Practical Electrical

Understanding DO-160.

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Owner Built and Maintained (OBAM) aircraft are largely immune to the effects of micromanagement rules and regulations regarding system performance. This happy condition is one of several reasons why they are economically attractive—OBAM aircraft are burdened less by regulatory overhead costs of dubious value.

The study of certified aircraft offers powerful history lessons and tools for achieving OBAM aviation design goals. Goals sought not because they are required…rather because they are supported by common sense and a history of successful recipes. This month, I’d like to discuss the ingredients that have gone into countless recipes for success in the crafting of electrical and electronic accessories for aircraft.

For the Common Good

There is an agency in Washington, DC whose full name is somewhat arcane, the “Radio and Technical Commission for Aeronautics.” It has been shortened to “RTCA.” Few individuals remember what the letters stand for.

Founded in 1935, RTCA is a think tank of individuals with practical experience in the art and science of crafting devices that meet performance goals while being good neighbors with other systems on an airplane. A family of RTCA Environmental Tests for Airborne Equipment go back to the 1950s.

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DO-160, formally titled DO-160, Environmental Conditions and Test Procedures for Airborne Equipment (and its predecessors), are not requirements. DO-160 is a constellation of tests intended to verify compatibility for a particular piece of equipment by demonstrating (1) it will not be damaged by normal and predictable operating conditions aboard the airplane, (2) it functions as expected in the presence of normal noises from other systems, and (3) it does not itself generate noises deleterious to performance of other systems.

DO-160 was authored and is maintained by a committee of representatives from industry, aviation user groups, and government. The document is a well-considered, elegant solution for tests that balance what is necessary against what is practical. It is a cookbook, if you will, of recipes for success. DO-160 is extensive, over 400 pages. Let’s discuss the features in DO-160 most relevant to the OBAM aircraft industry.

Documenting Compliance

When a manufacturer declares compliance with DO-160, they may not (and in fact, probably won’t) accomplish every test in the book. Let’s look at some of the tests commonly used and how they affect us in the real aviation world.

Power input

Unlike regulated, filtered, plug-in-the-wall power supplies, DC power from an alternator/battery system is not clean and stable. Devices intended for use in airplanes should be demonstrated to function as advertised over a range of 13.0 to 15.0 volts (alternator operating) and function with perhaps degraded, but still useful, performance down to 10.5 volts (end of battery life). Double these numbers for 28-volt systems.

Bus Noise

Only a solitary battery is a noise-free source of energy. Add alternators, blower motors, strobe supplies, etc. and the power bus becomes polluted with noise trash. Expect noise on the bus ranging from 10 to 100 Hz, ramping upward from zero volts pk-pk to 1.5 volts pk-pk. Then from 100 Hz to 1000 Hz, at a constant 1.5 volts pk-pk. Finally 1000Hz to 10,000 Hz with the amplitude ramping downward from 1.5 volts pk-pk to zero at 10KHz. Again, double these numbers for 28-volt systems. The point of designing and testing to DO-160 is an acknowledgement of the fact that no vehicular DC power source is “clean” and the prudent designer will accommodate it.

Interruptions

Sadly, this test is not high on the list of design goals for many manufacturers of processor-based devices for OBAM aviation. This test defines all manner of interruption and brownout. The device first should not be damaged by any excursions of power supply down to and including zero volts for any duration.

The device can fail to function below 10.5 volts, but should come back to normal operation in an orderly manner and without pilot intervention when the bus returns to normal voltage levels. Many processor-based devices on the market misbehave or take a long time to recover from brownouts due to starter inrush loads on the battery.

DC Power Surges

A qualified device should tolerate 20 volts for 1 second or 40 volts for 100 milliseconds without damage. Double these numbers at the same time intervals for a 28-volt system. Meeting these design goals is pretty easy using legacy input power conditioning techniques.

(My first full-blown DO-160 qualification was about 30 years ago for the first general aviation, multi-speed pitch trim system. The system was installed on the Lear 55 and ultimately retrofitted to the 30-series fleet. In this 28-volt world, I had to stand off 40 volts for 1 second, and 80 volts for 100 milliseconds. With a judicious selection of parts, I was able to crank up the power supply from a nominal 28 volts to 80 volts while the trim system was running…the motor speed didn’t change a bit. I turned to the FAA inspector who was witnessing the test and asked, “Is that long enough?” I demonstrated 80 volt operation for several seconds and well over 28 volts for 10 seconds. I got no arguments about the DC Power Surge test!)

300V Spike

The term “spike” strikes fear into the hearts of owners of expensive avionics. The same term is a convenient catch-all for bench technicians who have repaired a radio but really haven’t a clue as to the root cause for the failure. A favorite assertion states, “I guess a ‘spike’ got it.”

In fact, the 300-volt spike test has little significance to the suppliers and users of electronic devices on light aircraft. First, this is a low-energy event and is easily mitigated with a small capacitor across the power input lead to the device under test. Second, while the ‘spike’ might be observed in the wild on aircraft with very long DC power feeders (Like a DC-6 or B-29), the potential sources for such spikes and natural mitigation features common to all DC power systems drove this stress into extinction. Over the years, I’ve had dozens of opportunities to watch the bus on everything from C-150s to Beechjets for all manner of operations including engine cranking. I’ve never been able to capture a noteworthy spike in the field.

Nonetheless, the DO-160 test calls for a short-duration spike (100 microseconds) of up to 300 volts to be delivered through a 50-ohm source impedance into the 14-volt input of the device. A simple 10uF capacitor (rated for a 40-volt surge) right across the input stops the spike dead in its tracks. Many years ago, I blew a couple-hundred dollars of my boss’s money at Electro-Mech, building a fixture for spike test generation and using it one time. Once the significance of the little spike beasty was identified, all new products got the capacitor and the fixture went up on the shelf.

(I visited Electro-Mech a few years ago and saw the fixture sitting on the shelf and smiled about that lesson learned 20 years earlier. I was tempted to inquire about it—Could anyone tell me what it was?—but that wouldn’t have been nice. They probably would have let me have it for my collection of antiques, but I’ve got too much of that stuff already!)

Lesson learned: Be wary of assertions citing root cause for a failure to be the infamous radio-killing spike. The phenomenon is more imagination than reality.

Temperature & Altitude

There are lots of categories for this test. Cabin-mounted accessories for OBAM airplanes would typically be rated and demonstrated for up to 15,000 feet and operating temperatures of -40° to +55° C. Aside from issues surrounding forced air cooling, I’ve never encountered a concern for altitude effects. When com transmitters needed 300- to 1000-volt power supplies, one might have concerns about high-voltage arcing. Collins addressed this concern on single-sideband transceivers for the 1960’s B-52—the radio’s enclosure was a pressurized barrel!

Be wary of any notion that solid-state devices are less needy of external air for cooling due to low power requirements. Power density sets cooling needs. A stack of tiny avionics may consume less energy while dissipating heat into a still smaller volume, thus producing a higher temperature rise than its vacuum tube ancestors. Case in point: Consider the elaborate cooling systems for the CPU in modern computers. Tiny chunk of silicon, big fan, lots of fins, even heat-pipes!

Vibration

There are lots of categories here too, but unless you’re going to mount the device directly on the engine or landing gear, ordinary fabrication techniques will suffice. In this era of surface mount components, it’s really easy to build for mechanical robustness.

Vibration testing in the lab is accomplished on equipment similar to this. The shaker table is like the voice coil to a 10,000-watt speaker, minus the cone. The companion amplifier and controller will excite the shaker table to energetic vibrations over a range of frequencies, intensities and vibration spectrum tailored to conditions where the device will be located on an aircraft.

Items intended for installation on an instrument panel are not tested nearly so severely as an item that mounts on an engine or an aileron of a supersonic target.

Gunks, Goos, Grit, Bad Gas and Death by Athlete’s Foot

A prudent designer considers all forms of wetness, including water, hydraulic fluid, fuel, and oil. The designer may choose simply to keep the bad stuff out, or install the device where it doesn’t matter.

There is a test for fungus. There was a time when common insulations would support fungus growth. Nowadays, a statement in the qualification document routinely buys off the test that “no materials that are nutrients to fungus are used in the fabrication of this device.”

How about ozone? Used to be lots of it under the cowl that would eat up many forms of plastic finishes and insulation. Today—not so much.

Sand and dust is an interesting test. You operate your device in a cloud of calibrated dust. ISO 12103 has replaced the material I first knew as “Arizona Road Dust.” I am not disappointed for never having needed to run that test!

Radio Emissions

Testing the full range of DO-160 frequencies in a lab can cost big dollars. However, the OBAM aircraft electronics supplier would benefit from a quick look-see with a handheld VHF com and GPS receiver. Do any of these critters complain or seem to hear noise when operated in close proximity to the device?

Some effects may not be discovered until after the antagonist and potential victims are paired on the panel. Some models of vacuum tube VHF nav/com receivers radiated their local oscillators with sufficient harmonic strength to interfere with the naturally weak GPS signals. Fortunately, these instances are rare. Interestingly enough, the vacuum tube equipment was DO-160 qualified in a time before GPS. Later revisions to DO-160 recognized vulnerabilities of the new navigation technology and DO-160 radiation limits were adjusted accordingly.

If you encounter an unanticipated antagonist in your OBAM aviation endeavors, know that there was never a noise problem that couldn’t be whipped.

Radio Susceptibility

A handheld transmitter with antenna held about 12-inches away from a potential victim will often expose the most common susceptibilities.

An episode of Mythbusters attempted to address an urban myth concerning operation of a cell phone aboard an aircraft with a perceived risk of bent metal and broken people. Had they studied and understood the real hazards to onboard systems and the testing done to mitigate them, they would have known that their made-for-TV experiment was bad science. Potentially vulnerable systems are taken to the lab and radiated at anywhere from 20v/m to 200v/m of RF at 100Mhz to perhaps 18GHz. Cell phones don’t even begin to radiate at these levels. The problem is that folks watching the show were victims of “erroneous enlightenment” with respect to the science.

A number of popular instruments and electronics produced for the OBAM aviation industry are sadly susceptible to moderate RF stimulus (LED Indicators are one example). For some products it’s a condition that has been known for years, but the marketplace seems willing to put up with the effects. I’m aware of no upgrades to these devices that eliminates the susceptibilities.

Electro-Static Discharge (ESD)

This is a handling issue. Early solid-state devices were vulnerable to striking a machine or body static discharge to a connector pin. Formal testing for ESD can be accomplished with this handheld tool that generates 2,000-30,000 volt sparks to potentially vulnerable input/output pins on a device. You can zap flies with it, too. It’s easy to design for ESD immunity in a piece of avionics, but flies— not so much. I’ve seldom needed to test for ESD immunity.

Lightning

This is a big thing with the FAA, but I choose to ignore it for OBAM aircraft projects. It’s not terribly difficult to design qualified lightning protection into a device, but it adds a lot of volume to a small accessory, while driving up costs and parts count to guard against a rare event.

Further, it’s reasonable to believe that the pilot who finds himself at risk for taking a strike faces risks far greater than worrying about whether his avionics will take a hit and keep on ticking. The image below is one of several from a narration by Paul McCallister describing a strike to his Europa.

Damages to his aircraft and installed systems exceeded anything that DO-160 qualification techniques would have demonstrated.

High Power RF Radiation (HIRF)

This is also a big thing with the FAA. There is perceived risk for flight in vicinity of high-powered ground-based systems or being painted by either ground or airborne radar capable of delivering very high levels of RF energy—albeit for short periods of time. If you’re on a coupled approach, it would be really exciting if the local weather radar transmitter excited your autopilot into spontaneous aileron rolls!

Serious HIRF testing involves radiating the device under test with hundreds-to-thousands of watts of energy from a beamed antenna. HIRF testing is usually reserved for potential victims that navigate (GPS) or fly the airplane (autopilots).

Doing this Stuff for a Living

In the trade, we figured about $100-$200,000 in round numbers to write a test plan, build test fixtures, do a DO-160 sweep of a device’s characteristics and vulnerabilities, write a test report and shepherd the work through a growing maze of bureaucratic hoop-jumping and sand-pounding.

My writing often pays homage to repeatable experiments and recipes for success. An interesting feature of the DO-160 qualification exercise is that individuals who have designed devices in this venue for years enjoy 99% probability of passing every qualification test the first time because they have “been-there-and-done-that.” They’ve done their homework with pre-qualification engineering tests.

The goal for this article has been three-fold: (1) The educated consumer of cool gizmos for the OBAM aircraft industry should be able to lean on the counter at OSH and intelligently query and discuss the “story behind the story” about pretty screens and cool knobs. (2) Individuals who have some idea for a device, or perhaps a DIY article, will reduce market risk and add value to their work product by considering ingredients for success offered by designs that would qualify to the spirit and intent of DO-160. Many times, actual testing is unnecessary if the historically successful techniques and processes are used.

Finally, (3) There is an ancient myth prevalent throughout aviation hangar-lore that suggests it’s a good idea to turn all the radios off during engine cranking, lest some electro-gremlin find its way into the radio and render it inoperative. Better yet, airplanes should be fitted with a special switch (avionics master) used to protect all radios from this diabolical entity. For many years, manufacturers who faithfully complied with DO-160 testing still recommended the avionics master switch and/or turning off their device during engine start. The whole point of product qualification to DO-160 is to demonstrate that no feature of electrical system performance will put a qualified device at risk for damage. The only piece of electrical system hardware capable of inflicting severe damage to other components is the alternator. That’s why alternators get over-voltage protection systems.


Bob Nuckolls retired from Beech Aircraft in 2007 after more than 45 years of work in certificated aviation and over 25 years of support for the homebuilt aircraft industry. Bob publishes “The AeroElectric Connection” from his website at www.aeroelectric.com He also hosts the AeroElectric-List on Matronics.com. This special interest forum serves approximately 1600 participants.

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