Originally Posted by
Derfred
Your first image is normal (above FL370. Not at FL370 - should be 7.8).
Your second one isn't. In this case, my next step would be (after tapping the gauge!) to check the cabin vertical rate. If it is climbing, then consider whether you have just arrived at cruise altitude at a high rate of climb. If so, then the cabin is still catching up and the auto controller will resume normal diff shortly. Give it a few minutes. This is ok, no further action required.
If the cabin rate is descending, then there is a problem and you are not far away from pressure relief valves opening (9.1psi). In this case, the AUTO FAIL light should already have illuminated (>8.75 and auto not responding correctly), so this is an unlikely scenario. Have you seen this or are you posing a hypothetical?
In this case, I would conduct the "unscheduled pressurisation change" checklist, which will attempt the alternate controller first, and if that doesn't solve the problem, it will lead you to manual control. Take it slow. No rush here.
As an aside, I don't know whether the gauges get the cabin alt from the same sensor as the auto-controllers, or whether there are multiple sensors. So I don't know whether you could end up in this situation with a faulty gauge but yet the auto-controller is operating normally. Maybe someone can answer this.
But what I do know is that the Cabin Altitude Warning Horn does have it's own sensor, so you aren't going to unknowingly depressurise with a faulty gauge in manual mode. So in the case of a faulty gauge, I would try to keep cabin rate at zero until the outflow valve is fully open during descent, then land depressurised.
Had this during cruise flight after we had been level at FL370 for about 45 minutes. The AUTO FAIL light was not on.