Hobbs Meter
Encyclopedia
The Hobbs Meter records the time a device is used. It is frequently used in aviation
applications to record the time that the electrical power (battery master switch) is "on". Hobbs is a genericized trademark
for products generically called "Engine Hour Meters". The meters run electrically, indicating hours and tenths of an hour, but there are several ways a meter may record the "Hobbs Time":
, and many Fixed-Base Operations that rent airplanes charge an hourly rate based on Hobbs Time. Tach Time
is recorded in the engine's log books and is used, for example, determining when the oil should be changed and the time between overhaul
s. Tach (tachometer) time differs from Hobbs Time in that it is linked to engine revolutions per minute
(RPM). Tach Time records the time at some specific RPM. It is most accurate at cruise RPM, and least accurate while taxiing or stationary with the engine running. At these times, the clock runs slower. Depending on the type of flight, Tach Time can be 10–20% less than Hobbs Time. Many organizations such as flying clubs charge by Tach Time so as to differentiate themselves from Fixed-Base Operations by the fact that 10-20% less time recorded makes it 10-20% cheaper to fly (if the hourly rate is the same).
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
applications to record the time that the electrical power (battery master switch) is "on". Hobbs is a genericized trademark
Genericized trademark
A genericized trademark is a trademark or brand name that has become the colloquial or generic description for, or synonymous with, a general class of product or service, rather than as an indicator of source or affiliation as intended by the trademark's holder...
for products generically called "Engine Hour Meters". The meters run electrically, indicating hours and tenths of an hour, but there are several ways a meter may record the "Hobbs Time":
- It can measure the time that a master switch is on. This tends to overstate Hobbs Time.
- It can be activated by oil pressure running into a pressure switch, and therefore only runs while an engine is running. Many rental aircraft use this method to preclude flying with the master switch off to improperly reduce Hobbs Time.
- It can be activated by another switch, either an airspeed sensing vane under a wing (as in the Cessna Caravan) or a pressure switch attached to the landing gear (as in many twin engine planes). In these cases, the Hobbs Meter only measures the time the aircraft is actually flying. This is for those who wish to record Time In Service. Metrics such as Turbine Actual Runtime are kept to monitor overhaul cycles, and are usually used by commercial operators under Federal Aviation RegulationsFederal Aviation RegulationsThe Federal Aviation Regulations, or FARs, are rules prescribed by the Federal Aviation Administration governing all aviation activities in the United States. The FARs are part of Title 14 of the Code of Federal Regulations...
Parts 135, 121, or 125. - It can be activated when the engine alternators are online (as in the CirrusCirrus DesignThe Cirrus Aircraft Corporation is an aircraft manufacturer that was founded in 1984 by Alan and Dale Klapmeier to produce the VK-30 kit aircraft....
SR series).
General aviation use
For general aviation, Hobbs Time is usually recorded in the pilot's log bookLog book
A log book, in records management, may refer to:* A book of log tables* Logbook a log of important events in the management, operation, and navigation of a ship; or of the important events of a trip or expn* Inventor's notebook...
, and many Fixed-Base Operations that rent airplanes charge an hourly rate based on Hobbs Time. Tach Time
Tach Timer
The Tach Timer is an instrument used in aviation to accumulate the total number of revolutions performed by the engine. The unit of measure is equivalent to the number of hours of running at a certain, specific reference speed of rotation. If the reference speed of rotation is 2400 RPM then the...
is recorded in the engine's log books and is used, for example, determining when the oil should be changed and the time between overhaul
Time between overhaul
One important measure of an aircraft engine's overall economics is how often it has to be overhauled, the so-called time between overhaul, typically seen as TBO or TBOH....
s. Tach (tachometer) time differs from Hobbs Time in that it is linked to engine revolutions per minute
Revolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
(RPM). Tach Time records the time at some specific RPM. It is most accurate at cruise RPM, and least accurate while taxiing or stationary with the engine running. At these times, the clock runs slower. Depending on the type of flight, Tach Time can be 10–20% less than Hobbs Time. Many organizations such as flying clubs charge by Tach Time so as to differentiate themselves from Fixed-Base Operations by the fact that 10-20% less time recorded makes it 10-20% cheaper to fly (if the hourly rate is the same).