Whether on the ground or in the air, shock and vibration testing is essential to the safe operation of aircraft from takeoff to landing. Everyone from Original Equipment Manufacturers (OEMs) and parts makers to mechanics and pilots needs to know that the machinery they’re flying is safe and reliable, with limits and thresholds that are pre-quantified and capable of being accounted for.
Shock and vibration testing also plays a major role in monitoring aircraft systems during flight, and troubleshooting/remedying aircraft issues once the flight is over. Problems whose causes are invisible to the eye can often be diagnosed using these testing methods.
A Wide Range of Testing Options
Because shock and vibration testing covers so many elements of aircraft design, manufacture, and maintenance, the industry offers a wide range of testing options through a variety of companies.
TÜV SÜD focuses on ground-based shock and vibration testing for the entire airframe. “We provide testing services to simulate the environments that parts of an aircraft, or the equipment used in it, will be subjected to during its lifecycle,” said Martin Foley, TÜV SÜD strategic account manager. “This includes shock and vibration testing, as well as other conditions such as temperature, altitude, pressure, fluid contamination and loss of pressure. Our testing services also include those that meet aircraft-specific qualification requirements, as well as generic standards such as RTCA DO 160 and ED-14.”
As the designer and manufacturer of the PBS-4100+ series of engine vibration analyzers and balancing systems, MTI Instruments is a very specialized company in this industry, but no less important. “The PBS-4100+ family is considered the ‘gold standard’ for all of aviation,” said Ken Ameika, MTI Instruments’ executive director of global sales. “They are called into service when an aircraft indicates high vibration or when the engine is to be tested during manufacturing or maintenance.”
GPMS’ approach to shock and vibration testing is airborne. Its core product is Foresight MX, a Health Usage Monitoring System (HUMS) used by rotorcraft in flight.
Using data generated by a helicopter’s various onboard components and systems, “Foresight MX conducts an infinite amount of inspections to maintain their reliability,” said
Eric Bechhoefer, GPMS’ CEO and chief engineer. “We pull flight data off at 8 Hz (8 times/second) and vibration analysis every two to three minutes on the aircraft. This provides us with tremendous insight into the operation and performance of the aircraft, its engine, and its vital components. Really what we are doing is trying to help ensure the design reliability of the platform, by looking inside the gearbox and seeing and knowing what is happening.”
The primary purpose of Foresight MX is to detect problems before they become serious and reveal causes that might otherwise be undetectable.
“The problems that we are solving are very hard problems: They have very low signal to noise,” Bechhoefer said. “Analyzing more data allows us to integrate the signal better and give customers and operators more insight into things like degradation. This data also allows us to project and predict the remaining useful life of a component. Ultimately, the goal is to detect and prevent situations before they reach the chip light, so customers can avoid unplanned maintenance and scenarios where the aircraft is out of service away from home base.”
Who Uses Them and Why
Given the range of shock and vibration testing services offered by the three companies noted above, it’s no surprise that their aviation customer bases differ (although they do overlap).
For instance, TÜV SÜD provides testing services across the entire aerospace supply chain. The aircraft equipment it tests includes everything from avionics, actuation systems, flight control systems and aircraft structural systems all the way to cabin systems, galley equipment, lighting, seating and customer experience equipment. This testing can be done in various different phases of an aircraft’s pre-service life, from initial development through to Safety of Flight testing and finally to full qualification.
“All equipment used on aircraft needs to have some level of shock and vibration testing completed to ensure its suitability for use,” said Foley. “There is of course an initial qualification requirement: Is the equipment safe to use on an aircraft and will it affect any other aircraft systems? But it’s also vital to consider if the equipment will continue to operate for its expected lifetime, particularly given the potentially harsh environment some equipment may be operating in.”
MTI’s customers are anyone and everyone in the aircraft propulsion segment who needs testing information to keep their machines flying. “They include the engine OEMs, the airframe manufacturers, MRO organizations, and aircraft operators,” Ameika said. “The operators are both military (USAF, U.S. Navy and foreign military) as well as commercial (freight and passenger).”
GPMS’ customers are helicopter owner/operators in the civilian, parapublic, and military sectors. “Their primary goal with vibration monitoring is enhanced safety, but second, they have a mission, and Foresight MX provides them with more availability and allows them to move unscheduled events to scheduled events and spend much less time doing rotor track and balance,” said Bechhoefer. “Many of the operators we talk to only get paid when they fly, so the ability to know the physical condition and availability of their aircraft is huge. The condition and vibration monitoring data we collect and the analysis we provide gives them the confidence in their aircraft and their ability to make their missions.”
Creating Meaningful Tests
There is an old phrase in the computer programming world whose acronym is GIGO. It’s short for “garbage in, garbage out.” If the data quality going into a software program is flawed and unreliable (garbage in), the output from that program will also be suspect (garbage out). For that reason, the quest for accurate and reliable input data is a top priority for shock and vibration measurement firms, to ensure that the tests they conduct deliver meaningful and useful results.
For TÜV SÜD, one primary consideration when conducting shock and vibration tests on aircraft systems is “to accurately simulate the mounting arrangements that will be utilized once the components/systems are actually installed in an airframe,” Foley said. “Another primary concern is the choice of measurement transducers being used to record both the shock and vibration input, plus the resulting measurements that will be used for analysis and design.” Choosing the right transducers is critically important to drawing appropriate conclusions if the collected data is modeling the equipment’s reaction to various phenomena via finite element analysis (FEA, the process of simulating the behavior of a part or assembly under specific environmental/operating conditions).
“Consideration should also be given to the other environments that the equipment will be operating in whilst subjected to shock and vibration inputs, such as rate of temperature change, humidity. altitude and icing,” Foley said. “Other challenges include developing a test program that will accurately model the anticipated lifecycle of a piece of equipment or system, so that mean time before failure can be calculated and required maintenance planned accurately.”
For GPMS, creating meaningful aircraft tests means answering some fundamental engineering questions first. “Vibration inspection is not a physical inspection, so some of the challenges we face are setting the parameters for data collection,” Bechhoefer explained. “When do you take data? When do you set thresholds? When is it appropriate to trigger an alert? Pilots are focused on flying the aircraft, so we want to do these things in an automated way — and we want to make the insights easily accessible for operations and maintenance teams.”
At MTI Instruments, the focus is on vibration analysis for APUs, turbofans and turboprops. “Our chief challenge is to provide a vibration measurement and a corrective result as quickly as possible,” said Ameika.
Fast, accurate answers save money. “We estimate every hour of maintenance carries costs to the operator starting at $2,500/hour,” he said. “A computer-generated result enables an imbalanced aircraft fan to be corrected inside of 90 minutes. The most critical attribute of the PBS-4100+ series is that it ensures the aircraft is back into revenue service (or military availability) as soon as absolutely possible.”
Computer-Driven Testing Advances
Aircraft shock and vibration testing has come a long way from the basic methods used in World War II. The advent of computers, onboard diagnostic systems and artificial intelligence- enhanced (AI) software capable of processing “Big Data” (vast amounts of real-time data inputs) has realized this science’s full potential, giving aircraft owners/operators the ability to detect and remedy problems as soon as they start to manifest themselves.
Like the range of tests available today for aircraft and their components/systems, the advances in this field are widespread and varied.
“Significant progress has been made in measurement transducers and digital data capturing, in addition to more powerful FEA modeling tools allowing developers to minimize the qualification programs of equipment by more accurately predicting its lifecycle by using digital analysis,” said Foley. “Also, the deployment of non-contact transducers to minimize measurement errors has developed in both accuracy and use. These technologies can be incorporated into the final aircraft so that contact monitoring and reporting of equipment environments and condition supports this preventive maintenance model.”
For some companies in this space, software-driven technology not only enables their work today, but made it possible in the first place. For instance, “computer simulation was a tool employed by MTI Instruments early in the design life of our core algorithms,” Ameika said. “Occasionally, such tools are used to verify ongoing updates and enhancements to the engine vibration/balance engine.”
For GPMS, the leaps in computer processing power that made AI possible have also allowed the company to up its HUMS-based testing game. “Improvement in processing power has meant that we can pull more data off at higher frequencies, which has the practical benefit of improved detection,” said Bechhoefer. “And connectivity infrastructure, together with the cloud, has made vibration monitoring systems much easier to use and access.”
A case in point: “We mathematically model the dynamics of the gearbox so we can do analysis on it,” he said. “We identify the signals and use that information to pose the hypothesis: ‘Is the component no longer good? Is there enough evidence to show that this component is no longer nominal?’ For example, we make models of the transmission to hypothesize what the nominal transmission would look like and then take the evidence from vibration data to make a decision whether it is no longer normal.”
What’s Coming Next
Today’s aircraft shock and vibration testing industry is operating at a breathtaking level of technical and analytical sophistication. What’s coming next?
The answers from the three companies noted above vary widely. MTI Instruments’ Ameika avoided specifics. “MTI is always investigating the ability to better ‘prognosticate’ failures, as such intelligence saves our customers both in effort and in money,” he said — and left it at that.
TÜV SÜD’s Foley was more forthcoming in his predictions. “The accuracy of measurement systems and real-life monitoring techniques have allowed equipment developers to more accurately design and qualify equipment for use on aircraft,” he said. “This is now extending to more complex test systems becoming available to test in multi-axis, more combined environments to better simulate a given product’s service life and qualification program. This will result in shorter duration test programs, quicker time to market and longer product lifecycles.”
GPMS’ Bechhoefer offered the most in-depth forecast for the future of aircraft shock and vibration testing. “Reduction in cost and weight, and improved connectivity are two areas that we see on the horizon,” he said. “This is why we are focused on making Foresight MX more accessible to more customers through further reduction in cost and weight. This makes the business decision around equipping your aircraft with a vibration monitoring system easier.”
Bechhoefer also foresees “connectivity” supporting advances in this area. “For example, everyone has a smartphone,” he said. “So what we want to do is improve connectivity of the aircraft to devices like smartphones so that operators can do more diagnostics with what they have in hand. Imagine being able to do rotor track and balance on a smartphone!”
One thing is certain: The ever-expanding capabilities of aircraft shock and vibration testing will continue to translate into better-maintained aircraft with less time spent in the shop — and more time in service.