Diagnostics Techniques: The physician in all of us

Component history documents often provide a valuable key to what components to approach first.

The physician in all of us
By Jim Sparks

Many have had the situation where a visit to the doctor was required to resolve some abnormality with bodily functions. In some cases the problem is beyond the scope of the general practitioner and a specialist is required. Many years of study provide these experts with unique insight into specific areas of the human body.

Unfortunately a mistake in the diagnosis can result in a patient's increased suffering or even death. To successfully analyze a problem sophisticated and often-expensive test equipment is frequently required. Sometimes a test may be accomplished that reveals no problem and to the educated professional a test that reveals no problem may in fact provide significant insight to what the dilemma might actually be. The parallels between the aviation maintenance technician and the medical diagnostician are numerous. With the main one being an inaccurate conclusion could result in loss of life.

Problem resolution on aircraft is often considered a gray area. Regulations created by various airworthiness agencies often address standard practices utilized in the normal scope of aircraft maintenance and inspection, however, they almost never focus on diagnostic techniques. Some airframe and engine manufacturers include a troubleshooting section in their product documentation. Conversely, in my experience the problems encountered are not listed in the book.

One thing most aircraft maintenance programs do include is system operational and/or functional checks. These provide the technician with insight as to what the system should really do. As with the medical profession we in aviation maintenance need the ability to identify hypochondriacs. One of the reasons doctors spend many years in school is to learn the specific functions of each of the body's systems as well as their interaction. This is also the key to success in problem resolution on aircraft.

A technician has to have the ability to review the symptom and determine if it is in fact a problem. If a true discrepancy does exist it is then essential to narrow the problem down to the appropriate system.

One example of this was an auto pilot problem that occurred on an aircraft with a split bus electrical system. During a refurbishment this machine had a new interior and galley installed along with a new digital auto pilot. Everything appeared to work flawlessly during the test flights as well as trips that were under two hours. The aircrew reported that when the aircraft went on long legs generally over two hours the auto pilot would disconnect. In all cases the crew was able to initiate a reset. Eventually all of the components involved in the new auto flight system were systematically replaced but the problem persisted.

As it turns out, on long flights a dinner was served to the passengers. When the newly installed galley oven was selected on the high electrical current, the draw caused a voltage drop on the bus. The auto pilot was connected to the same electrical power distribution bus as the oven. By swapping the auto pilot power source to the second electrical system the problem was resolved.
By understanding the limitations of the system ideas can be formulated and a logical plan created to tackle the malfunction.

Many airframe manufacturers report that in excess of 50 percent of returned parts are found to be defect free. Unfortunately much of today's problem resolution involves replacing all of the line replaceable units in any system that is operating abnormally in the hope that one of the devices will resolve the issue. This means if five components are changed and only one is defective there are four parts that are returned to service with no defect found.

As an aircraft technician I still cringe when I am trying to return an aircraft to service and the replacement part has documentation that the component had been returned to stock and no fault was found.

Successful troubleshooting involves detailed planning. Someone much wiser than I once said, "Failure to plan is a sure plan to fail."

Analyze the problem
Being a winner at problem resolution requires several elements. First is an accurate interpretation of the defect. Aircraft that require more than one pilot should involve a debrief of all crew members that observed the problem. Often a second solicited observation may provide the key to the resolution. Questions should include: Has the problem been noticed before? What configuration was the aircraft when the problem occurred? Was it ground or flight, climb, decent, or cruise?

Did you do anything to remedy the problem and how did it affect the situation? Was anything recently done to the aircraft? This may include major or minor maintenance, the installation of new systems or components, or even a revision in operating procedure or technique. Weather is often a factor in system failures - things like "Cold Soak" or exposure to high temperature.

Does aircraft altitude or state of pressurization have an effect? Rain or high humidity will often aggravate a problem. Never discount the obvious and never "assume" anything. I recently had a telephone call from a pilot complaining that when the engine start switches were selected neither engine start system would work. This particular aircraft uses a pneumatic start system that is normally supplied air from an auxiliary power unit (APU). The first question I asked was, "Is air coming into the aircraft through the air-conditioning system?" This apparently jogged this pilot's memory as he usually flew aircraft with electric starters. After a long pause his response was, "I'll call you back." He never did. I later found out that he had forgotten to start the APU. The point is that just because a problem is reported does not mean one always exists. So in some cases even the most basic questions can have technical merit.

Know the aircraft and systems
Knowledge of the aircraft as well as the specific system in question will provide the troubleshooter with the ability to determine which components are most accessible and what information can be retrieved during testing. As an example, a fuel boost pump system typically requires the pump which may be remotely located within a fuel cell, a pump relay possibly located in a relay rack in a maintenance compartment, a switch, and a circuit breaker located on various panels in the flight compartment. When the switch is selected "on" it may supply an electrical ground to the relay which in turn supplies power from the circuit breaker to the pump. By accessing the relay, checks can be made of the circuit breaker, switch, and the relay itself. Obviously if all these components check as they should then it is time to go after the pump. The point is by going after the relay most of the system can be tested at one point that has easy access.

Contact the manufacturer
Making contact with the aircraft, engine or device manufacturer may provide significant information about history or similar fault situations. Many aircraft manufacturers include a schedule of component servicing recommendations as part of their maintenance and inspection schedule. This document can also play a beneficial role in problem resolution. Many component manufacturers publish the anticipated useful service life for their devices.

In addition various checks may include such things as inspection of electrical brushes for wear or special lubrication intervals.

This information can be used possibly as a decision-maker as to where to go first when more than one item may be causing a fault. After all, if one part has four-fifths of its useful life already used and the other suspect device has only been in service for several hundred hours and both devices are subject to internal wear odds may favor the high time device as the culprit. This theory may also have a negative side as in some cases a component freshly out of repair or overhaul may develop some in-service glitches. It is often worthwhile to do a bit of research to determine the reliability of suspect components plus a telephone call to the component repair center who returned the suspect unit to service and speak directly to one of the specialists who deal with these devices on a daily basis.

Method and tools
Once the decision is made as to which component is a likely candidate, the selection of a troubleshooting method and appropriate tools needs to be made.

In my opinion the most important consideration here is technician familiarity. The most expensive and complex test equipment is often designed to be used by a qualified person to perform perhaps one specific check. When an aircraft is in an AOG situation it is usually a situation that is not conducive to a learning environment. When troubleshooting an electrical problem a multimeter is most frequently my tester of choice. Fluency with this device can enable a technician to conduct a multitude of tests.

Record and review
Once the plan is complete implement it step by step while recording all findings. A common mistake is to assume that once an abnormal value is spotted troubleshooting is complete. On the contrary, frequently a perceived discrepancy may in fact be the result of a device way down the list, yet the specific problem may impact findings on other devices in the system.

Before taking any action review all findings from the various troubleshooting procedures. Review them individually and then look at the big picture. Results can often be confusing and may in fact call for a second plan to be developed to try to sort them out.

In a situation where a lamp in an electrical circuit fails to illuminate all of the components should be considered suspects. If the circuit contains a circuit breaker, switch, lamp, socket, and wiring, of course the lamp would be a leading presumption. If the light element had failed, an easy verification could be made by installing a voltmeter or some other appropriately rated testing device across the contacts of the lamp socket. If power is detected with a voltmeter it is not an absolute given that the circuit is healthy.

After all, a corroded ground or a burned set of switch contacts may still show the presence of voltage in a no flow condition. By using a voltmeter in parallel with the lamp and utilizing an independent ground for the tester, voltage can be observed coming into the lamp as well as on the ground side. If significant voltage is noticed on the ground terminal then replacing the lamp probably won't resolve the discrepancy. This voltage may be the result of a poor electrical connection to aircraft ground or may even indicate a broken wire.

Similar diagnostic methods are easily applied to a variety of systems. A hydraulic problem can be discovered using the same method as solving an electrical problem. All that changes are the tools and the type of tests.

Once the troubleshooting is finished and the culprit is dealt with the job is still not finished. A thorough testing of the repaired circuit or system is essential to verify the fix does not have any adverse side effects.

And when all is said and done and to stay true to form with our medical counterparts, payment is expected when services are rendered!