Originally Posted by
737aviator
1) We forget, as I pointed out before, and trigger an off scheduled descent master caution...Thereafter play with the flight altitude setting (action the QRH) and totally confuse the pressurisation controller.
An off schedule descend is a non-issue in most airlines, no big deal...
Originally Posted by
737aviator
2) We reset the pressurisation system to FL370, and therefore trigger a climb in cabin altitude as the differential pressure reduces, and this seems to be bad to have in your opinion.
Correct course of action, the pressurisation system has maintained a cabin altitude below maximum for the flight until now, resetting it to a lower level as further climb is no longer foreseen or planned by crew triggers the system to now use a lower pressure differential, causing the cabin to climb while aircraft is in level flight until the maximum pressure differential for the cabin has been reached.
From a technical point of view, in your option 2 all the pressurisation controller does is open the outflow valve a little bit and let some air escape to raise the cabin altitude.
Conversely when
not selecting as described highest expected flight level pre flight, from a technical point of view, we would now be asking the aircraft to pump more air into the aircraft cabin at altitude to lower the cabin altitude inside the pressure vessel (your engines thus working harder due additional bleed demand) while trying to climb to the newly selected level at the same time.
Next time you try this, have a good look at climb performance when aircraft is adjusting cabin altitude and notice the difference when it has completed this task, you can see the difference in climb performance.
There are multiple ways to try and explain it to you, multiple reasons why Boeing is suggesting you do it as per advertised materials, including warning about flying above selected altitudes (as the controller is playing catchup to adjust to your newly created cruise level scenario which can be avoidable by following their guidelines), but please realise that unless you have flown the NG you are never and will never be confronted with this design anomaly of a pressurisation change due to differential changes in design when passing FL370...
If you choose to interpret the documentation of your manufacturer the way you do, please feel free to do so, hopefully you will not be a statistic one day when the system fails as the system is operating as advertised.
Meanwhile, please allow others including me to operate it more safely (according to Boeing) by selecting the highest expected flight cruise level pre flight and remove the risk to a more manageable one as no further action on the overhead panel is required...
As a side note, Boeing has placed by design, anything on the overhead panel which should be set pre flight. Unless directed to do so (eg by a checklist or changes in ambient conditions), no changed should be made to them. Just a thought