Diagnosing mysterious avionics challenges
By Jim Sparks
Did you ever encounter one of those problems that surface at random and have the ability to hide when the aircraft is in the shop?
During my years working as a tech rep for an aircraft manufacturer, I would periodically hear from an operator that their machine has a "Gremlin." I can honestly say that I think I saw these little varmints on several occasions in my earlier years of turning wrenches (usually after 24 to 36 hours of continuous troubleshooting).
Those who have seen the movie Gremlins may recall in Hollywood’s rendition, a Gremlin is a cute, furry little critter that thrives on creating mischief and disrupting the lives of all those in the immediate area. Also, there are both good gremlins and bad gremlins. The good gremlins attempt to make temporary repairs of the systems that the bad gremlins disrupt. This is one reason the job of diagnosis and ultimate problem resolution is made more challenging.
Unfortunately for us in the aviation business, this mischief may result in a potential life-threatening situation. Gremlins are creatures of habit, that is, they only come out under certain conditions. Exorcising these little varmints in a timely fashion is a feat that generally requires a complex plan of attack and in some cases, voodoo rituals.
Frequently, atmospheric conditions such as rain have an effect on gremlin activity. Sometimes temperature, or possibly pressurization, will play a role in when a gremlin appears.
The first step in gremlin eviction is to determine lifestyle characteristics. The first action is to determine when they come to life. Are they active on the ground, or only in flight? It is essential to identify when the problem will occur. Many aircraft utilize extensive ground/flight sensing systems that can have a dramatic effect on the operation of numerous aircraft systems. This is, however, only part of the consideration of ground flight. One example is a pilot-reported discrepancy where the crew could not use the #2 communication radio at certain locations on their home airport. However, as soon as the aircraft was in the air, the radio operated as it should. After numerous man-hours were invested troubleshooting and most all the components had been replaced, it was determined that with the location of the #2 communication antenna as being under the belly of the aircraft, in certain positions on the airfield, the airframe structure would effectively block the radio transmission.
Another situation that can create system disturbances is the build up of electro-static charge, and this is not just limited to accumulation on the fuselage exterior. Often, glass instruments in the flight compartment are excellent storage devices for static charge. Other anomalies may be the result of operating the aircraft with the landing gear retracted instead of extended. Frequently, advanced aircraft will employ the use of electronic proximity switches to sense the position of various landing gear components. The proper operation of these devices require them to produce an internal high frequency alternating current. Sometimes this can result in a feedback through the aircraft electrical system. De-energizing a landing gear circuit at various positions in its cycle while monitoring the operation of a suspect faulty system may provide direction to a faulty component.
Environmental conditions are another consideration. Some Gremlins thrive in very cold conditions while others prefer it warm and wet. Duplicating these circumstances often provide some unique challenges to those of us tasked with resolving the discrepancy. Sometimes these ornery little critters will only awake when exposed to extreme cold for significant time periods, while others can be excited using an environmentally-friendly aerosol-cooling product. One example of the extended cold soak dilemma had to do with an abrupt drop in aircraft fuel quantity during extended high altitude flight. This would typically occur after about three hours flying at altitudes over 35,000 feet. The crew would notice a drop of about 1,200 pounds of fuel in one wing tank. The aircraft did not roll nor did the crew have to introduce any aileron trim. Often, when the aircraft descended to a much lower altitude or shortly after landing, the fuel quantity indications would return to normal. The particular type fuel quantity indicating system utilized here contained two silicon diodes installed on each fuel quantity probe. These diodes were imbedded in an epoxy housing. As the probe was exposed to extremely low temperatures for a sustained time period, the epoxy housing expanded at a rate faster than the diodes and at least one of the diodes had its anode lead pulled from the diode casing. Then, when the aircraft descended to lower temperatures, the epoxy block would return to its original configuration and restore the electrical connection within the diode. The only way this problem was resolved was by freezing the fuel probes overnight then testing the diode circuit.
In another case, an intermittent problem with an electronically-controlled landing gear system would periodically prevent the landing gear from extending when the gear handle was selected "Down." This aircraft utilized sequenced landing gear doors that were supposed to open fully before the landing gear would unlock from the retracted position. In this instance, the doors would not even open. In all cases, the gear was successfully deployed using a back-up system. Each time the aircraft was placed on jacks to perform an operational test, no fault could be duplicated. By applying an aerosol spray coolant to all the suspected landing gear uplock microswitches, an open circuit was detected in one. It seems that some moisture had worked its way into the switch and when it froze enough movement occurred to create an open circuit in the landing gear extend system.
Temperature, and specifically cold soak, can have other mechanical effects such as straining wiring that may have been installed with too little slack or possibly changing clearances that can affect the proper rigging of switches or other types of electrical sensors.
On the other extreme, heat can be as much a culprit as cold when it comes to stimulating gremlin activity. Auxiliary Power Units (APU) are often located in aircraft equipment areas, which are frequently limited in size. One thing APU operation is sure to produce is plenty of heat. In many aircraft, these Auxiliary Power Units are small, self-contained turbine engines. An Electronic Control Unit (ECU) regulates most of these devices and, as the name implies, this component is full of electronics. In one case, as the aircraft sat on the ground sometimes for several hours at a time with the APU operating, an uncommanded shutdown would occur. This was most prevalent on very hot days. The ECU had been replaced three times and in each case, appeared to rectify the problem.
It was only after the third ECU change that some further diagnostics were employed. A temperature sensor was installed in the immediate area of the Electronic Control and the APU was operated. When the aircraft environmental systems were activated, a sharp rise in ECU temperature was noted. It seems that hot, APU bleed air was leaking from a coupling in the supply line to the environmental cooling systems and was directed on the ECU. On hot days, this small amount of very hot air was enough to take the ECU out of its design limits and promote internal failure. Fixing the leak was enough to make that gremlin move on.
Motion is another source of stimulation for the gremlin community. Any wire bundle such as that found on a landing gear may be subject to bending and flexing during a landing gear extension or retraction. Sometimes, exposure to the airstream may cause excess vibration. In either case, the resulting work hardening of the internal conductors will inevitably lead to failure. Testing for this type of fault will often require passing a current through the suspect wire while imposing physical movement on the wire bundle. Continuity testers are not always the best devices for isolating this type of problem — this is due to the very small current they typically produce. Ideally, testing should be accomplished using current loads that are nominal for the circuit in question.
Vibration is another form of motion that can provide significant frustration to troubleshooting efforts. One such example involved the operation of an anti-skid system. The flight crew reported that even on dry runways the aircraft had a tendency to pull to the right. After checking the basics such as tire condition and alignment as well as nose steering, a landing was conducted with the anti-skid system deactivated. In this state, no adverse pull was observed. This led to performing functional as well as operational testing of the wheel brake and anti-skid systems. The devices used to sense wheel velocity are mechanical generators that produce a DC output proportional to wheel speed. Internal resistances of these devices were tested, as were the output voltages at specific RPMs.
All values were well within the system manufacturers prescribed limits. After a while, uneven brake wear began to show. Of course by now, the aircraft operator was growing tired of the problem and wanted all the components changed "Under Warranty." At that point, the heavy artillery was dragged out and the wheel speed generators were once again removed for another operational test. This time, however, the voltage test was accomplished while a rivet gun with a padded set was applied to the case of the transducer. All generators but one responded well to this adverse vibration situation. The suspect generator exhibited significant voltage fluctuations as the brushes "bounced" against the commutator. Once this generator was replaced, the problem disappeared.
Induction is yet another area where our gremlin friends thrive. The principle of induction is not as easily observed as vibration, motion and temperature, but it is every bit a formidable foe. Typically, AC electricity can be induced in a conductor in close proximity. This is one reason for providing a conductive shielding for most wires carrying AC electricity as well as an incentive for synchronizing AC Generators/Power-supplies on many aircraft. This phenomenon will generally stimulate gremlins to attack systems utilizing low voltage AC or DC power. One case in particular had an effect on another Anti-Skid system. The reported discrepancy was in fact, similar to the one discussed earlier. The flight crew reported weak brakes on one side of the aircraft. This however, was a very infrequent problem and would occur maybe on one in five landings. Switching off the anti-skid system only had a minor effect and the problem was still identified. Once again, all of the components were removed and tested and once again revealed no significant fault. In this particular system, the anti-skid computer would supply a low voltage DC signal to an anti-skid servo valve, which would result in a decrease in brake pressure on either the left- or right-hand brakes. Troubleshooting got to a point where an Oscilloscope was installed to monitor the signal to both anti-skid servo valves. After spending several hours of doing every type of test imaginable, the troubleshooting crew was just about ready to throw in the towel and shotgun all suspect components. An aircraft cleaner happened to walk into the aircraft and turned on the cabin fluorescent lights. The technician monitoring the Oscilloscope happened to notice a significant jump on the screen followed by numerous voltage fluctuations. When the cabin lights were selected "Off," the stray voltage went away. Further investigation revealed that a breakdown in shielding had occurred at an electrical connector that was shared with the anti-skid valve. The induced voltage from the high-voltage fluorescent lighting was adequate to cause one anti-skid servo to reduce brake pressure.
There are many areas where Gremlins thrive but they are more often than not, creatures of habit. Once their specific habitat is explored, it makes ridding the aircraft of the nuisance that much easier.
I have had many people tell me "Life in aviation would be much better if all the Gremlins were vanquished." Still, I believe that without these cantankerous little critters, many of us would be driven to an early retirement due to lack of a good challenge.