I read "bridge clearance" as clearance from the ATC to detatch from the bridge and was so confused lol
this post was submitted on 04 Aug 2025
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xkcd
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Sometimes it do be like that
I was on a flight to Richmond, VA, many years ago. Delayed delayed, mechanical something. We finally board and depart, and right in the middle of the climb after takeoff, the plane turns real hard and heads back to the airport. People were kind of freaked out, they weren't telling us anything.
Back in the airport, we learn that there was some issue with the cabin pressure. They thought they had fixed it, they hadn't. Eventually, they decided, "Fuck it, we're going to fly across the country in this 737 at 7000 feet." Maybe a third of the passengers got back on the plane, I was one. It was cold as fuck.
"How many of y'all haveheard of the Dambusters?"
why was it cold, because they couldn't maintain an internal cabin atmosphere, including temperature?
Also, I had to look it up, but just wanted to confirm 7000 feet is much lower than typical 737 cruising altitude, which is usually 30,000 - 40,000 feet.
7000 feet is still pretty high and around where oxygen saturation decreases - could you tell any effect? I just assume they were able to still oxygenate the cabin even if they couldn't go as high 🤷♀️
I frequently fly at 10 to 12 thousand feet msl and have no issues. Supplemental O2 is only required from 12500 to 14000 for durations longer than 30 minutes. It is required above 14k for the crew according to the regs. At 15k, everyone on board needs oxygen or a pressurized cabin.
why was it cold, because they couldn’t maintain an internal cabin atmosphere, including temperature?
No pilot or Jet mechanic but afaik the pressurisation is happening with bleed air from the turbines that is then cooled down to comfortable levels. So there is no "heater" for the cabin so to speak as for a warmer cabin you just cool down the bleed air less. As such without pressurisation there is also no cabin heat.
You just causally knew that offhand, and it's not even your industry??
I am weirdly nerdy about planes. Always wanted to be a pilot as a kid.
prolly went down a wikipedia rabbit hole or something
7000 ft is relatively common for small planes like cessnas, they don't even keep pressurized cabins, but It's fine if you stay low. So there's nothing wrong with that altitude, but it's awfully low for a jet. The jet will be flying at low fuel efficiency the whole way and it certainly won't have much wiggle room if something really goes wrong. (You can lose 7000 ft of altitude very quickly)
ah, I wasn't sure because I was reading this:
The oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around 2,100 metres (6,900 ft) above sea level, the saturation of oxyhemoglobin begins to decrease rapidly.[2] However, the human body has both short-term and long-term adaptations to altitude that allow it to partially compensate for the lack of oxygen. There is a limit to the level of adaptation; mountaineers refer to the altitudes above 8,000 metres (26,000 ft) as the death zone, where it is generally believed that no human body can acclimatize.
https://en.m.wikipedia.org/wiki/Effects_of_high_altitude_on_humans
it seemed like between 7,000 and ~~8,000~~ 26,000 the air is not suitable, but I figured the pilot wasn't likely to be taking risks like that, so there must be some explanation
edit: mixed up feet and meters
You're mixing up feet and meters. The death zone is at 8 km, i.e. 26k ft.
2100m is barely mountaineering, you can bring grandma and the newborn hiking there and at most you'll notice a mild shortness of breath.
In fact normal cabin pressure at cruising altitude is equivalent to 7000 ft. Besides a lot of ear popping most people don't even notice it, though mild altitude sickness (i.e. a small headache) is possible, but ultimately harmless.
ah, that explains my confusion - thank you for clearing that up!
7-8k is fine. There are many towns in the U.S. at or above that elevation.
it seemed like between 7,000 and 8,000 the air is not suitable, but I figured the pilot wasn't likely to be taking risks like that, so there must be some explanation
Yeah, I mean the atmospheric pressure at altitude varies, it's definitely not black and white. And it's true, pressure does start to really drop off around 7000ft, but it's just starting to drop significantly at that point, that just makes 7k a good ceiling.
So yeah, this all seems pretty much consistent.
It also seems low for crossing mountains.
~~actually, cabins are typically pressurized to around 10,000 ft, not sea level, iirc. so this was actually a higher pressure than is typically experienced in a pressurized cabin.~~
I'm wrong
I got mixed up. the limit for unpressurized cabins is 10,000 ft, so if you're flying a small plane without supplemental oxygen you can't go higher than that
Do you have a source for this? My only reference here is hiking at > 10 000 ft (3000 m), and from that I can say that this seems very unlikely: If you stay at 3000 for a couple hours without acclimating first, you will definitely start to feel the effects. To be fair, you're usually not moving around a lot in an aircraft, but a couple hours at 3000 m can make you feel sluggish and weak, and even a bit light-headed, you could even get a mild headache from oxygen deprivation.
Note that not everyone will see severe symptoms already at 3000 m. Plenty of people can go to 4000 m before seeing significant symptoms. However, given that I've never heard of anyone experiencing altitude sickness in a properly pressurised aircraft, it seems unlikely to me that they're pressurised to 3000 m.
I'm wrong
just assume they were able to still oxygenate the cabin even if they couldn't go as high
Aircraft are typically pressurized to the equivalent of about 5000-7000 feet altitude.
They do not oxygenate the cabin. The oxygen supplies on board are usually in the form of chemical generators, sometimes known as "oxygen candles", and can only provide about 15 minutes supply. That should be plenty of time to descend below 10,000 feet, where everyone can come off oxygen. They don't "burn" those chemical generators except in actual emergencies.
Pilots and crew have a sufficient supply of bottled oxygen. Pilots and crew are required to go on oxygen if they spend more than 30 minutes above 12,500 feet cabin pressure, or any time over 14,000 feet. Passengers are required to be on supplemental oxygen above 15,000 feet cabin pressure.
Above 35,000 feet flight altitude, at least one pilot must either be on oxygen, or have a mask that can be donned in less than 5 seconds.
The oxygen requirements only come into play when you fly depressurized. The reason that passengers aren't on oxygen when you are flying pressurized at 30,000 feet (probably 5-7,000 ft pressure altitude inside the plane) is that the air they are pressurizing (thin, high altitude air) is still the same ratio of nitrogen/oxygen/CO2 even though there are smaller amounts of each.
The FAAs requirement of people being on supplemental oxygen only matters in non-pressurized flight, like small Cessnas and pressurized aircraft experience a depressurization emergency.
The oxygen requirements only come into play when you fly depressurized.
That is true for the 12,500, 14,000, and 15,000 feet cabin pressure numbers I gave, yes.
However, the oxygen requirement associated with the 35,000 foot number I gave applies to both pressurized and unpressurized flight. If your aircraft is at or above 35,000 feet pressure altitude, at least one of the pilots must be on oxygen, or must have a quick-donning mask available at all times.
No oxygen problems that I recall. I don’t think there’s a hypoxia risk until 10K feet? Cold I’m sure because it was like flying with a window open. The heaters for the cabin air probably couldn’t keep up.
The heaters for the cabin air probably couldn’t keep up.
This made me curious if planes use "free" excess heat from the jet engines, same as a car would, or if they need to burn fuel to heat the cabin. And it turns out the latter is the case, so not only could the heaters probably not keep up, attempting to even do so would have wasted a ton of fuel.
Fuel that was already being burned at a high rate because of flying down in thicker air.
Hypoxia risk is usually around 14k feet. 7k feet is like skiing town tree line altitude, cold as fuck but not dangerous. Fascinating story, thanks for sharing.
Anecdotally, the highest altitude I've ever been at hiking was around 11,000ft, and the group I was hiking with started feeling it a little bit around 9000-10,000 ft.
It wasn't dramatic, where we really noticed it was after we'd made camp, we had a little downtime and there was a bit of an open area, so we started throwing around a Frisbee, and running around chasing the Frisbee we could feel we were getting winded a bit quicker than we usually would.
Age, fitness, genetics, etc. will of course factor into that, but I suspect that most reasonably healthy people sitting in an airplane probably wouldn't feel much at 7000ft. Maybe they would if they were jogging up and down the aisle of the plane, but even then it probably wouldn't be anything too obvious, and if they didn't know anything about altitude sickness they'd probably chalk it up to being tired from the mental stress of air travel- getting to the airport, making the connection, security, lost baggage, etc.
I was in Ecuador recently, Quito which is 9350ft. The second the plane door opened, I felt it. Leaving the plane the air was so thin I could barely stand up. The altitude sickness was alot more for me than "being tired". And I had started altitude sickness pills 2 days before my flight.
and the group I was hiking with started feeling it a little bit around 9000-10,000 ft.
Hiking. Physical activity. You'll feel it a lot sooner than if you're just sitting in your seat, bored out of your skull.
Legally, the FAA doesn't require passengers to be on oxygen until cabin altitude is above 15,000 feet. Most aircraft are pressurized to the equivalent of 5000-7000 feet pressure altitude.
I think it was a co-worker who told me about flying once from one of the main NYC airports down to Raleigh or Charlotte and they’d been delayed, delayed, delayed by weather. Finally the pilots decided to Leroy Jenkins it and do the entire flight under 10k feet and using the weather radar to maneuver around storms that were dotting the east coast that day. I’m sure it was more formal than I’m remembering but he said it was by far the most unusual commercial flight he’d ever taken. This was within the last 15 years or so, well past when that was common.
Did you get a window seat?
Yeah, I will need an explanation on this.
Do they plan to taxi all the way to the destination? (Are those bridges clearance charts about height clearances for trucks on the kinds of bridge that passes over a road?)
You know, those jet engines are LOUD. Sure, they can technically move the plane on regular roads too, but the noise is just astounding.
In a big airport you might not have noticed, because you were either in the terminal or inside a plane when that happened. However, if you’ve visited a smaller airport, you may have stood just outside the fence when someone reved up their jet engines. Imagine that, but even louder, and moving on normal roads. Should be fun for everyone.
Sure, they can technically move the plane on regular roads too
Well, not really. Or technically, they can if they move fast enough. If they stop, wherever they are will stop being a road quickly.
And there's that entire problem of making room for the wings. It's one of the largest problems when designing airports.
"Beep beep I'm a plane!"
Yes, the wings are a huge problem. Not that many roads can handle something as wide as a plane.
Imagine that, but even louder, and moving on normal roads. Should be fun for everyone.
Of course it'll be fun for me. Im in the airplane, getting to my destination on time.
If it were the sound of the Concorde, may she RIP, I think it’s a loud I could tolerate. Sonic booms from like a city block away would relax my nerves.
That's how I understand it. Return to bus.
They might be referencing this American Airlines “flight” that is actually a bus.
https://www.nerdwallet.com/article/travel/american-airlines-bus-service-landline-guide
Yes and yes.
11-foot-8! 11-foot-8!
plane arrives at its gate without the top two feet of its tail fin
Can opener bridge ~~strikes~~ is struck again!
Yeah, drive the plane on the ground since the storm makes it hard to fly!
I think they plan to ignore ATC, turn up their engines and fly away
Haha, this is just about the Charlotte airport...
About to go through there tomorrow. Is it as bad as I've heard? Only flown twice before (to Mexico and back from Tennessee), went through Knoxville and Atlanta on the way in, then Detroit and Knoxville on the way back.
I see Randall has been listening to Cabin Pressure.