Tuesday, May 01, 2007

Avionics Upgrade - Part 7

So far we have disassembled the panel, fabricated some filler panels, and sent the exposed parts out to be painted, covered with leather, etc. It will take awhile for the pieces to be returned, so at this point we can focus in on wiring.

Unlike computers or most anything you buys now days, avionics require that most of the wiring be fabricated manually. It is a tedious process and if you are not careful costly avionics can be destroyed. It is absolutely critical that the installation manuals are read cover to cover before attempting wiring on your own.

If you are not comfortable doing the tedious wiring I highly suggest outsourcing the core construction of the harness to a professional. NOTE: Businesses that focus on the Experimental Aircraft market such as Pacific Coast Avionics will custom build the core harness for a nominal fee. This is well worth the few hundred dollar investment.

The tools required to build the harness depend on the avionics you are installing. 36G's project has products from Garmin, King and PS Engineering. King used to be the industry standard, but Garmin can now make that claim. If you are installing all new, I would recommend staying with the Garmin line. PSE has products that will work with either. The PMA-7000 that I have uses the King standard connectors whereas the PMA-8000 line follows the Garmin. Changing crimp connection types can add hundreds of dollars to a project since you will have to purchase the appropriate tools.

The first section of most installation manuals detail the tools and dyes required to do the job. Before starting a project like this it is important to read the manuals. If you find that the tooling is too costly then have someone else build your harness. You cannot do this without the proper crimp tools and connectors. You also need quality pin insertion and extraction tools. It is very easy to put a pin in the wrong location. When they are in, they are in. It is nearly impossible to remove them without the proper tool to do the job.

The remaining tools are standard wire tools. You will need sharp diagonal wire cutter, wire stripper, soldering iron/solder, heat shrink tubing and various crimp tools. You will also need an assortment of crimp connectors and other aviation approved pieces parts -- some of which may be hard to find. Aircraft Spruce sells some stuff, but I've found that Pacific Coast and Chief Aircraft have a better selection. I recommend calling some approved parts distributors and have them send you a catalog well before starting a project like this. It is easier to shop for parts when you have a book to reference.

You will, of course, need lots of wire. It is absolutley critical that you use aviation approved MIL SPEC wire. You cannot buy this stuff at Radio Shack. Aircraft Spruce, PCA, etc. all stock wire. Just make sure you purchase the appropriate size specified in the install manual. NOTE: Wire size is contrary to how most think. The larger guage number is actually the thiner wire (e.g. 18 guage wire is thicker than 20 guage). I generally go for about the middle of the range specified by the Install manuals to be safe. You don't want to go too large since it will add unnecessary weight and girth to your wire bundles.

Approved Data
Installation of avionics into a certificated aircraft requires that you use approved data. Most installation manuals qualify, but they do not cover everything required such as electrical load testing or how to solder a wire. The FAA Advisory Circular AC43.13 is the approved source for wiring standards. The booklet is very specific about how to wire an aircraft. Follow it to the letter unless the Approved Installation Manual says otherwise. The Install Manuals are always the one to follow if there is ever a conflict.

The installation will also require that an FAA Form 337 is completed and approved by an Aircraft Mechanic that holds an Inspection Authorization (IA). Again, if you are not an IA, you need to connect with someone approved to return the aircraft to service and work though the details well before starting a project like this. You do not want to find out you can't get someone to sign-off on your work or that they require you to change something that took you hours or weeks to complete.

Communication Protocols
36G's Extreme Avionics Makeover calls out for installing a Garmin GNS 430W GPS, Garmin MX-20 MFD, Garmin GTX-330 transponder, upgrading the KX-165 to one with a glideslope board, and integrating everything with the existing PS Engineering PMA-7000 Audio Panel, King KAP-150 Auto Pilot and Flight Director/HSI system. Most of the systems were built by different manufactures and what we are installing didn't even exist when the legacy system install manuals were written. Even systems from the same company have a different evolution. For example, the Garmin MX20 used to be owned by UPS AT; however, the GNS430 was built by Garmin Corp. The manuals do not always detail exactly how to hook the systems together. When you do, you create a Technically Advanced Airplane (TAA). Traditional avionics were primarily standalone and/or didn't rely on other equipment to function.

The key to being successful is to clearly understand how the systems will communicate. There are several types of protocols used in aircraft and the components have to be configured to use the right one or they will not function. I generally focus in on the most capable unit and figure out what it can and cannot do then expand outwards. In 36G's case, this would be the GNS430W GPS. The 430's and 530's are remarkable. They work with just about anything, which is good and bad. The good is it will probably work, the bad is that it is not always straightforward on how to get it to work.

To digress a moment, I thought I should discuss a bit about the GNS430W, which is a WAAS certified unit. WAAS certification is an entirely new standard that enables aircraft to fly ILS-like approaches solely using GPS signals (e.g. /G designation). The downside to installing a 430W/530W, however, is that they are only certified to work with external components specified in the Installation Manual. Even if they will talk to something else you cannot connect it and be in compliance with the STC. There are also very specific requirements about where the system is mounted, the antenna type, wiring, etc. Carefully read the manual before deciding to install one of these units. The GNS-430 and GNS-430W may look the same, but they are not the same. If you have components that are not certified and you intend on keeping them then you cannot interface them with the 430 without getting FAA field approval (good luck with that ;)

The next step in the communication protocol game is the MX-20, which was initially manufactured to work with UPS systems. The manual doesn't even mention Garmin who was a competitor at the time. The primary communication channel on the MX20 for position awareness is RS232. Those familiar with computers will recognize this format. It is a standard serial connection protocol that is found on most computers. For the MFD to function it must receive a continuous feed of longitude/latitude information in order to plot the aircraft position. The MX20 is useless without external input. The GNS430 has three RS232 ports. Any port will work with the MX20 as long as the 430 port is set to the Aviation protocol without altitude information.

The MX20 also requires a serial input from an external altitude encoder, which is why altitude information should not be sent form the GPS. Most legacy encoders use a graycode wire configuration, which is basically 10 wires that when grounded represent a hexadecimal number. Altitude is derived depending on what combination of wires are grounded. I will be installing a new Trans Cal serial/graycode encoder for 36G's Makeover. The Trans-Cal is an excellent unit and has 2 separate RS232 connections that can be independently configured (it technically has 3, but the third is for programming only). The key take-away from interfacing the encoder to the MX20 is that it uses the UPS Protocol for altitude information whereas the GTX-330 Transponder requires the Garmin protocol. They are both RS232, but the ASCII code string is different. The Trans-Cal, similar to most serial encoders, requires you to connect a PC directly to the unit in order to program the RS232 ports to operate different protocols. Read the manual carefully or you will not receive altitude information!

The next Technically Advanced System is the Garmin GTX-330 Transponder. This is a highly capable unit and will receive Traffic Information Service reports from the ground radar stations and display them on the GNS430 as well as the MFD. The GTX-330, however, communicates on the ARINC 429 bus, which is not the same protocol as the RS232. The GTX-330 also requires bidirectional signaling. In other words, the connected units need to talk to it and it needs to talk to them (e.g. requires two wires instead of one). The GNS430 has four ARINC 429 communication ports. The GTX330 can be connected to any of them so long as the 430 setup is configured for the GTX-330 with TIS at the same speed as the GTX330 is configured to send (generally set to high on both). Connecting the GTX330 to the MX20 requires that the MX20 has the Traffic Module installed (not all of them do). The basic difference is that the standard MX20 does not support ARINC 429. The GTX330 can also be connected to an Outside Air Temperature probe that will enable the unit to display pressure altitude. It will also send the OAT information to all units on the ARINC 429 bus. NOTE: the OAT used for the GTX-330 needs to be dedicated to the unit. You cannot connect it to an existing unit. Be careful here. Interfacing with the wrong type of probe will fry the 330.

The final technically advanced system I will be installing is a Garmin GDL-69A XM Radio/Weather unit. I haven't spoken much about this system since it is mounted on the remote avionics shelf, but in my opinion it is worth its weight in gold. It receives near real-time weather information and will display it on the GNS430 and/or the MX20. The A designation in the GDL-69A name indicates it will receive XM Radio as well -- All piped into the audio panel -- Wow! The GDL69 also requires a bidirectional signal, but uses the RS232 protocol. The 69A has 3 RS232 ports, so it can be connected to three control units simultaneously.

HSI and Autopilot Connections
The next consideration in connecting the GNS430 is interfacing it with the legacy systems for CDI indication and autopilot functionality. I will be integrating 36G's systems to the existing King KI525A HSI and KAP-150 autopilot. The legacy systems are connected using the tried and true variable resistance for course deviation. What this means is that the 430w will change the ground and/or power voltage, which will cause the needle to move in the opposite direction. Surprisingly enough, this is a very accurate system. The NAV flags are similar, but they receive positive voltage. When there is no power, the system is flagged. When there is power, the NAV flag will be lifted, the To or From will be displayed, etc.

The King 525A HSI is stil the industry leader. It is a very reliable and accurate unit. The key to getting it to work is to make sure that all connections are made exactly how the GNS430W says to connect them. Initially with 36G I had some issues getting the glideslope to operate. It turns out that in legacy KNS80 RNAV installations not all of the wires were connected. I removed a KNS80 and neglected to notice that the wire connections did not go completely through the system. Heed my warning. Read the manual and follow it to the letter.

The KAP-150 autopilot is about as easy as it gets. It has 4 primary wires that need to be connected and the system will follow it. Left & Right Course Deviation and Up and Down glideslope. In order to get the KAP-150 autopilot to follow the glideslope, however, you also need to connect the ILS Energize from the GNS430 to the Approach Active in the KAP-150. This tells the system that the approach is active. Without this wire connected the autopilot will not follow the glideslope.

Reference Material
I have found some links on the web you may find useful. They are not all approved data, but are helpful nonetheless.

King HSI 525A Test Procedures
HSI 525A Interface to GNS430 Schematic (PDF)
KMA-24 Audio Panel interface to GNS 430 (similar to PSE)
Various Avionics pin-out diagrams (Includes GNS430, KX165, etc)
Essco Aircraft Manuals Website -- great source for difficult to find Installation Manuals
FAA AC43.13-1B Advisory Circular
L J Aero's Aviation Links -- About every aviation related business is listed here
Stein Air -- Good source for miscellaneous parts and connectors

Now that the initial wire harness is complete, the next step will be installing everything in the Airplane!