For any operator, particularly the ag aviation operator, having an engine go out-of-service unexpectedly is not only frustrating, but potentially revenue-impacting, or worse, life-threatening. It’s not possible to predict all engine failures in advance, but it is possible to detect many of them through the use of a full-spectrum vibration survey.
A full-spectrum vibration survey, or vibe survey for short, is a procedure that uses engine-mounted sensors to measure the frequency and magnitude of vibrations emitted by an aircraft engine. Certain frequencies of vibrations map to specific problems or potential problems. In other words, a vibe survey can tell you a number of things about what’s going on inside your engine based on the presence of vibrations. Identifying these vibrations early, when they’re relatively small, allows the operator to address them before they result in a failure. For this reason, a vibe survey is an excellent preventative maintenance tool.
Conducting a vibe survey doesn’t take long, and most of the time is spent setting up the airplane with a phototachometer, reflective tape and an accelerometer to measure the vibration. Once the airplane is set up, the engine is run up to operating RPM and with the push of a button, the vibe survey data is captured. The photograph below shows the set up on a Grumman Ag-Cat with a Garrett TPE331 engine and a 3-blade prop:
Vibe surveys measure vibration in Inches Per Second (IPS). In general, the maximum, acceptable level of vibration is .15 IPS. However, vibrations at this level probably escape the notice of the operator, but a vibe survey can detect a vibration that an operator can’t yet feel. Vibrations of .50 IPS are substantial enough that, over time, they can damage the engine, airframe or instruments.
Most of the time, vibrations are the result of propeller assembly imbalance, a condition corrected by dynamically balancing the prop. Typically, a vibe survey is done as part of dynamic propeller balancing. The table below shows the actual result of a series of prop balancing runs performed using a DynaVibe GX prop balancer and vibration analyzer on the Ag-Cat.
The second column in the chart above shows the vibration magnitude for each successive prop balancing run. Subsequent columns show the recommended weight and location for adding that weight to balance the prop. At the end of the third balancing run, the vibration was almost eliminated, and was at a low enough level to not require further balancing runs.
Sometimes vibrations are more complex, and the value of a vibe survey is revealing the source of the vibration, particularly if the cause is something other than the prop. In the case of this particular Ag-Cat, the post-prop balancing vibe survey looked like this:
After prop balancing the only significant vibration remaining is the 3-per vibration (100 Hz) caused by the 3-bladed prop passing near the ground. This vibration is not a concern as it will go away once in the Ag-Cat is in the air.
Vibe surveys are a great way to do engine trend monitoring. Conduct a vibe survey on a new engine or anytime an engine has been overhauled. This baseline vibe survey then serves as the benchmark for comparing future vibe surveys done at regular intervals. The approach allows detection of vibrations when they’re relatively slight, before the operator can feel them. The operator is alerted to their presence and can continue to monitor them and address them before they grow to the point where they can damage the engine, or worse, result in a failure.
Matthew Mitchell, who operates Mitchell Flying Service based out of Perry Municipal Airport (F22) in Oklahoma, has been flying since 1990 and puts about 250 to 300 hours per year on his Grumman Ag-Cat. Mitchell has two TPE-331 engines that he rotates as needed. Anytime Mitchell puts an engine together, he likes to have a baseline vibe survey done. This allows him to monitor the health of that engine while it is in use. Regular vibrations surveys can help operators like Mitchell to identify problems before they cause a shutdown or result in a failure.
The DynaVibe GX system is the most affordable way to do dynamic propeller balancing and vibration surveys for engine trend monitoring. To learn more, click the yellow "Contact Us" button below, or call the DynaVibe team at: 405.896.0026. You can also enter your email address in the form below if you want us to contact you. If you'd like to shop for a DynaVibe GX2, visit our online store.
Click here to watch a short video showing how to conduct a vibe survey with DynaVibe.