3 ms·

You can't really go above the Armstrong limit (~60k feet) in a passenger airplane without adding huge complexity in handling the event of cabin pressure loss.

You can't really go above the Armstrong limit (~60k feet) in a passenger airplane without adding huge complexity in handling the event of cabin pressure loss. https://en.wikipedia.org/wiki/Armstrong_limit https://en.wikipedia.org/wiki/Armstrong_limit

taf2 7y ago
I see that does making exceeding 60k pretty scary. I was from London to Denmark and we had cabin pressure loss I think we were maybe at 30k and dropped pretty fast to 20 if I recall correctly. The sensation that I remember was slow motion and shock from the flight attendant as the masks did not deploy. Later after landing the captain told us during preflight they set the cabin pressure incorrectly such that as we went too high an emergency system kicked in the prevent the cabin from exploding... I was maybe 12 so the details are fuzzy but definitely the take away is cabin pressure loss is no joke
- sokoloff 7y ago
  Hard to puzzle the details there without knowing the aircraft type, but in the pressurized piston plane I fly, there is a main outflow valve used to regulate pressure that's under my (indirect) control and a safety outflow valve that's set just over the rigged max pressure differential (such that it partly dumps the cabin if that pressure is exceeded). Given that your flight didn't get the "rubber jungle" of deployed masks, the cabin pressure probably never dropped precipitously low, but rather started to oscillate as the safety valve dumped and closed, but never dumped enough to deploy the masks and the crew declared and descended in order to sort things out. A small number aircraft (non-airliners) are lost each year due to pressurization issues. It's a serious business, even in the high 20s and 30s (of 1000' MSL). Single pilots are required to continuously wear O2 mask at/above FL350 (35000') and one pilot of a two pilot crew must continuously wear O2 over FL410. Between FL350 and FL410, two pilot crews may rely on quick-donning masks. This is believed to be a commonly violated regulation (in that air crews regularly do not wear the required mask when things are operating smoothly).
  - VBprogrammer 7y ago
    > This is believed to be a commonly violated regulation I believe that is an understatement. I've heard it described as the most commonly violated rule in aviation. There are a number of youtube pilots who commonly violate this rule seemingly without too much worry.
Robotbeat 7y ago
Pressurized cabins are going to be essential for electric flight to have any kind of reasonable speed (and range). It shows that the most impressive electric aircraft specs are for the pressurized Eviation Alice (1000km, minus margin for contingency, and 250 knots cruise). If you stay low enough to not require pressurization, then you have to compromise the lift to drag in order to have a decent cruise speed or you have to tolerate a really low cruise speed. Altitude is essential. Bite the bullet and build a pressurized cabin so we can get on with replacing fossil fuel aviation with full electric. https://www.eviation.co/alice/ https://www.eviation.co/alice/ Doesn't have to be above the Armstrong Limit, but it sure does help to be above 10,000 feet. Eventually we'll have supersonic electric aircraft. To have sufficient efficiency, they'll need to be at or above the Armstrong Limit, like Concorde. (And perhaps higher, like the 96,000 feet record holder for horizontal powered flight, NASA's Helios... which just happens to be electrically powered. https://en.wikipedia.org/wiki/Helios_Prototype https://en.wikipedia.org/wiki/Helios_Prototype .) EDIT: High altitude enables you to use an extremely efficient airframe with sailplane-like lift to drag but STILL achieve high cruise speeds. For instance, the Perlan II glider actually has no engine and is able to soar higher than any towplane, above 76,000 ft where it flies at about 250 knots (actual airspeed). Without any engine at all. https://www.youtube.com/watch?v=NnpE5xS1g80 https://www.youtube.com/watch?v=NnpE5xS1g80 And here's a solar electric aircraft aiming for similar goals: https://www.solarstratos.com/en/challenge/ https://www.solarstratos.com/en/challenge/
- semi-extrinsic 7y ago
  All passenger jets have pressurized cabins, but they are working at lower pressure differentials than what you need above 60k feet, and they sometimes fail, with oxygen masks being the fail-safe. If you go above 60k, and the plane experiences a rapid loss of cabin pressure, you have 60 seconds to restore cabin pressure before the passengers start dying. So the failsafe system will have to be massive. That increase in weight and complexity isn't worth the efficiency gains. You mention Concorde, and indeed it had a very substantial failsafe system even though it only touched the lower end of the limit. Concorde had really small windows, so even with two windows gone it took some time to equalize pressure. The pilots had positive pressure oxygen masks, and the plane had the ability to drop altitude immensely fast in an emergency.

taf2 7y ago

I see that does making exceeding 60k pretty scary. I was from London to Denmark and we had cabin pressure loss I think we were maybe at 30k and dropped pretty fast to 20 if I recall correctly. The sensation that I remember was slow motion and shock from the flight attendant as the masks did not deploy. Later after landing the captain told us during preflight they set the cabin pressure incorrectly such that as we went too high an emergency system kicked in the prevent the cabin from exploding... I was maybe 12 so the details are fuzzy but definitely the take away is cabin pressure loss is no joke

sokoloff 7y ago
Hard to puzzle the details there without knowing the aircraft type, but in the pressurized piston plane I fly, there is a main outflow valve used to regulate pressure that's under my (indirect) control and a safety outflow valve that's set just over the rigged max pressure differential (such that it partly dumps the cabin if that pressure is exceeded). Given that your flight didn't get the "rubber jungle" of deployed masks, the cabin pressure probably never dropped precipitously low, but rather started to oscillate as the safety valve dumped and closed, but never dumped enough to deploy the masks and the crew declared and descended in order to sort things out. A small number aircraft (non-airliners) are lost each year due to pressurization issues. It's a serious business, even in the high 20s and 30s (of 1000' MSL). Single pilots are required to continuously wear O2 mask at/above FL350 (35000') and one pilot of a two pilot crew must continuously wear O2 over FL410. Between FL350 and FL410, two pilot crews may rely on quick-donning masks. This is believed to be a commonly violated regulation (in that air crews regularly do not wear the required mask when things are operating smoothly).
- VBprogrammer 7y ago
  > This is believed to be a commonly violated regulation I believe that is an understatement. I've heard it described as the most commonly violated rule in aviation. There are a number of youtube pilots who commonly violate this rule seemingly without too much worry.

Robotbeat 7y ago

Pressurized cabins are going to be essential for electric flight to have any kind of reasonable speed (and range). It shows that the most impressive electric aircraft specs are for the pressurized Eviation Alice (1000km, minus margin for contingency, and 250 knots cruise). If you stay low enough to not require pressurization, then you have to compromise the lift to drag in order to have a decent cruise speed or you have to tolerate a really low cruise speed. Altitude is essential. Bite the bullet and build a pressurized cabin so we can get on with replacing fossil fuel aviation with full electric. https://www.eviation.co/alice/ https://www.eviation.co/alice/ Doesn't have to be above the Armstrong Limit, but it sure does help to be above 10,000 feet. Eventually we'll have supersonic electric aircraft. To have sufficient efficiency, they'll need to be at or above the Armstrong Limit, like Concorde. (And perhaps higher, like the 96,000 feet record holder for horizontal powered flight, NASA's Helios... which just happens to be electrically powered. https://en.wikipedia.org/wiki/Helios_Prototype https://en.wikipedia.org/wiki/Helios_Prototype .) EDIT: High altitude enables you to use an extremely efficient airframe with sailplane-like lift to drag but STILL achieve high cruise speeds. For instance, the Perlan II glider actually has no engine and is able to soar higher than any towplane, above 76,000 ft where it flies at about 250 knots (actual airspeed). Without any engine at all. https://www.youtube.com/watch?v=NnpE5xS1g80 https://www.youtube.com/watch?v=NnpE5xS1g80 And here's a solar electric aircraft aiming for similar goals: https://www.solarstratos.com/en/challenge/ https://www.solarstratos.com/en/challenge/

semi-extrinsic 7y ago
All passenger jets have pressurized cabins, but they are working at lower pressure differentials than what you need above 60k feet, and they sometimes fail, with oxygen masks being the fail-safe. If you go above 60k, and the plane experiences a rapid loss of cabin pressure, you have 60 seconds to restore cabin pressure before the passengers start dying. So the failsafe system will have to be massive. That increase in weight and complexity isn't worth the efficiency gains. You mention Concorde, and indeed it had a very substantial failsafe system even though it only touched the lower end of the limit. Concorde had really small windows, so even with two windows gone it took some time to equalize pressure. The pilots had positive pressure oxygen masks, and the plane had the ability to drop altitude immensely fast in an emergency.