In its most basic form, measuring flight time can be achieved easily. Unfortunately, there are too many ways to go about it. However, of all the methods, the most misunderstood are Hobbs and tach time. Hobbs time is the time between an engine starting and stopping, whereas tach time consists of three components.
At cruise RPM, tach time counts like a standard clock. At an RPM below cruise, tach time counts slower than a standard clock. At an RPM above cruising speeds, tach time counts faster than a standard clock. While this may sound simple to comprehend, not fully understanding the differences between Hobbs and tach time can get you in a lot of trouble.
The accuracy of a flight logbook and the aircraft’s maintenance is highly dependent on a pilot’s ability to log flight time with increased accuracy. To ensure that you know the unique distinctions between Hobbs and tach time, this blog will cover each in more detail, as well as which is more advantageous.
What Is Hobbs Time?
Invented by John Weston Hobbs in 1938, Hobbs time is measured by a Hobbs meter. The Hobbs meter is either turned on or off by one or more of the following: an oil pressure switch, a master switch, and/or an airspeed indicator. Nonetheless, the most common way to control the Hobbs meter is with an oil pressure switch.
Generally, Hobbs time is measured in hours and tenths of hours. For instance, two hours and a half would be recorded as 2.5 hours. Due to its simplicity, general aviation pilots use this method to log flight hours.
What Is Tach Time?
The tach time method can be compared to an odometer in a car. Measured by a tachometer, tach time is utilized to determine when maintenance should be carried out on an aircraft. To put it simply, tach time can be thought of as “engine time” because it is a time measure of how much strain the engine has been subjected to. It is important to note that tach time is usually 20% less than Hobbs time based on the type of flights undertaken.
Both tach and Hobbs time should always be logged into the aircraft logbook or “flight folio.” This ensures that, if either the tachometer or Hobbs meter were to malfunction, the differences between the two times would not go unnoticed. Pilots must then confirm that these two measurements correspond to the indications in the aircraft before and after a flight.
Legal Requirements for Tachometers and Hobbs Meters
14 CFR 91.205 details that a tachometer is a staple for all general aviation aircraft. With this in mind, you can understand the importance of accurate engine maintenance monitoring, since the only other mandatory engine monitoring instruments are the oil pressure and oil temperature gauges. The Hobbs meter, on the other hand, is not a legal requirement. Despite this, most general aviation aircraft feature a Hobbs meter.
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