Piston engine troubleshooting

Piston Engine Troubleshooting

By Thomas Ehresman

March 1998

You work in a shop that specializes in turbine equipment. The few cabin class piston twins for which the FBO is responsible get their maintenance done elsewhere. From time to time, however, piston engine problems do arise that demand your attention. And since everyone else is busy, guess who the boss chooses to evaluate the problem?

If you don't do this every day, it can be a little intimidating; trying to rattle the rust out enough to remember the systems and what to do first. So let's examine some troubleshooting fundamentals for piston engines that will both simplify and speed up the process.

Clarity of Information
This is the first and most important principle. A common mistake is to rely on unclear or vague descriptions of the symptom. Often, there are subtle events or symptoms which occur that someone else may miss, especially if they are otherwise occupied flying the aircraft. It is important to get information on as many other parameters as possible, even ones that seem unrelated. Many things are better sorted out later, in the shop. A good situational awareness is a great asset in this phase of troubleshooting.

The best way to get a good read on the symptom is to experience it yourself. Do your best to emulate the same conditions and duplicate the problem. This way you will find the problem and not create a different one; if you end up creating a different one, the old one will come back to haunt you. Remember, the same problem can give different symptoms in different conditions.

Sometimes it will be necessary to rely on secondhand descriptions; e.g. "gremlins" that come and go at the most frustrating and inopportune times (Murphyism). It is then imperative that clear and concise information is exchanged both ways. The person experiencing the symptom, usually the pilot, needs clear and definite instructions to troubleshoot when the symptom occurs again (provided safety of flight is not compromised).

Avoid wasteful speculation. This is different from hypothesizing. Speculation is where your mind sort of goes off without you and causes you to start working on things before a test has confirmed that what you are doing will actually fix the problem. Whereas, hypothesizing is based upon educated guesses that presupposes that one is well educated. This brings up the next important principle.

Systems Knowledge
Systems knowledge is somewhat similar to clarity of information in that good system information is a must when diagnosing a symptom. You don't have to have mastery of all systems to be effective at this. Accurate systems information can come from any number of sources. Maintenance manuals, training materials, experienced personnel, or manufacturer customer service are some good starting points. Caution: Don't let unqualified sources lead you down the wrong path.

Diagnosis
At this point, sit down with your systems knowledge (whatever the source) and go through the system operation step by step. Note which component and situation would cause the symptoms you're experiencing. List these possible causes and move to the next step.

Testing
Start with the most likely and simple causes first and test to eliminate or confirm them. Tests should be limited in effect to the suspect component. Don't apply a test that involves too many components or you won't narrow your field of suspects. Use as specific of a test as possible; one that will eliminate possible causes. Accurate information is essential throughout this process. Reevaluation may be needed after testing a suspected cause.

If no test indicates the source of the symptom then the dreaded shotgun may be pulled out. Again, limit the replacement of components to those which will affect your symptom. Start with the most inexpensive ones first. If you have access to these components without having to purchase them, or have the ability to return them if not needed, the process will be much cheaper. This one, however, is a time eater. It may take a lot of shop time and shipping charges if it starts to go very far.