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….

That Sling Smile

Yesterday I was fortunate enough to be able to take my friend Ari flying. He may not know it but he’s actually my aviation mentor – one heck of a nice guy who has a knack of thinking about general aviation in ways I haven’t thought of. He’s a commercial instrument rated pilot with more than 1500h and he flies a FIKI Cirrus SR22T with his father.

One reason we are buddies is that we have similar outlooks on general aviation safety and doing GA correctly. My feelings on these are borne out of lots of reading – his I suspect are from lessons learned in years of flying. It can be daunting flying with a significantly more experienced pilot in a non instructor/examiner situation (OK, it can be very daunting flying with an examiner) because one feels like one’s skills are on show – this was the first time I was to be PIC in my aircraft with him (I’ve flown in his aircraft many times). You don’t want your first flight with a new passenger to be your last flight with that passenger..

Special Sling preflight pose – lifting the elevators to view the control rod attachment bolt

I must confess to feeling that the Sling was a little shabby compared to the extremely well equipped G5 Cirrus but hey, an airplane is still an airplane – and I’m a proud owner.

We had a secondary objective and that was to test the Aerox system I acquired recently – it seemed prudent to do this with an experienced supplemental oxygen user. We came prepared – he had 2 oximeters and I had 1. Of course, true to form mine was DOA, although after 10years of hard use in my day job meant I shouldn’t be surprised. It is time for a new one anyway as the LCD display isn’t very readable in the bright cockpit. In addition we had an emergency cylinder in case the main cylinder didn’t perform as expected.

For a change the weather was playing along really nicely and although the visibility wasn’t really good, the air was smooth as we climbed energetically away from BaraG – had a good 750-800fpm climb rate which impressed my passenger. There was no other traffic in the training area and we started our climb after exiting from under the Johannesburg TMA. Information approved our climb and handed us over to the area controller (Centre) and I handed control over to Ari. For the last day in March it was still pretty warm – was showing an OAT of 21 Celsius at 10000ft but the Sling trundled on at 400fpm through 11000ft with the Airmaster Prop in Climb and 34” manifold pressure on the throttle.

We routed south as we climbed, out over the Vaal River and Parys, enjoying the views (despite the limited vis) and the smooth air. Climbing through 10000 feet I popped the pulse oximeter on and was quite alarmed to find my sats were 89%. I wasn’t expecting it to have dropped so low at a relatively low altitude. I didn’t feel any adverse effects although I don’t believe time of useful consciousness is an issue at this altitude. However, it seemed like a good idea to get the oxygen on which we did.

Looking silly with a poorly applied Oxysaver

Let it be said that one looks a little ridiculous wearing an oxysaver cannula – but you look a lot less ridiculous with the cannula on than slumped over the controls with two fighter jets on your wing. The oxysaver allows pretty low flows – <1litre per minute at altitude up to 15000ft – by using a small reservoir so it’s relatively economical to run once you’ve forgotten the initial outlay for the equipment!

Above the inversion @11500ft south of the Vaal River

Area control came on asking us to stop our climb at FL125 due to other traffic on the airway above us which was a little disappointing but the density altitude was already well over 14000ft by this time, so I didn’t think with a service ceiling of 16000ft we’d get much higher anyway.

Heading back into the smog…

At this point we noticed that the oil pressure had decreased to 1.6bar from 2.5bar. Lower limit of normal is 2bar on the Rotax 914UL. This was obviously not something to take lightly. Initially we thought it could have been due to the warm conditions – oil temp was normal and CHT’s were within limits. Still, we arrested the climb and throttled back with no improvement. The best call at this point was a 180deg turn and route back to home base keeping a good lookout for potential off airport landing sites.

We had to descend to avoid busting airspace and as we descended below 9500ft the oil pressure recovered to normal and stayed there, going up to 3bar as we leveled off at 7000ft. Given that there was no other indication of abnormality we felt it was reasonable to do some maneuvering while keeping an eagle eye on the engine gauges.

I did two steep turns (PPL standard @ 45deg) and thereafter Ari did 2 at CPL standard (60deg). That man can fly an airplane – made the wake on the second attempt in an unfamiliar aircraft. He was raving about the control responsiveness and the inherent balance of the Sling – I think he was quite impressed – high praise indeed from someone who has hundreds of Cirrus hours. “It feels like I have an RV Grin,” he said as we cruised back for the overhead at Baragwanath. I dubbed it the “Sling Smile”

Steep turns – fun!

One touch and go then a full stop landing and it was time to put the plane away. I did get some constructive criticism of my crosswind landing – I need to try and land upwind of the centreline rather than ON the centreline – this makes sense as it gives one a few more metres to deal with any squirrelly behaviour after touchdown.

All in all a successful morning’s flying. The oil pressure reading remains a concern but I’m waiting on the AMO to comment. We suspect it may be a grounding issue or a problem with the oil breather pipe. We had a good look around the inside of the cowling and there is absolutely no oil leaking anywhere and no oil stain on the belly of the plane. Time will tell – the joys of aircraft ownership…