cost estimate using dry rate

[eman1200]

a local club has published DRY rental rates. can y'all give me approximate average hourly lesson fuel consumption rates? figure a 172 and your typical hour long lesson, is it in the 10-12ish gallon/hour rate?

also, if it's a wet rate, I'm assuming whatever fuel u leave in the plane you lose?

thanks!

 

[dukeblue219]

a local club has published DRY rental rates. can y'all give me approximate average hourly lesson fuel consumption rates? figure a 172 and your typical hour long lesson, is it in the 10-12ish gallon/hour rate?

MUCH less than that, especially if you're flying in the pattern and doing lots of taxiing. I think about 6 gal per hobbs hour is probably a good ballpark for a 172 (160 or 180hp?) during a typical lesson.

 

[dukeblue219]

also, if it's a wet rate, I'm assuming whatever fuel u leave in the plane you lose?

No, in fact it's the opposite. With a wet rate you pay a straight hourly cost and pay no attention to fuel. You can add as much as you need (and get reimbursed for it) and the next guy can just run it empty if he wants and you both pay the same hourly charge. The downside to this is that there's no incentive for anybody to fly economically.

With a dry rate you generally are expected to fill a certain level (full, or to the tabs for example) so that everyone pays for the exact amount they used. In this case if you over fueled you're just gifting that fuel to the next guy unless you have other arrangements.

 

[eman1200]

actually this list doesn't specify tach or hobbs. hobbs is actual engine run time, correct? what do u mean by tach time?

I went to the club yesterday after work but the guy had left for the day so I didn't get to ask him. but there was a G4 parked out back, which looked pretty sweet!

 

[dukeblue219]

actually this list doesn't specify tach or hobbs. hobbs is actual engine run time, correct? what do u mean by tach time?

Hobbs is "normal" clock time, usually from engine start to shutdown, but it can be wired in different ways (such as whenever the Master is on). This is how most flight schools seem to do wet rates.

Tach time is the engine time, and runs at a different speed depending on the RPM. If you're in normal cruise it's the same rate as Hobbs time, but if you're going slow or sitting on the ground taxiing, it will run much slower. This is more common with ownership groups or flying clubs I think.

A lesson at a busy towered airport can be 1.5 on the Hobbs but just 1.0 on the tach, while a long cross country they'd be about the same.

 

[Brad Z]

actually this list doesn't specify tach or hobbs. hobbs is actual engine run time, correct? what do u mean by tach time?

On most trainers, the hobbs time is the actual time the engine is running. The clock moves the same whether you are idling at 700 rpm or crusing at 2300 rpm.

Tach time adjusts the speed of the meter based on the RPM of the engine. Depending on how it's set, the time will be 1:1 at 2300 rpm, while the clock will run significantly slower at idle.

It's hard to compare which accounting method is better without knowing how much time is spent taxiing versus cruising.

 

[eman1200]

On most trainers, the hobbs time is the actual time the engine is running. The clock moves the same whether you are idling at 700 rpm or crusing at 2300 rpm.

Tach time adjusts the speed of the meter based on the RPM of the engine. Depending on how it's set, the time will be 1:1 at 2300 rpm, while the clock will run significantly slower at idle.

It's hard to compare which accounting method is better without knowing how much time is spent taxiing versus cruising.

understood, thanks! and yes, this makes it a little difficult to compare one price dry vs another price wet.

 

[gismo]

understood, thanks! and yes, this makes it a little difficult to compare one price dry vs another price wet.

And just to clarify, 2300 isn't a universal number but it's probably close for a 172. The RPM where tach time changes at the same rate as your watch varies among airplane types and potentially model years but it's usually close to what you'd get cruising at 65-70% power (a typical power setting). Tach time is really counting revolutions of the engine with the gearing chosen so it advances one hour in an hour on your watch at a specific (magic) RPM.

Hobbs time is an electric clock that changes like a watch but only runs when power is applied. On some airplanes power is applied through an oil pressure switch so the clock runs when the engine does. Others run if the master is on whether or not the engine is running (and might stop if you turned the master off in flight) and some use some sort of airspeed sensing switch to power the clock whenever you're going fast enough that you are likely to be flying. On some retractable gear airplanes the Hobbs runs when the gear is up and a few (mine included) power the clock through a switch on the landing gear strut that activates once the wheels aren't holding the plane up.

You'll typically find that tach time runs around 20-30% slower than Hobbs time in an airplane with a fixed pitch prop (e.g. 172) if you're mostly doing pattern work or maneuver practice. But in cruise, especially up high it will likely run faster than Hobbs because the RPM will be higher than the "magic" RPM unless you slow way down (it takes more RPM to go fast up high because the air is "thinner"). In the pattern the RPM is much lower on the average, taxiing is obviously with a very low RPM, during takeoff and climb the RPM doesn't get real high because the airspeed is low and you typically keep the RPM well below the "magic RPM" once you level out to avoid building too much speed, coming in to land on final the RPM is even lower, and then you're back on the ground at idle.

With a constant speed prop (e.g. C182) tach time in the pattern will be higher than with a fixed pitch prop and might even exceed Hobbs time if you're doing touch and go's but it might run slower than Hobbs in cruise flight since you can dial back the RPM and still keep the speed up pretty high.

 

[Henning]

understood, thanks! and yes, this makes it a little difficult to compare one price dry vs another price wet.

For comparison on a 160hp 172 use 6.5gph and 180hp use 8.5gph and multiply by the price of fuel. Just remember that the price of fuel changes.