Settle a tach recording argument

Tachometer-based time recording remains a persistent source of operational ambiguity in flying clubs and shared-aircraft environments, and the scenario described — a needle falling between two tenths on a mechanical tach — is one of the most common points of contention in club administration. Unlike a Hobbs meter, which measures elapsed wall-clock time, a tachometer measures engine RPM-referenced time, running slower than real time at cruise power and faster at high RPM settings, though most training and club aircraft operate in a range where tach time runs roughly 15–20% less than Hobbs. When billing is tach-based on a wet rate (fuel included), even a single tenth of an hour carries real monetary consequence — at a typical club rate of $150–$180/tach hour, the difference between .1 and .2 is $15–$18 per ambiguous return.

The club's stated policy — round up — is operationally sound and administratively defensible. Rounding up on ambiguous tach readings is consistent with how most maintenance tracking systems handle uncertain values: it errs on the side of conservatism with respect to engine time accumulation, ensuring that TBO and inspection intervals are not inadvertently understated. From an aircraft-owner or operator perspective, underreporting engine time is the more dangerous error. Billing a member for an extra tenth of a tach hour is a minor inconvenience; failing to capture true engine time can push an aircraft past a maintenance threshold without triggering the appropriate inspection.

The broader issue the debate surfaces is the need for explicit written policy governing tach recording — a gap that exists in many informal club environments. Professional Part 135 and corporate flight departments resolve this ambiguity by either using Hobbs exclusively or requiring pilots to record the exact tach reading to the displayed digit with no rounding, letting the billing system apply consistent rules. Flying clubs that operate on tach billing without a documented rounding convention leave room for inconsistency between members, which erodes trust and creates disputes exactly like the one described. A simple one-line policy addendum — "when the tach needle falls between two displayed tenths, record the higher value" — eliminates the argument entirely.

For pilots operating in Part 91 club environments, this episode also highlights the importance of understanding how your aircraft's time-tracking method affects both your logbook entries and your billing. Most pilots log flight time (Hobbs or block-to-block), but when a club bills on tach time, the two figures diverge, and members who don't understand the difference may perceive billing errors that are actually the result of legitimate tach-vs.-Hobbs discrepancy. Clubs are well-served to educate members on exactly what the tach measures, how it differs from real elapsed time, and what the rounding policy is — ideally during the checkout process rather than after a billing dispute arises.