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Hobbs/VDO, tach time and airswitch?

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Hobbs/VDO, tach time and airswitch?

Old 12th Apr 2018, 11:45
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Hobbs/VDO, tach time and airswitch?

I (think that I) know the difference between these two times however when referring an aircraft to a 50 or 100 hourly maintenance, which time should be considered? Hobbs or tach?

Same question regarding overhauls. If your aircraft manufacturer states TBO: 2000 hours, is that in Hobbs or in tach time?

Also:
It would seem that in Australia the Hobbs meter is usually called VDO. Is that correct?
Is the tach time the same as an airswitch?
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Old 12th Apr 2018, 12:08
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Originally Posted by Okihara View Post
I (think that I) know the difference between these two times however when referring an aircraft to a 50 or 100 hourly maintenance, which time should be considered? Hobbs or tach?
50 or 100 hourly maintenance is usually measured as hours 'time in service'. 'Time in service' is usually measured from wheels off the ground to wheels on the ground.

That's not tach time.

It could be Hobbs time, but only if the Hobbs is connected to an air switch. Many Hobbs meters start counting and keep counting while ever the battery (and maybe master) switch is on.

Same question regarding overhauls. If your aircraft manufacturer states TBO: 2000 hours, is that in Hobbs or in tach time?
That sort of time is usually for engines. If it's for an engine, tach time is the measure.

Also:
– It would seem that in Australia the Hobbs meter is usually called VDO. Is that correct?
Dunno.

The confusion arises because a Hobbs meter is merely a brand. That brand can be connected to a master (and maybe master) switch and counts time while ever it's switched on, or can be connected to an airswitch and measures time in excess of a particular airspeed (based on the assumption that the aircraft is flying while ever that airspeed is exceeded).

– Is the tach time the same as an airswitch?
Nope. Never.

Tach time measures the time the engine is running. (Although it's worth noting that the time recorded on a tach is only accurate when the RPM is at the RPM for which the tach is calibrated. Below that RPM an hour of actual operation will be recorded as a shorter time on the tach, and above that RPM an hour of actual operation will be recorded as a longer time on the tach.)

Airswitch time measures the time from wheels off to wheels on, or pretty close to that.
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Old 12th Apr 2018, 12:38
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Quick, someone post that GIF of Michael Jackson eating popcorn....
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Old 12th Apr 2018, 12:43
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It can be simplified further ...

Maintenance time is the time between the wheels leaving the ground and the wheels touching the ground. How this is recorded is up to the operator but could be taco, air switch or watch time.

What you pay for the aircraft is also an operator decision but is usually startup to shutdown time. Again, how this is recorded is up to the operator but a “Hobbs” (brand) meter is common, it starts when the oil pressure activates it and stops when the oil pressure stops.

The difference between the two is around 0.2 per sector or lesson. This “guestimate” is 6 minutes till airborne and 6 minites after landing till engine off. Pilots log time in command which is usually considered as startup till shutdown.
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Old 12th Apr 2018, 12:59
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Maintenance time is the time between the wheels leaving the ground and the wheels touching the ground. How this is recorded is up to the operator but could be taco, air switch or watch time.
This depends on the aircraft type. Some do not allow engine maintenance off the air switch.




Last edited by StickWithTheTruth; 12th Apr 2018 at 13:16.
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Old 12th Apr 2018, 13:00
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Well lets see.

Pilots logbook is chock to chock.

Engine and/or airframe on tacho, Hobbs connected to battery or oil pressure or torque gauge line or air switch - it goes on.

All of these times are significantly different when recorded.

Friendly FOI visits and peruses the books.

Notes discrepancy in times recorded.

Does what FOI's do.

Operator now insists hours recorded must all line up.

Now which way do you think it will go?

Pilot logbook now on air switch? Or aircraft time in service chock to chock?

Hint - some of these recording means are very adjustable.
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Old 12th Apr 2018, 13:11
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Aircraft 'time in service' has never been and never will be the same as the time recorded as time in command of the pilots of that aircraft.

Without going into the tedious detail and exceptions, the general rules are:

- aircraft time in service is measured from wheels off to wheels on

- pilot time in command/dual is measured chock to chock.

Accordingly, pilots can and should be recording more time in their logbooks for a flight than the 'time in service' recorded for the aircraft for that flight. The pilot is in command/dual for a longer period of time than just the period from wheels off to wheels on.

If some FOI reckons the numbers should match, the FOI is an arseclown.
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Old 12th Apr 2018, 13:23
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It has been thus, longer than I care to mention.

Also, how the air switch is adjusted is shall we say far from standardised or tamper proof. Some may stop logging at TOD.
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Old 12th Apr 2018, 15:19
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That sort of time is usually for engines. If it's for an engine, tach time is the measure.
And if there is no tachometer recording engine time, then flight switch for recording engine TIS?
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Old 12th Apr 2018, 20:21
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Originally Posted by currawong View Post
Well lets see.

Pilots logbook is chock to chock.

Engine and/or airframe on tacho, Hobbs connected to battery or oil pressure or torque gauge line or air switch - it goes on.

All of these times are significantly different when recorded.

Friendly FOI visits and peruses the books.

Notes discrepancy in times recorded.

Does what FOI's do.

Operator now insists hours recorded must all line up.

Now which way do you think it will go?

Pilot logbook now on air switch? Or aircraft time in service chock to chock?

Hint - some of these recording means are very adjustable.
Really? Is that an Orstralian thing, or just a retarded ops inspector? Anyplace I've ever flown which had an Ops Inspector we noted block out time, wheels off time, wheels on time and block in time. Pilot times were, of course, block to block, Mx times were wheels off to wheels on. I assumed that was the way it was done everywhere.
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Old 12th Apr 2018, 20:22
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Originally Posted by StickWithTheTruth View Post
This depends on the aircraft type. Some do not allow engine maintenance off the air switch.
Which types, and what is the reason for that?
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Old 12th Apr 2018, 20:25
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Originally Posted by currawong View Post
Also, how the air switch is adjusted is shall we say far from standardised or tamper proof. Some may stop logging at TOD.
How do you adjust an airswitch so that it stops logging at TOD. Seems like you could conceivably set one so the it *only* logs time when the airspeed builds in the descent, but I'm not seeing how you could do it so that it *stops* logging when you push the nose over.
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Old 12th Apr 2018, 20:28
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Originally Posted by Okihara View Post
Also:
It would seem that in Australia the Hobbs meter is usually called VDO. Is that correct?
For what it's worth, VDO is, like "Hobbs", just another brand name of instruments.
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Old 12th Apr 2018, 21:48
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Originally Posted by A Squared View Post
Which types, and what is the reason for that?
Jabiru.

Because some schools had *massive* differences between the two meters.
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Old 13th Apr 2018, 00:03
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It's all academic, most use their watch or other time piece. Talking only of GA here.
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Old 13th Apr 2018, 04:55
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Quite a few years ago now, I flew about four different Chieftains at various times. All had Hobbs meters fitted.

None of them worked!

BTW, MR time was wheels off to wheels on.
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Old 13th Apr 2018, 05:18
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Every flight school I have dealt with has used only two separate timers for their maintenance and student (or hire & fly) billing.


For charging of students and hire & fly customers it was always hobbs time (engine start to engine stop).


The time recorded in the MR for maintenance purposes was always airswitch time (even on aircraft which also had tachometer). I have flown G1000 C172 aircraft which have the tacho time on the G1000 panel but we always recorded airswitch and hobbs only.
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Old 13th Apr 2018, 06:11
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So the answer to the OP's original question is....

You can pretty much do whatever suits you.

If you are an owner or operator seeking to maximise returns for hire, use a system that tends to over read.

If you are an operator seeking to minimise costs, use a system that tends to under read.

Using a combination of both? Pure genius

If you are a line pilot seeking to comply with regs but stay employed, well, that can be awkward.
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Old 13th Apr 2018, 06:17
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Canny students need to be aware of how his flight is charged. if the VDO is based upon when the master switch is first turned on, he is being ripped off.
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Old 13th Apr 2018, 06:29
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And possibly finishing their training woefully short of actual flying hours.
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