Having done a fair bit of air travel over the winter, I have been wondering about the following questions :
Is there any regulation as to what the minimum cabin air pressure needs to be during normal operation of passenger aircrafts ?
At what cabin pressure do oxygen masks deploy ? Do they deploy automatically like air bags in a car ? Or does the aircraft computer gives the alarm/recommendation to the pilot who then decides whether to deploy them or not ?
Is there a government approved pressure monitor & recorder that a passenger can buy and carry to record cabin air pressure for the entire duration of the flight ?
Pilot here, but not of pressurized planes (more Cessnas, Beeches, etc.).
I have a notion about your first two questions, but will wait for the board’s experts to weigh in. Hopefully they’ll be along soon.
As to your third question: I often fly at high altitudes in unpressurized planes wearing O2 cannulas, and instead of the cabin pressure, I’m concerned about oxygen saturation in my blood. So I wear one of those fingertip devices while up there, and monitor to ensure it never drops below 90% (number advised by my Doc.). It might be that a pulse oximeter is what you really need. Any device that measured ambient pressure would need set before flight, and continually updated along the path to be accurate. And probably would generate a lot of interest from security before boarding.
An answer to your last questions: many, perhaps most, smartphones have a pressure sensor (it makes the GPS navigation more reliable). To use it you will need a specialized app, take a look at androidappsforme.com/barometric-pressure-apps/
Common electronic “weather stations” (you can search for it om Amazon) will tell you the pressure, but won’t make a recording of it.
Government approved? Why should government be bothered with these gadgets? A government approval does not really tell you what you want to know. If you are concerned about the accuracy of your measurements, you could spend some serious money on professional scientific instruments, and learn how to use them. That will give you lots more information than an official blessing.
The Boeing 787 can boast a 6000’ cabin altitude partly because its carbon fiber fuselage can withstand resulting stress without being too heavy. And I understand some bizjets can be pressurized to a cabin altitude of 4500’.
As to question 2. Automatic deployment or the crew can order it. Also extreme turbulence or hard landings can warp and dislodge the mask compartment doors. Some aircraft have the crew hand out the masks which are then plugged into a receptacle.
Note that you are not getting compressed oxygen; the oxygen is from a chemical generator. See wiki for an overview on oxygen systems.
I’ve flown two that could reach 45000’ and the cabin would pressurize to around 8000’. My current jet’s ceiling is 41000’, and I usually see an 8000’ cabin as well.
To the OP, the masks in my plane will drop automatically at a certain pressure which I’m supposed to know and escapes me at this moment. But I’m not working right now, so I’m not looking it up! That assumes the arming switch is in the correct position. Said switch can also be used to drop them manually.
One of the jets I’ve flown had the manual drop switch in a bad spot, near the pilot’s arm. And it was a pull-up switch. So a common embarrassment was for the pilot’s headset cord to catch under the switch and activate it, leading to a momentary mayhem in the cabin. This was mentioned to me on day one of training, so I never did it, thankfully.
The following story might be an urban legend, but I once read a somewhat amusing story about a certain airline (American Airlines if I remember correctly) flying their inaugural flight to El Alto Airport near La Paz, Bolivia, which is one of the highest altitude commercial airports in the world at 4,061.5 m (13,325 ft) above sea level per Wikipedia. This supposedly is actually above the altitude at which the oxygen masks automatically deploy, so when they parked at the terminal and opened the door, the masks all deployed. According to the story they had to specially modify the planes used on that route to prevent this from happening every time.
Although now that I think about it, planes have sensors that detect when there’s weight on the landing gear, right? So the plane’s computers would know that it’s on the ground, and I would think it wouldn’t deploy the masks while on the ground, so that casts a bit of doubt on that story. Is that how it actually works, or would the masks deploy at a certain altitude no matter what, even if on the ground at an airport high in the mountains?
I’ve flown out of El Alto a couple of times but hadn’t heard of this. I also flew in and out of Cuzco on a business jet (a Gulfstream G4), which is nearly as high—11,200 feet vs El Alto’s 12,300 feet. I was in a jump seat with a headset on (I’m a big plane nerd and the pilots were nice) and I don’t recall anything but the standard landing checklist items. And I happen to know for sure that the G4 wasn’t “modified” in any special way prior to that trip.
I’m sure the commercial pilots on this board can say a lot more about this, but my limited experience makes me think this is an urban legend.
The part about “special modifications” seems especially suspect to me. I mean, maybe somebody screwed up a checklist on that inaugural flight (thereby causing the masks to drop after landing) but I’d be surprised if there was more to it than that.
Some modern aircraft avionics will have a mode for high-altitude airports. My last jet had this, and I don’t remember the details. But I think it involved a different pressurization schedule and a few other items - could have involved the masks maybe. But again, I’m on my off days and not cracking open the manuals.
Yes, that really sounds like an urban legend.
The automatic oxygen mask systems need to be turned on to be activated by low pressure. And it seems likely that they are not normally turned on during takeoff, but only after the plane is airborne. Similarly, I’d think they would be turned off before landing.
After all, fire is one of the big risks in an airplane accident, and you wouldn’t want to have any source of extra oxygen available during either takeoff or landing. Pilots have checklists they follow to turn various systems on and off; possibly one of the pilots on the SDMB can tell us just when this occurs.
I have been testing aircraft oxygen systems for over 30 years. mostly on the 737 but all passenger aircraft operate in the same manner. There are 2 ways to activate the oxygen system, automatic or manual. The manual method is easy, someone in the flight deck flips a switch, some relays close and down come the masks. The automatic system works off of a vacuum switch set to close at an set altitude of around 14,000 feet, plus or minus 250 feet. We use a certified vacuum pump to test the system.
As far as the urban legend above, there are aircraft designed to land at high altitude airports without deploying the oxygen masks. On these aircraft the oxygen system is tied into the aircraft air/ground system. While in the air, the oxygen system is activated but will not deploy unless the airplane loses cabin pressure. As soon as the airplane touches down on the runway, the plane goes into ground mode and the oxygen system is disabled. The flight crew can also turn off the oxygen system by pressing a switch.
I use to hate testing the high altitude oxygen systems, the tests took about 12 hours to complete. They are gaseous systems and don’t use cannisters. That means pressure testing the system before the electrical portion of the test. The normal oxygen test on a 737 can be done by 2 people in 15 minutes. Drop the left side onto the test stops, drop the right side on the test stops, close a couple doors on PSU’s on each side then perform the auto portion of the test. Then relatch all the oxygen doors.
That assumes the arming switch is in the correct position. Said switch can also be used to drop them manually.