Definition
A type of aircraft tachometer that measures engine RPM by counting the number of engine revolutions over a precise, fixed time interval and displaying the result, rather than by sensing rotational force or speed continuously. It typically combines a digital or stepped-needle RPM display with a built-in clock-style timing mechanism, and it also records engine running time to a high degree of accuracy.
Plain English
A tachometer that uses a built-in timer to count how many times the engine turns in a set period, then shows that as RPM. It also keeps an accurate record of how long the engine has been running.
Context Anchor
Seen in aircraft engine instruments and maintenance discussions about engine RPM indication.
Derivation
‘Chronometric’ comes from the Greek ‘chronos’ meaning time, combined with ‘metric’ meaning to measure. So a chronometric tachometer is literally a ‘time-measuring’ tachometer — it works by timing the revolutions rather than reacting to them mechanically.
Why Pilots Care
Chronometric tachometers give a more accurate count of actual engine revolutions, which is important because maintenance intervals, engine life limits, and rental billing are often based on recorded engine time. A pilot reading this instrument is seeing a precise, time-based record, not just an approximate speed reading.
Analogy
It is like counting how many times a wheel turns in a fixed number of seconds, then using that count to figure out how fast the wheel is turning.
Intuition Check
Do not read chronometric as meaning this is simply a clock. It is a tachometer that uses a timed counting method to measure engine speed.
Example Sentence 1
During the preflight, the pilot noted the chronometric tachometer reading so the flight time could be logged accurately at shutdown.
Example Sentence 2
During the run-up the mechanic verified that the chronometric tachometer matched the propeller blade count over a timed interval.