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Hobbs vs Tach

WingedFrog

Well Known Member
As the Tach is not displayed under my set-up I rarely look at it. I checked yesterday and found the difference between the two quite significant:
Hobbs: 77.8
Tach: 55.1
I wonder what is the reading like on other RV-12 E-LSAs. (I assume the S-LSAs may have closer readings as people flying them tinker less than we, builders do)
 
My S-LSA has about the same numbers......about 25% difference which only means that I spend 25% of my engine time sitting on the ground and taxiing vice flying.

I think the TBO hours advertised by Rotax are Hobbs hours so it really doesn't make much difference; you're gonna overhaul by the Hobbs in any case. Ditto oil changes and 100 hour inspections.
 
More confirmation ...

I'm hangared at a towered airport where we often have to taxi 6000' or more to get to the RW in use. I show closer to 30% difference between tach and Hobbs time.
 
the calibration value is based on flying at a max. continuous cruise power setting (5500RPM).
Any time your are operating below that (economy cruise power setting, etc.), recorded tach time will not match hobbs time.
 
the calibration value is based on flying at a max. continuous cruise power setting (5500RPM).
Any time your are operating below that (economy cruise power setting, etc.), recorded tach time will not match hobbs time.

Then the Tach value is not of great interest in my opinion. I usually get to 5500 rpm at the end of a climb when I level and even, in case of turbulence I have to throttle back below 5500 rpm to stay in the green. Now I understand the big gap I have between Hobbs and Tach as I was initially overpitched and rarely hit 5500 rpm.
 
Tack time on the RV-12 is not of much value anyway. That is why it is not configured to display on the EMS page.

Rotax specifys that all maint. be done on hobbs time intervals.
A primary reason is that they do not consider low RPM operations to be less wear on the engine because that is the speed range that the gear box is actually more vulnerable. Particularly if the engine is being abused (a lot of ground idle below 1800 RPM, etc.).

Most people who operate at high cruise power settings set it slightly under 5500 (I use 5450-5460) this gives you a little cushion to avoid over speed in turb., etc. This small of an amount below 5500 will have very little error effect on tach vs hobbs (but like I already said, on a rotax, tach time is of little value)
 
Hobbs is best. Logbook entries should be hobbs too.
Hobbs only reads when the oil pressure is above 15 psi and stays at a constant time recording. The tach many times can come on when the power switch is turned on and even if you have by-passed that part, many times it varies with rpm. Low rpm the tach records slower and high rpms it records faster. These are usually off by at least 20%

Stay with Hobbs with your Dynon.
 
Ok stupid question, but is the flight record on the 496 the Hobbs you are referring to? I checked mine after a trip last week and found the difference to be about 20%.
Dick Seiders
 
In most aircraft, Hobbs time is real time, active anytime the engine is running
(can't remember at the moment if that is how Dynon has it configured).
If I remember correctly, flight time recording on the 496 is activated when it senses that you are above local ground level and deactivates when you return to the local vicinity ground level.
 
Ok stupid question, but is the flight record on the 496 the Hobbs you are referring to? I checked mine after a trip last week and found the difference to be about 20%.
Dick Seiders

The 496 flight log is based on a ground speed threshold. Hobbs time is engine run time controlled by an engine sensor. Don't know what Rotax uses but Cont/Lyc engines typically use oil pressure.
 
Getting back to the basics, originally, 30+ years ago, the Hobbs was activated when the engine was running, the "tach" time on aircraft and industrial engines was connected to revolutions of the engine, thinking that was better correlated to service duty. It was a geared instrument that directly reflected the number of revolutions. This was related to piston travel, thermal cycles etc. The ratio of "hours" those was to some engine reference rpm. That was typically the rpm where the engine was approved to operate at full load continuously. With the application of electronics, the definitions of hobbs and tach started to go astray.

Apparently, we can trigger time by several methods.

1. Power on
2. engine running - oil pressure
3. over xx kts airspeed
4. altitude

Maybe there are more trigger options. Personally, for engine related matters I like the #2 (hobbs), but if I were renting (out) a unit, tach time would be a better comparison of how the engine was operated. I am not sure how that is obtained with my G3X.
 
There are only two types of hourly times relevant to flying and piloting.
From FAR 1.

Flight time means:

(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing; or

(2) For a glider without self-launch capability, pilot time that commences when the glider is towed for the purpose of flight and ends when the glider comes to rest after landing.

and

Time in service, with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.

So - - Flight Time goes in your pilot log book and Time in Service goes in your aircraft's journey log and maintenance records.

This from AC 43-9:
8. RECORDINGTACHOMETERS.
a. Time-in-service recording devices sense such things as electrical power on, oil pressure, wheels on the ground, etc. , and from these conditions provide an indication of time-in-service. With the exception of those that sense aircraft lift-off and touchdown, the indications are approximate.

b. Some owners and operators mistakenly believe these devices may be used in lieu of keeping time-in-service in the maintenance record. While they are of great assistance in arriving at the time-in-service, such instruments, alone, do not meet the requirements of section 91.4 17. For example, when the device fails and requires change, it is necessary to enter time-in-service and the instrument reading at the change. Otherwise, record continuity is lost.

g. Section 91,417(a)(2). Requires six items to be made a part of the maintenance record and maintained as such. Section 43.9 does not require maintenance personnel to enter these items. Section 43.11 requires some of them to be part of entries made for inspections, but they are all the responsibility of the owner or operator. The six items are discussed as follows:
(1) Section 91.417(a)(2)(i). Requires a record of total time-in-service to be kept for the airframe, each engine, and each propeller. Part 1, section 1.1, Definitions, defines time in service, with respect to maintenance time records, as that time from the moment an aircraft leaves the surface of the earth until it touches down at the next point of landing. Section 43.9 does not require this to be part of the entries for maintenance, preventive maintenance, rebuilding, or alterations. However, section 43.11 requires maintenance personnel to make it a part of the entries for inspections made under parts 91, 125, and time-in-service in all entries.

The record of Time in Service is a required part of the Aircraft Records and should be auditable.

I'm using a Excel generated form that you can see and copy for free at:
http://www.twisted-wrench.com/files/2_-_GA_Aircraft_Times_and_Landings.pdf

You can modify this form to your likes but it will satisfy the requirement for a Time In Service Record.

Skyview has a time tracking feature that records Time in Service pretty close. From the Skyview pilot's manual:

The Total Air Timer - labeled AIR TOTAL when displayed on an Engine Page - runs any time that SkyView determines that the aircraft is in flight. SkyView uses the presence of significant airspeed or GPS speed to decide whether the aircraft is flying. This timer can only be reset or adjusted in the Aircraft Information menu within SkyView Setup. See the SkyView System Installation Guide for information about using SkyView?s Setup Menu.
The Air Timer - labeled AIR TIME when displayed on an Engine Page - runs any time SkyView determines the aircraft is in flight. SkyView uses the presence of significant airspeed or GPS speed to decide whether the aircraft is flying. Resets as SkyView detects that the aircraft has transition to ?in the air? This time is also optionally used to calculate the Switch Fuel Tank Reminder.
The Air Trip Timer - labeled AIR TRIP when displayed on an Engine Page -runs any time that SkyView determines that the aircraft is in flight, just like the Total Flight Timer. Unlike the Total Air Timer, it can be reset by pressing ENGINE > CLR TMR.
 
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