L-1011 Trainer Installation: INU CDU and ATC Transponder Power-Up

L-1011 Litton INU CDU

Today Jim Blasco and myself installed a number of items into the simulator. We installed several computer components that will be used to drive the digital I/O as well as the ARINC 429 I/O. We also put power to all 3 of the Litton INU CDUs in the cockpit. There are two CDUs in the forward center console as well as a third on in the aft center console. You can see the #1 and #2 INU CDU in the image above. The picture below shows the cabling in progress.

L-1011 Litton INU CDU Cabling

We also powered up the ATC Transponder head as you can see in the image below. The ATC Transponder head that we have has facilities for TCAS. However, we will initially not build out the simulator for TCAS.

L-1011 ATC Transponder Power-Up

Avionics Bending: Inertial Navigation System (INS) - Part 3

Litton LTN-72RL CDU with Simulator Data

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:

1. Translate each character from ASCII to the Litton Character set (this is done on the INS simulator side)
2. Package characters in groups of 4 into the 32bit ARINC 575 word. 
3. 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

Avionics Bending: Inertial Navigation System (INS) - Part 2

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.

Avionics Bending: Inertial Navigation System (INS) - Part 1

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. 

Avionics Bending: Litton LTN-72RL CDUs (Pinout)

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.

Litton LTN-72R Pinout

L-1011 Navigation: INS CDU (34-43)

This is the CDU as it appears with the test switch pressed. The intensity of all controls which are shown illuminated is controlled by the DIM knob on top of the data selector. The illumination of the keyboard is controlled through the center pedestal rheostat (external to the device).