Tacho Time

[In Altissimus]

[I did do a search before asking this...]

OK - I just discovered what the difference between a tacho and a Hobbs meter is :ooh:

Now I have two questions:

Is there a standard setting for the revs at which one tacho hour equals one clock hour? (C172N)

If the tacho is set to 100% at say 2400 revs, is it fair to assume that it ticks at 20 seconds per clock-minute at 800 revs?

[Kolibear]

Sideslipping dramatically here, but what is the difference between tacho & Hobb's?

[In Altissimus]

Ah - that's the bit I do understand...

The Tacho 'ticks' somehow proportionately to the work the engine does (i.e. slower when idle), the Hobbs ticks at 1 minute per real minute as long as the engine is running.

[Dude~]

http://www.pprune.org/forums/showthread.php?s=&threadid=89548&perpage=15&pagenumber=3

Read page three of my old thread. There are some thoughts, not facts on tacho times. It seems to be generally accepted that 1.0 on tacho = 60 minutes at roughly cruise rpm, and that tacho records slower at slower rpm settings.

Also bear in mind that Hobbs meters generally record in increments of 0.1 (6minutes) whereas tachos record 1/100ths of an hour, so what you pay for vs what you fly, is far more accurate. It always used to annoy me when renting club aircraft that sometimes as soon as you started the engine, the hobbs would increas by 0.1 thereby costing me £9.50. People say 'it all evens out' but I dont think I always managed to shut down just before it recorded another 0.1!

Seems strange to me that while we plan flight to the nearest minute, most of us record flight tiime in our log books to the nearest 6?!

[Mark 1]

I seem to recall posting on this subject before.

There can be quite a bit of variation on the hour gearing of some tachometers.

For instance, the standard fitting on the PA28 is 2300RPM, but for Cessna 152/172 its 2566RPM - about 10% different.

You can find a listing on http://www.aircraftspruce.com/catalog/inpages/mech_tachs.php

But whichever you use the Hobbs time will usually be the biggest number. Thats why hire aircraft use them, the operators would have to quote a higher hourly rate otherwise. I personally advocate tacho hours for group owned aircraft, much fairer overall.

[In Altissimus]

Thanks for the links guys.

My main question remains though: Is there a 'straight-line' relationship between tacho time and revs?

[Mark 1]

The answer to that question is yes.

The hour meter is directly geared to the tachometer cable - e.g. for a 2300rpm tacho:

1 tacho hour = 2300*60 engine revolutions

So it will flip over .01 hours for every 138 engine revs

[IO540]

I think the answer should be "it depends".

With a mechanical tacho, the thing normally just counts revolutions. With an electric one, it normally counts engine running time, but it could be running time above a certain RPM.

You can also bring in airspeed (to measure just airborne time), retractable gear status (again to measure just airborne time), oil pressure (to measure engine time but only above e.g. idle) etc etc. All these and probably others have been done.

[FormationFlyer]

Dude~ said...Seems strange to me that while we plan flight to the nearest minute, most of us record flight tiime in our log books to the nearest 6?!

Um..if you are recording hobbs in your personal flying log then you are not recording time in accordance with the ANO....

Article 129 quite clearly states an aircraft flight time is brakes off to brakes on....so being sat on the pan for 20 mins after engine start shouldnt be going in the log book....

Article 129
(2) An aircraft shall be deemed to be in flight:
(a) in the case of a piloted flying machine, from the moment when, after the
embarkation of its crew for the purpose of taking off, it first moves under its own
power until the moment when it next comes to rest after landing;


or JAR FCL...

JAR–FCL 1.001 Definitions and Abbreviations
Flight time:
The total time from the moment that an aircraft
first moves under its own or external power for the
purpose of taking off until the moment it comes to
rest at the end of the flight.


And whats more....

JAR–FCL 1.080 Recording of flight time
(See IEM FCL 1.080)
...
(b) The record shall contain the following
information:
...
(2) For each flight:
...
(iii) Place and time of departure
and arrival (times (UTC) to be block
time)


On the other hand if you note the hobbs time directly before brakes off - and precisely on brakes on you should be ok. But hobbs time cannot be logged directly.

(Edited to add JAR FCL ref)

[WestWind1950]

@FormationFlyer

when flying in the USA we used only hobbs times.... and all the Jeppesen logbooks I know of don't allow any room for take-off and landing times. These hobb times are the flight times used for the records (I always got laughed at when I wanted the clock-times to use as flight times).

Until we had JAR-FCL (began this past May1), we recorded here in Germany as flight times the actual take-off (lifting from the runway) to landing (on runway) times. These times were recorded in our personal logbooks and in the airplane logbook. The hobbs were also recorded for our payment (running usually 1.00 = 1 minute).

Now my question to all you JAR-FCL experienced people: do you look at your watch as soon as you leave your parking position to taxi toward the runway and again after you park and then record these times in your logbooks? The tower obviously only records actuall take-off and landing. I agree that I'm the responsible person as soon as that aircraft moves (PIC), so it should be in my books. And I also know that some pilots even record times when they're a passenger :* but there will always be people misusing the system.

Looking forward to some answers....

WestWind1950

[Mike Cross]

For the purposes of the aircraft technical log the ANO says

11 (1) A technical log shall be kept in respect of every aircraft registered in the United
Kingdom in respect of which a certificate of airworthiness in either the transport or in
the aerial work category is in force.
(2) (a) Subject to sub-paragraph (b), at the end of every flight by an aircraft to which the
provisions of this article apply the commander of the aircraft shall enter:
(i) the times when the aircraft took off and landed;...........


In our group we record and pay for take-off to landing times. In respect of the personal flying log book the times to be recorded are different, as FF says.

It is my practice to add 5 mins at the beginning and end of each flight (total 10 mins per flight to allow for taxying, holds runup etc. I believe many flying clubs also use the same system, i.e. the figures in the a/c tech log are 10 mins less than what is paid for (most charging chock to chock).


Mike

[RodgerF]

Question:

Now my question to all you JAR-FCL experienced people: do you look at your watch as soon as you leave your parking position to taxi toward the runway and again after you park and then record these times in your logbooks?

Answer

Not quite. I take the time just as I am about to move. Not a good idea to be looking at your watch whilst moving in a possible confined space.

When I have stopped and before I start the shutdown checks.

[FormationFlyer]

RodgerF Me too....

Ive always been happy with the fact that what I get charged and what is logged are two different things.

With regard PIC and responsiblity - they are both defined in the ANO/JAR-FCL as well - and you are actually PIC/responsible before flight time starts :)

The RAF use airbourne times - which always seemed sensible to me - I never have worked out why civilian world uses block time.....but thats the way they want it logged....

On another note - I actually used a Jeppesson logbook (and still do) pre and post JAR. However, I have always written down the times of departure & arrival.

Just because your logbook doesnt have a column for it doesnt mean you cant write it down - and therefore be JAR compliant regardless of weather or not the logbook is JAR compliant. The law mean requires you to keep a logbook and to maintain certain details about the flight...

Regards,
FF

[BEagle]

Do what the grown ups do and log off-blocks to on-blocks as your 'flight time'.

What the Hobbs meter records is for your bean counters' purpose only. But there again, in the land of the Crimplene clad hamburger backside, they can't even work out duodecimal time, let alone understand that charging by Hobbs' time leads to fast taxiing with cold engines....

[Dude~]

Um.. Formation FLyer, I believe you misinterpreted my post. I clearly did not say I logged hobbs time, merely that I log time on 1.10ths of an hour. I record 6 times when I fly, for various records, they are start up and shut down, brakes off and brake on and take off / landing times.

[FormationFlyer]

Dude~ Quite right. My humble apologies. Should have thought a bit harder :)

BEagle I quite agree. Hobbs merely encourages folks to get airbourne without warming an engine properly - in winter this is particularly nasty - surely it is actually more economical to charge BO/BO or airbourne+10 and encourage better treatment of the engines?