No, not airtight. Airliners always leak a bit, by design.
Indeed if they did not leak, with all the pressurized air constantly coming into the cabin from the engines, the aircraft would quickly become very much over-pressurized, and perhaps eventually pop like a balloon. But that cannot happen. Fortunately there are many small vents on an airliner that release internal air constantly and normally on certain schedule.
For example, these are some of the small vents in an Airbus A-320 that routinely release internal air outward into the atmosphere:
- Avionics ventilation – A computer and two fans cool the avionics and exhaust that air overboard.
- Battery ventilation – A venturi in the skin of the aircraft draws air from around the batteries, and vents it overboard.
- Lavatory & Galley ventilation – An extraction fan runs continually with electrical power, drawing ambient cabin air through the galleys and lavatories, and exhausts it overboard.
- Pressure relief valves – these valves will open in case the cabin accidentally becomes over pressurized.
- Outflow valve – This is the largest opening – as pictured below. It is a relatively large valve that a pressurization computer will normally and constantly control its opening and closing, and therefore cabin altitude, automatically. As an aircraft climbs, the outflow valve will modulate to maintain and control on a set schedule the highly pressurized air coming from the engines, to keep the cabin at a comfortable cabin altitude. While different aircraft will have different pressurization schedules, the outflow valve will be slightly open to allow somewhere between 6,000 feet (1,828 meters) to 8,000 feet (2,438 meters) maximum cabin altitude while at cruise altitudes of 35,000 feet (10,668 meters) to 40,000 feet (12,192 meters).
It may also be interesting to note the Airbus aircraft have a “Ditching Switch’ that when pressed, automatically closes all these valves and a few others to aid the aircraft to float for some period of time after ditching in water. Ironically, this switch was never activated in the 2009 ditching of Flight 1549 in the Hudson River. Nevertheless the aircraft while somewhat damaged, managed to float for some period of time easily allowing everyone to evacuate and be rescued.
Author – John Chesire
