One of the frustrating aspects of trying to solve the problems with the erratic pressure sensors is that it doesn't happen all the time, and we seem to have well behaved airplanes in our own fleet, so nothing to experiment on. That changed a couple of weeks ago when we got access to a customer airplane that was having issues with oil pressure indication. We made the "standard" changes to remove the mate-n-lock connectors, swapped out the sensor etc. It looked like it was fixed, but about 5 hours of flying later, the problem was back.
As frustrating as this can be for the customer and maintenance personnel, it is a goldmine for us as engineers to have a badly misbehaving airplane in our hands to do a bunch of troubleshooting and various experiments on.
Over the course of a couple of days, we did a few experiments and flight checks.
Flight 1: We didn't touch the existing pressure sensor, but we installed a secondary oil pressure sensor on a different port, powered it from an independent power source, and monitored its signal via oscilloscope in the aircraft.
Flight 1 Results: Erratic behavior of the ship gage was seen again, but the secondary source remained stable throughout the flight. Based on this we concluded that it was a sensing or indication problem and not a problem with the engine
Flight 2: Ship probes signal and supply voltage from the GEA were monitored by the oscilloscope along with the secondary probe signal and supply voltage.
Flight 2 Results: Erratic gage performance again, but supply voltage was stable throughout the flight. Only the signal output was erratic. Based on this the problem is either in the connector to the oil pressure sensor or the sensor itself. It is unlikely to be the sensor as this was the 3rd or 4th one installed on this aircraft.
Flight 3: We installed a "splint" type of support on the sensor body to hold the connector and its incoming wire harness fixed together to minimize relative movement between the connector and the pressure sensor.
Flight 3 Results: A few isolated events, but vast majority of flight had smooth and stable indications. Based on this flight it seems that minimizing relative movement between the sensor and its connector and wire harness appears to have a clear stabilizing affect on the signal.
Flight 4: Same configuration as flight 2, in other words, we removed the splint.
Flight 4 Results: Erratic gage performance was again observed. Based on this flight, the strain relief device alone was responsible for the performance improvement.
Based on these flights and experiments, we then did a few more experiments that refined our "splint" from a crude "Popsicle sticks and duct tape" type of splint, to a purposefully designed metal support with adele clamps and zip ties, re-did some experiments, tore apart some sensors and connectors etc. We learned a lot from this airplane.
We had always suspected that wire and connector security at the sensor was an issue, but until now we had not been able to prove it.It is invaluable to have an airplane that was not fixed by our typical repairs, as we learn a lot more from it. I will have more to say about this as we implement permanent changes into production and develop a service bulletin for the field.
