8
$\begingroup$

I have done research into the question of pressure differences between the cabin and cockpit and the theoretical answer I receive the most is that the pressure difference would equalize almost instantly, as cockpit doors are not capable of an airtight seal — and it makes sense, but I recently came across an incident that happened a couple years ago: Sichuan Flight 8633.

On that flight, the windshield actually exploded, and based on the passenger recordings, the cabin seemed a little messy from objects flying around, but they could still hold onto their phones and the flight attendants were able to stand up.

The only comment I could find on that matter had to do with insulation and sealing that protected the passengers from the low temperature and pressure loss. So with that information in mind, is it possible for a cockpit to experience extreme loss of pressure while the cabin suffers minor loss of pressure as on Flight 8633 (an A319)?

$\endgroup$
2
  • 1
    $\begingroup$ As far as stews standing up or passengers holding their phones, that wouldn't be effected by pressure loss, but by wind. Since the cockpit door held enough to keep the temp in the main cabin up, it was also resistant enough to wind to keep from blowing things (or people) around. $\endgroup$
    – FreeMan
    Apr 28 at 14:29
  • $\begingroup$ As the answers have pointed out, the premise of this question is incorrect: there was no pressure differential on flight 8633. $\endgroup$
    – Ian Kemp
    Apr 29 at 11:37
18
$\begingroup$

The internal walls in an aircraft are not designed to withstand any significant pressure difference. Since the accidents of American Airlines flight 96 and Turkish Airlines Flight 981 where explosive decompression in the cargo hold caused the passenger cabin floor to partially collapse and damage the control cables running below it, the internal walls and floors are intentionally designed so that the pressure has a way to equalize without causing damage to any of them (with enough openings or loosely fitted panels that will blow out if needed). Therefore it is not possible for pressure loss in cockpit not to extend to the cabin, or the cargo hold.

The information I found about Sichuan Flight 8633 only says the passengers were not exposed to cold, nothing about pressure. During decompression, the air rushing out is not doing much work, so it does not lose much energy, and therefore only cools down a bit. What is going on is not an isentropic expansion, but something closer to free expansion. The air still has to do some work to push the outside air out of its way, but not as much as if it was expanded gradually from the 8,000 ft cabin altitude (in which case it would get down to similar temperature as outside).

The only reason for exposure to cold would be if the cold outside air was rushing into the cabin. Which the wall between the cabin and cockpit minimized, plus the engines were still running and supplying warm air.

$\endgroup$
4
  • $\begingroup$ When air depressurizes, it does get colder, but a 0.3 bar difference wouldn't be a heck of a lot. Work around a mechanic's shop sometime - the compressor gets hot and has an intercooler, and the air vented from air tools is cold even though the air line to the air tool is room temperature or a bit warm. Fact is, air conditioning would work even without the enthalpy of vaporization :) $\endgroup$ Apr 29 at 17:16
  • $\begingroup$ @Harper-ReinstateMonica The troposphere is in thermodynamic equlibrium and that means that if you take air from one level and adiabatically expand it to a pressure at higher level, it will match the temperature at the higher level too. The cabin air is a bit warm for the 2 500 m level, but it would still cool down to -40°C to -50°C at FL360 adiabatically. Fortunately, decompression isn't adiabatic, so the air will not cool down to anywhere near that. $\endgroup$
    – Jan Hudec
    Apr 30 at 5:28
  • $\begingroup$ @Harper-ReinstateMonica, the tools in contrast do expand the air (approximately) adiabatically, because the pressure is used to drive the tools and that is what cools the air. The air-cycle machine works similarly. It takes the hot pressurized air—compression always heats the air, because the compressor must do the work and that ends up as heat in the compressed air—cools it down in the intercooler and then expands it through a turbine to make it adiabatic expansion and cool it further below the initial temperature. But decompression isn't adiabatic. $\endgroup$
    – Jan Hudec
    Apr 30 at 5:35
  • $\begingroup$ adiabatic → isentropic. Free expansion is still adiabatic, because there is still no transfer of heat; the difference is that it increases entropy. $\endgroup$
    – Jan Hudec
    Apr 30 at 6:36
7
$\begingroup$

Great question with an interesting history.

The barrier between the flight deck and the cabin of commercial aircraft is engineered to blow out in a controlled fashion upon sudden decompression like the other barriers discussed in Jan's answer.

However, after 9/11, the FAA mandated reinforced doors to prevent unauthorized access. These doors are much heavier than their predecessors, and the security requirements basically dictate that they be one single hunk of material.

Now, consider a decompression event in which the big, heavy door gets rocketed one direction or another. Bad news for anything or anyone in its path:

The pressure differential and the force on the sealed cockpit door might be too much for the frame structure to handle if the cockpit depressurizes first. Passive and active internal venting (dado panels) may malfunction, get clogged by flying debris, or simply not react fast enough to relive [sic] the pressure differential on the security door. A loose, heavy, armored door can be much more dangerous than no door at all (or light door), as it could seriously injure or kill the flight crew, jam and/or destroy flight controls, damage and destroy flight instruments, and create mayhem in the cockpit. Such a scenario is incredibly dangerous, as it may prevent emergency descent. The rise of pressure differential and the force across the cockpit security door may well be much quicker than the designed venting system is capable of handling.

A solution was needed to nearly instantaneously unlock the door when a major pressure difference was felt. Enter the decompression sensing module, some variant of which is almost universal at this point. It integrates with the other components of the flight-deck access system and can unlock the door in a small fraction of a second:

In 2001 the commercial aircraft OEMs needed a solution to rapidly unlock the cockpit door in the event of a decompression event. NAT Engineering developed an electrical solution that responded to a decompression event in the millisecond range. In a matter of months it was taken from concept, through engineering design, computer modeling, prototyping, R&D testing, formal qualification testing, and into production for delivery to the OEMs. The NAT electrical device included integration of control for the keypad, thus providing for a secure keyless entry solution.

A fun thing about the DSM is that one way to test it is by shooting it with a high-powered handgun loaded with a blank cartridge. Makes for an interesting day in the lab.

$\endgroup$
3
$\begingroup$

There are two different factors to consider: First, there is the ambient air pressure - cabin and cockpit are not really separated in regard to pressure. So both would suffer from the "leak" nearly the same. On the other hand, there are wind effects - and these are considerably stronger close to the leak, especially when in front of the airflow.

$\endgroup$
2
  • $\begingroup$ And what's going to fail first - the cockpit door, or some of the other internal structures around it / near the bulkhead - that possibly weren't hardened... $\endgroup$
    – Mr R
    Apr 28 at 6:38
  • 1
    $\begingroup$ @MrR, nothing; there is enough completely open space, or space covered only by loosely fitted panels, to allow the pressure equalize without damaging anything. $\endgroup$
    – Jan Hudec
    Apr 28 at 11:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.