Simulator Integration Planning: ATA 21 - Air Conditioning and Pressurization (Label Assignments - Part 1)

To integrate the L-1011 pressurization panels with Lamina Research x-plane we are able to us a number of datarefs that are already in place for aircraft pressurization. The datarefs are:

sim/cockpit2/pressurization/actuators/cabin_altitude_ft sim/cockpit2/pressurization/actuators/cabin_vvi_fpm sim/cockpit2/pressurization/actuators/dump_all_on sim/cockpit2/pressurization/actuators/dump_to_altitude_on
sim/cockpit2/pressurization/indicators/cabin_altitude_ft sim/cockpit2/pressurization/indicators/cabin_vvi_fpm sim/cockpit2/pressurization/indicators/pressure_diffential_psi

While the datarefs are not complete for the pressurization implementation they are a good starting point.   Lets take a closer look how each dataref that is already present in x-plane can be used and which system elements have to be created as a separate code element for the Air Conditioning and Pressurization subsystem:

1. actuators/cabin_altitude_ft

This dataref sets the cabin altitude that is command in feet. On the L-1011 pressurization panel we have an analog dial that computes the commanded altitude for a given flight level. 

L-1011 Pressurization Panel

The flight level is set with the ALT SET knob and the the cabin altitude is read on the right hand side of the dial. For example, we set FL330 and the L-1011 is capable of safely holing a cabin altitude of 5000ft. In order to set the commanded altitude in x-plane we need to read the analog value from a potentiometer attached to the dial and translate it into a flight level. 

2. actuators/cabin_vvi_fpm

The pressurization module on the L-1011 has several ways of setting the vertical speed for the cabin pressurization. typically any pressurization of 500ft/min in ascent and 300ft/min in descent will be experienced as uncomfortable by passengers. The L-1011 differentiates between a NORMAL and STBY MODE for pressurization. Normal is a fully automated mode and STBY is a somewhat manual mode. 

L-1011 Pressurization Panel - Mode and VVI Select

The mode is selected by the rotary switch on the left and. When the pressurization system is in NORMal mode the preset rates of 500fpm up and 300fpm down can be increased by moving the middle rotary switch to the right. When the system is switched to STBY then both the ascent as well as the descent rate for the system is set with the rotary switch on the right. In order to associate the VVI speeds of the L-1011 to the cabin_vvi_fpm dataref in x-plane we need to convert the analog data represented by a potentiometer in the device for either the NORM or the STBY switch to a number value. X-Plane does not understand VVI for NORM or for STBY ... so I need to create a piece of code that will read the switch position of the MODE SEL switch and then send the respective selected VVI rate from either the NORM RATE or the STBY RATE rotary switch to x-plane. 

3. indicators/cabin_altitude_ft

Depending on the panel configuration, the L-1011 has either two or three gauges relate to aircraft pressurization. Regardless of the number of instruments, the gauges show a) differential pressure, b) cabin vertical speed and c) cabin altitude. The first label to look at is the cabin altitude in feet:

L-1011 Cabin Altitude

Delta used the configuration of gauges as seen above, TWA used a 3 instrument configuration as seen below:

L-1011 Pressurization Panel - TWA

The altitude data needs to be send from x-plane to an ARINC 429 compatible altimeter or output as fine/coarse synchro signals. 

4. indicators/cabin_vvi_fpm

Typically the VVI for the pressurization system on the L-1011 is an actual pressure rate change instrument. In other words, it is fully analog. I will not be able to simulate an actual rate of pressure change with air and therefor will substitute this instrument with a servoed vertical speed indicator.

L-1011 Cabin Vertical Speed - TWA

Depending on the type of servoed instrument the x-plane computed Cabin Vertical Speed needs to be output as analog value between -10VDC to +10VDC.

5. indicators/pressure_diffential_psi

The third and last instrument for the pressurization system of the L-1011 is the Pressure differential indicator. The Laminar Research x-plane simulator computes pressure differential for the hull. 

L-1011 Differential Pressure - TWA

Just as with the Cabin Vertical Speed Indicator the Cabin Pressure Differential gauges is fully analog and will need to be replaced with a servoed version. The x-plane computed pressure differential the needs to be output as an analog value between -10VDC and 10VDC. 

The data value of the pressure differential will also drive a number of additional indicators on the L-1011 pressurization panel that are not covered by the x-plane and/or a x-plane dataref. When pressurization exceeds that maximum permissible pressure differential the SAFETY VALVEs on the aircraft will open to prevent he hull from over pressurizing. There are two annunciators on the L-1011 panel that will illuminate when the safety valves are open. 

L-1011 Pressurization - Alert Annunciators

Safety valve opening is also accompanied by an alarm sound that can be silenced with the HORN CUTOUT momentary switch. The SAFTEY VALVE annunciators are 24VDC relay outputs and the HORN CUTOUT momentary switch is a digtial I/O port. The HORN CUTOUT switch will also be used for an alert that sounds when rapid de-pressurization is sensed.