HkCaGu

Your phone doesn't have a BAROMETER to measure pressure i.e. "cabin altitude". It likely has a GPS that measures actual altitude.

Wrong. Cabin pressure is usually set to a maximum/constant differential with the outside. During cruise phase, you can detect altitude changes by looking at pressure changes. In my experience, in FL280-FL340 you'd usually see pressure altitudes 4000-6000 feet. The 8000 feet isn't usually seen until the plane reaches FL390-FL410 or the service ceiling.

In my experience (been wearing a Casio triple sensor for decades on hundreds of flights), the worse is ERJs where it easily gets to 8K. I actually had a medical emergency landing flying AA/MQ ELP-LAX nosediving into PSP because someone fainted in the aisle.

PacificWave

[QUOTE=HkCaGu;25313049]Your phone doesn't have a BAROMETER to measure pressure i.e. "cabin altitude". It likely has a GPS that measures actual altitude.

My phone has very sensitive and reasonably accurate barometer, along with GPS, which reports actual altitude of the aircraft more or less.

The captain of my LAX OGG flight kindly called out cabin pressure for me over the PA as being around 6000 ft (the analog gauges in the cockpit are approximate) when level at 37000. My phone GPS showed around 37800 and the barometer showed 5780. So close enough.

HkCaGu

[QUOTE=PacificWave;25313556]I had never heard a barometer as a feature advertised, but it appears that the Galaxy series has it to aid the GPS!

So as to GPS, planes fly at standard flight levels (FL) based on standard pressure or altimeter altitude based on designated ground pressure reading. Both are not directly tied to "real" altitude. In a warmer-than-standard atmosphere (most of the time LAX-OGG), your real altitude will be higher than standard, above or below FL180.

As to the barometer/barometer, I wonder if it is something you can reset (like Casio or similar watches) as pilots set their altimeter. This can explain the difference between standard pressure altitude and whatever your device says.

PacificWave

http://1drv.ms/1hUoI25


Check this out... click on link above... from the cockpit of AA 161 LAX-OGG, and thanks to two great pilots of American Airlines who spent a few minutes going over the functioning of the cabin air pressure systems. They were very pleased that forum readers would be interesting in these arcana of airplane operation.

On the left, is the difference, in PSI, between inside and outside air pressure; in the middle, the cabin altitude, and on the right, the rate of change of cabin pressure in thousands of ft per minute. 8 psi pressure difference between inside and outside gives about a 6,000 cabin at 34,000 ft, approximately. At 45,000 ft, 9 psi will give a 7,000 cabin, and per the yellow and approaching red zone on the gauge, that's going to be about it.

They are mainly automatic, so apparently the crew on a 757 can't adjust the rate of change. If the cabin pressure starts heading towards 10,000 ft, an alarm goes off, and at 12,500, the oxygen masks fall out, although the pilots would have been wearing them earlier.

For very high altitude airfields, such as in Bolivia or Tibet, the pilot can manually drop the cabin air pressure so that it matches the very low outside air pressure. This prevents the doors from blowing open.

Per HkCaGu, the GPS altitude always showed higher than the nominal FL standard pressure altitude, just as you pointed out. Of course, from FL 370 the aircraft may gradually float up to FL 390 as fuel burns off (42,500 pounds required for the TransPac flight; leaving around 12,000 pounds, which is enough to divert to any other Hawaiian island and then some), but the GPS definitely showed the "real" altitude to be higher than the FL nominal altitude.

So it look like the GPS was working accurately after all. Looking at the Cabin Altitude dial, it only shows approximately values, so the phone barometer reading of 5,780 while the captain said 6,000 could be quite accurate. No way of knowing, really.

According to a Wiki on the topic, the A380 has a cabin altitude of 4,990 ft at 35,000; 787 is around 6,000. Boeing said that they experimented with various cabin altitudes, and there wasn't much perceived benefit with lower pressures; however, higher humidity was greatly appreciated.

Aircraft relative humidity is very low, often around 4-5%, with the minimal comfort level being about 30%. Economy humidity levels will be perhaps 10-15% (more sweating bodies, and glasses of beverages), while in first, 5%. The 787 apparently supplies 15% along with HEPA filters. Real humidity levels seem hard to find directly from manufacturers, and there are lots of complicating options available...

Airbus 350 offers humidification options for first and business to increase levels to 20-25%. Increased cabin humidity requires dryers to get rid of condensation on the fuselage. Both the water and the dryers add weight...

Insulator-King

Great info PacificWave.

When I was an NFO in Naval Aviation, I learned to 'valsalva'. That is the method of opening up the inner ear to the sinuses. As mentioned, most people do it by closing their mouth, holding their nose, and [gently] blowing out.

You can feel your ears pop when you do it. Others have recommended vigorously chewing gum.

Kevin AA

I wish there was a way for planes to fly from one sea level airport to another (e.g., JFK-LAX or LAX-HNL) with a cabin altitude of 1,000 feet. Would the plane explode? I don't have ear problems but I don't like the pressure changes either.

fpc552

Driving up into the mountains is probably not a good proxy, simply because climbing from sea level to 6000' on a mountain road takes at least an hour, while in a pressurized aircraft, it just takes 10 minutes.

If you really want to do this experiment (and cheaply), I'd go my local flight school and book a flightseeing trip. You can probably get exactly what you'd need for around $100. Explain to the pilot what you're asking. Most light aircraft used for training and flightseeing are unpressurized, so all you'd need to do is takeoff, climb to 6000' or 8000' and fly back.

There are also physicians who subspecialize in aerospace medicine and undersea and hyperbaric medicine who might be good to talk to about middle ear problems during air travel.

fpc552

The aircraft hull is designed for a certain pressure differential between the outside and inside. To contain and higher differential would require more structure, thicker skin, more weight, less fuel efficiency.

Of course, you can maintain a sea-level cabin by flying at a lower altitude (say, 10,000'), but that also requires more fuel.

Kagehitokiri

wow, great post

although link requires login?