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Hobbs
The hobbs meter is a digital clock-like device that operates whenever the
engine is making oil pressure. It measures time in "real-time", exactly the
same as your watch. From the moment you start the engine, the clock starts
ticking. This is the time you would log as your total flight time.
Tachometer
The "tach" or tachometer has an odometer-like display, which measures time
whenever the engine is running or specifically whenever the propeller is
turning. It measures time proportionate to the speed the propeller is turning.
The tachometer method produces a significant difference that is generally
beneficial to the pilot, since quite simply, the time passes more slowly when
the engine is running slower.
This variance in measurement evens out around 2300 RPM, when the meter equals a
real-time clock measurement. Below that power setting, time will accumulate
slower than a clock, and above that setting, it will accumulate faster. This is
a significant benefit to the pilot during periods of taxiing, waiting for
clearances, or take-off. For pilots performing training operations, such as
touch & go's, stalls, slow-flight, etc., it has been found to offer between
a 15 and 25 percent advantage. In operations such as cross-country flight, this
benefit disappears as power settings are usually in the 2300 RPM range
throughout most of the flight.
Higher power settings will produce the opposite effect, but generally to a much
lesser degree. It has been determined that power settings in the 2500-2600 RPM
range will cause the tachometer to register an extra tenth of an hour for every
actual hour flown.
Wet
Wet means fuel is included in the rental price.
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