Low Cabin Altitude Operations

James Albright (a former G450 driver)

Updated: 2015-06-24

There isn't a published procedure on how to do this, but if you have any experience in a GIV you should be able to figure this out. The AUTO pressurization schedule, shown above, is going to bring the cabin altitude up at what it thinks is an optimal rate so you end up with a minimum cabin altitude at high pressure altitudes. But lets say you have a reason to keep the cabin altitude low. If you keep the airplane below 26,700 feet you can keep the cabin as low as sea level.

Everything here is from the references shown below, with a few comments in an alternate color.

Figure: Cabin Altitude vs. Differential Pressure, from G450 Airplane Flight Manual, §05-01-20, figure 9.

Cabin Pressure Selector Panel

[G450 Aircraft Operating Manual, §2A-21-30, ¶2.D.] The CABIN PRESSURE SELECTOR panel is located on the aft right side of the cockpit center pedestal. Panel feature functions are contingent on the mode of operation of the pressurization system:

With the pressurization system in the AUTO mode, the CABIN PRESSURE SELECTOR panel digital Liquid Crystal Displays (LCDs) display FMS and ADS data currently in use by the CPC. The rotary selector knobs are disabled.
With the pressurization system in the SEMI mode, the rotary selector knobs are active and are used in conjunction with the LCD displays to enter airplane / cabin altitude, barometric correction, landing field elevation and cabin rate of climb / descent.

In AUTO mode, this panel simply reports to you what the system is doing, based on inputs it gets from the FMS. In SEMI mode, this panel enters the inputs shown, removing the FMS inputs.

Manually Controlling Cabin Altitude

Figure: Cabin Pressure Selector Panel, from G450 Aircraft Operating Manual, §2A-21-00, figure 9.

If you need to keep the cabin altitude at sea level or some other value at or below 26,700 you must select the SEMI mode since AUTO will want to fly at the schedule shown on the diagram at the top of this page. In SEMI mode you can manually dial in the desired cabin altitude in the Cabin Pressure Selector Panel in the right side of the window, the left side will show you the corresponding maximum aircraft altitude. If you've never done this before:

Preflight: select SEMI, set departure field altimeter setting in the BARO COR window, select the desired cabin altitude in the right side of the AIRCRAFT / CABIN window.
Prior to landing: select LANDING, set landing field elevation and altimeter setting. (Unlike the GIV, the pressurization system should automatically select LANDING mode after you descend 1,000 feet, even in the SEMI mode. But you should keep an eye on it.)

Reference:

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013.

Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013.