Datcom hour? vs Tacho hour?
Joined: Feb 2007
Posts: 4,598
Likes: 0
From: Amsterdam
I'm not familiar with "datcom" per se, and google comes up with 250.000+ hits, but I guess it's similar to a "hobbs" meter which runs only when the engine is running. It probably also runs linearly, meaning that if there's exactly two hours between engine start and engine stop, you are charged exactly two hours. Doesn't matter whether you spent those at the hold or in a fast cruise.
A tacho also counts the hours the engine is running, but is dependent on the actual RPMs. So while you're idling at the hold, it might take two hours or more for the tacho to increase by 1.0, while in a very fast cruise, that same 1.0 on the tacho might only take 50 minutes. Normally a tacho is calibrated to count "true" at the normal cruise setting.
What most, if not all, not-for-profit clubs/schools/groups do is divide the total annual costs, by whatever the hobbs/tacho/whatever increased over the year, and use that as the basis for the hourly fee to pay, again calculated based on that same hobbs/tacho/whatever. So normally, at the end of the year, it all averages out and there really is no difference.
However, if your flying style differs greatly from the other pilots in the group/club (e.g. you fly long and high IFR legs while the others just do short, low-level bimbles) you might have a relative advantage or disadvantage.
One advantage of a hobbs meter, I find, is that whatever is on the hobbs, can go straight into my logbook, as long as you move the aircraft a bit as soon as you started the engine, and don't wait too long before shutting it down once you've set the parking brake. Can't do that with a tacho.
A tacho also counts the hours the engine is running, but is dependent on the actual RPMs. So while you're idling at the hold, it might take two hours or more for the tacho to increase by 1.0, while in a very fast cruise, that same 1.0 on the tacho might only take 50 minutes. Normally a tacho is calibrated to count "true" at the normal cruise setting.
What most, if not all, not-for-profit clubs/schools/groups do is divide the total annual costs, by whatever the hobbs/tacho/whatever increased over the year, and use that as the basis for the hourly fee to pay, again calculated based on that same hobbs/tacho/whatever. So normally, at the end of the year, it all averages out and there really is no difference.
However, if your flying style differs greatly from the other pilots in the group/club (e.g. you fly long and high IFR legs while the others just do short, low-level bimbles) you might have a relative advantage or disadvantage.
One advantage of a hobbs meter, I find, is that whatever is on the hobbs, can go straight into my logbook, as long as you move the aircraft a bit as soon as you started the engine, and don't wait too long before shutting it down once you've set the parking brake. Can't do that with a tacho.
Joined: Sep 2004
Posts: 1,684
Likes: 3
From: Down at the sharp pointy end, where all the weather is made.
Group tacho flying
In our Group we pay by tacho time. I think our tacho records one hour for full-throttle running for an hour, so 70% rpm will cost 70% of an hour. We record these tacho times and chock-to-chock on our booking in/out sheet.
Many years ago we had CAA approval to use the tacho times as equivalent to airborne times for the purposes of the aircraft log book, but when we changed maintenance organisations a while back they wanted to see a separate tech log with actual airborne times recorded in it, so we keep the two systems running side by side; a bit fiddly but with only one a/c, not a disaster. In fact, looking long-term, with a full-power tacho and sensible cruise power settings, the two still run pretty close.
We also like this tacho arrangement because it encourages people to set a reasonable cruise power; I always felt that the Hobbs encouraged people to thrash the engine. Most of our members want to enjoy watching the countrysidee slip by, not see how quickly they can get from A to B, so the tacho as we operate it suits their inclination and their wallet.
The other organisation where I work use neither system; one records the airborne time, then add 6 mins either side for the cost of the flight. This is regardless of having spent 30 mins explaining how the checklist works after engine start and before taking off. Quite a good deal for the customer, really. I've become adept on trial lessons at being airborne for EXACTLY 48 mins!
TheOddOne
Many years ago we had CAA approval to use the tacho times as equivalent to airborne times for the purposes of the aircraft log book, but when we changed maintenance organisations a while back they wanted to see a separate tech log with actual airborne times recorded in it, so we keep the two systems running side by side; a bit fiddly but with only one a/c, not a disaster. In fact, looking long-term, with a full-power tacho and sensible cruise power settings, the two still run pretty close.
We also like this tacho arrangement because it encourages people to set a reasonable cruise power; I always felt that the Hobbs encouraged people to thrash the engine. Most of our members want to enjoy watching the countrysidee slip by, not see how quickly they can get from A to B, so the tacho as we operate it suits their inclination and their wallet.
The other organisation where I work use neither system; one records the airborne time, then add 6 mins either side for the cost of the flight. This is regardless of having spent 30 mins explaining how the checklist works after engine start and before taking off. Quite a good deal for the customer, really. I've become adept on trial lessons at being airborne for EXACTLY 48 mins!
TheOddOne
Joined: Jan 2004
Posts: 585
Likes: 0
From: London
A system which records airborne time plus a fixed amount to cover taxying is probably the most sensible.
Recording Hobbs time especially encourages hirers to rush through their checks and get airborne before the engine is properly warmed.
Recording Hobbs time especially encourages hirers to rush through their checks and get airborne before the engine is properly warmed.
Joined: Oct 2005
Posts: 3,218
Likes: 2
From: USA
Tach hour recording (odometer) measures correctly with respect to time at maximum RPM. It has nothing to do with throttle position or the power the engine is producing; only with RPM. Operate at a lower RPM, the tach is turning slower, and it's recording time more slowly. It's strictly a mechanical function of the instrument. Turn the instrument faster, the time turns over faster. This is a good measure of the work the engine is doing, and consequently a good way to keep track of time for maintenance purposes.
Datacon and Hobbs are simply brand names of electronic hour meters. When they begin recording time depends on the installations. Some begin when the battery master relay is energized, some when the engine oil pressure activates a pressure switch to start meter. Others are attached to landing gear squat switches, or pressure switches that activate when enough parasitic drag develops as airspeed increases.
Generally billing is done by the electronic meter, while maintenance is scheduled by the tach.
If you're rushing your checklists to save a dollar (or euro or pound as the case may be), then you're flying under the wrong set of priorities.
Datacon and Hobbs are simply brand names of electronic hour meters. When they begin recording time depends on the installations. Some begin when the battery master relay is energized, some when the engine oil pressure activates a pressure switch to start meter. Others are attached to landing gear squat switches, or pressure switches that activate when enough parasitic drag develops as airspeed increases.
Generally billing is done by the electronic meter, while maintenance is scheduled by the tach.
If you're rushing your checklists to save a dollar (or euro or pound as the case may be), then you're flying under the wrong set of priorities.
Joined: Sep 2003
Posts: 0
Likes: 0
From: UK,Twighlight Zone
SNS, sadly there are many that fly this way especially with the cost of fuel and the hire rates in Europe.
I fly a twin that used to be rented out by the datcon hour, we had an observed problem with people not doing checks or warming up properly to save themselves a few quid. We changed it to an honesty system of brakes to brakes and the problems went away and strangely nobody cheated us. I used the GPS downloads to verify the flights against the tech leg and they were always accurate within a few minutes which is close enough for peace of mind.
I fly a twin that used to be rented out by the datcon hour, we had an observed problem with people not doing checks or warming up properly to save themselves a few quid. We changed it to an honesty system of brakes to brakes and the problems went away and strangely nobody cheated us. I used the GPS downloads to verify the flights against the tech leg and they were always accurate within a few minutes which is close enough for peace of mind.
Joined: Jan 1999
Posts: 6,209
Likes: 2
From: north of barlu
The aircraft that my company leases are all charged "take off to landing" this way there is no proffit to the pilot leasing the aircraft to rush the checks or take off with a stone cold engine.
In the long run it is better all round but Occasionaly I do get people being very economical with the truth over flight times. The thing that gives them away is the fuel burn.
I am thinking of fitting the loggers that glider pilots use to give my customers "by the minute" billing, i am sure that it would save the honest guys who lease my aircraft a £ or two when they log to the nearest five minuites and would catch red handed the occasional joker who thinks he can have me over.
In the long run it is better all round but Occasionaly I do get people being very economical with the truth over flight times. The thing that gives them away is the fuel burn.
I am thinking of fitting the loggers that glider pilots use to give my customers "by the minute" billing, i am sure that it would save the honest guys who lease my aircraft a £ or two when they log to the nearest five minuites and would catch red handed the occasional joker who thinks he can have me over.
Joined: Sep 2004
Posts: 1,684
Likes: 3
From: Down at the sharp pointy end, where all the weather is made.
Tach hour recording (odometer) measures correctly with respect to time at maximum RPM. It has nothing to do with throttle position or the power the engine is producing; only with RPM.
Yes, you're quite right, understood, merely sloppy phraseology on my part! I think we agree on the basics, though.
Cheers,
TheOddOne
