503
xkcd #3124: Grounded
(imgs.xkcd.com)
Title text:
We should have you at the gate in just under two hours--two and a half if we get pulled over.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3124/
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 🤷♀️
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 casually 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:
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.
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.
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.
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.
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.
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.
~~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
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.