Gearbox Oil Problems

[pilot and apprentice]

This is probably a little thread drift, but this is where I was reminded. (decided to start a new thread)

An issue with Cougar 491 was the lack of high MGB oil temp after oil loss. This is explained by the system reading (when dry) air temp in the box.

I've had issues with the 212 C-Box twice. Once a blockage causing no flow, resulting in zero pressure indicated. The second was a loss of oil to zero pressure (and post-flight confirmed it was gone). In both cases, the temp went full hot with the cooresponding light.

I'm not seeing why the system differs enough that we didn't see similar indications: normal or no temp light with no oil flowing past the switch in case #1, and no oil in the sump to cover the bulb in case #2.

[Arm out the window]

Wild guess, but I'd think it would depend on the characteristics and mounting / positioning of the sensor. If it's directly mounted in or on the metal of the gearbox sump, then without cooling oil it would tend to read very hot. If it's in a line or not directly attached to the gearbox metal it might not get that heat soak and, as you say, read the air temperature.

A bit like how if you stick your hand in the engine bay it's warm, but if you touch the hot metal it's bloody hot!

I might be totally off track there, too, but it sprang to mind.

[riff_raff]

p&a-

As AOTW noted, the oil temp sensor is only detecting the local temperature at the sensor tip. I don't know what particular type of sensor was used on the examples you describe, but one common type of temp sensor is the thermocouple. The thermocouple uses two different types of metal conductors that produce a temperature dependendent voltage at their junction point. The junction point is usually at the sensor's tip. An oil temp sensor is usually mounted such that the tip protrudes some distance beyond the housing/manifold wall and into the oil flow, so that the bulk oil temperature is being measured rather than the housing/manifold wall temperature.

While your question about MGB oil temp readings is valid, I would suggest that we take a step back and look at the bigger issue of why MGB oil temps are important. All MGBs are essentially oil cooled devices. The friction heat generated at the gears and bearings is removed almost entirely by heat transfer to the recirculating lube oil flow. There is very little heat transfer directly from the bearings and gear meshes to the housing due to the limited thermal conduction paths. With regard to MGB temps, what is of primary concern are the operating temps of the gears and bearings themselves, and not so much the oil temps. But since there is presently no practical method to directly measure the operating temps of gears and bearings, we use the indirect method of measuring lube oil temps.

Hope that helps give you a better overall perspective of the issue.

riff_raff

[Arm out the window]

The temperature indication is of great value to operating crew not only to confirm normal operation, but also as a secondary indication of an abnormality, so it's useful to know what will generally happen in a particular type.

Sounds like the Bell situation is to read hot with oil loss or very low pressure, whereas the S92 didn't apparently. No doubt there would be other secondary indicators like warning captions etc, but I guess it's good to know what your type can be expected to do in a given emergency (although as we all know, Murphy's law can be relied upon too, so it might not always go that way).

[pilot and apprentice]

I agree R-R, we are looking at a symptom to find a cause.

The reality is though, that in the air those symptoms are all we have. If I had access to the 2 types in question right now I'd be taking a look to see what is different about the installations, sensing technique, and anything else I could find, that could explain the readings. As much for curiosity as anything else. Then I would apply that to the type I'm on now to predict how it would react in real life.

As a pilot, I have used temps and pressures often to make critical decisions about the real severity of a malfunction: is it a gauge, a sensor, or a component that is in trouble? Very little of my flying has been over what could be called non-hostile terrain, and I take very seriously the trust that the people in the back put in me to make the right decision.

As an engineer, I want to provide a safe aircraft, and to know the machine well enough that when a pilot asks my opinion on it, I am confident of my answer.

I took a look at my old 212 notes and trying to think back. The notes are more reliable :-) A temp bulb for the gauge and a temp switch in the cooler output line for the light. I guess I'll have to wait until I can put my hands on them to really answer the question.

Cheers guys.

[riff_raff]

p&a-

If you can provide a lube system schematic for the MGB design in question I'd be happy to take a look at it and see if I can provide you any additional insight.

As was noted, how one interprets what a high/low oil temperature or pressure reading might indicate depends upon where the temp/pressure sensor is located in the lube circuit. And as you inquired in your OP, different combinations of abnormal pressure and temperature readings can indicate different problems. However, without knowing the specifics of the MGB lube system design in question, there are still a couple of conclusions we can draw about what abnormal oil temp/pressure readings might indicate, since most MGBs now have certain common design features.

For example, assuming we believe the oil temp/pressure readings to be valid, a combination of high oil temp and normal pressure would likely be due to some problem with the oil heat exchanger. Something like a loss of airflow over the heat exchanger core or a malfunctioning thermostatic bypass valve. While high oil temps can eventually cause gear/bearing scuffing failures, as long as there is still adequate oil flow (as indicated by oil pressure readings above minimum threshold), the MGB would not likely suffer a rapid catastrophic type failure.

On the other hand, a combination of very low (or zero) oil pressure and normal/low oil temperature would likely be very problematic, and could be caused by a number of things. It's this type of condition that emergency lube systems are designed to address. The gears and bearings of an MGB will continue to operate for a while as long as they are provided even a very small amount of oil. Even a small amount of oil at critical gear meshes and bearings will greatly reduce frictions and reduce the heat generated.

In short, high MGB oil temps are bad. But loss of MGB oil flow/pressure is far worse.