Landing Altitude

[HiFly 16]

In the latest upgrade to the B744, setting the Landing Altitude to "AUTO" sets an altitude below the field elevation.  Not too critical for takeoff  because doors are closed before aircraft is pressurized but after landing is different story.  It used to be common to keep the cabin altitude slightly above the landing altitude.  This created a negative pressure that made opening the doors much easier but so slight to be undetected by the passengers.  This was correctly modeled in the NGX and also in the first B744 v3, but this latest upgrade does the reverse.  Is there some new logic to this or is this an oversight?  Just curious.  Fortunately my computer is in an unpressurized room so it doesn't affect my ability to fly the simulator!  LOL!

 

Craig

[thibodba57 462]

47 minutes ago, HiFly said:

In the latest upgrade to the B744, setting the Landing Altitude to "AUTO" sets an altitude below the field elevation.  Not too critical for takeoff  because doors are closed before aircraft is pressurized but after landing is different story.  It used to be common to keep the cabin altitude slightly above the landing altitude.  This created a negative pressure that made opening the doors much easier but so slight to be undetected by the passengers.  This was correctly modeled in the NGX and also in the first B744 v3, but this latest upgrade does the reverse.  Is there some new logic to this or is this an oversight?  Just curious.  Fortunately my computer is in an unpressurized room so it doesn't affect my ability to fly the simulator!  LOL!

 

Craig

How much lower?  The 747 lands with slight positive pressure for touchdown, so the altitude will be lower than that of the airfield.

[HiFly 16]

Thanks, Brian.  That explains it then.   The newest upgrade has that simulated correctly.  NGX, MD11, and former B744v3 had auto setting for landing altitude always above field elevation.  If I recall correctly, back in my day (circa 1980's) it was SOP to set pressure altitude above landing altitude by a couple 100 feet.  And the reasoning was that the doors opened inwardly and negative pressure after landing allowed them to be more easily opened by the crew.  I need to see if I can dig up one of my old FCOM's to verify that but I recall it was a particular point of discussion in ground school at the time. But if you are an active FO on the B744 in real life then I will take your word.  I do wonder about the logic though.  Maybe you could shed some light on that change in procedure.

 

Craig

[thibodba57 462]

12 hours ago, HiFly said:

Thanks, Brian.  That explains it then.   The newest upgrade has that simulated correctly.  NGX, MD11, and former B744v3 had auto setting for landing altitude always above field elevation.  If I recall correctly, back in my day (circa 1980's) it was SOP to set pressure altitude above landing altitude by a couple 100 feet.  And the reasoning was that the doors opened inwardly and negative pressure after landing allowed them to be more easily opened by the crew.  I need to see if I can dig up one of my old FCOM's to verify that but I recall it was a particular point of discussion in ground school at the time. But if you are an active FO on the B744 in real life then I will take your word.  I do wonder about the logic though.  Maybe you could shed some light on that change in procedure.

 

Craig

I'd have to look thru my other FCOMs to see if it addresses the reason behind it.  I would venture to say it's to keep positive controller of the pressure/outflow valves in case of a go around.  But once you are on the ground the outflow valves are triggered to full open either way so there should be 0 pressure on the aircraft anyways at the gate.  Having any actual negative pressure on the airframe is undesirable.  0 pressure works best for the doors as positive or negative will place tension on the cam locks of the doors.

[HiFly 16]

Thanks, Brian!

Craig

[JRBarrett 3,038]

10 hours ago, thibodba57 said:

I'd have to look thru my other FCOMs to see if it addresses the reason behind it.  I would venture to say it's to keep positive controller of the pressure/outflow valves in case of a go around.  But once you are on the ground the outflow valves are triggered to full open either way so there should be 0 pressure on the aircraft anyways at the gate.  Having any actual negative pressure on the airframe is undesirable.  0 pressure works best for the doors as positive or negative will place tension on the cam locks of the doors.

The landing altitude is only an approximation in any case, so that the pressurization controller can calculate an appropriate rate-of-change schedule during descent, so that the cabin differential pressure will be close to zero upon landing.

All pressurization controllers sense the actual outside air pressure at all times, either by a connection to a dedicated static port, or by taking the pressure from one or both air data computers. The pressure altitude at the landing airport will equal the physical MSL elevation only when the barometric pressure is 29.92 in/hg, most of the time it will be higher or lower.

Most aircraft have a slight positive pressure on touchdown, but once weight on wheels is sensed, the outflow valves will fully open so that the aircraft is completely unpressurized by the time it gets to the gate.