Simulator Integration Planning: ATA 21 - Air Conditioning and Pressurization (Label Assignments - Part 3)

L-1011 Air Cycle Machine Temperature Ranges

In order to program the AC behavior of the L-1011 correctly we will have to go a bit deep on the various parameters in the system. First, lets take a look at the normal temperature ranges we should observe on the ECS Monitor in cruise flight. The numbers in the table below correspond to the numbers in the image above:

• [1] Pack Inlet Bleed Air Temperature ... 160º-200ºC in cruise
• [2] Compressor Discharge Temperature ... 60º-80ºC in cruise (depending on Turbine bypass valve setting)
• [3] Turbine Inlet Temperature ... 15º-20ºC in cruise (will vary with RAM air setting)
• [4] ACM Discharge Temperature ... 15º-20ºC in cruise. The lowest possible ACM discharge temperature is 2ºC
• [5] Hot Air Manifold Output Temperature .... 110ºC

The normal Pack Flow outflow for Pack #2 and #3 is between 2000 and 2500 CFM (#1 has 300-400 less). The spring loaded valves for the Pack Flow Control and the Hot Air Manifold will not open if the duct pressure is below 15 PSI.

Warning Indicators

• Pack Flow Control Switch OVHT light illuminates when the Compressor Discharge Temperature [2] exceeds 200ºC or when the ACM Discharge [4] temperature exceeds 96ºC.
• DUCT OVHT ... illuminates when the Hot Air Manifold Duct temperature has reached 145ºC. This causes both Hot Air Isolation Valves to close until the temperature falls below 120ºC.
• Hot Air Isolation Switch OVHT ... illuminates when the respective valve has reached 135ºC

Valve Position Values

There are a number of default valve position values that the system simulator needs to take into account.

Turbine Bypass Indicator - The turbine bypass value is positioned full warm until the RAM AIR louvers have reached the full cool position as indicated on the RAM AIR indicator (9 o'clock  on the ground or 1 o'clock position in flight). Once this status has been reached, the turbine bypass valve will now be positioned to hold the required temperature. With the pack turned off, the turbine bypass valve will be set to the 11 o'clock position by default.

RAM Air Indicator - With the Pack turned off the the louvers are automatically set to the 9 o'clock position on the ground and the 1 o'clock position in flight.

Turbine and RAM Positions on Ground.

Turbine and RAM Positions in Flight.

In part 4 of this series we are going to start developing a model that will allow us to compute the approximate temperatures and cabin zone temperatures base on the relationships established in normal cruise flight. Again, the idea is the create a systems simulation of the L-1011 air conditioning and pressurization system that is as real-world as feasible for this project. 

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. 

Simulator Integration Planning: ATA 21 - Air Conditioning and Pressurization

ATA 21 is the Systems chapter for Air Conditioning and Pressurization (a complete listing of ATA chapter numbers can be found here). I decided to work through the simulator system integration according to the ATA chapters and that way everything stays in a somewhat logic order. The upper panel of the flight engineer console on the L-1011 is largely consumed by control and indicator elements for ATA 21 and ATA 36 (which is Pneumatic).

Below are images from the original documentation of the L-1011 simulator created by Singer Link Division in the 1980s.

Environmental Control System (ECS) Monitor Panel (*791/A9)

The simulator implements all component pieces of the ECS monitoring panel.

L-1011 Trainer - ECS Monitoring System (Component Labels)

Implemented Components

Gauges
M10 ... Pack Flow 
M11 ... ECS Temperature
M12 ... Cabin Temperature

Switches
S10 ... Pack Select 
S11 ... ECS Temperature Select 
S12 ... Cabin Zone Temperature Select    

Switch Lights
SDS26 . Cool Air Overboard
SDS27 . Cargo Heat Forward
SDS28 . Cargo Heat Mid
SDS29 . Cargo HEat Aft
SDS30 . Instrument Cooling

Annunciators
DS14 .. Avionic Air Forward
DS15 .. Avionic Air Mid

Cabin Pressurization (*791/A9)

Several of the gauges for the pressurization panel are photo simulations on the original Singer Link Division L-1011 simulator. These instruments will be replaced with the real flight hardware as part of the simulator outfit at the museum. However, several switch and annunciator elements are already fully implemented.

L-1011 Trainer - Pressurization (Component Labels)

Implemented Components

Gauges

M1  ... Forward Outflow Valve 

M2  ... Aft Outflow Valve

Switches

S1  ... Forward Outflow Valve Manual Control 

S2  ... Aft Outflow Valve Manual Control 

S9  ... Cabine Altitude Horn Cutout

Switch Lights

SDS1 .. Forward Outflow Valve Manual/Automatic

SDS2 .. Aft Outflow Valve Manual/Automatic

Annunciators

DS12 .. Forward Safety Valve 

DS13 .. Aft Safety Valve

Cabin Temp Control (*791/A9)

The Cabin Temperature and Engine Bleed Control panels are combined. The lower portion of the panel is the cabin temperature control panel. For the ATA 21 integration we will only cover the lower portion of the panel. 

Implemented Components

Gauges
M4  ... #1 Turbine Bypass Valve Position 
M5  ... #1 RAM AIR Valve Position
M6  ... #2 Turbine Bypass Valve Position 
M7  ... #2 RAM AIR Valve Position
M8  ... #3 Turbine Bypass Valve Position 
M9  ... #3 RAM AIR Valve Position

Switches
S1  ... #1 Valve Cool
S2  ... #1 Valve Warm
S3  ... #2 Valve Cool
S4  ... #2 Valve Warm

S5  ... #3 Valve Cool
S6  ... #3 Valve Warm

S7  ... Area Overheat

S8  ... Area Overheat Test

Switch Lights
SDS10 . #1 Pack Flow Control
SDS11 . #2 Pack Flow Control 
SDS12 . #3 Pack Flow Control
SDS13 . #1 Valve Control Auto and Manual
SDS14 . #2 Valve Control Auto and Manual
SDS15 . #3 Valve Control Auto and Manual
SDS16 . Left Hot Air 
SDS17 . Right Hot Air
SDS18 . Left Isolation Valve
SDS19 . Right Isolation Valve
SDS20 . Ground Cont
SDS21 . Flight Station Open Close
SDS22 . Forward Cabin Open Close
SDS23 . Mid Cabin Open Close
SDS24 . Aft Cabin Open Close
SDS25 . Galley Open Close

Annunciators
DS7 ... Area A Overheat
DS8 ... Area H Overheat
DS9 ... Area B Overheat 
DS10 .. Area C Overheat
DS11 .. Planum Duct Overheat

L-1011 ECS: Cabin Pressure Control Panel (21-17/18)

The pressure control panel is located on the FE/SO upper control panel. The pressure control panel provides selection of the cabin pressure planned parameters. On the panel, the selectors and controls asset in the programming of the required cabin pressure, while indicators monitor the outflow valve position. The resulting cabin altitude levels and rates of pressure change are provided by aircraft indicators in order to monitor passenger comfort levels. 

The profile displays the cabin altitude in relation to normal operating differential pressure. As can be seen, the cabin can be maintained at sea level up to an aircraft altitude of 22,000 feet and will not exceed a cabin altitude of 8,000 feet when the aircraft is at 43,000 feet when maintaining a max. operating pressure differential of 8.44 psi. 

L-1011 ECS: Cabin Altitude And Differential Pressure Indicator (21-23)

The main function of the cabin altitude and differential pressure indicator is to monitor the cabin altitude and cabin differential pressure throughout the flight.

The indicator has two scales (or two instruments in case of a TWA or Eastern panel configuration), the outer scale and larger pointer represents cabin altitude in thousands of feet. The barometric knob and window are used to set the correct barometric pressure in inches of mercury to correct for non-standard day conditions. Only this gauge is affected by the setting.

The inner scale and small pointer indicate cabin differential pressure. The amber range represents the maximum differential pressure controlled by the outflow valves. The red range represents the maximum  differential pressure which is limited by the positive pressure safety valves. Note that the maximum differential pressure for takeoff and landing is 0.15 psi and is placarded above the indicator. A pressure lower than 0.15 psi will allow all exits to be able to open in case of an emergency evacuation. 

L-1011 ECS: Outflow Valves (21-22)

Pressurization is maintained by two electrically driven outflow valves. Both outflow valves are located on the lower left side oft he fuselage. The outflow valves maintain pressure by restricting the outflow of air from the aircraft. With one outflow valve closed, the remaining outflow valve is sufficient to control normal pressurization.

Each outflow valve has an AC and DC motor which drives the outflow valve through common push rod linkage and gearing. The AC motor is utilized for normal (automatic) operation and standby (semi-automatic) operation. The DC motor is used as a backup in case of failure of the normal or standby mode. Manual operation is selected by latching in the MNL switch lights and controlled using the three-position toggle switches.

The position of the outflow valve is displayed by two indicators on the pressurization panel. 

L-1011 ECS: Normal Rate Selector (21-21)

The normal rate selector controls the cabin altitude rate of change when the mode selector is in the NORM position. When set a the DOT, the cabin rate of change is approximately 500 feet per minute up and 300 feet per minute down. The desired rate of cabin change can be changed by selecting different positions with the rate knob. Here are the pressurization rate limitations:

Ascent

• Minimum 50 to 400 FPM
• Dot Indexed 400 to 850 FPM
• Maximum 1250 to 2400 FPM 

Descent 

• Minimum 50 to 350 FPM
• Dot Indexed 150 to 550 FPM
• Maximum 750 to 1450 FPM

L-1011 ECS: Cabin Pressure Control Panel (21-20)

Cabin pressure is controlled electronically and is regulated electromechanical. The pressurization control system regulates and maintains cabin pressure and the rate of cabin pressure change as a function of settings on the pressure control panel. This is accomplished by regulating the exhaust airflow from the cabin through two motor-driven, variable-gate cabin outflow vales. The pressurization control system generates the necessary controlling signals to regulate the position of the outflow valve gates. This can be accomplished in any of the three modes of operation: Normal (NORM) Mode, Standby (STBY) Mode or MANUAL. The pressure control panel also provides a check (GND CHK) mode that conduct checks of the system while on the ground. 

Avionics Bending: Cabin Vertical Speed Indicator (Analysis)

There are a number of different configurations of the Cabin Pressuizaiton control panel for the Lockheed L-1011. Most L-1011's had a combination instrument that would show cabin pressure and cabin altitude (similar to what you would find on the overhead of Boeing 737 panel). However, versions of the L-1011 had a discrete VSI, Pressure Differential and Cabin Altitude instrument .... very much like the Boeing 747. The pressurization panel that I have happens to be one that takes all 3 instruments. Today, we are going to look inside the fully analog CABIN VERTICAL SPEED Insrument.

Top View: The inside of the instrument is dominated by the large copper disk that flexes under different air pressure and controls the needle. Air will either enter the instrument at a fixed rate or leave it at a fixed rate ... hence, the differential will show the cabin climb or descent in thousands of feet per minute.

Avionics Bending: Cabin Pressure Control Module (Analysis)

I described the function of the Cabin Pressure Control system of the Lockheed L-1011 in a previous posting: Original Flight Hardware: Cabin Pressure Control (Sub-Panel) In this analysis we will discover that the actual control panel is a rather simple device consisting of a number of switches and two tapes for ALTidude SET and BAROmetric pressure SET. The pressurization control module on the L-1011 was built/designed by the engineering company Hamilton Standard. (By the way, they also designed the life-support systems for NASA's space suits amongst many many other things).


The only enclosure component that can be easily removed is the top panel.


Top View


The tape mechanism in the cabin pressure control module is significantly simpler than the tape used in the trailing edge flaps indicator. In essence, the tapes are the same size as 35mm motion picture film and use a similar transport mechanism (one perforation side only) with a supply and a take-up reel. For position encoding, the tape is linked to a potentiometer (see image below).



The original flight hardware had a number of pressure switches that would actuate different functions of the instrument. The pressure switch and the pressure transducer in the image below will not serve any function once we re-work the module to connect to the flight simulator.

Original Flight Hardware: Cabin Pressure Control (Sub-Panel)

After a few days of rest (I took a brief vacation), I am continuing my blog postings with the last and final panel of the 4 sub-panels of the pneumatics panel of the Lockheed L-1011. The pressurization system of the Lockheed L-1011 is electronically controlled. Generally, pressurization is controlled by the control of cabin exhaust.

Cabin pressure is controlled by a selective, automatic system with manual override control. Major components include the cabin pressure control panel (discussed here), the cabin pressure controller, and two electrically operated cabin outflow valves.

Pressurization control is normally operated in the automatic mode. In this mode, the flight crew selects the desired cabin or aircraft altitude prior to takeoff, and sets barometric correction and landing filed altitude prior to descent. The pressurization system is fast responding and pressure transients are controlled below normal threshold of detection. In the automatic mode, cabin altitude is controlled at a fixed rate of change until the selected altitude is reached. For increasing altitude, the rate is 500 feet per minute; and for decreasing altitude, it is 300 feet per minute. Other fixed rates of change may be selected by the flight crew with complete automatic operation. The selectable range for increasing altitude is 200 to 1500 feet per minute and for decreasing altitude the range is 120 to 900 feet per minute. The selectable rang for cabin altitude is from 1000 feet below sea level to 10,000 feet above sea level.

The controller automatically controls the outflow valves to limit cabin pressure differential to a nominal 8.44 PSI. This provides a cabin altitude of 8000 feet at an aircraft altitude of 42,000 feet. A sea level cabin altitude can be maintained up to an aircraft altitude of 22,000 feet without exceeding the maximum pressure differential.

In the top left corner are indicators for the safety relief system of the L-1011 pressurization system. The two annunciators are "SAFETY VALVE FWD" and "SAFETY VALVE AFT". If OPEN is illuminated, the respective pressure safety relief valve is open. The CABIN ALTITUDE HORN CUTOUT SWITCH is a momentary push button which silences the warning horn when cabin altitude reaches 10,000 feet.


Right below the safety relief system indicators are 3 instruments (one is missing in my current collection). The three instruments are:

• Cabin Differential Pressure
• Cabin Rate of Climb Indicator
• Cabin Altimeter (missing from the picture)

In the image above I have highlighted the ALTITUDE SELECT INDICATOR AND KNOB. The ALT SET knob sets selected cabin flight altitudes. FLT shows present aircraft altitude. CAB shows cabin altitude when pressurized to maximum differential. The BAROMETRIC CORRECTION INDICATOR AND KNOB is used to set the barometric pressure and program pressurization controller.

In the image above we can see the three main system selector knobs. From the left to the right they are:

• MODE SELECT SWITCH ... GRD CHK - permits normal system check on the ground. NORM - selects fully automatic pressurization at rates selected by normal rate knob, provided manual switches are not illuminated. On the ground the outflow valves are fully open. STBY - selects cabin altitude rates selected by standby rate knob without isobaric inputs to pressurization controller, provided manual switches are not illuminated.
• NORMAL RATE KNOB ... permits selection of automatic cabin altitude rate of change when mode select switch is in NORM. In detent, the cabin will ascent at 500 FPM and descent at 300 FPM. Rotation the knob out of detent increases or decreases cabin rate of change. Cabin altitude will level off at preselected cruise altitude.
• STANDBY RATE KNOB .. permits selection of manual cabin altitude rate cahgne when mode select switch is in STBY. HOLD - no cabin rate change, DSCD - increase cabin altitude rate of descent from 0 to 1000 FPM. ASCD - increases cabin altitude rate of ascent from 0 to 1000 FPM. The preselected altitude function is not operative.

The right hand side of the CABIN PRESSURE CONTROL panel is dedicated to manual operation. On top we have the AUTO FAULT LIGHT which, when illuminated indicates a problem with the automatic pressurization system in respect to out-flow or cabin altitude limits. The two outflow valve position indicators can be controlled with the toggle switches if the respective valve has been put into MANUAL mode (MNL will be illuminated).