I was flying a Fokker 100 a few months back over central Europe. We had probably just reached cruising altitude when the cabin temperature suddenly skyrocketed without reason, with the air coming out probably around 30 degrees (Celsius). After complaints by fellow travellers it became cooler. Is there any reason why it would do this?

I guess the pilots might have done it, but I see no reason for it, so could the aircraft have done it on its own accord?

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    $\begingroup$ Your flight attendant probably got cold and turned up the temperature until people complained. $\endgroup$ – casey Feb 25 '14 at 22:32
  • $\begingroup$ @casey maybe it could be so simple. just seemed like the temperature was maxed out however. felt really strange.... $\endgroup$ – Thunderstrike Feb 25 '14 at 23:02
  • $\begingroup$ @Manfred It's definitely not unknown to happen, it happened on Scandinavian Airlines Flight 751, but that was on the ground, not in the air... $\endgroup$ – flyingfisch Feb 26 '14 at 1:11
  • $\begingroup$ @casey: Sounds like me at some of the places I've lived! $\endgroup$ – Vikki Jul 18 '19 at 0:31

Over Central Europe in the winter time, air becomes tremendously cold. Since warm bleed air from the compressor stage of a jet engine is used for cabin heating, the pilots might have set temperatures to maximum in order to warm the cabin. But being in a climb, almost all power of the engines is required for lifting the aircraft, thus leaving only little excess bleed air to be used for heating. During normal days and outside air temperatures this should be enough, though.

By levelling out the aircraft, the engines are spooled back (less power required than in climb -> hence more excess bleed air available -> higher ability to warm the cabin in cruise than in climb). If the pilots/cabin attendants just didn't realize this circumstance rapidly enough, the cabin is being flooded with maximum heating air from the engines.

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    $\begingroup$ That makes perfect sense. The F70/F100 climate system is extremely sensitive to engine settings I've noticed on several flights. $\endgroup$ – DeltaLima Feb 26 '14 at 9:51
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    $\begingroup$ I've never flown a Fokker, but every jet that I've ever flown produces more bleed air at higher power settings. The amount of bleed air produced is relative to how fast the engine/compressor is turning. $\endgroup$ – Lnafziger Feb 26 '14 at 11:34
  • $\begingroup$ In addition, some engines have multiple bleed air taps at different stages of the N2 compressor, and engine setting may change which bleed source is being used to supply the packs. $\endgroup$ – casey Feb 26 '14 at 20:27
  • $\begingroup$ Yet another additional factor is adiabatic expansion of the cabin air. On climb out the cabin altitude approaches 8000 feet (lower in some airplanes) and for dry air this expansion alone would result in 24C cooling of the air from sea level. You probably wouldn't notice anything that dramatic depending on residence time of cabin air, humidity and the temperature of the incoming air, but it is something, at least in part, to overcome during the climb. Once the cabin altitude is stable, you lose this effect and if you the now unneeded heat input may start to warm the cabin. $\endgroup$ – casey Feb 26 '14 at 20:32
  • $\begingroup$ Bleed air from the engines is mixed with cold ram air (-50°C) from the atmosphere, in a variable ratio, according to wikipedia and the temperature is controlled when pressure varies. How to explain such temperature increase when the aircraft levels off? $\endgroup$ – mins Dec 15 '14 at 2:56

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