Daniel Jonas’ 1978 Duchess

 Until Daniel  Jonas taxied up to our hanger here in Santa Maria We had never done a complete installation with Custom Panels, Custom Lights, and all the rest of the stuff Avionics West is noted for on a Beech Duchess before.  Usually it’s the Cessna 182s and the 210s that get our treatment.  So when Daniel agreed to the upgrade I just had to add it to our feature projects.   I have to admit that a Duchess is one of the most enjoyable twins to fly in because of its excellent visibility.  From the front seat you can’t see the nose of the aircraft because of its downward slope.  Looking out the huge front window all you see is the world coming at you. Anyway------ Back to the project.  

When the plane arrived it was pretty well equipped with avionics:  Garmin 340 Audio panel, GNS 430, KX 155, and a Garmin GTX 320.  Not too shabby really.  What more could one want in a light twin like this.  Well,  Daniel wanted a lot more.  How about getting rid of the KX 155 and adding a second 430 for back up, and, how about a Ryan 9900B TCAD so you’ll know where everyone is up there, and,  maybe a GDL 69 Datalink Receiver so you won’t have the need to consult with the flight service station for your weather and TFRs before you leave,  and,  as long as we have a lot of data to display to the pilot,  we might as well just put it all together on an MX 20 I/O with Chartview Great! Now we don’t need any sectionals or airport directories.  I tried to talk Daniel into getting the GDL 69A with XM radio instead of the Vanilla model but he insisted on not having anything installed that would distract him from his duties of flying the airplane???????????  Maybe I should have suggested an autopilot!

 In order to make the new radios more useable and make the MX 20 more viewable to the pilot it would be necessary to move them to the left.  The only way that was going to happen was to relocate the huge engine cluster over to the right side of the panel and put the left hand radio stack in its place.  Now if we move the Century III autopilot and Radio coupler over to the left side of the instrument panel where the pilot can control it with his left hand we open up a large enough area to install a second radio stack with the MX 20 being at the top and the transponder below.  By the way,  Daniel also wanted to replace the GTX 320 transponder with a Garmin GTX 327. Good looks and no knobs!  Actually I find it better for me to have one with knobs. Having a knob gives me something to hang on to when I’m bouncing around up there in my Cessna 150.    

Well now its time for the fun part of the job, tear apart time!  You are looking at is the original aircraft instrument panel that was modified to accept the last configuration of radios and an overlay panel was fabricated to match the instrument holes in this original.  Someone with  a file and a lot of time on their hands had to duplicate the instrument holes in the overlay panel so they would fit just right to mount the instruments correctly.  As it was, the instruments were mounted to two thicknesses of .080” of material.  This caused the instruments to be set back so far that they weren’t even flush with the panel front.  We will not do that with our new panel even though the instruments are not to be moved from their present locations.  The plan here is to cut out the left side of the panel like the right side is leaving enough material around the perimeter to attach our new one.  This will bring the panel thickness back to the original dimension.  Working in an avionics shop and doing these installations is like being a detective in forensics.  As you dig down into the menagerie of wires, connectors and butt splices a story unfolds telling the airplanes’ history of avionics and electrical modifications.  It is especially revealing when there are layers of major upgrades still in the airplane.  Anytime a major avionics upgrade has been done without taking out all of the old equipments associated wiring, switches, mounting racks etc,  you end up with layers of installations and a much degraded wiring system as a whole.  Its like having a mess on top of a mess.  It finally becomes impossible to give the customer a quality product unless the whole kit and caboodle is taken out and redone.  I have had many disgruntled customers with airplanes I have refused to work on because of this condition.  Sorry!  The wiring in the Duchess was actually not too bad and since the only piece of equipment that was being removed was the KX 155 most of the wiring would stay.  The power, Ground, and audio wires for the old KX 155 would be connected to the new GNS 430 through an interface connector making it a plug-and-play ordeal.  All remaining interfaces in the aircraft wiring will be made into an 29 pin circular connector.  The mate to it will be on the new harness assembly for the MX20, transponder and data lines which will be fabricated on the wiring bench along with the mounting racks for each radio.  The idea is to do as much work on the wiring before it goes into the airplane as you can and when it does, make it as easy as possible to connect to all of the interfaces.  Some places where you must reach in general aviation aircraft are extremely hard to reach. There are many tricks of the trade that are only learned through experience.  The ideal avionics installer is 3 feet tall, ambidextrous, and can stand on their head for extended periods of time!

The Antennas are installed.  With the addition of the second GPS, the Ryan TCAD and the GDL 69 Weather Datalink Receiver we ended up with quite an array of antennas on the roof.  We always try to arrange them is some king

of order so as not to create a just-stick-it-anywhere look like so many planes have nowadays.  The top mounted Ryan TCAD antenna shown above is similar to a transponder blade antenna as it should since it receives transponder reply signals from the airplanes around you.  The ideal location to mount the top Ryan TCAD antenna is at the highest place on the fuselage so that is has an unobstructed view of the forward horizon.  This will allow for the systems best performance in looking forward for incoming traffic threats.  In the Ryan Traffic system the top antenna is responsible for sensing the targets forward and aft for Multifunction display purposes. 

The bottom antennas are responsible for sensing the left and right positions of the traffic.  The TCAD processor takes the signals from each antenna and is able to determine exactly what  bearing the traffic is positioned with respect to your airplanes heading..  The processor now calculates the vertical separation between your airplane and other aircraft by comparing the mode C transponder signal received from the other airplane with the your onboard altitude encoder which is hardwired to the processor.    So far we have bearing and

Vertical separation but what about distance?  How far away is it?  This is the grey area of the system and I haven’t had a straight answer from any of the Ryan engineers about how this works.  Since the Ryan 0900B TCAD is a passive system, it only receives.  Therefore the only way it can determine how far away an RF signal was originated, it would have to assume that the signal power was at a certain level at the time of transmission.  Now, us Avionics types know that all transponder out there don’t transmit at the same power level, in fact,  there can be as much as 200 to 250 watts peak difference between them!  Maybe we can get one of the Ryan engineers to write and explain it a little better.   

Coax Lengths are Important

Each one of the antennas is connected to the processor with two lengths of Coax.  Four in all.  Each one of these lengths of coax must have a 3db signal attenuation at 1000 Mhz for the correct loading on the antennas and amplifiers of the system.  Any more or less than that will cause the system to be degraded.  WE have found that 17 ft of RG 400 works just fine for just about any GA aircraft installation.  If the distance between the processor is less than 17 ft then coil the excess coax and stow it neatly by the processor.  Do not use RG 58 for short runs. RG 58 is inferior to these installations and Remember when coiling up coax or routing it in general    do not exceed its minimum bend radius which is 10 time the diameter!  I’m starting to sound like an aerospace quality inspector now.  Sorry! 

Installations like these takes good planning, innovativeness, and absolute attention to details.  There is probably not another profession which deals in such complexity and tedious tasks.  It usually takes 3 to 4 years to train an Avionics tech to be proficient at all of these things.  The reward of doing a good  installation   is the feeling you get when you turn on power for the first time after the install and everything starts up and works like its suppose to.  As far as Avionics is concerned, there is nothing worse than rework. By Steve Walker, Owner of Avionics West, Inc. For more information Drop Steve a Note.