This blog documents a project to build a functional Lockheed L-1011 cockpit replica
using real flight hardware, NI labVIEW and the x-Plane software platform.
The Litton INU display units are terminal units, often referred to as a Central Display Unit (CDU), that use a variant of the ARINC 429 standard to display information and read character input. A few months ago I started writing software for these INU CDU units. Today we installed three of these units in the L-1011 simulator. Typically L-1011 routes across the Atlantic would have had 3 of these units installed, and this is what we are simulating here.
Just with any of other ARINC 429 compatible instrument in the simulator, we are hoping to drive the Litton LTN-72RLs with the SIM429-11 interfaces.
Today we took on the installation of the pilot's overhead panel and the installation of the CB2 overhead circuit breaker board. The overhead panel is a massive piece of hardware and it took 4 of us to lift it and mount it in place. The panel interconnects with all other pieces of the simulator and gives it a large degree of stability. In the image above and below you can see Jim Blasco working on the interconnecting bolts between the overhead panel and the CB2 breaker panel.
L-1011 CPT Overhead Panel Installation
Each sub-panel of the CB1, CB2 and CB3 breaker panel is hinged and opens up. In the image above you can see Jim's head obstructed by the lowered left hand panel of the CB2 breaker panel. The image below shows the mounted overhead panel with all of the instrumentation removed. The panel is split into three major groups: a) the two eyebrow panels with the light controls, b) the pilot's overhead panel with four sections of instruments (or columns) and c) the CB1 breaker panel (which is after on the overhead)
L-1011 CPT Overhead Panel Installation
The image below shows the aft section of the overhead panel which is occupied by the CB1 breaker panel.
L-1011 CPT Overhead Panel Installation
The following two images illustrate how the overhead panel connects to the CB2 panel which then connects to the flight engineer's station. All three panel elements together make up the main structural overhead pieces of the L1011 cockpit procedure trainer. The images also show a lot of the real flight hardware already mounted into the overhead panel and the second officer console.
L1011 CPT Overhead Panel Installation - Pilot's Overhead and CB1
L1011 Overhead Panel and CB2 Panel Installation CB1, CB2 and SO Station Interconnect
Now that all the structural pieces of the sim are in place we can start placing the instrumentation in the correct places. The images above and below show the start of the instrumentation outfit of the L-1011 Cockpit Procedure Trainer. Unlike the original trainer, we are using actual flight hardware from several L-1011s to allow us to illuminate the instruments and to interface them with the flight simulation software as the real instrument. Below you see the Litton LTN-72 CDUs mounted on the forward center pedestal.
The simulation of the Litton INS system would not be complete without the simulation of the INS alignment. The Litton LTN-72 uses ultra precise gyroscopes with ring-lasers that need to be aligned with a known position prior to aircraft movement. The status of this alignment process is indicated by an "ALIGN" indication on the status page of the LTN-72RL CDU. The image above shows the the Mode Control Unit (MCU) that is typically mounted on the overhead panel of the aircraft. It is put there so it is out of the way because it really only needs to be touched during alignment prior to aircraft movement.
The INU starts the process once the switch is lifted out of a locked Standby (STBY) position and moved to the Alignment (ALIGN) position. In the image below you can see that the process starts with a status of 90 and counts down to a status of 2. The INU also estimates the time for alignment to be 14.6 minutes. This value also counts down as we work towards status 2. For the simulation it is important that the simulator does not report aircraft movement or the alignment will not complete or will be wrong.
Once the INU reaches Status 2, the last alignment status, the green READY NAV light comes and indicates that the INU can now be put into navigation mode. Navigation mode should not be entered until we are ready to move the aircraft.
Once the INU is in navigation mode (see image below) the timer starts counting up to give us an indication for how long the INU has been running. For a precision INU, such as the LTN-72, i think the average error is 2NM over a 4 hour period.
Below is the display sequence as published by Litton in the LTN-72 Pilots Operating handbook.
Litton LTN-72RL CDU Development Setup with X-Plane
Still working on the Instrument Data page of the Litton LTN-72RL CDU. The data page actually incorporates a great number of X-Plane data refs and I finally got them all to work. A few of them, like drift angle, turned out to be a bit of head-scratcher.
Here is the list of X-Plane data refs that feeds the CDU's Instrument Data page:
The first image shows the aircraft sitting on the runway end, ready for take-off. You see that the "Wind Data" line is blanked because we have not reached the minimum target speed of 100KT yet. The wind data is only really relevant in flight and is used on this page to compute the Drift Angle. The track heading that is dialed into the INS is a course from Seattle Tacoma direct to JFK (a bit unlikely ... but good enough for the test.
Litton LTN-72RL - On Ground
In the image above you see that on the HSI we are expecting the course pointer to point to 063º for our track and the the compass card to indicate a heading of 161º. Both are magnetic as indicated by the "M". You see that we have a ground speed of 0 but a True Air Speed indicated of 2KT ... that's our headwind component that points directly into the pitot :)
Once we are airborne the Instrument Data page can be used as a cross reference to the HSI, the Airspeed Indicator and the Altimeter. In the real aircraft installation, the INS would actually send all this data to the HSI if it is selected as the source for the HSI.
X-Plane ADI and HSI showing the corresponding INS data.
In the image above and below you see the direct correlation between the X-Plane simulated ADI and HIS as well as the output on the Litton LTN-72RL CDU. This is a few minutes into flight and we are climbing through an altitude of 5600ft on a heading of 034º magnetic towards our track of 064º magnetic. I tried to take the two images as close to each other as possible, but you see that the LTN-72RL indicates a ground speed of 244KT and X-Plane shows roughly the same (246KT).
Litton LTN-72RK showing heading and speed data.
The Instrument Data page of the LTN-72RL has more than 5 lines of data. To reach the additional data the display needs to be scrolled down with the down arrow. The additional lines give data for body acceleration along the track (G's) as well as Drift Angle and Altitude in Feet and Meters (this data is not corrected for pressure). The image below shows the X-Plane rendered altitude, data and the corresponding data output on the Litton LTN-72RL Instrument Data page.
X-Plane ADI Altitude Band Output
LTN-72RL Altitude Information on Instrument Data Page
On climb-out and as we are approaching our target altitude of FL200 (20,000ft) the simulated weather outside gets smoother and the LTN-72RL indicates a longitudinal axis acceleration of 0.095 Gs. You are probably familiar with this when you fly on a regular commercial jet, it's that gravity pull that makes you feel just a little heaver in your seat. However, 0.095Gs you would probably not feel and it would be quite comfortable. Given the wind at 18,400ft we have also computed a Drift Angle (DA) of 6.4º nose right ... as indicated by the "R" in the DA line.
Litton LTN-72RL Track Acceleration and Drift Angle
Writing the integration for the LTN-72RL is really a lot of fun. Again, the device uses ARINC 575 as the data transfer from the computer simulated INS to the display. The INS simulation runs on the same computer as x-plan ... so all it really requires is my laptop and the ARINC 429/575 interface.
Worked on the implementation of the Instrument Data page. The instrument data page shows the INS data that is sent from the INS to the flight displays. There are still a bunch of challenges with the display refresh method ... i am thinking about using line blanking which is available to the CDU via a specific data label.
Litton LTN-72RL Data Field Examples
The instrument data page can show heading and track data as Magnetic or True. All speed data is shown in Knots. Here is the data page from the Litton manual showing the flow of ANS data to the HSI as well as the layout of the first 5 lines of the Instrument Data page:
Litton ANS Data and the HSI (from Litton LTN-72 Pilot Reference)
Litton LTN-72 Instrument Data Page (from Litton LTN-72 Pilot Reference)
Below is a short video showing the Instrument Data page in action:
First Litton LTN-72RL CDU Integration with X-Plane
So far all of the data you have seen on the Litton LTN-72RL CDU was test data. Today I finally got around to integrate the Litton CDU with X-Plane. For this first setup i chose the data that is displayed on the POSITION DATA page of the CDU; this data is Latitude, Longitude and time in GMT (or Zulu).
Here are the X-Plane data refs that I feed to the CDU via the ARINC 429 interface using ARINC 575 data words:
While X-Plane does not directly talk to the CDU (it talks to a CDU device driver that does the ARINC formatting and data transfer via UDP) I still chose to implement the data extraction out of X-Plane following the ARINC 429 label standard. The labels are 0310, 0311 and 0150. Below is the code snipped from the X-Plane plugin that builds the extract data:
Lets put all this in motion. Watch the short video below to see the LTN-72RL CDU display the LAT, LNG and GMT data from X-Plane in real time via the ARINC 429/575 data stream:
Took a few days off over the holidays. Hope everybody had a few nice days as well and is getting ready for 2013! I am now back to working on the Litton LTN-72RL implementation with the flight simulator using an ARINC 429 interface and the ARINC 575 characteristic. Before the CDU can talk to the simulator, however, the functions of the INS and IMU need to be simulated in software as described in my previous blog posting.
The Litton INS is arranged into pages. Each page can have multiple lines and can also have sub-pages. The CDU, however, has room for 5 display lines with 16 characters each. In order to reach lines below the 5th line the up and down arrow keys are used. Litton referred to this as slewing the page up and down ... I guess today we would probably call it scrolling. The Litton CDU does not use ASCII code for the symbols but has a 6-bit character set defined by Litton. Therefore, in order to paint data on the screen the following needs to happen:
Translate each character from ASCII to the Litton Character set (this is done on the INS simulator side)
Package characters in groups of 4 into the 32bit ARINC 575 word.
Transmit the ARINC 575 word to the LTN-72 CDU via the ARINC 429 interface.
The same, of course, has to happen in reverse for every key entry. Again, for key entries, Litton encoded each key on the keyboard as a 6 bit character.
In Litton's documentation, each screen and scroll sequence is sometimes represented by a stack of images (as seen below). So far I have worked on the database validation page as well as the position entry page. If you want to know more about the LTN-72RL document that I am using here for the blog, you can find it here: LTN-72RL Pilots Guide
Database Validation
From TP72RL-73-00 Litton Aero Products
Litton LTN-72RL CDU Simulator - Database Data Validation Entry
Once the database validates the INS will be ready for use. The LTN-72RL was originally not fully Y2K (there is blast from the past) compatible. However, it is now understood that "12" for the years does not stand for 1912 but for 2012. I also added the liberty of calling the version "72-73-CZ" instead of "72-73-00" as would have appears on the actual INS to assure that the screen I produce can not be mistaken for actual Litton screens.
Initial Position Data Entry
Prior to INS alignment, the initial aircraft position has to be entered as a set of longitude and latitude coordinates. The POSITION DATA page allows for entry of this latitude and longitude data as well as GMT.
From TP72RL-73-00 Litton Aero Products
The GMT line is actually on line 6 which is initially hidden from view. To get to the GMT line the display has to be moved up one line.
LTN-72RL CDU - Position Data Page
LTN-72RL CDU - Position Data Page scrolled to see GMT
The Litton LTN-72RL implements a complete Area Navigation System (ANS). On the actual aircraft, the ANS consists of:
Inertial Navigation Unit (INU)
Computer Display Unit (CDU)
Mode Selector Unit (MSU)
Litton LTN-72RL CDU and MSU
For the INS simulator integration I will emulate the INU and attach to the actual CDU and MSU. In a typical installation the CDU would be mounted on the center pedestal or the forward electronics pedestal. The MSU would be mounted on the overhead panel. Each INU on board (most aircraft have two or three INUs) has a corresponding MSU and CDU.
To create a simulation of the LTN-72 my simulation software needs to interact with the CDU in a way that appears natural to the way pilots interact with the LTN-72RL. Talking to the LTN-72RL CDU is actually quite simple. The CDU sends data in the form of keystrokes to the INU and the INU in return sends data on how to paint the screens back to the CDU. See the image below:
INS Simulator Topology
The Litton LTN-72 system has quite a large number of screens that need to be painted by the simulation software. Here are the data pages that need to be simulated and integrated in a way that they can interact with the flight simulator:
Position (POS) - shows present position in Longitude and Latitude; Bearing and Distance from station; GMT and others
Course (CRS) - displays the TO and FROM waypoints; the desired track and the cross track
Track Leg Change (LEG) - permits the entry of FORM and TO waypoints; leg change entry; automatic and manual selection
Waypoint (WPT) - provides pages for waypoint entry and route entry
Flight Plan (FPL) - here we will enter waypoints and compute the flight distance
Direct To (DIR) - enter data to fly direct to
Instrument Data (DATA) - shows the INU data for heading; track; groundspeed; true airspeed; wind and along track acceleration
Catalog (CAT) - provides pages displaying a station catalog; airport catalog; waypoint catalog; route catalog; location stations and a way to update catalog data
Remote Ranging (RMT) - enables remote data from another INU unit, or the cross-filling of data from one INU to the others.
Status (STS) - Provides information on IMU alignment status, time to NAV mode and other action codes
Below is a short video showing a simple READ and WRITE test from the ARINC interface to the CDU. Data is being read on ARINC 575 label 030 and written with 21 labels starting at label 0200.
Area navigation on the L-1011 was accomplished by the use of an Inertial Navigation System. Some L-1011s also used the now dismantled OMEGA system for area navigation. OMEGA was more or less the first global position indication system that used a network of terrestrial antennas to create signals that could be triangulated into a position with roughly 8 NM accuracy. OMEGA was replaced by the public version of GPS around 1996.
Inertial Navigation Systems (INS) are still very much in use today. I will not go too much into detail on how INS technology works, but trust me, it is super cool. Read about it on Wikipedia by following the link. Present day Inertial Reference Units (IRUs) use lasers and are ultra precise. One of the main manufactures of INS and OMEGA systems was Litton Aero Products which got acquired by Northrop Grumman in 2001.
The image below, taken from airliners.net, shows three Litton CDUs installed in the RAF L-1011 tanker.
from. airliners.net (C) Duncan Monk
Over the next few weeks I will be creating a simulation of the Litton LTN-72RL INS. The LTN-72RL CDU basically acts as a terminal for the INS unit and in a triple system installation, as in the image above, all 3 INS units can be synchronized with each other. Here is the test setup on the workbench:
Litton LTN-72RL CDU and INS MODE SELECTOR on DZUS Rails
Litton LTN-72RL CDU and INS MODE SELECTOR on DZUS Rails
Litton LTN-72RL CDU and INS MODE SELECTOR on DZUS Rails
Litton LTN-72RL CDU and INS MODE SELECTOR on DZUS Rails
For this series of avionics bending I will be using the ARINC 575 protocol format. ARINC 575 is electrically identical to ARINC 429 however the general word format is different. ARINC 575, just like ARINC 429, uses an 8bit label and a 24bit data filed. However, unlike ARINC 429 the 575 format does not compute parity, does not have a SID or a defined SSM filed. The Litton CDU uses ARINC 575 words for data between the CDU and the INS unit in both directions. In the next posting we will take a look at the general system architecture and how I will go about creating and INS system and then integrating it with x-plane.
Mostly heads-down programming on the device driver for the SFENA HSI today, however, I also received some cool new toys. I got a set of Litton LTN-72RL Control Display Units (CDUs) from Lee King in Arizona today. Litton Aero Products, the original manufacturer of the CDU and the INU it was connected to is now part of Northrop Grumman. The units are really neat and it will be lot of fun trying to make them work (of course after the HSIs are done).
Litton LTN-72RL CDU
The CDUs are vintage 1981 and have spent a really long time powered off ... first in the desert and then in storage. They are really clean and, despite their age, powered right back up. The units are driven by a 28VDC source. The next few steps will be to get into the digital protocol that the units used between the INU and the CDU.
I actually found the pinout for the CDUs in a technical reference manual for the KC-10.
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