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Which is the better system for recording an aircraft's Total Time in Service (or pilot's flying time for that matter)?

 

There are at least two ways of recording an aircraft's TTIS - Tacho time or Hobbs Meter time. Although aiming for the same target, the two methods record different things.

 

Tacho time is a recording of the time an engine has been operating. It is linked to engine revolutions per minute (RPM). Tacho 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. That might be an accurate measurement of wear and tear as one would expect the engine components to be less stressed around idle speed.

 

Hobbs time is time recorded by an electrically powered clock. The Hobbs meter can be activated in a number of ways:

 

  1. It can measure the time that the electrical system is on. This maximizes the recorded time.
  2. It can be activated by oil pressure running into a pressure switch, and therefore runs while the engine is running. 
  3. It can be activated by another switch, either an airspeed sensing vane under a wing  or a pressure switch attached to the landing gear. In these cases, the meter only measures the time the aircraft is actually flying.

 

With method 2, the Hobbs reading will be closest to the Tacho time, but will always be higher because  of the slower reading rate of the Tacho when the engine is at low revs. With method 3, the Hobbs meter records the time that the airframe was subjected to in-flight forces. With method 1, leave the Master turned ON when you hangar the plane and your maintenance period will be short, without any flying being done. 

 

What is the best way to record TTIS to balance the needs of good maintenance and economical running? Each method introduces inaccuracies. Perhaps the compromise is to wire the Hobbs meter to the oil pressure switch so that Hobbs time and Tacho time are more closely aligned. The Hobbs meter will account for the under reading of the Tacho on the ground to account for engine use, and allow for structural wear to the airframe during movement on the ground.

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 Whatever you pick you are supposed to stick to and there are (sometimes) adjustments within the definition(s)  ie  Use different servicing times.  From engine start to engine shutdown would be good enough for me. It's one that's widely used. If it's related to engine RPMs as well there's a good relationship to hard use and light use (idling and taxi (which are often the same).  Having it begin co incident with with electrical power ON isn't too realistic for airframes/engines etc. . Nev

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I use flight time. From start of take off run to standstill after a flight. That includes taxi back to start point or wherever, but not taxi to start of take off run.

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I use tacho time as it is the only meter I have. VDO don't provide any data on the speed it runs at at low/idle RPM. I always assumed it was just a clock activated when the tacho began to read RPM when the engine is running.

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The information I have is that RAA aircraft should run on hobbs and GA on tacho.

That not particularly being my opinion, good or bad, but from what I've seen.

There was a local aircraft that was changed from ga to RAA rego.

It was maintained by a LAME under tacho hours in GA.

RAA wanted it maintained under hobbs, so it "gained" a couple of hundred hours unfortunately. 

Note, it contained both hobbs and tacho meters.

If it was me, I would have probably just continued on tacho, and not sought clarification....😎

Edited by Downunder
  • Agree 2

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Rotax require engine hours recorded from start to stop, puts engine hrs about 10% above airframe hours. My old Cessna had  the Hobbs  with a vacuum air switch which cut in at around 25 kts. IAS. Vacuum provided by an external horn.

Edited by Mewp

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Agree with Mewp - Rotax (and I believe other engine suppliers) specify Hobbs/engine time - Makes sense that an engine should be serviced and recorded for all time it is running anything less is a misrepresentation. Propeller time is also best recorded as engine time.

 

In small, simple, fixed wing, aircraft, engine time is the significant measurement - airframe hours is not usually a factor of note. I would go so far as to suggest calendar (and possibly no, landings) time may have more significance to airframe than Hobbs/engine or other measurements.

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 Individual interpretation is not actually available and the authority would have the final say. On pressurised aircraft pressure cycles are recorded and are significant in airframe life but not so with firebombers where they would operate unpressurised and are subject to a lot of landings and rough air operation so are in no mans land  where a lot of inflight failures are happening. Special use aircraft (aerobatic) have more rigid inspection schedules and lifed parts and  a "G" meter. Heavy landings require inspections and that applies to any aircraft( even your Optimistic MK1). It's really a lot of commonsense. Aeroplanes aren't made like bulldozers as they have to FLY and things that fly well are LIGHT. Nev

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4 hours ago, old man emu said:

Even fewer pressurised bulldozer owner/builders.

I’m not so sure....many bulldozer drivers I’ve seen look like they’re going to pop, usually their shirts already have. 

  • Haha 1

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In a hired plane, you don't want the hirer to feel under financial pressure to taxy out quickly and possibly interfere with other aircraft. So for this application, a hobbs type that only cuts in at 25 knots is better, using this as the cost basis makes taxy time free.

For changing the oil, tacho time is probably better.  I have thought about oil additives and concluded that frequent oil changes ( 25 hour tacho time ) is probably a better way to go. The best Jabiru engine I have ever seen ( based on leakdown ) has an owner who uses 12.5 hour oil change time.

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