Thursday, 08 December 2011 00:00

Air Conditioning System

Here's a few words on the 737 air conditioning and temperature control system that we are making.


 Since there's no real feed back in a simulator whether it's cold or hot in the cabin, it's   not entirely obvious that you bother with modelling it even for a detail sim like the one  we're building. But our tech consultant and 737 captain, Jan, asked that we'd build it  and since he provided some very detailed ideas on how it could be done, we kind of had  to do it. It's not an entirely marginal system either. At FL370, everyone on board would  probably freeze to death without it and Jan says the air cond controls are used a lot in  the real aircraft during everyday operations.

The system appears simple but as always when you want to represent something like this in computer code things tend to get interesting anyway. This is particularly true since we, to the extent that it's possible, like to model our systems "from bottom up". By this we mean, rather than just hard coding responses to switch positions etc, we try to model the underlying components and relevant parameters of a system and let them play out, hopefully resulting in accurate behaviour. The advantage of this approach is that you tend to get more detailed accurate behaviour and failure chains for free. The downside is that, when "modelling nature", it can be tricky to get the exact results for a given set of conditions and you often end up with a more complicated model than you anticipated.

The code deals with the vital components of the system. We model the amount of airflow supplied by the the air cond packs, with dependencies to bleed air system availability and pressures. We also model the basic cooling and heating capacity of the packs. Then, to get realistic control of temperature, we model the mix valves which determine how much of hot pack airflow is directed to the mix manifold and supply ducts, versus how much is routed via the heat exchangers and cooled before it's mixed with hotter air again.

The mix valves have both auto and manual modes. In auto, the pilot sets the   desired temperature for flight deck and passenger cabin and the system automatically positions the mix valves to provide hotter or colder air as required. A quirk in the system is that, while the right air cond pack supplies the passenger cabin with air, the left pack supplies both flight deck and passenger cabin. The feedback temperature sensors which the auto mode uses to control temperature, however, are positioned in the separate supply ducts to flight deck and passenger cabin. This leads to the effect that, while trying to maintain requested temperature in the cockpit, the left pack mix valve also affects the temperature of the air going into the passenger cabin, which in turn, according to Jan, make a subject for constant fighting between cockpit and cabin crew. ;)

The manual mode simply means that the pilot controls the mix valves directly to achieve the temperature hevdesires. In this mode however, there is no protection against excessive duct temperature, which makes it possible to drive the mix valve too far and overheat a duct. This in turn causes a PACK TRIP OFF. When this happens, the pack valve closes, the mix valves go to full cold and warning lights on the overhead panel lights up. To re-engage the tripped pack, the pilot can press the TRIP RESET button on the bleed air panel, which will open the pack valves and make the tripped pack contribute to pressurization and temperature control again.Another detail worth mentioning the the temperature select knobs. We use a lot of custom 


logic manipulators on the 737 and these are no exception. When the knob is in the 8 to 4 o'clock position, it sets the target temperature for auto mode. At the 8 and 4 o'clock positions there are detents that you can pull through with the mouse. When you do, the knob becomes spring loadedto the 6 o'clock position.

Moving it left and right of this causes the mix valves to move in manual mode.To complete the air cond and temperature model, we take the current cabin and flight deck temperature, subtract a fraction of the ambient temperature differential and add a fraction of our supply duct temperature multiplied with pack airflow. The the result is a cabin temperature that realistically adjusts to outside temperatures and the air condition system.

Oh btw, did I mention that we model the cockpit being heated by the avionics and that cabin temperature changes to ambient faster if the doors are open? :)


  • Comment Link Monday, 18 June 2012 18:23 posted by Dozer

    Larry - yes it is - using a Teensy USB board. (This is not 737-related. You could make a fan run from any dataref using Teensy.)

  • Comment Link Friday, 15 June 2012 23:42 posted by Larry

    Is it possible to interface the air conditoning to run a small usb fan to simulate real air conditioning?

  • Comment Link Monday, 30 January 2012 08:54 posted by Programmable Controls

    After reading the blog given by you,I just want to say that it gives a useful information about the A.C system which helps us alot...Thanks for the post...

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