I understand the B737-800 has two air conditioning packs that handle the temperature of three zones: the cockpit, the fwd cabin and the aft cabin.

I understand that which pack is turned on is controled from the cockpit.

My questions are:

  • can the temperature be regulated for each zone seperately?
  • where are the switches located that control the temperature in each zone?
  • who operates these switches?

1 Answer 1


The 737-400, -800, and -900 have two cabin air conditioning zones plus the cockpit. The rest have one plus the cockpit.

Both packs are on in normal operation because they are slightly more efficient at lower flow rates. It also is safer as no immediate action is necessary to prevent depressurization if a pack fails.

Temperature is regulated separately. The packs cool to the lowest temperature demanded, and unconditioned bleed air, called trim air, is mixed in for the warmer zone.

The dual cabin zone models have three knobs which set the temperature of each zone, from a low to high temperature with a default detent.

The single cabin zone models have a knob for the cabin and one for the cockpit zone. The knob is divided into two ranges, an auto range, where a temperature is controlled, and a manual range, that selects the position of the hot air mix valve in each pack.

The temperature control dials are located in the overhead panel in the cockpit. The pilots operate this, often in response to complaints from the cabin crew relayed over the interphone. (The A320 has an option for cabin crew adjustment of temperature, eliminating the hassle)

  • $\begingroup$ So can this trim air (and hence temperature) be regulated for each zone separately? Can a flight atendant ask the flight crew to adjust e.g. only the fwd cabin temperature? $\endgroup$ Commented Apr 23, 2019 at 18:34
  • $\begingroup$ Ah, I see in the link from @fooot that there are separate dials for each zone. Thank you all very much. $\endgroup$ Commented Apr 23, 2019 at 18:37
  • 1
    $\begingroup$ @PaulOgilvie Updated my answer slightly to describe the knobs. $\endgroup$
    – user71659
    Commented Apr 23, 2019 at 18:47
  • $\begingroup$ If I may add one more question: am I correct to understand the temperature setting(s) manage the temperature of the air provided to the cabin, but there are no temperature sensors in the cabin that regulate this output air? So a cold hull will not receive hotter air, even though the heat is already "consumed" after the first few rows? $\endgroup$ Commented Apr 25, 2019 at 9:16
  • $\begingroup$ @PaulOgilvie No. The 737 has one temperature sensor per zone, which measures cabin air temperature independent of output air. Just like a home thermostat. The heating isn't "consumed": air ventilation systems on aircraft are designed not to have much forward-aft flow. $\endgroup$
    – user71659
    Commented Apr 25, 2019 at 15:05

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