Here is some information on a long-lost subject that could be of interest to pilots who fly pressurised aircraft or those who hope to do so in the future.
The cockpit window
Aircraft manufacturers know that fire could break out anywhere on an aircraft, including the cockpit. For this reason, pilots are provided with procedures and checklists that cover fire suppression and smoke removal for various zones.
The cockpit window may need to be opened to help clear smoke and fumes. Due to the aerodynamics of an aircraft’s nose section, the in-flight opening of a cockpit side window results in the suction of air out into the free airstream. No high pressure air will enter the cockpit.
What if no cockpit window
Not all aircraft have cockpit windows that can be opened in flight. Those that don’t have a smoke removal hatch. Checklists that deal with the removal of smoke or fumes from the cockpit always have a warning note that states “Caution! Window should not be opened unless the source is confirmed to be on the flight deck.”
Training used to cover the opening of cockpit windows
During the 1960s and ’70s, when most airliner conversion training was done in the aircraft itself, the actual opening of a cockpit window was an exercise that was included in the syllabus and was accomplished by every single pilot.
Conversion training is now done in sophisticated simulators and the opening of side windows as an exercise has fallen away. The result is that if a pilot is faced with a procedure or situation that requires a window to be opened, he or she could lack the full knowledge and confidence to comply.
Until very recently, I had not opened a side window in any pressurised aircraft for some 35 to 40 years. On an occasion that I was scheduled for a ”return to service” test flight in a Boeing 737 – 800, I was able to re-visit the exercise.
But first, some “Gee Whiz” info to help your understanding…
- Since most of the 737’s measurements and pressurisation data is presented using the Imperial system, I’ve used inches for lengths and breadths and pounds per square inches for pressure.
- Very roughly, the side window has an area of 15.74 inches multiplied by 18.89 inches equalling an area of 297.33 inch².
- At about 35 000 feet, with a pressurisation differential pressure of 7.8 lb/ inch², the force on the window is 2 320lb or 1 050kg!
- We had to get the differential pressure down to zero. Even when it’s thought that the aircraft is completely depressurised, there’s always some residual air pressure in the fuselage shell. Even the very slightest amount of pressure from the flow of air through the aircraft will place a force on the window that could be as much as a typically enthusiastic traveller’s suitcase. This would still make it difficult or maybe even impossible for some to “crack” the window open.
We descended to 9000 feet and depressurised the aircraft
Starting the exercise at 35 000 feet, we obtained a clearance to descend to FL 90. A landing altitude of 9000 feet was selected so that while we were descending to that height, the cabin was climbing upwards to the same level.
The differential pressure reduced steadily throughout the descent. On reaching 9000 feet, the aircraft altitude and cabin altitude were the same and the differential pressure was zero.
The following instruction was read from the checklist. “Establish normal holding speed. High airspeed may prevent opening the window”.
The first attempt to open the window was at 210 knots and I simply was not strong enough to open it, even with two hands. At 180 knots, I was able to move the window somewhat, but realised that this was a job for a far stronger pilot! Eventually, having selected flap and reduced the speed to about 150 knots, I got the window open,
There was about as much of a draught or wind in the cockpit as you would experience flying a Harvard with the canopy open, or what you’d experience in an open-cockpit biplane. From this point of view then, it was a non-event.
However, the noise level was extremely high. VHF communication was almost impossible. We had anticipated this and had told ATC that we were going to be off the air for a few minutes during accomplishment of the exercise.
Closing of the window was also done at 150 knots and this required a fair amount muscle, but not as much as when the window was being opened.
Why it’s good to know this…
One never knows when this procedure might need to be applied. In the early 1980s a crew flying a McDonnell Douglas DC9 had been almost totally overcome by smoke and fumes. A passenger had thrown a burning cigarette butt into the waste basket of the front toilet, located just behind the cockpit door. A fire had broken out and the smoke it generated spread rapidly through the aircraft.
Having lost most of their instruments and systems and with their oxygen masks and goggles already donned, the pilots diverted immediately to where they could perform an emergency landing. The situation worsened dramatically. Eventually they opened their side windows and stuck their heads outside so they could breathe fresher air and also see where they were going and what they were doing. This course of action ended up being the most practical tool that was left for them to use.