# In aircraft ventilation, what is meant by temperature increasing/decreasing for calculating the on-ground threshold?

In an open-circuit configuration, in normal operation, the A320 FCOM states:

The open-circuit configuration operates when skin temperature is above the on-ground threshold. On-ground threshold = +12C (53F), temperature increasing, or +9C (48F), temperature decreasing.

I am confused as to what is meant by the "temperature increasing" and "temperature decreasing". Does it mean that the open-circuit configuration is in operation when the skin temperature is more than 12C or less than 9C?

• Your title question doesn't really match the question in the text. You really seem to be asking about the meaning of increasing/decreasing, not on-ground/in-flight. Want to edit the title to match the question (since you've got a good answer). Feel free to ask a new question about on-ground/in-flight if you are confused about that, too. Commented Dec 24, 2021 at 15:17
• Well in my opinion the increasing/decreasing forms part of the threshold. But I can see how it could be confusing. I changed the title to be more specific, as I'm also only referring to the on-ground threshold in the text. Commented Dec 24, 2021 at 15:44
• Not intentional, fixed. Commented Dec 24, 2021 at 16:18
• Makes much more sense now. Thanks! Commented Dec 24, 2021 at 19:14

Transitions based on temperature (or other measured physical quantities, for that matter) run into the problem that the reported value will vary slightly due to gusts of wind, imperfections in the probe, stray signals in the wire, etc. Thus, if the temperature is very near the transition temperature, the measured value will fluctuate above and below, causing the system to switch rapidly between closed- and open-cycle mode. This could cause premature wear on the valves, a lot of electrical transients, etc., and is just a bad thing in general.

The solution is to add what we in the computer science world call "hysteresis", which is just a fancy way of saying we consider what the temperature reading was in the past as well as what the current value is.

If the temperature is currently increasing, the plane switches to open-circuit configuration at 12°C. It then stays in that mode even if the temperature drops back below that value. If the temperature is decreasing, it would have to reach 9°C before switching back to closed-circuit.

So, in short, if the temp is above 12°C, then it's in open-circuit mode. If the temp is below 9°C, it's in closed-circuit. If the temp is between those two values, the mode doesn't change, but just keeps whatever mode it was last in.