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Plane Answers: Can passengers survive an explosive depressurization?
We've all heard the standard spiel about oxygen masks and flotation devices. Likewise, we've all seen the cartoonish drawings of proper positioning of one's body in the event of an emergency (the "brace for impact" pose), etc... Two things I've heard people say are that:
a) the air temperature outside the cabin at most cruising altitudes on jet engine planes is sufficient to instantly freeze all bodies on board solid within literally seconds;
b) the change in air pressure is likely to be so disruptive to one's ear drum, putting on oxygen masks and taking the fetal position is difficult to impossible due to disorientation.
As to the first one, I've flown many a Delta flight where on screen displays indicate the temperature outside the cabin to be extremely low (far far below zero). Likewise, I recall reading an article about a jet crash in Greece (I think) where the plane was supposedly depressurized in flight and crashed into a mountain. The report indicated that rescue workers arrived in a relatively short time, but everybody on board was in fact frozen solid. The report indicated this happened in the air, and w/in seconds of depressurization, not on the ground. So there seems to be some credence to this one.
As for the second one, about air pressure and disorientation, I'm of the understanding that though the need arises very rarely, passengers have been able to take action to put on oxygen masks when necessary. Of course, I don't know how many times (if ever) that need has arisen when at any significant altitude.
So are these frequent flyer myths, exaggerations, based in some fact or accurate descriptions of the reality of jet travel?
The most common cause of depressurization on an airplane is from the loss of both of the air conditioning and pressurization "packs." There are two of these units that pressurize the cabin on all airliners and one of them is allowed to be inoperative, although it's not a common occurrence. Should the airplane lose the remaining pack, the cabin altitude, which normally allows for a comfortable 6,000 feet when the airplane is flying above FL 300 (30,000 feet), will slowly climb to the same altitude the airplane is flying.
So it's imperative that the pilots descend below 14,000 feet, the altitude that the masks will deploy, as soon as possible and to level off at 10,000 feet or lower.
This situation recently happened to my brother. He was able to descend to a lower altitude and the cabin altitude never exceeded 10,000 feet, so no passenger masks dropped from the ceiling.
In the case of an explosive depressurization, like that of Aloha flight 243, these masks will be extremely important. Those passengers as well as the people aboard a United 747 that lost a cargo door, were able to don the masks and remain warm enough to survive until the airplane reached a lower altitude. Both those cases were near Hawaii, however. So it could be a rather cold descent anywhere else. But the initial explosive depressurization didn't result in so much disorientation that they couldn't put their masks on.
And you're right, it's common to see minus 40 to minus 60 degrees Fahrenheit (-40 to -50 Celsius) when at altitude. At temperatures of minus 40 (C or F), skin freezes almost instantly, although the temperature warms quickly as you descend.
Finally, the Greek Helios 737 flight that you mention was never pressurized after takeoff, because of a mistake that was especially tragic. The pilots inadvertently departed without noticing the pressurization controller was in the manual position. They missed the ear-popping cues, the temperature cues, the warning lights on the overhead, and they misdiagnosed a cabin altitude warning horn for the horn that notifies pilots that the airplane is unsafe for takeoff because of incorrectly configured flaps, trim or speedbrakes. Interestingly, the sound of the horn is identical in both situations.
On a side-note, I've talked to the Boeing engineers who worked on an early version of a 'text message' system called CPDLC that air traffic controllers can use to provide instructions to pilots. I asked these engineers what sound they would be choosing to alert the pilots of an incoming message.
As I suspected, they explained that they would be using the same sound that flight attendants use to call the pilots. And that chime is used for FMC wind and route uplink notifications among other things. They claimed that studies have shown that people have difficulty differentiating between more than five types of sounds.
The Helios pilots failed to understand this warning horn and subsequently failed to don their masks, resulting in the masks dropping in the back of the airplane while the pilots were trying to simply silence the warning horn.
Oxygen is vital for a pilot to be able to troubleshoot an abnormal situation as this amazing recording between a cargo flight that lost pressurization and air traffic control demonstrates. Note the altitude warning horn in the background of this ATC tape with the flight:
On a similar, but far less morbid topic, Steve asks:
What is the average temperature inside commercial airliners? I was told 82 degrees F by a pilot who was seated next to me in first class. This is to put everyone to sleep. At 35,000 ft. the temperature outside is -60 F, correct?
Yes, it's often nearly that cold, as I mentioned above. According to our indications on the Boeing, we shoot for around 70 to 72 degrees. Unfortunately, it's very difficult for that indicator to be perfectly calibrated. And when the flight is nearly full, pumping 70 degree air into the cabin can be too warm. Fewer passengers on board means we need to increase the selected temperature.
But by far the biggest driver of the temperature is the flight attendant. Typically they like it a bit cooler while they're working hard to get a meal service accomplished, and afterwards, when they're not as active, they'll need it to be warmer.
So on your next flight, see if the first part of the flight, during the meal service, is cooler than the latter part.
If it were up to the pilots, the controls for the cabin temperature would be in the back, with the flight attendants. The 777 has some control over the temperature provided to the flight attendants, resulting in far fewer calls to the pilots asking for warmer or cooler temperatures.
And contrary to the belief by some cynics out there, we're definitely NOT keeping the cabin cooler to sell more blankets.
Do you have a question about something related to the pointy end of an airplane? Ask Kent and maybe he'll use it for the next Plane Answers. Check out his other blog, Cockpit Chronicles and travel along with him at work. Twitter @veryjr
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