Troubleshooting Low Oil pressure

Oil. It’s quite important stuff in engines, especially in aeroplane engines. Considering the only thing keeping us up in the air is the engine it makes sense to keep a close eye on the oil.

He who is without oil, shall throw the first rod

Compressions 8.7:1

I have an issue with a low oil pressure reading. It would be less irritating if there was some oil floating around in the cowling, or a stripe down the fuselage, but no. Not a drop.

Initially it seemed like the pressure dropped with altitude which made us suspect that there may be an issue with the breather pipe, but the oil system on the Rotax is somewhat more complex than the wet sump on most aviation pistons. The engine is a dry sump system. The oil resides in the oil tank (perhaps unsurprisingly) and is pumped from there to the oil filter via the oil cooler by the oil pump which is driven by the camshaft. Oil returning to the sump is pumped back to the oil tank by blow by gases, the Turbocharger has oil pumped via a secondary line and the oil drains from the turbo sump to the oil tank.

The oil pressure is measured by a sensor ‘distal’ to the pump and there is a pressure regulator. The pressure regulator may be the source of the issue – it contains a spring and a ball bearing which regulates the pressure. When we looked at the engine analysis graphs again it appears that there is a better correlation between oil pressure and oil temperature, rather than altitude.

Oil pressure in blue plotted against oil temp (Purple ) and Altitude (green/yellow)

This may mean the problem is easier to solve. Maybe. As temperature increases the viscosity of oil decreases – which may make it harder to pressurize if there is some issue with the seal, pump, oil cooler bypass etc etc.

What this really needs is someone with real know-how to test fly the aircraft, possibly also inserting a mechanical Bourdon tube type gauge into the system and comparing pressure readings. What I wanted to do was to try and replicate the problem with the altitude factor removed – and this would mean flying in the pattern at high power until the oil temperature rose enough to decrease the oil pressure.

So this is what I planned to do. Of course, having an issue to solve on the aircraft almost certainly means that no time will be available to fly so it is 10 days later and time to test fly. That was the plan anyway. Of course, on arrival at the airfield I found it blanketed in mist despite the late hour (9:30am). Not a problem, I had another job to do and that was to try and get a fuel stick calibrated.

I had a great idea for this job – take a hose pipe, stick it into the tanks, siphon out as much as I could then sump the rest out. Well, that plan did not work at all. Firstly I could only siphon 25 litres from the tanks before the hose was uncovered and I couldn’t push it further into the tank (admittedly, I was loathe to force it in in case it did damage to the float gauge). Also, siphoning petrol is no fun at all. I got quite a lot of fuel in my mouth and in my nose which caused me to smell like I was living in a fuel tank for the rest of the day.

I decided to compromise and drain until the area directly below the filler port was dry ( ‘to tabs’ if you will) and then measure how much fuel it would take to fill the tanks to the brim again. With a known tank capacity (84 litres) I could subtract this amount and have the fuel level at tabs. As it turns out, if the tabs are dry, there is less than 35litres in the tank. Good to know. I like having some kind of scientific estimation of fuel tank volume. I used a big piece of wood, marked it as I filled the tank 12.5 litres at a time and then transferred these marks onto a wooden dowel stick.

Primitive Dipstick

While I was sorting this out, the fog had lifted to low overcast but that seemed compatible with circuit flying so I started up and headed out. On climbing out from the field I couldn’t help but notice the rather large storm approaching about 30km away – this was going to put a spanner in the works. Still, I was able to do about 5 circuits at high throttle settings, getting the oil temp up to 82 Celsius / 180 Fahrenheit- without causing a low oil pressure alarm – the pressure did decrease though – as shown in the Savvy graph below.

Top graph is Oil Temp against Pressure – seems to correlate somewhat doesn’t it?

The incoming storm put paid to the 30min flight I was hoping for and I put her down very quickly – into the teeth of the Cb outflow – not my best ADM moment but safely down and we crammed the plane into the hangar just as the first drops were starting to fall.

It looked worse than this in real life!

There will be no flying for the next two weeks but after that I’m popping IBM across to the AMO for the oil change and they will check and possibly change the oil pressure regulator – which will hopefully sort out the issues. To be honest I think it needs a climb to altitude and thereafter a throttle back to allow oil Temp to decrease a bit to fully troubleshoot. Time will tell. In the meantime, I need to wash my hands. Again. Blood petrol….

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