B737NG Cruise Alt and early return logic???
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Join Date: Mar 2006
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B737NG Cruise Alt and early return logic???
Here is the question:
Cruise altitude set in the pressurization controller is 400. After takeoff we see that we are too heavy for 400 for quite some time.
This is a 4 hour flight from Cancun to Denver.
So we request and get 380 as the final cruise altitude. My partner goes right ahead and sets 380 in the pressurization controller as we are climbing thru about 10,000 feet. I then seem to remember something about an "early return logic" where the pressurization will "know" the departure airport elevation in case we have to return to the field of departure. Remember that Denver landing altitude of 5450 is set in the LAND ALT window.
Two part question:
When does the logic witch over to my destination airport and what would happen if we returned to Cancun (at sea level) after setting 380 and without that early return logic being there anymore.
If one looks at the OFF SCHEDULE DESCENT QRH, it has you do nothing at all if you land at the airport of departure. Do you have to set zero in the case of Cancun or what happens when you touch down?
Cruise altitude set in the pressurization controller is 400. After takeoff we see that we are too heavy for 400 for quite some time.
This is a 4 hour flight from Cancun to Denver.
So we request and get 380 as the final cruise altitude. My partner goes right ahead and sets 380 in the pressurization controller as we are climbing thru about 10,000 feet. I then seem to remember something about an "early return logic" where the pressurization will "know" the departure airport elevation in case we have to return to the field of departure. Remember that Denver landing altitude of 5450 is set in the LAND ALT window.
Two part question:
When does the logic witch over to my destination airport and what would happen if we returned to Cancun (at sea level) after setting 380 and without that early return logic being there anymore.
If one looks at the OFF SCHEDULE DESCENT QRH, it has you do nothing at all if you land at the airport of departure. Do you have to set zero in the case of Cancun or what happens when you touch down?
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Cruise mode activates when the differential pressure is within 0.25 psid of the target differential pressure. If you stopped climb at FL380 and then reset the cruise altitude on the panel to FL380, the "automatic abort capability' is lost. Note that Boeing's recommended way of handling step climbs is to set the highest expected FL on the ground, and then as you step climb, the pressurisation controller will take care of the rest. If you divert when at cruise level already, you should re-set the landing altitude on the pressurisation panel.
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Nothing special will happen when you touch down, although you'll be depressurised from roughly 5,500 ft, which may be uncomfortable! In the opposite scenario, i.e. landing at a high alt airfield after offsked descent from a low alt airfield, you would of course be landing with a cabin delta (px controller expecting lading at sea level), and therefore the aircraft would depressurise after touchdown.
