Do you know the difference between Hobbs and tach time? As an aircraft pilot or owner, the accuracy of your pilot and engine logbooks depends on your understanding of these two measurements of aircraft time.
Hobbs Time Explained
The Hobbs meter measures the hours the aircraft is being operated, and it was invented in 1938 by John Weston Hobbs. The switch controlling the Hobbs meter can be set to turn on when the Master switch is turned on, via oil pressure after the engine is started, or multiple other methods. The Hobbs meter records time as hours and tenths of hours, and it is a true measurement of how long the airplane is being operated. In other words, when an hour passes on your watch, an hour has passed on the Hobbs meter. Pilots should use Hobbs meter time when logging flight hours.
Tach Time Explained
Tach time is measured via the tachometer and can be compared to a car’s odometer. However, instead of measuring tire revolutions, the tachometer measures the number of propeller revolutions. The tachometer’s primary use is to measure engine hours, which are recorded in your airplane’s engine logbooks and used to determine when your airplane needs its 50 and 100-hour inspections.
Hobbs Time Measures Flight Time
As a student, pilot or flight instructor, you always want to use Hobbs time when you make entries into your pilot logbook as Hobbs time is a true measure of the time you were flying the aircraft, which is essential when building your flight time for your private pilot’s license and additional pilot certificates and ratings. If you use the tach time for your pilot logbooks entries, you could be shorting yourself on your flight hours because tach time can be as much as 20 percent lower than the Hobbs time.
Tach Time Measures Engine Time
The tachometer only matches the Hobbs meter when the aircraft is operating at cruise. When you pull back the throttle and lean the mixture, the propeller’s RPM slows, which slows the tachometer. The difference between the Hobbs meter and tachometer is greatest when descending and landing. In an effort to reduce engine wear and tear from students and pilots, some FBOs and flight schools charge by the tachometer hour instead of the Hobbs meter hour.
Tach Time and Aircraft Maintenance
Aircraft that are operated for hire are required to have 50 and 100-hour inspections. The timing of these inspections is calculated via the tachometer. For example, if your last 100-hour inspection was performed when the tachometer read 2,500 hours, you would need your next inspection would need to be performed at 2,600 hours. The FAA gives an additional 10 tachometer hours to travel to a certified aircraft maintenance center.
As an alternative, some aircraft owners opt to get progressive inspections, which can be done every 25 tachometer hours. These inspections are often much shorter than the traditional 50 and 100-hour inspections and can result in your airplane sitting in the hanger for less time, which means you’ll have more time to fly.
Aircraft Requirements for Tachometers and Hobbs Meters
FAA Part 91.205 lists the minimum equipment list for general aviation aircraft with standard airworthiness certificates. Under this part, all general aviation aircraft must have an operational airspeed indicator, altimeter, fuel gauge and a magnetic direction indicator. Applicable aircraft must also have a landing gear indicator and manifold pressure gauge. Each engine on the aircraft must also have a corresponding oil pressure gauge, temperature gauge and tachometer as well as other basic equipment for the safe operation of the aircraft.
General aviation aircraft are not required to have a Hobbs meter, but many aircraft used for hire and rental have them installed. If the aircraft does not have a Hobbs meter installed, the tachometer usually includes an hour meter.
Aircraft Inspections at Double M Aviation
Here at Double M Aviation in Lakeland, Florida, we can perform all the needed engine and airframe inspections on your general aviation aircraft. We offer 50-hour, 100-hour and annual airframe inspections as well as progressive inspections. If your airplane needs any repairs or maintenance, our experience A&Ps can notify you of the needed repairs and perform the required repairs in order to restore your aircraft’s airworthiness.
To get your questions answered about Hobbs and Tach time or to schedule your aircraft inspections and maintenance, call us at 863-940-3450.