By BOB ECKES
7930 E. Mabel Dr.
Tucson, AZ 85715
THE LONG-EZ is an outstanding cross-country aircraft having a range of over 1000 nm. You can cruise in pure comfort at 140 to 180 KTAS (depending on engine size), at altitudes up to the Positive Control Area. Having flown to the Sun 'n Fun and EAA Oshkosh fly-ins, as well as numerous trips in the southwest from my home base in Tucson, I was convinced that my Long-EZ needed additional and improved instrumentation and avionics. The ultimate choice would be to upgrade to full IFR capability, including a loran. Read on and find out about the decisions and compromises that faced me.
Since I am an "old" (but young at heart) fighter pilot, I am spoiled from having used the best: inertial navigation, autopilot, navigation/weapons data computer, projected map display, heads up display, etc. Transitioning from this to a VOR and whiskey compass has been a real "culture shock". I was compelled to do something about it.
The immediate obstacle (even before you empty your wallet) was limited panel space in the Long-EZ. Uncle Sam is not buying my toys now, so limited panel space helped to limit my spending habits. I found an elegant and straightforward solution to the very common problem of not enough instrument panel space. It is a nifty new instrument that combines several instrument functions into a single small package. This freed up enough existing panel space to allow loran installation and simultaneously upgrade the existing instrumentation. And I did not have to alter the existing instrument panel.
The II Morrow, Inc. 618C loran that I wanted to add fits right in a standard 3-1/8" instrument panel hole. There was no space for another hole in the panel. To add the loran I needed to free up an existing hole.
Rocky Mountain Instrument came to my rescue to solve the space problem. Their microENCODER was exactly what was needed. It solved both the budget and space dilemma. The instrument combines a mode C altitude encoder with a graphic/digital vertical velocity indicator, digital airspeed indicator, sensitive digital altimeter, and digital outside air temperature. This single instrument occupies a standard 3-1/8" instrument hole.
Since the microENCODER contains VSI and ASI functions, I was able to take the existing instruments out for both of these functions. With two holes to work with I was able to put the microENCODER in one hole and the loran in the other.
VFR only pilots will find the Rocky Mountain microENCODER cost competitive compared to purchasing the individual instruments that it replaces. Additional functions available at the touch of a switch are: true airspeed, altitude alert, density altitude, pressure altitude and true air temperature. Also, altitude and airspeed warning limits can be easily set. In addition, the microENCODER has an RS232 serial port output that gives VNAV capability when coupled to the II Morrow 618C loran.
Rocky Mountain Instrument's literature states, "The microencoder MUST NOT be used to replace more than the vertical speed indicator in aircraft certified for IFR flight." This is not just a disclaimer, it is sound advice. Since I will not be flying IFR at this time, I decided to go ahead and replace both the VVI and the airspeed indicator. This provides a spare hole for the loran. Later, and with more motivation, I shall do a major panel modification and then add a gyro compass, attitude gyro, and reinstall the VVI. I propose to accomplish this by talking Rocky Mountain Instrument into housing their Engine Instrument Monitor in a standard 3-118" panel mount instrument. Four of my 2-1/2" engine and electrical instruments can then be replaced by this one. A very nice instrument panel design should result.
Pitot pressure, static pressure and outside air temperature are measured by sensors and converted into digital information on one of two large Printed Circuit Boards (PCB's). The other large PCB is the computer that processes the digital information, sends it to the display section, and then on to the instrument panel readout. The instrument is housed in an amazingly small 3.2 x 3.2 x 7.5 inch package.
Building an electronic kit is great therapy. It rewards you with a feeling of accomplishment every time your completed creation is used.
There are just three simple qualifications that are necessary in order to build this electronic kit: the ability to read and follow instructions and some basic soldering skills.
The Assembly Manual provided with the kit includes all the necessary information for the first-time kit builder. Illustrations accompany the text, and a step-by-step checkoff procedure will ensure that everything is done in proper sequence. Installation, Programming, and Operations Manuals cover the remainder of what is needed in order to be able to use the new creation.
The art of soldering should be successfully mastered before assembly is started. Printed Circuit Boards and delicate components can be quickly destroyed by improper soldering techniques. Reading two excellent articles on soldering, authored by T. E. Gilland, in the October and November 1990 issues of Kitplanes magazine, and some practice will ensure that you learn these skills.
A most important consideration when handling semiconductor devices is protection from electrostatic discharge (ESD), a significant cause of device failures. Many of the components in the kit are semiconductors and require proper handling precautions to prevent shortened operating life or being rendered inoperative.
A good way to prevent ESD damage to semiconductors is to ground everything and everyone that may be a source of, or a path for, ESD. An inexpensive and effective way to provide for a work surface that is properly "static dissipative" is to cover the work surface with aluminum sheet metal or heavy gage aluminum foil. This aluminum covering must then be grounded. I used the third ground line on our electrical outlets, after making sure it was grounded at the junction box.
At this point it would be negligent if a lesson in electrical safety were not included! In order to prevent a potential lethal electrical shock to anyone who touches the aluminum covered work surface, a resistor having a resistance of about 1 megohms must be installed in series with (between) the work surface connection and the point of connection to the building electrical system ground. The purpose of this resistor is to "throttle" the current through you to a lower than lethal level, should you happen to come in contact with anything that is inadvertently "hot" on your 120 VAC equipment (such as the cord on your soldering iron that may have been accidentally burned through, exposing the 120 VAC). As an extra measure of safety, be sure that the connections to the resistor are insulated to prevent a scrap piece of wire, solder, etc. from bypassing the connections through the resistor, thereby defeating it. Anyone approaching the work surface must put on a conductive wrist strap (bracelet) that is connected to a cord that has a 1 megohm resistance, which is connected directly to the static dissipative work surface. The purpose of this arrangement is to "bleed" any static discharge current that would otherwise "zap" your ESD sensitive components.
Simple tools that are found on any electrical technician's workbench are all that are necessary when building the kit: longnose and diagonal cutting pliers, Phillips and blade screwdrivers, soldering iron having proper wattage and small blade tip, solder sucker or solder wick for correcting soldering connection mistakes. A headband binocular magnifier is particularly useful for inspecting your solder connections. I also used a small equipment convection fan (that I kept lying around the house) to waft away the smoke from the soldering flux. Keep the airflow low and away from the solder joint, it is not good to prematurely cool the solder joints.
Construction of the kit is straightforward. The complete instructions ensure that you won't have to second guess. Resistors and capacitors are packaged in containers clearly marked with their value and a reference number that matches a corresponding number printed on the PCB. In addition, each component is marked with its own colorcode or part number. This prevents guesswork when "stuffing" the PCB's, assuring that the proper component goes in the correct location. I found long-round-nose pliers particularly useful for bending component leads; they ensure nick-free, radiused bends that will not fatigue and break later from vibration. I used a 60/40 tin/lead alloy rosin-core solder that is commonly used in commercial electronic assembly. The flux provides good wetting but must be removed. Any good commercial defluxer solvent may be used, as mentioned in the articles in Kitplanes, or as sold by Radio Shack. Whatever solvent you use, you must keep scrubbing until inspection with your magnifier reveals that all residue has been cleansed. Residual flux contamination can cause corrosion as well as "leakage currents" that will deteriorate reliability. While cleansing, you have an opportunity to re-inspect each and every soldered joint. When soldering and cleaning is finished the completed PCB's are ready for bolting to the chassis - then "cross your fingers" when initially powering-up.
Manufacturers of high reliability electronics traditionally subject their finished product to an operational burn-in at elevated temperature. A high percentage of electronic components that fail do so in the first hours of operation. The purpose of this burn-in is to increase component stress to induce these infant failures at a more opportune time (prior to actual use). RMI recommends a burn-in program that includes both a cold and hot operational test. The cold test takes place in the freezer, and the elevated temperature test is conducted in a homemade oven.
The oven consists of a box made out of scrap boards, 100 watt light bulk and socket, and thermometer. The oven is fired up and given time for the temperature to stabilize. Holes are then drilled in the top and sides to adjust the stabilized temperature to 140 degrees Fahrenheit. The microENCODER is placed inside the oven and powered up for 48 hours. My unit had good fortune: there were no component failures.
Do not be tempted to use the kitchen oven for this burn-in. There is too much danger in overstressing the instrument due to poor temperature control and the possibility of an inadvertent increase in the oven's temperature control. Also, 48 hours is a long time to have the kitchen strewn with wires, a battery and charger.
While the microENCODER was being burned-in was a good time for reading through the Operation and Programming manuals. There are options I needed to consider (such as whether to have the altitude barometer setting in inches of mercury or millibars). Also I needed to determine the values to enter for the different airspeed and altitude warnings.
Calibration and programming was an exhilarating part of this project since I had that warm feeling of accomplishment while watching the instrument work. I first thought this step would be difficult. Not so. Set two voltages using a Digital Voltmeter or Multimeter, program the computer, then head for your closest FAA Certified Repair Station for their calibration and correspondence tests.
Programming, as used here, is more akin to choosing COOK or DEFROST on a microwave oven. It is nothing more than a series of menu selections using the front panel controls.
The first programming step is to enter altitude and airspeed sensor data into the unit. RMI measures the response curve of the individual sensors, and provides data that characterizes each sensor. These data are entered into the computer to assure accurate display information.
Next, numbers are entered into the computer for airspeed range warnings (red, yellow, white, gear, flap, stall), altitude alert windows (hold, approach, converge), individual audible alarms (ON or OFF), and other display type options (A/S in knots, MPH, or MACH; InHg or Mb, etc.). Once these are set, they remain so until re-programmed.
I will jump ahead in the sequence of events and discuss the calibration and correspondence which takes place after installation. Make sure you go to an FAA approved facility that has equipment capable of doing "in the aircraft" calibration and correspondence. Some shops don't have this capability. This requires removal of the instruments from the aircraft for bench testing. With the right equipment, all they do is hook into your static line and run the tests.
After connecting the test set to my static system, we adjusted the altimeter settings to 29.92 inches of mercury. We then compared the microENCODER altitude readout with that of the test set. Wow, it was only 30 feet off! Not too surprising since I had used a six digit voltmeter to set the reference voltage. Also, RMI had done a very accurate calibration of my altitude sensor. This error was corrected by putting the RMI computer into the "program mode" and entering the altitude of the test set altimeter. This stores a correction factor that is used by the RMI computer in all future altitude calculations. Next, we ran the test set down to sea level to compare the altitudes, then up to 20,000 feet. Along the way we cross-checked the test set and microENCODER altimeters to see that they tracked each other. Also, the altitudes being transmitted by my transponder were compared to those of the test set. It was exciting to see my altimeter track the calibrated test set altimeter from sea level up to 20,000 feet! We found only a 10 foot error after stabilizing at 20,000 feet. No further programming was necessary for this step.
Airspeed calibration should be done in the aircraft before using the microENCODER on its first flight. The accuracy of the airspeed indicator is checked using a homemade water manometer. Instructions for constructing this device are included in the Programming Manual. If a correction is necessary it is accomplished in a similar manner as done previously when correcting the altitude error.
Once the microENCODER kit was built, burned-in, and programmed, it was time to install it in the aircraft. Since I wanted to complete as much work as possible in the warm confines of my home, I decided to build an installation kit. I determined the routing and length of each wire, then made up the wiring harness at home. What I ended up with was an installation kit that could be installed into the aircraft with just a screwdriver.
A great deal of thought was put into the design of the wiring harness. Since a loran was to be installed, I was concerned about interference signals. have not recently been involved in electrical system design, so I discussed the concepts with a few of my electrical engineer associates, then called Bob Nuckolls, the author of the Aero Electric Connection, a homebuilders guide to the design and construction of aircraft electrical systems. We discussed the design and, as would be, we did not totally agree on a best design. By the way, if you don't subscribe to the Aero Electric Connection and you are building or restoring an aircraft, consider this subscription a "must". You can write to Bob at P. O. Box 206, Benton, KS 67017.
Interference signals can generally be grouped into four categories: conductive, common impedance, electrostatic, and electromagnetic. Conductive interference enters an instrument by way of its own wiring (noise on the power line interfering with the normal operation of an instrument is an example). Common impedance interference takes place when more than one instrument share the same conductor (using the same ground return for example). Electrostatic interference takes place between different signal or power lines due to stray capacitive coupling (parallel unshielded wiring is the biggest culprit). Electromagnetic interference comes about by signals induced in a wire while in the presence of a magnetic field. These interference signals must be minimized as much as possible.
Some of the techniques incorporated in aircraft electronic systems to minimize these signals are the use of linear voltage regulation, grounded conductive shields, twisted pairs, concentric cables where the center conductor carries the signal current and the surrounding shield carries an equal return current, physical separation of the wiring and, of course, the use of the shortest possible wires. Many times when minimizing the effect of one of these interference signals another one is increased.
The wires of a system can be grouped into one of two categories: antagonists or victims. The antagonists generate unwanted interference signals, while the victims receive them. In this installation the antagonists include the power supply wiring, the transponder antenna, and to some extent the digital data wires from the microENCODER to the other two instruments. The victims are the loran antenna and the digital data wires. Note: the digital wires appear in both categories. If they were only antagonists, a better wiring design would result. The power leads could then be routed with the digital wires inside a shield used as the ground return, but in this case the digital wires must be separated from the power wires.
I protected the digital data lines from electrostatic coupling by housing them and their signal ground return in shielding which is grounded at one end only. Twisting these lines will also lower the effects of electromagnetic coupling. The power supply wiring need only be "twisted pairs" (power and ground return twisted together about one turn per inch) routed from the breaker panel to the encoder as a triple of' twisted pairs", then bundled with the digital cable to their respective instruments. This reduces the electromagnetic interference. Antenna wiring should be that recommended by the respective instrument manufacturer, be routed away from antagonists (power lines especially), and have isolated ground planes. The result is shown in the wiring diagram.
There is no right wiring harness design; it is a compromise at best. The design depends on the particular aircraft, its existing wiring layout, and the instruments to be installed. Choose the combination of cancellation techniques that works in your aircraft.
The RMI warning system audio line to the intercom was not included in my harness. I speculated, from my bench tests, that the loudspeaker on the unit was loud enough to be heard over the sounds of the aircraft, even with my earphones on - this proved correct. Since I have not yet installed the loran, those leads are insulated, bundled, and tied off for now.
Flying with a digital instrument is like reading a digital watch: you look at it and round off the answer when accuracy is not required, or you read it to the last digit when appropriate. I found that it took two flights to become comfortable with the display, except for the VVI. I am going to have to practice some more, forcing myself to use it by covering up the back-up VVI. Once the VVI has been mastered, I am confident that my cross-check will improve. With practice, three pieces of flight information should be picked up with a glance.
After two test flights at my home airport in Tucson, I was off to Salt Lake City, UT. Before takeoff I set my desired cruise altitude into the computer as the Alert Altitude. As I approached this altitude, the microENCODER emitted an audible BEEP and the altitude display started to blink. I will have to admit that on a long climb-out this feature is only a novelty, since there is plenty of time to cross-check the instruments. However, it sure came in handy in the TCA, at my destination. Approach control loves to give decent level-off altitudes. Also, on leaving Salt Lake, my departure altitude snuck up on me and the microENCODER gave me a gentle hint that I was about to overshoot my altitude.
Once in level flight, the microENCODER warned me, with one BEEP and a blinking display, when I strayed off my altitude more than 100 feet (1/2 the Hold Alarm distance programmed during calibration). If I persisted in drifting farther off, I would get an alarm consisting of a continuous series of BEEPs with the blinking display. I could easily silence the unit by either returning within the Hold "window", or just pressing the Acknowledge or Alert Switch. This Hold function is my favorite feature since I always use flight following, where Air Traffic Control is monitoring my altitude hold accuracy.
Prior to installing this instrument, my altitude seldom agreed with that of Center. I had a blind encoder that did not stay in calibration. With the microENCODER, pressing the PALT function changes the display altitude to pressure altitude. Continuing to hold this switch allows the encoder altitude, being emitted by the transponder, to be read. With this feature, and the back-up altimeter, I am confident that Center has the correct information. Of course, the current altimeter setting must be entered in both instruments.
Another function used a number of times while on the trip was true airspeed. You reach down, activate the TAS switch, and up pops the true airspeed - this is great. In this condition, IAS is replaced by TAS while the springloaded switch is held in the TAS position.
With more practice, proficiency in the use of this instrument will improve. There are things yet to be tried. I may even program the microENCODER to present the airspeed in MACH, so that I can "hangar-talk" with the "big boys".
While doing this project, I was thinking, "Maybe I can install a HUD, all the IFR goodies, and an autopilot coupled to my loran." Of course, I will have to incorporate an alarm clock to wake me up at my destination!
RMI Note: The above unedited article appeared in Sport Aviation (EAA) Magazine, April 1991