westjetforlife From Canada, joined Jun 2005, 814 posts, RR: 1 Posted (3 years 8 months 1 week 6 days 3 hours ago) and read 3849 times:
Hello, everyone. It's Nik again.
Once again, something has been boggling me, so I might as well ask the professionals as opposed to guessing on my own. Here goes.
Now, say for example, a Boeing 737-700/800 is planned to cruise at FL390 as its final cruising altitude. However, because it is too heavy with fuel and payload (let's assume a MTOW/ETOPS situation, say YVR-HNL or YYZ-BGI), it has to cruise at a lower altitude to burn off fuel, say FL330.
Now, from what I understand, the flight crew is to set the cruise altitude in the CRUISE ALT window on the overhead pressurization panel before takeoff from the origin airport, as well as on the PERF INIT page of the FMC. My questions are: do 737 flight crews set the CRUISE ALT window setting to the INITIAL cruise altitude, or do they set it to the FINAL cruise altitude and allow the aircraft to pressurize itself on schedule, say when the MCP altitude is reset from FL330 to FL370 or 390? Likewise goes with the FMC: do you enter in the initial cruise altitude into the PERF INIT page, or the final cruise altitude?
Mir From United States of America, joined Jan 2004, 21425 posts, RR: 56
Reply 3, posted (3 years 8 months 1 week 5 days 4 hours ago) and read 3616 times:
Quoting 26point2 (Reply 1): Not sure about your 737 but in Honeywell equipped a/c we set final cruise altitude in FMS. It also knows landing field elevation. Set it and forget it. I would imagine others the same.
The 737 has two knobs on the overhead to set cruise altitude and landing altitude - that's what the OP is asking about. The 737NG is that odd mix of a very advanced avionics setup with a much older overhead panel, and I'm not sure how much the FMS actually talks to the various systems.
In my limited reading on Smartcockpit.com, it says that the cruise altitude knob should be set to the planned cruise altitude, but I'm not sure whether that's the planned initial or planned final altitude. I do know that if you don't reach the altitude in the window before you start a descent, you'll get a off-schedule descent caution message or something like that, so my guess would be that you set the knob to your initial altitude, and then change it if you step climb later on in the flight.
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AAR90 From United States of America, joined Jan 2000, 3462 posts, RR: 47
Reply 4, posted (3 years 8 months 1 week 5 days 3 hours ago) and read 3598 times:
Quoting westjetforlife (Thread starter): My questions are: do 737 flight crews set the CRUISE ALT window setting to the INITIAL cruise altitude, or do they set it to the FINAL cruise altitude and allow the aircraft to pressurize itself on schedule, say when the MCP altitude is reset from FL330 to FL370 or 390? Likewise goes with the FMC: do you enter in the initial cruise altitude into the PERF INIT page, or the final cruise altitude?
Not sure about other airlines, but for AA 738's we set the PLANNED MAX cruise altitude in the pressurization panel and the INITIAL cruise altitude in the FMS (usually done automatically anyway).
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DogBreath From United Kingdom, joined Mar 2008, 259 posts, RR: 0
Reply 5, posted (3 years 8 months 1 week 4 days 13 hours ago) and read 3517 times:
In my airline we set the initial planned cruise altitude into the FMC and to the overhead pressurisation panel always. When a further climb or step climb is carried out, the FMC and overhead panel are reset to the new Flight Level.
With regard to the pressurisation system, there's an Off Schedule Descent mode fitted. If the aircraft begins a descent before reaching the cruise altitude that is set to the overhead pressurisation panel (actually greater than 1000ft before reaching cruise) it will trigger the off schedule descent mode. Which is an Off Schedule Descent warning (with Master Caution) and the auto controller will return the cabin altitude to the altitude of the departure airfield minus 300ft. This is designed for a turnback in the departure phase.
So with that in mind it's best to set the planned level to the overhead panel, as if a higher level was set and due to ATC a climb to that higher level isn't possible an off schedule descent will occur at the end of cruise phase. If departing from a sea level airfield and flying to a destination that is 1800ft high, the off schedule descent mode will return the cabin altitude to -300ft, whereas you'll be landing at 1800ft. Off course the QRH should be consulted and would reset the system but why put yourself in that situation in the first place.
The planned cruise level is always set into the FMC and reset later when further climbs are approved by ATC. If a higher level is set (ie. final cruise level) at the preflight stage. The aircraft will always climb to the level set into the MCP window and the FMC will then remain in the climb phase until it reaches the level set. Using your example, if FL390 was set into the FMC and for performance reasons only FL350 was possible, on reaching FL350 the aircraft will level off to 35,000 as set to the MCP and the FMC will remain in the climb phase. Therefore the FMA on the PFD's will show Altitude Hold and the speed window on the MCP will be open at the current Mach Number. Sure the aircraft will fly all day like this, however that's not the way it's intended to be operated.
Occasionally we depart with a planned FL of say FL380 and due to traffic, ATC can only give us FL360. No problem reset the FMC and the Overhead Panel to FL360 and continue climbing, sit back and have a coffee.
113312 From United States of America, joined Apr 2005, 569 posts, RR: 1
Reply 6, posted (3 years 8 months 1 week 4 days 3 hours ago) and read 3441 times:
The fact remains that the pressurization controller will be maintaining the normal maximum pressure differential, in the Auto mode, regardless of intermediate or final cruise altitude. In older aircraft with more manual control of pressurization, a pilot might set the cabin altitude at the highest cruise altitude that was planned so that there would be no pressure changes in the cabin as the plane climbed from initial to final cruise altitude. In most modern systems, the automatic system controls the cabin and will adjust cabin altitude with each level change. However, the logic that the cabin should be descended to the destination elevation is a function of the FMS on some, and final cruise cabin altitude setting in others.