I have always found troubleshooting to be an enjoyable challenge in applying logic.  It requires fashioning a path of investigation through a malfunctioning system that systematically checks every possible cause of a problem, preferably progressing in steps beginning with the least intrusive and expensive.  That’s the general theory, although a slightly alternative approach could progress from most to least likely, regardless of other factors.  Whatever the case, discipline and diligence are key, both in adhering to the plan, and in interpreting the results of each step.  With long practice afforded by operating submarines as well as airplanes, my innate skill at troubleshooting has been well exercised, making it all the more disconcerting on those occasions when I find myself having gotten off track.  The failure of the tachometer was one such case, and to rationalize the expensive lesson, I blame that mantra of instrument training, that pilots should always trust their instruments.

The problem began with RPM indications that failed to match what was expected from adjustments in propeller control.  The errors were greater at higher power settings, and they would sometimes wander in a way that seemed to suggest the propeller governor was having difficulty holding pitch.  The fact that the governor had not been overhauled in an exceedingly long time, further focused my suspicion, which is where, in hindsight, I lost objectivity.  Sent out for overhaul, it was only when the governor was found to be working fine that my troubleshooting path circled back to the less intrusive test of the tachometer.  Once the governor was reinstalled, this would involve measuring the spinning propeller directly.  However, failing to observe oil pressure upon engine start, the shop, without consulting me, began troubleshooting the new problem and had a number of engine components disassembled before I was made aware and intervened.  The lack of oil pressure on startup, after an extended period of inactivity, was something I had experienced before as a loss of pump suction.  The unorthodox, but highly effective fix, was to start the engine with the oil filter cracked off its seat to allow the pump to prime, and then immediately re-tighten it.  Though messy – and undoubtedly tense for the one holding the filter – it instantly restored oil pressure, much to the astonishment of the experienced mechanics who moments before had been consigned to engine teardown.  With the engine now operational, an easy check of the propeller with a strobe confirmed the fault, indeed, lay with the tachometer.

So it was, that an initial hasty assumption had derailed the logic of our troubleshooting plan and incurred several weeks delay and the cost of much unnecessary effort.  Fortunately, the flaw in the subsequent oil pressure troubleshooting was caught before more serious investment had been made.  It’s worth being wary in troubleshooting, as sometimes a little bit of knowledge provides just enough confidence to inspire bad assumptions.  Combined with the perennial pressure to get our airplanes flying again as soon as possible, it can lead to extensive missteps and other undesirable outcomes.