Originally Posted by
BFSGrad
There is a mindset that any decompression—fast, slow, explosive—will frost the windows. I agree that there is evidence that is not always the case; e.g., 2022 OE-FGR.
I think Dan Gryder is off the mark on this. “He doesn’t know what he doesn’t know.”
I’m not a pilot, but am a mechanic, with many year’s experience working on air conditioning and pressurization systems on small, medium and large jet aircraft.
A loss of pressurization at altitude will
not (necessarily) inevitably lead to the windows frosting. It depends on what caused the loss of pressurization.
All jet aircraft use heated engine bleed air to condition the cabin air. In larger aircraft, that have a full ACM (Air Cycle Machine) the bleed air temperature can be modified to as low as 32 degrees F by using an expansion turbine combined with heat exchangers. (The bleed air cannot be pumped directly into the cabin from the engines as it is far too hot.)
The Citation 560 does not have a full ACM. Like many smaller GA jet aircraft, it uses bleed air and heat exchangers to set the incoming air temperature for heating. For cooling, (typically only needed on the ground on hot days, or at lower altitudes), it has a standard freon vapor cycle refrigeration unit, little different from a home or automobile air conditioner.
Larger aircraft, with a full ACM, can provide heated or cooled air (as needed) from a single unit.
Cabin air temperature will usually be automatically controlled by adjusting the mixture of heated bleed air and refrigerated air to keep the temperature at a comfortable level.
The incoming conditioned air is what pressurizes the cabin. The degree of pressurization is controlled by outflow valves, typically located at the rear of the pressure vessel, which permit the cabin air to discharge to the external atmosphere. On the ground, the outflow valves are fully open, and all incoming conditioned air flows right back out again, so the cabin pressure remains the same as external ambient air pressure.
Upon takeoff, the outflow valves begin to close under the control of the pressurization system, which permits the cabin air pressure to increase above ambient external pressure as the aircraft climbs. At high altitude, the outflow valves will be almost completely closed.
If a loss of pressurization is caused by a failure of the outflow valves - either because of a fault in the valves themselves, or the pressurization controller, (or failure by the pilot to enable the pressurization system in the first place), then the cabin pressure will drop to external ambient pressure.
But, in this scenario, the cabin temperature does not automatically drop to ambient temperature. The air conditioning system is still dutifully pumping heated bleed air into the aircraft. If the outflow valves are wide open, all that air is discharged to the external atmosphere so the cabin pressure remains at, or near external atmospheric pressure - but the air temperature inside the aircraft will not necessarily fall to ambient as long as a continuous supply of heated air continues to flow.
A pressurization loss caused by by a failure of the incoming air supply is another matter. In this case, the outflow valves might be working perfectly, and would quickly go fully closed, but with no incoming heated air, cabin altitude will rise to ambient,
and the cabin temperature will quickly fall to extremely cold levels. The ambient air temperature at FL 340 is typically between -45 to - 55 Celsius (depending on the season).
In this scenario, the cockpit and cabin windows will likely frost over relatively quickly.If the cockpit windows are electrically heated, they might not frost up completely. If they are heated by the conditioned air supply (like an automobile defroster), they likely will frost up.
A third scenario is explosive decompression, which is anything that results in a major breach of the pressure vessel. That could be caused by a window or door blowing out, or structural failure of the pressure vessel. In this case there will obviously be loss of pressurization, and if the “hole” is large enough, even the best cabin air supply system would not be be able to maintain the temperature.
TL / DR
Pressurization failure caused
only by a problem with the outflow valves will not necessarily cause cabin temperature to drop to sub-freezing levels, if the heated cabin air supply is otherwise working.
Pressurization failure caused by a total loss of incoming cabin air supply
will cause the cabin temperature to fall to very cold levels quickly, with accompanying frosting of cockpit and cabin windows. Likewise for an explosive decompression event caused by a major breach of the integrity of the pressure vessel.
Jim Barrett