If you’ve been reading or watching the news lately, you’ve probably run into multiple stories about Alaska Airlines Flight 1282. During this flight, the Boeing 737-9 MAX aircraft involved lost one of its fuselage door plugs. Rapid depressurization followed and the skilled crew got the aircraft on the ground with haste. Thankfully, there were no serious injuries and, after repairs, the aircraft will fly another day. But what are door plugs? And what are plug doors? You, like me, have probably seen both terms used interchangeably in news media lately, but they aren’t the same thing. Let’s dig in.
On January 5, Alaska Airlines AS-1282 departed Portland, Oregon, bound for Ontario, California. The Boeing 737-9 MAX, registration N704AL, carried 171 passengers and 6 crew. The aircraft was on its downwind-leg climb from Portland International Airport’s runway 28L when a bizarre incident struck just ten minutes or so after departure. The door plug located at row 26 blew off of the aircraft, taking its interior panel and the fabrics of the window seats with it. The force of the aircraft’s depressurization was severe enough to tear the shirt off a boy seated in seat 26C, who was being held by his mother in the same seat. Reportedly, the depressurization also blew several phones out of the aircraft.
Oxygen masks dropped from the aircraft as the pilots arrested the aircraft’s ascent at 16,000 feet. After a descent and a quick loop, the pilots got the aircraft back on the ground only about 20 minutes after departure. As you may have read in the news, passengers were shaken, and some sent what they thought would be their final text messages to their loved ones. A rapid depressurization event is a dramatic situation, and as the passenger you almost certainly have no idea what’s going on. Thankfully, planes are more than capable of flying after incidents like these, and the skilled pilots in the cockpit will get you on the ground as soon as safely possible.
Alaska Airlines says door plug failures are rare. I searched a few databases of aviation incidents and came up empty-handed. That doesn’t mean a plug failure has never happened before, but it’s so rare you’d probably have better luck winning the lottery and witnessing a Sharknado on the same day.
If you’ve been following the news about this incident, you may have heard the terms “plug door” and “door plug.” Some news outlets have seemingly confused the two, which only complicates matters because a plug door and a door plug aren’t exactly the same thing. They serve two different functions. So, what exactly blew off Alaska Airlines Flight 1282?
Plug Doors
The entry and exit doors you walk through when you board a modern pressurized airliner are known as plug doors, and a plug door literally plugs the opening it fits within.
To better understand plug doors, let’s take a look at an aircraft door that isn’t of a plug design. As illustrated below, the door may rely on a system of locks, cams, and hooks to stay closed. You aren’t going to board a plane through a door like this, but you will see cargo doors like this. The FAA shows offers this as an example of a typical aft cargo door:
There are plug-type cargo doors, but as they open inward, they take up space that could be occupied by cargo. Instead, they open out and allow cargo to be swallowed in. During the early decades of jet airliners, cargo doors sometimes blew off, with varying levels of destruction caused afterward. Design changes to these doors made them safer.
While an outward-opening door braces itself with mechanical locks, plug doors use our good friend physics to seal the door to the fuselage. Plug doors come in different designs, but they are built so that the pressure differential between the aircraft’s interior and the outside pushes the door into the fuselage, creating a seal so strong that even Dwayne ‘The Rock’ Johnson won’t be able to open it in flight. Plug doors also prevent those nasty door failures mentioned in the above paragraph. Watch the process of closing the cabin door of a Boeing 757:
Plug door designs can vary between manufacturers and even aircraft types. A common plug door design is a door that’s larger than the opening it has to fit through. To open these types of doors, you would first open the door inward, turn it, and slide it through a gate that allows it to open outside of the aircraft. Other plug doors may slide into the ceiling rather than require the previous dancing act but still use the same concept of pressure to keep the door closed.
There are other doors that are still considered to be plug doors but do not rely on the door being larger than the opening. Instead, these doors lower into strong tabs, and then use cabin pressure to lock the door into the tabs. You’ll find these on Airbus aircraft as well as aircraft like the Boeing 777. Below, I’ve provided an example of what these tabs look like. This blurry screenshot comes from an Airbus A320 emergency door video, the tabs are the blue structures I circled in red:
You can watch the video for yourself. The door-opening action happens at 2:30 and 10:20, respectively:
No matter the design, these doors form a strong seal against the fuselage that ensures a safe, uneventful flight for the vast majority of flights these planes will ever fly. As I said before, mere mortals aren’t opening these doors when the aircraft is at altitude. Cabin doors are locked during the flight. Even if you beat these locks, you’d have to figure out how to pull the door inward to make any sort of difference.
Unless you have superhuman strength, that’s unlikely to happen. The exception to the rule is when the cabin equalizes with the outside pressure. Then the doors can open with mere human force. And that has happened before! Last year, a man reportedly opened a cabin door during an Asiana Airlines flight. However, at the time, the reported altitude of the aircraft was just 700 feet.
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The smaller emergency exit doors located closer to the center of the fuselage are also plug doors. These doors will also often seal using tabs and stop fittings rather than being larger than their openings. Once again, these doors will be locked while the aircraft is in flight.
Door Plugs
That brings us to what failed on Alaska Airlines Flight 1282. Airliners must be able to be evacuated in 90 seconds during an emergency. To help facilitate this, the aircraft must have enough emergency exits. The Boeing 737-9 MAX can carry a maximum of 220 passengers. However, Alaska’s aircraft was certified for up to 189 passengers and the aircraft involved had just 178 seats. Thus, it did not require all of the emergency exits provided on 737-9 MAX aircraft.
Since emergency exit doors were not required in that location of the aircraft, door plugs were installed there instead. Door plugs are a neat solution to this problem. They fill the space of an emergency door, but they technically are not a door. You will not find any opening mechanism, nor will you find an emergency slide or a pressure vent. If you are a passenger sitting in that row, you may even get a full-size window. If you didn’t know any better, you would not know you are sitting in what would otherwise be an exit row. Check out the NTSB photo below:
Your clue would come when inspecting the outside of the aircraft. The door plug isn’t obvious, but you can spot it by looking at the outline of where an emergency door would otherwise be. As Chris Brady, an airman with over 20 years of experience with 737s, explains, 737 door plugs are held in place with 12 stop fittings and stop pads around the plug:
As I mentioned before, the stop fittings are what take the pressure loads and are what classify these door plugs as plugs. These plugs are neat because not only do they solve the problem of covering up unneeded emergency exits, but should the airline reconfigure the aircraft and need to use those exits, the structure is still there. The plug can be removed and an emergency door put in its place.
Additionally, the door plugs have lower hinges and straps. Yes, a door plug can open, but only for maintenance, and short straps hold the plug slightly open.
Otherwise, the door plug is sealed shut. Since the door plug is not supposed to be used as a door, there isn’t any mechanism to move the door out of the track that holds it closed. Instead, there are four bolts that lock the plug in place from moving vertically and outward. These bolts work by blocking the plug’s rollers from moving out of their closed position. A castle nut with a pin is used to prevent the bolt from backing out. Think something like an axle nut.
Alaska Airlines Flight 1282
So, with that in mind, how did the door plug fail on Alaska Airlines Flight 1282? The investigation is ongoing and as of right now, a cause is not known. However, images taken after the incident do provide some information.
Brady notes that the roller pins and stop pads, at least the ones visible in the aftermath photos, are intact. He believes that a part of the investigation may focus on those locking bolts. Some news publications have reported that the two-month-old aircraft had been giving pressurization warnings on three earlier flights.
If these reports are true, it’s possible there was a problem before the failure occurred. Reportedly, Alaska Airlines responded to the pressurization warnings by banning the aircraft from making flights to Hawai’i in case it needed to get to an airport for a failure. Again, it’s early on in the investigation, so it’s unclear whether those warnings were related to the incident.
The event also marks another bit of trouble for Boeing supplier Spirit AeroSystems, which is unrelated to Spirit Airlines. Spirit AeroSystems provides Boeing with the fuselages for its MAX aircraft, including doors. Last year, Spirit AeroSystems and Boeing discovered improperly drilled fastener holes in the aft pressure bulkheads of MAX fuselages. These bulkheads are dome-shaped structures at the end of the cabin. Reportedly, these defects did not cause safety issues but caused enough of an internal scandal that Spirit AeroSystems ousted its CEO.
At any rate, on January 6, Alaska Airlines grounded all 65 of its 737-9 MAX aircraft to conduct inspections. About a quarter of these inspections were reportedly completed “with no concerning findings.” Also on January 6, the FAA issued Emergency Airworthiness Directive 2024-02-51, which grounded all Boeing 737-9 MAX aircraft in the United States. These aircraft will be able to take to the skies again only after they’re inspected and after any necessary corrective action is taken.
Aside from that, we’ll just have to wait and see what caused this door plug to take flight. The investigation is ongoing and it’s unlikely the National Transportation Safety Board will have a clear answer until about a year from now. Until then, now you know how plane doors work. They’re a wonderful part of why commercial aviation is so safe and unless you’re looking for it, you probably won’t even notice.
Update: As ABC News reports, United Airlines has found loose door plug bolts in an unspecified number of Boeing 737-9 MAX aircraft:
United Airlines said Monday that it has found loose bolts during inspections of its 737 Max 9 fleet in the wake of a door plug getting blown out of an Alaska Airlines plane over the weekend.
United won’t say how many planes had loose bolts.
“Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening,” United said in a statement. “These findings will be remedied by our Tech Ops team to safely return the aircraft to service.”
Update: If you’re curious, Lewin did some rough back-of-the-napkin math to find out how much force was being exerted on the plug. Also, how much force you’d need to produce to open a door at altitude. His conclusion is that the plug was holding back roughly 3,683 pounds of force. As far as opening a cabin door goes, you’d need to beat 10,371 pounds. Those are all rough numbers, of course, but underscores why a drunk isn’t opening a cabin door at 33,000 feet.
One would hope that that particular aircraft would be scrapped. Personally you won’t find me on a 737 Max ever again. Too many problems in too short of a period of time.
About United Airlines statement…
“we have found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening,”
Were the bolts (in question) found to be below Boeing’s manufacturer toeque specs.?
Or
Did Boeing do some additional calculations and advise the original torque specs. we’re insufficient and Boeing provided updated torque specs. for the bolts holding the door plugs in place on all 737 Max9 aircraft?
If the former was found… why were the bolts found to be loose?
I realize investigation is ongoing, though above are the immediate (possibly obvious) questions that come to mind…
Although there are some changes to the fuselage between the NG and MAX, they’re mostly surrounding the engine pylons and nacelles, and landing gear. Considering the NG has been in service for more than 25 years with no door plug failures, and the likelihood that no changes were made to the design of that particular part for the MAX, I’m going with the former.
The problem right there, why not leave the damn exit doors in place? Save a few bucks?
I mean it looks like FAA allowed them to just install a piece of plywood, in the place of a door, securing it you know with 4 screws.….
I find it a bit ridiculous that despite having 12 pads, and cable straps and hinges ….only 4 screws hold it into place.
Also to that point – they detected air pressure loss – I would think a normal door would have some sort of vibration, maybe pressure sensors and such – but this shed plug obviously does not, they now discovered other airlines had them loose supposingly without any warnings being triggered.
I wonder if those fake even go thrpugh testing at all
The fuselages are pressurized internally to check for leaks as part of the final testing process.
However, we know contractors and maintenance facilities don’t always follow procedure to the letter, with disastrous consequences. See also: ValuJet 592, American 191, China Airlines 120, etc.
If they left emergency doors in place, then the airline could legally cram even more people on board.
On the bright side, at least Boeing know now that if they are banned from using the things for passengers, they can sell it as a sky dive plane…….
Probably be cheaper than a Cessna too…..
The force of the aircraft’s depressurization was severe enough to tear the shirt off a boy seated in seat 26C
Poor Joey isn’t safe anywhere…
https://www.youtube.com/watch?v=n2A194yTWoQ
Looks like United just found a bunch of “oopsies” on some of their fleet. Yikes.
https://abcnews.go.com/US/united-finds-loose-bolts-737-max-9-planes/story?id=106204513
CNN reports it too.
https://www.youtube.com/watch?v=CkDZ88Q7scg
Torque and safety wire those bolts kids.
Make damn sure it doesn’t move. Red threadlocker and weld the bolt to the nut. Then smack it twice and say “that ain’t going anywhere.”
I love how weird this site is. Thanks guys!
Upcoming David Tracy article:
“I just scored the cheapest 737 MAX9 on Craigslist. It’s mostly fine except the seller says it’s got a little ventilation issue. I’ve only got 3 days, a couple tubes of JB Weld, ratchet straps, and plywood to get it ready for Oshkosh. Am I in over my head?”
You should be suspicious of the black electrical tape over the “pressurization failure” light in the cockpit.
It was a Boeing aircraft. Always prepare to die if you are on one. That’s where the age-old saying “If it’s a Boeing, I am not going” comes from.
The old saying is “If it’s not a Boeing, I’m not going”, and dates back to when they actually had a famously solid engineering and safety culture. Reportedly that started to change after the merger with McDonnell Douglas with some old-time senior employees still holding the line, but in the decades since pretty much all of them have retired
It has been “if it’s a Boeing, then I’m not going” for decades now. It used to be what is now unthinkable, but that was a long, long time ago.
Is that back when they used to innovate and focus on world class products rather than stock buybacks and juicing the stock price for shareholders?
>on January 6, Alaska Airlines grounded all 65 of its 737-9 MAX aircraft to conduct inspections. About a quarter of these inspections were reportedly completed “with no concerning findings.” Also on January 6, the FAA issued Emergency
They Inspected roughly 16.5 aircraft in 48 hours? I have concerns about how thorough those inspections were.
You should maybe concerned about how simple the fix to the problem is that could easily have resulted in loss of life.
That could also mean that some or all of the other inspections may have concerning findings.
I’ve seen pics on fb from aviation techs, it looks like the structure is designed to be opened and inspected.
Do you care to explain what you are basing this groundless assertion on?
Before spouting off some uninformed ideas, I would recommend reading the US Code of Federal Regulations, namely 14 CFR part 39 (legal basis of Airworthiness Directives) part 65 (licensing of aircraft maintenance technicians -AMTs), part 43 (the rules for aircraft maintenance), part 121 (airline operations), part 91 (general operating rules of flight)
EVERY bolt ever touched on an airplane airframe or engine has a logged event in a permeant record of what was done with the signature of the AMT (FAA licensed A&P mechanic), with their name and license number. As the author noted, the FAA issued an emergency Airworthiness Directive, meaning the plane is not flyable until a licensed A&P performed the exact steps in the AD (the engineering parts of which were approved by the FAA).
Once the process is completed, the licensed A&P legally signs the plane off for Return to Service. His name and number are there in black and white forever. I have seen log entries from 50+ years ago, it’s no joke of an event. There is NO A&P in the world who is just signing off stuff for the heck of it. The above regulations will also point you to the severe civil and criminal penalties for falsifying a aircraft maintenance record.
Getting an A&P license is a very regulated process and to go to a school to get one is 1900 hours of timed attendance (two years, full time), and a series of difficult written and practical tests. I got a graduate degree quicker than I got an A&P.
If you really have concerns about how thorough those inspections were, read the AD and contact the FAA department AIR-520, Continued Operational Safety Branch. Every AD is open to public comment, and visit the CFR dot gov site to post your informed feedback.
The part that makes me question how they could inspect the planes so quickly and determine that there is nothing wrong is the fact that they haven’t determined what caused the failure:
>The investigation is ongoing and as of right now, a cause is not known.
Mercedes should combine her love of planes and trains to write about how Spirit gets the fuselage from Kansas to Seattle on a train that goes through tunnels with some TIGHT clearances.
Also maybe those Boeing trucks that have someone driving in the rear axle: (2) Boeing’s long-load steer car helps move large aircraft parts from Frederickson to Everett : Truckers (reddit.com)
A few years ago a train carrying 737 fuselages derailed in Montana, sending them down an embankment and marking the first ever plane crash recorded BEFORE the plane was built.
Bonus fact: the first task at Boeing’s receiving yard in Renton is to inspect those train-ferried fuselages for bullet holes. Apparently they make good rolling targets for bored Plains residents.
Bonus fact: the first task at Boeing’s receiving yard in Renton is to inspect those train-ferried fuselages for bullet holes. Apparently they make good rolling targets for bored Plains residents.
My dad once escorted a Polaris ICBM booster on a train through Texas, terrified the locals would do the same thing
So just how many corners did Boeing cut on the Max series of planes? What other issues are going to pop up in the next decade? Can we kick out the accountants and put the engineers back in charge?
Remember as passengers we’re cattle. The likely only way Boeing leadership will give a shit is if they are significantly financially impacted by additional FAA (or other flight safety orgs.) forced grounding or accidental groundings (i.e. additional crashes).
Given Boeing “business leadership” is now physically located separately from their manufacturing and engineering departments… I am curious to hear of examples of companies that have intentionally made business structure changes like this (separating their business leadership from their Engineering leadership) that have been able to successfully reversed the change to again be Engineering focused?
This sure sounds like someone forgot to install or pin those castle nuts during installation and the bolts backed out from normal vibrations.
I’ve watched quite a few Airplane Disaster shows and these types of things happen fairly often. They don’t normally cause problems due to redundancy, but every so often it will cause a big problem. Unfortunately often the production/maintenance procedures don’t always have a truly detailed checklist and things like cotter pins or locking screws/bolts can be missed. Auto manufacturers probably do a better job these days since every part is bar-coded and tracked.
Also, because Boeing is pretty much the last USA passenger plane builder, every single piece of bad news is going to involve them.
If United is finding these bolts loose on their planes it makes me think Boeing didn’t use cotter pins. I wonder how much that saved them.
https://www.click2houston.com/news/investigates/2024/01/09/loose-bolts-on-boeing-737-max-9-planes-discovered-by-united-airlines-what-attorney-is-saying-to-kprc-2-investigates/
Not enough apparently. It’s never enough savings for them.
Frankly, I can’t understand why they would use anything but a Fail Safe fastening system that couldn’t release even if the nuts/bolts were left off. Spring loaded safety pins. Not that complicated. But it would cost more…
Saving a few bucks on hardware is not really a thing in aerospace (unless you’re SpaceX). A plane delayed for delivery to the customer can cost Boeing $100,000 a day, so bean-counting at this level makes little sense. In fact, most would be amazed at ‘belt, suspenders, AND duct tape’ approach to almost anything fastened to anything on a plane. Serial number plates are riveted, adhered, and have a bead of caulk applied to their perimeter to fair them in.
More likely it was a checklist oops or faulty part, as Crank suggested.
https://sfeldmanhousewares.com/cdn/shop/products/CLEAR_01bd9861-a5ff-42d1-b49f-337a6498bdc3_500x.jpg?v=1571500816
All i can say is: “If it’s a Boeing, I’m not going! ☜ (◉▂◉ )
A rubber plane should eventually get you there. Boeing, Boeing, Boeing…
Well, at least the front didn’t fall off ヽ(•‿•)ノ
I thought that was oil tankers
It was.
Well, cardboards out
And…
No cardboard derivatives
No paper. No string. No cello tape
It could go on and on… still one of the more memorable skits there is
Goodyear? No, it was the worst! ( ͡°⊱ ͡°)
Except as I mention above, if Boeing and Airbus are the only companies making planes, every incident is going to be about them. Avoiding them means avoiding all air travel (not a bad thing).
Boeing has turned into a Sh*t show of incompetence, coverups and deceit. They only care about getting the 737’s built and delivered.
They deserve to go out of business! IMHO.
Indeed
https://www.theatlantic.com/ideas/archive/2019/11/how-boeing-lost-its-bearings/602188/
Well, there’s also Comac and United Aircraft
You are more than welcome to fly on their planes. I personally would choose a Boeing every single time over either of those manufacturers. Boeing may currently be a shitshow, but China and former USSR? No thank you.
Gulfstreams are made in Georgia.
True, but nothing very large. I would however fly in a Gulfstream. 🙂
Lots of Embraer out there that you’re forgetting.
Ironically, only Embraers fly out of PAE (as commercial, non-chartered flights), which is across the street from a huge Boeing assembly plant.
Maybe it’s only in Canada and abroad that I’ve really flown them. Plenty out there though.
I think maybe I didn’t express the thought clearly. I was saying that, as far as commercial aircraft that fly out of one specific airport (Paine Field), they are all Embraers. And that particular airport happens to be across the street from Boeing’s widebody manufacturing facility in Everett, WA.
Embraers are extensively used as regional airliners in the States.
Mercedes, if you haven’t already heard of aviation safety write Admiral Cloudberg (Tyra Dempsey), I highly, highly recommend her plane crash series on Reddit/Medium. She covers a ton of fascinating topics in depth–for example, the following article that discussed cargo door failures on the DC-10 and many of the mechanics you have mentioned here: https://admiralcloudberg.medium.com/a-legal-and-moral-question-the-crash-of-turkish-airlines-flight-981-and-the-dc-10-cargo-door-saga-d22f0b9fa689
Also, as a fellow trans person, I can’t mention her without also mentioning that she has a new podcast called “Controlled Pod Into Terrain” that is a lively and entertaining look into plane crashes. It’s hosted by a lovely group of queer folks absolutely nailing the previously unfilled niche of meme humor as it intersects with rather dry aviation safety.
I hope the poor kid at least got a free Alaska Airlines souvenir t-shirt.
Exactly! It’s the least that they could do. After all, the poor kid gave them the shirt off of his back! . .･ヾ(。￣□￣)ﾂ
Ba dum tssss https://www.fesliyanstudios.com/soundeffects-download.php?id=5893
Thank you, thank you……I’ll be here all week…….(͡o‿O͡)
And a $50 voucher for a future Alaska Airlines or Hawiian Airlines domestic flight (not valid for Alaska or Hawaii)
And a million air miles.
I love how so many firms are so heavily invested in Boeing that they didn’t allow the stock to fall like it should have after this. It dropped 8% after multiple failures of its newest airframe.
Fucking ridiculous.
Capitalism, where too big to fail is how you actually win.
Monopolies suck. Lockheed left the commercial airliner market and Boeing gobbled up McDonnell-Douglas, so here we are.
What’s great about that story is that Lockheed basically set out to make the best plane they could with the L1011 Tristar while McDonnell-Douglas phoned in certain aspects of the design on their competing DC-10, such as the cargo door that showed as being locked when it in fact was not locked. Because Lockheed and MD ended up splitting the market for three-engined planes, ironically neither ended up selling enough units to actually turn a profit on them. As a result, Lockheed gave up on civilian airliners while MD went on to pollute Boeing’s safety-first culture with its own cost-cutting culture later in the 90’s.
So in a way, I blame the DC-10 for the way Boeing is today; it screwed us out of Lockheed and then allowed MD to limp along long enough for them to then mess up Boeing.
I miss the TriStar. I’ve flown transatlantic on all three classic jumbos — 747, DC-10, and TriStar. The TriStar gave the 747 some serious competition for comfort and smoothness. And in its day, it was the most technologically advanced with an early form of auto-descent and auto-landing capability.
Yes, the TriStar versus the DC-10 is a classic case of competitors trying to take over a market that turned out to be much smaller than expected. Douglas took the bulk of the sales with a parts bin plane they could poop out for lower cost, but still didn’t sell enough to make a profit. In a fairer world Douglas would have realized they couldn’t turn a profit in that segment and the TriStar would have sold in the numbers it deserved to. I never got to fly in a TriStar, but my dad said they were a real treat. I flew in several DC-10s and MD-11s and they were about as memorable as riding a bus.
Why would they fly it when getting pressurization warnings previously ?? this a bit more than driving with a check engine light on
$$$
Yes, Alaska considers restricting the plane to airspace where the pilots have a solid chance of successfully landing in an emergency good enough. Even planes that really should be grounded can be told to get back in the skies and make daddy some money.
Wonder if Alaska Airlines and I use the same brand of black tape
I would assume a CEL on an aircraft would also be non-trivial. Perhaps a ‘low blinker fluid’ warning might be less of a worry.
…
Another article I read said the issues were reported and inspected by maintenance and no issues were found for the previous occasions. Unfortunately, the light didn’t tell them where they needed to look. If they even inspected the plug, it was probably a quick visual.
It appears that inspecting the plug means removing the interior panel, which means the seats have to go as well. Probably these things get checked when they hit a milestone that requires a fleet maintenance facility. Otherwise, Boeing screwed up the out the door final inspection before delivery.
Aviation maintenance probably was all over the outside surface of the plane previously looking for cracks or structural faults and did not find any that could have made the pressurization light come on. But we’ll see what the investigation says. Obviously, moving from Seattle to Chicago to Washington DC has not been kind to the operations of the company. Chasing Congresscritters and the Pentagon around for contracts means you’re not keeping an eye on the prize at the manufacturing facilities.
You’d think they’d have a way to just pressurize it to +4PSI or so on the ground and trace the leak, but maybe it was intermittent or flying changed temperatures and stresses just enough that it didn’t leak on the ground.
Soapy water in a spray bottle
That would be great but it sounds like a fleet maintenance ability. A plane less than a couple months old would be seen to have a ‘bug’ and according to the rules, would fly under restrictions. Civilian and military aircraft with ‘errors’, if they fit certain criteria, are authorized for flight under proscribed restrictions.
The crazy thing is I had a plane grounded for a faulty parking brake once. I can’t imagine how you let one take off with pressurization issues.