Time is an essential parameter in aviation since it is used to track different important aspects of flight operations like flight hours and engine hours for the pilot and engine logbooks. For example, flight schools require their students to log flight hours to determine when they are ready to go for a license certification exam, and Federal Administration Administration (FAA) typically requires that your airplane is inspected every 50-100 engine hours.
But how are all these times measured? Are they all measured by the clock? And is it flight time the same as the engine time?
Well, if you want to find out the answer to all these questions, we invite you to keep reading as provide detailed answers.
What is Hobbs time?
Hobbs time is a measure of the total time that an aircraft has been in operation. In other words, it is the total number of hours the aircraft’s engine has been running. Therefore, Hobbs time is generally measured since the master switch is on.
Hobbs time is usually expressed in hours and tenths of an hour. For example, if an aircraft engine has been running for 1 hour and 15 minutes, the Hobbs time would be 1.25 hours.
The Hobbs meter
Hobbs time is measured using a device called a Hobbs meter. This is a device that is connected to the aircraft engine to record the number of hours that it has been running.
The device takes its name from John Weston Hobbs, who invented the device in 1938. John Weston Hobbs invented this device and manufactured it in his own electrically wound clocks for vehicle use factory located in Springfield, Illinois after World War II created the demand for aviation hour meters.
The Hobbs meter is often found in the cockpit so that the pilot can easily see how much time has elapsed.
There are modern aircraft with newer technology that are equipped with electronic Hobbs meters. These are able to record and store the data automatically, which is very helpful for pilots to track Hobbs time over a long period of time, or to share the data with someone else like flight schools or a mechanic.
The Hobbs meter is usually wired into one of three possible systems that will activate it as soon as they become active. Depending on the aircraft, the selected system could be:
The ignition system or master switch.
An oil pressure switch
Any other system associated with the engine operation such as an airspeed sensor.
In general aviation aircraft, the most common way to measure engine hours is to install the Hobbs meter by connecting it to the oil pressure switch. Also, a general aviation aircraft pilot normally uses the Hobbs meter or the Hobbs time to log flight time.
What is a Hobbs hour?
As we described above, Hobbs time is usually expressed in hours and tenths of hours. Therefore, it is common to use the term Hobbs hour when speaking about the readings of a Hobbs hour meter. Now, when using the term, it means that an hour of Hobbs time has elapsed, which is exactly the same time elapsed when a clock hour passes.
The fact that a Hobbs time matches clock time is the main reason to use it when logging flight hours. Now, flight time is different from engine time, also known as tach time which is the time used by the FAA in its guidance for inspection requirements.
Let’s see exactly what is the difference between Hobbs time and tach time.
What is Hobbs time vs tach time?
In aviation, there are two main ways to measure the time that a pilot has spent flying: Hobbs time and tach time.
You already know that the Hobbs meter records time during which the engine is running. Now, the main difference is that tach time refers to the number of hours that an engine has been running at a certain speed.
Another difference between Hobbs time and tach time is the way they are measured. While Hobbs time is measured with a Hobbs meter, tach time is measured by a tachometer, hence the name.
Yet, one of the biggest differences comes in the way tach time is expressed. Since tach time is measured by a tachometer, the first measure it gets is rotational speed or RPMs. And this is how many rotations per minute the propeller is making. Therefore, tach time is measured in hours, but it is not the same as how many hours pass on your watch or in Hobbs time.
This is because tach time is a measure of how long the engine has been running, and fuel consumption is directly related to how long the engine has been running and at what RPM the engine has been operating. In most cases, an airplane will be most efficient when it’s operating at or near its cruise speed.
Moreover, it is the time that is tracked to comply with regulations such as the inspection intervals provided by the FAA. This is because the tach time is considered a good measure of the wear and tear of the engine. Normally, the engine wear increases when it is running at higher speeds for longer periods of time, while it decreases when the propeller’s rpm slows down.
How is tach time calculated?
As described by J.P Instruments, tach time is determined by using the average cruise RPM of the aircraft. Tach time is accumulated once per second, using the following calculation: Tach time = Tach time + (Current RPM / Average Cruise RPM).
As a result, you can get one of three possible results when compared with a standard clock. This is how it works:
At cruise RPM, tach time counts like a standard clock.
At an RPM less than the cruise, tach time counts slower than a standard clock.
At an RPM more than the cruise, tach time counts faster than a standard clock.
There you have it. Now you know the most important aspects of Hobbs time. Remember this:
Hobbs time is a way of measuring the amount of time that an engine has been running.
It is usually expressed in hours and tenths of hours, which is exactly the same as when a clock hour passes.
The most common way to connect the Hobbs meter to a general aviation aircraft is by wiring it to the switch controlling the oil pressure.
Pilots often use Hobbs time to log flight time, as it is seen as a more accurate measure than tach time. This is related to the fact that the engine of a general aviation aircraft will be running at different RPMs at different stages of the flight.
Tach time refers to the number of hours that an engine has been running at a certain speed, and is measured by a tachometer.
While there are differences between Hobbs and tach times, they both have their uses in aviation. For example, most flight schools charge by Hobbs meter hour, but there will be others charging customers by tachometer hour since they believe it is more representative of the costs associated with engine wear.
Something similar happens with rentals and leasing. The most important is to have the difference clear to understand whether you are getting a good deal or not.
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Jet pilot @NASA
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