What is the difference between tachometer time and Hobbs metre time and how are they both measured?
The tachometer is actually a measurement of engine hours. For example, at sea level, your engine’s ‘red line’ may occur at 2700 RPM – this is the engine speed which develops 100% power. If you run the engine at 100% power for 1 hour, then you have used 1 tach hour. More usually, we use 75% power and less. At most high cruise altitudes, 75% power RPM settings are very close to 100% power setting RPM's at Sea Level, so tach time will be very close to real time in cruise configuration.
The Hobbs metre records real time, or 'clock time', and is activated by oil pressure. For many aircraft, this means that as soon as the engine is started, the Hobbs metre is activated. This measurement of flight time is completely independent of engine RPM settings. Most instructor fees are based on Hobbs time but it is more usually called 'Clock time' in this case.
The differences between the two measurements are noticeable when, for example, you are operating from a busy airport where ground delays are common. While the Hobbs metre is ticking over in real time, the tachometer will be ticking over in accordance with idling engine speeds. In this case, the tachometer will lag the Hobbs metre. On the other hand, if you spend very little time on the ground at idle and more time in the air, tach time and Hobbs time will be very close.
At Trim Flying Club, while tachometer and Hobbs metre are both recorded, the time spent on the flight is from wheels off to wheels on - this reduces the need to rush things in preperation for flight.