A nagging feeling turns out to be justified when an aircraft suffers a huge loss in power immediately after takeoff, as Bob Grimstead recalls…
A few years back, I was flying our Maule over the Pilbara: rugged, brown, mountainous outback country in the far northwest of Western Australia, as part of a continuing exploration of my newly adopted home nation. With me were my wife, Karen and my daughter, Lucy — precious cargo.
Before setting off, I spent a week carefully preparing our aeroplane, replacing any component that might be nearing the end of its life, and inspecting and cleaning everything else, so the Maule was as serviceable as a human could make it. For most of our subsequent two-week adventure I flew at low power settings and with the mixture well leaned out, because fuel installations are few and very far apart in this huge state. The aeroplane turned out to be surprisingly economical, averaging less than 35 litres per hour.
One day, thirty flying hours and more than week into our exploration, we were flying unusually high because we were beating into a strong, bumpy headwind, at full throttle and 2,300rpm. After many minutes of steady cruise flight, I thought I discerned a slight change in engine note.
Scanning the dials, I asked the girls: had they noticed anything unusual? Slightly startled, and forever thereafter critical of my passenger-communication skills, both assured me they had heard nothing. I shook my head to clear my oxygen-starved mind. I was sure something had subtly changed in our engine’s normally steady and reassuring note, but now all seemed well again. Or was it?
We were flying over hostile terrain. The tortured red compacted earth below had first been thrown into huge folds and creases by unimaginable tectonic forces, and then eroded by millennia of exposure to Australia’s harsh climate. There might have been some feasible forced-landing places down in the rugged Pilbara Ranges more than a mile below, but from up here, illuminated into stark relief by the low, soon-to-be-setting golden sun of these tropical latitudes, everything looked hilly, rough and universally uninviting.
I worried. This was just like when flying over water, so had I perhaps imagined the engine’s roughness? And yet, and yet… I was almost certain there was an infinitesimal change in the engine’s beat for a while. Nothing definite, just a slight deterioration in smoothness, and now it had passed (perhaps).
For the remainder of our two-week trip the engine ran as sweetly as any big-bore, four-cylinder Lycoming can, so I relaxed — although the girls no longer could. Nevertheless, for months afterwards, just every now and again I thought I detected this momentary change in engine note, as though one cylinder was occasionally not pulling its full weight. It was nothing you could really put your finger on, nothing definite; certainly not a proper misfire. Whatever I did, I could not reproduce this oddity on the ground, and the variation was so infrequent that I began suspecting it might all be a figment of my imagination.
After fifty years of exposure, I don’t trust piston engines, I don’t like high wingloading aeroplanes, and I am especially wary of high wing-loading, single-piston aeroplanes. I only have the Maule because it was suited to our air-to-air photography needs. Of course, the Lycoming O-360 is statistically the most reliable piston aero engine of all time, and our Maule’s wing-loading, while high, is far from being the highest among American singles. It just doesn’t glide as well as I would like.
But everything worked fine. Pre-flight magneto checks (at 2,000rpm) were always normal. Spark plug colours indicated a correct mixture, there were no apparent exhaust or intake leaks. During the next Annual Inspection all parameters were correct, and cylinder compressions were perfect. Everything seemed acceptable, but there was still that lingering doubt.
That was, until one particularly hot day the following year when, taking off for a local flight with my son James, at 200ft all hell broke loose. Out in front there was a terrific misfiring, heavy vibration and a huge power loss. Seriously startled, I instinctively throttled back, simultaneously pushing the yoke forward to maintain speed. I would have forced the Maule back on to the ground, but its lowered nose revealed the disappearing upwind end of the runway. At that airfield the terrain ahead is extremely uninviting, and an engine failure after takeoff has always been a serious concern.
Slowly, my brain absorbed that closing the throttle had stopped the rough running, so I tried opening it again. Unfortunately, as soon as I moved it more than an inch the misfire was back, coarse and ugly. Experimenting a little, while swinging us gently into a very slight left turn, I established that I could coax just enough power for level flight from our ailing engine, but not enough to climb.
So I flew a quick, low-level, ball-centred, ten-degrees-of-bank circuit. Turning final for a point one-third down the runway, when I knew for certain I could glide in, I tried opening the throttle again, but this only brought back the same dreadful roughness and serious power loss. Lacking the courage for further diagnosis, I unceremoniously plonked our Maule back on the deck.
After much investigation over several days it turned out that, despite there never once having been a rev drop during numerous magneto checks, two spark plugs had cracked central electrodes, were not working properly at high power settings, and had probably both been deteriorating gradually for ages. That was the occasional oddity I had heard, one plug in one cylinder quitting for a while. It caused just the subtlest change in engine note, but no significant power reduction.
The real problem was that these two suspect plugs were in the same cylinder, and when they quit completely we got the partial engine failure. When you think about it, it’s obvious. All those spark plugs were the same age, so I should have expected them to quit around the same time. Looking back on those occasions I had heard something unsettling, they were either when I was cruising full throttle and high altitude, or after an extended period at fairly high power settings on hot days. In other words, exactly those situations when the spark plugs are under greatest stress. What did I learn? Never change all your spark plugs at the same time. Do some this year, and the others in a couple of years so that, at any given moment, some of them are fairly new while others are ageing.
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