Gremlins!

Gremlins Diagnosing mysterious avionics challenges By Jim Sparks February 2001 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...


Environmental concerns
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 troubles
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 sickness
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.

Bad vibes
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.

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