed. I assume the continued airworthiness programs airlines use are predicated on starting with an airworthy aircraft.

It will be interesting to learn whether the pressurization messages were related or unrelated to this failure.

For those familiar with the pressurization system on this aircraft, I am assuming at the low altitude it wasn't near max diff, and if this happened in the high 30's at max diff, it could have been far worse to the airframe, leaving aside the passenger's oxygen needs.

The pressurization system has two independent controllers. They are identical and swap out each cycle on which is the “Master” and which is the “Backup”. “Auto” mode in this system isn’t like “Auto” mode on more modern designed systems. It requires manual input each flight for both cruise altitude and landing field elevation. Most newer designed systems do this automatically when you load the flight plan into the computer. If an abnormality is sensed within the controller that is acting as “Master” the system will automatically swap to the other controller making that one the new “Master”. This can be something as small as a voltage spike when transferring power during start, a full blown controller failure, or a spike of several sensed items within the controller. If BOTH controllers fail, you can still manually operate the outflow valve with a toggle switch to modulate the outflow valve on the right rear of the aircraft. Depending on the failure and what is sensed by the controller, you may not even immediately get an amber caution light. The only time you would get a WARNING (red light and aural tone) would be for a excessive cabin altitude sensed. This occurred in this case, but not until the cabin pressure was lost due to the door removing itself.

Pressurization spikes of any significance are truly noticeable in the cabin. It’s not uncommon to feel a slight “fullness” in your head as the power is brought up for takeoff. This is because most systems slightly pressurize the cabin prior to liftoff so a “bump” is not felt on climb out. It is a FRACTION of a psi. So if the cabin pressure would be fluctuating a significant amount...EVERYBODY would feel this. Especially the crew.

Nope. Not operating at max diff thru 16,000’. When you’re checking gauges on climb out (I’d check it after flap retraction, thru 10,000 when we dinged for sterile cockpit, at 18,000 when I turned the lights off, and at cruise level off) you’re normally NOT looking at diff, but cabin altitude which is really what’s important.

The final question? Remember the deHaviland Comet?

[GJM]

Entropy, pretty good explanation of the pressurization system for a retired guy! My wife looked up the N number, 704AL, and saw it went back and forth to Hawaii in December.

[entropy]

Entropy, pretty good explanation of the pressurization system for a retired guy! My wife looked up the N number, 704AL, and saw it went back and forth to Hawaii in December.

Lol. Thanks. It is was a good exercise in systems knowledge. While things have changed on the variants of this airplane from the time I was in diapers, the basics of how the systems work have never truly been redesigned. Think of it as a flying PC. Lots of patches. Many gauges and switches (and procedures) have not changed in 50+ years. The dial that shows cabin altitude and differential pressure is the SAME gauge used on both the 727 and the 707.

Yep. A buddy flew it across a few weeks back. No anomalies.

[0ddl0t]

It makes you wonder what else on these newer aircraft Boeing missed. I assume the continued airworthiness programs airlines use are predicated on starting with an airworthy aircraft.

Did you see the report of the missing nut in the rudder linkage last ~week? https://www.reuters.com/business/aer...aa-2023-12-28/

And Indian carrier inspected the linkages on their fleet of 40 737Max's and found 1 was missing a washer: https://theshillongtimes.com/2024/01...-carrier-dgca/

It will be interesting to learn whether the pressurization messages were related or unrelated to this failure.

Here are the criteria for tripping the warning light:

My understanding is that at least 1 of the 3 prior warnings occurred on the ground without any pressurization (suggesting at least that particular warning light incident was unrelated)

According to Blancolirio, even with the warning light the plane still met minimum equipment list requirements for flights to Hawaii (Alaska Airlines voluntarily restricted it to flights over land after the 3rd warning)

[entropy]

There is a distinct difference here between a “Warning” light and a “Caution” light and a “System Annunciator” light.

A WARNING light requires immediate action. Usually by memory. It is red. It is normally accompanied by a CONTINUOUS aural warning. Either a bell, claxon or something similar.

A CAUTION light does NOT require immediate attention. It normally signals a system abnormality. The light is amber. No memory items are required. The abnormality may constitute a failure, or it may simply be a system configuration issue. They occur regularly during preflight, start and flight. (For example you will get a caution light if both tha auxiliary power unit and an engine are supplying air conditioning air. You also get one when the the larger center fuel tank begins to run dry in flight. (Normal occurrence.)

Lumping every annunciation in the cockpit into a “warning light” is not indicative on how the aircraft is designed nor operated.

ETOPs (oceanic) rules are VERY STRICT when it comes to what may be inoperative. Generally speaking, a system must be at least triple redundant, or the item in question not safety or operationally required. An inoperative lavatory would be a simple example of this. So...when they say it was still OK for an ETOPs release...the issue that is in question would have to be considered pretty mild. This stuff is taken seriously.

The image you posted are the items that will trigger the AUTO FAIL annunciator on the pressurization controller. It can signal the malfunction of a single controller (there are two identical as previously stated) or it can signal the loss of BOTH controllers. Either way, it is a CAUTION not a WARNING. It depends on what other annunciations are involved. Having an “Auto Fail” system annunciation, although not a daily occurrence, would elicit a WTF?, a sigh, and putting down of the coffee cup and pulling out of the checklist. It’s not exactly something you spring out of your seat for.

1. The cabin is unpressurized at altitude. The AUTO FAIL light is the least of your problems. It’s just piggybacking on the Red WARNING light, the Red “CABIN ALTITUDE WARNING” light that’s literally in front of your face, and that obnoxious horn that in the heat of battle, everyone forgets to silence.
2. A cabin rate of climb that significant that quickly? Your ear drums will let you know before you notice any light.
3. Controller lost power. Not normal but stuff happens. Electrons are sometimes weird. Relays sometimes stick.
4. OVER pressurization. Mechanical poppet valves prevent damage.
5. Most common. It’s a computer. Computers break.

[DC_P]

Nope. Not operating at max diff thru 16,000’. When you’re checking gauges on climb out (I’d check it after flap retraction, thru 10,000 when we dinged for sterile cockpit, at 18,000 when I turned the lights off, and at cruise level off) you’re normally NOT looking at diff, but cabin altitude which is really what’s important.

Did you use 10-4 and 18-6 for differential pressure? I had a LCA tell me that when I did OE on the 88 and it works close enough on the 73 as well. Nobody I have asked with has ever heard of it.

[entropy]

Did you use 10-4 and 18-6 for differential pressure? I had a LCA tell me that when I did OE on the 88 and it works close enough on the 73 as well. Nobody I have asked with has ever heard of it.

Lol. Yep. Absolutely. That gauge is so antiquated in its depiction that you needed something quick at a glance. It’s amazing how the brain catches that stuff so quickly when it’s out of whack too. The bleed duct pressure gauge (yet another antiquity) was the same way. Normal was about 40psi which was depicted at about 11 o’clock on that gauge. That one was included in the scan of the cabin alt/diff gauge.

Edit to add: The above discussions are all what make (IMHO) the 737 such a PITA to fly. Instead of constantly flipping switches and checking gauges, the pilot should be left alone to fly and manage the systems. Every single modern transport category aircraft has a monitoring and alert system. It tells you if something is amiss, prioritizes it, and even brings up checklists for corrective action. These systems SIGNIFICANTLY reduce pilot workload. The 737 was designed in an era where there were still flight engineers. In fact, the original design had one planned. The FAA finally told Boeing that they would not certify the MAX10 without one of these systems incorporated into the design. Politics got involved, and magically that mandate has disappeared.

[GJM]

NTSB info on door/plug:

https://www.flyingmag.com/ntsb-ident...-on-737-max-9/