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Falling oil pressure

InstrumentPilot - 88/2011


I learnt about (instrument) flying from that


Alan South recalls an apparently minor mechanicalproblem that could have ended in disaster.

It was one of those moments. I couldn't be surę, but it looked like the oil pressure gauge had dropped a needle's width sińce I last checked. It was also one of those moments when in the face of some demanding flying, my subconscious was starting frantically to assemble all the Information collected during the day. A needles width drop was significant, as the green arc was only 6mm wide. On the other hand, the wiring on this 1982 Mooney was a bit dubious, and most of the other gauges in the cluster had given problems in the past. For the moment, I flew on. I got out the POH to try and learn a bit morę about the oil pressure indicating system. However, as soon as I saw the gauge drop another fraction of a needles width, I knew it was time to check this out on the ground.
Here is how it all started, just a few hours earlier. I was pre-flighting my group's Mooney 201, and F d just dipped the oil. It was down at 4 quarts. As the engine was getting on a bit, we tried to keep the oil level at about 6-6l/2 ąuarts, as any higher, and it breathed a quart or so out over the belly. Nonetheless, by the time the level was at about 6 ąuarts, things would stabilise and we would get fairly Iow oil consumption. I checked the tech log, and found out that the Mooney had flown in the day before from Lyon-Bron, so maybe the pilot couldn't get any oil and had headed home with a Iow starting level. It had never happened before, but thcrc's always a first time.
Anyway, I had to be getting on. My plan was to fly from Cambridge to Biggin to pick up a colleague, and then for both of us to head up to Liibeck for a business meeting the next day. Time was getting a little short, and the weather en route had a forecast of thunderstorms for much of the Low Countries and northern Germany. On days like this I always like to have plenty of time, fuel and daylight. I filled the tanks and put in 3 litres of oil. On the run up there was a persistent mag drop on the "first click to the left". This set of plugs for some reason was always morę prone to fouling than the others. Normally any fouling cleared after just a minutę of running at 2000rpm, but this time it took some prolonged running at 2500rpm before the mags would check OK at 2000rpm. Credible, but odd. I kept to my schedule, and the quick hop over to Biggin went fine, though Fm surę the engine coughed on very short finals to runway 21.
My colleague, Neil, was ready and we were soon airborne again on our IFR flight plan to Liibeck.
The weathermen were true to their word. As we approached the Belgian coast at FL90 there were huge build ups from about 5 miles inland, stretching as far as the eye could see. I heard one airliner ask for a 40-mile deviation "to avoid". We were in the clear as we routed up the coast, and there was nothing showing on the Stormscope.
Still, it was hard not to be a little preoccupied by the weather as we soon had to turn inland to cross Holland. There was certainly the prospect of a rough ride, and always the possibility of a diversion. There was the added factor that our business meeting started at 9am the next morning. Dropping oil pressure Somewhere across Holland, just as it was looking like we would have to start flying through some of the weather, I did a FREDA check. My habit was to carry out this check every 10 minutes, on the 10 minutę points of the flight timer. As I got to the E for Engine, I noticed that the 11 pressure gauge had dropped a needle's width. Five minutes later, the oil pressure indication had dropped another needles width. Despite an overwhelming urge to disbelieve the gauge and to get to Liibeck, something clicked inside, which is where this story began. These "credible but odd" instances were somehow stacking up to tell mc that this was a problem to be checked out on the ground. I decided it was best to turn away from the weather and reąuested a diversion into Rotterdam, about 10 minutes behind. During those 10 minutes, the oil pressure indication continued to fali, and by the time I got Rotterdam in sight, it had fallen through the green and was nów at the bottom of the yellow arc. Also during those 10 minutes, I reflected on how Dutch fields tended to be very long and narrow, just like a runway in fact, but today they were orientated pretty much at 90 degrees to the wind that was gusting to 30 knots, and had ditches on all sides.
As I taxied in, with the engine running perfectly smoothly, I was trying to think of all the ways the oil pressure gauge might fail, and the tests I might do, alone on an apron on a Sunday night.
First thing after shutting down was to dip the oil. There didn't appear to be any! We took the cowlings off and looked for oil leaks or excessive breathing. There didn't appear to be any, either. As the engine cooled, and the oil drained back from the galleries into the sump, there was just a trace, right at the end of the dipstick. Somehow 5 or 6 litres of oil had simply disappeared in Wi hours flying. We left the aeroplane in Rotterdam and drove all though the night to Liibeck. The city has a large number of churches, and they all chimed nine as we turned into our client's car park. Despite the total lack of sleep, we had a good meeting, and I put poor Neil on a scheduled flight back to the UK before driving back to Rotterdam to try and sort things out.
So what had gone wrong? It turned out that the piston rings on two of the cylinders had become gummed in their grooves. This meant that two cylinders were pumping a huge amount of oil and burning it — which causes even more gumming — a runaway effect. Virtually the whole contents of the sump were burned, therefore leaving no trace, in 1V'2 hours. I guess that there might have been a blue cloud of smoke behind, but I didn't have a rear view mirror to look for it. The falling oil pressure would have started as the oil level got so low that the pump started to draw in a mix of oil and air. The fouled plugs and possible cough were due to the cylinders having lots of oil inside. It's not hard to work out how much longer the engine would have run before seizing. I reckon it would have been 5-10 minutes.
So what did I learn about flying from that?
1. Most disasters happen as a result of a chain of events. After this episode, I was reassured by the way my subconscious pieced together a series of events as potential links in a chain.
2. On reflection, I flew the diversion with more trust in the engine than I should have done. I was still of a mind that I had an indication problem, not an engine problem. I should have planned my descent on the basis I had imminent engine failure and held FL90 until Rotterdam was within gliding distance. Dutch ATC, like all ATC in my experience, were incredibly helpful and offered me any routing and any profile. Next time, I resolved to treat all technical diversions as real until proven otherwise. Interestingly, in that very same aircraft, there was a "next time", and yes, I did keep that promise to myself.
3. I often worried that all I was doing was checking that a gauge was still there, not really looking at the indication. I'm convinced that the enforced rigour of the FREDA check helped here
4. As if we didn't have enough problems, when we went to hire the car at Rotterdam, I realised I didn't have my driving licence. Neil was 24, and no one would hire a car to him. I now carry my driving licence in my flight case on every flight.
5. Non-flying passengers can be very resilient and maintain their sense of humour. After an all-night drive, a long business meeting, and a scheduled flight home, Neil still had the sense of fun to draw this cartoon entirely from memory. He even got the fields in the right proportion. I think it is reasonable to forgive him for not quite getting the registration right!