The International Cessna^® 170 Association

Does anyone have either (1) a reliable, accurate estimate of the fuel normally expended during a normal start, taxi and takeoff or (2) figures from a fuel flow meter in the cockpit for what an O-300 is burning per hour when idling at 800 rpm, taxiing at normal speeds and during a runup at 1600 or 1700 RPM?

Reason for the question: I was brought up flying with navigation logs such as the Air Force Form 70, accounting for fuel burn leg-by-leg during a flight, with a standardized allowance for start/taxi/takeoff. In the C-17 that standard value is 1200 pounds. I'm guessing that the process from startup to liftoff takes about 1/2 gallon in my O-300 equipped 170B with my leisurely pace of checklists and slow taxiing, but that is entirely a WAG. I'd like to really know a better number based on real fuel flows. If it's really 1/2 gallon, then I'll base my future planning on 36.5 usable when spinning the winds on DUATS, etc.

It's a persnickity thing, I know. If I ever cut it so close that the last 1/2 gallon makes all the difference, I'm a fool. And taxi times and throttle techniques vary. It takes me about 6-7 minutes to get off the ground, with about 30 seconds spent at runup RPM, and about 15 more seconds at full throttle prior to liftoff. I just kinda want to nail down the number to keep my fuel planning habit patterns the same.

Scotty

What you seem to be asking for is the fuel burn average to about fifty feet in the air if there has been no delay en route----Correct?
My vast experience in things like this start on the Boeing B-29 where I spent a few years as as a flight engineer instructor and also as a pilot.
You can depend on the accuracy of my figures because I'm good!
A stock engined C-170 will burn, under the conditiones as outlined above, exactly 23.666 fluid ounces of av gas. Mogas will be damn near the same only your spark plugs will stay clean.

Anyone that can come up with any other figures for this problem is probably using a wild guess!

Let me tell you something, Sport, if you get down to where this gets important, you're in trouble!

Hi Scotty,

I can relate to the desire to figure in all the factors when estimating fuel requirements for a flight. It would be hugely satisfying to be able to predict the fuel used to within a fraction of a gallon, and do it consistently.

I have a fuel-flow meter on my O-300 powered 170, and for 5-7 minutes idling before takeoff, plus runup, I see about 0.3 gallons used. Your estimate of 1/2 gallon is a good one, but just a little bit high in my experience.

Happy flying!

John

Get out there and enjoy the flying don't make it a job.

There's more than one way to enjoy flying! Maybe almost as many as there are pilots.

John

I know this doesn't have anything to do with the fuel flow during taxi/runup, but Blueld, we'd sure like to hear some stories of your flt engineer ops and pilot ops in the B-29. While a lot of it may have been routine at the time hearing input about flying that Superfortress and managing those 3350's must have been really interesting.

Relying on fast-fading memory, Cessna planned on 2 lbs of fuel for start, taxi, runup. This would be a good question for Mort this summer.

Two pounds would be 1/3 gallon, close to the 0.3 that I'm seeing. Sounds right to me, George!

John

scottgsaunders wrote:Does anyone have either (1) a reliable, accurate estimate of the fuel normally expended during a normal start, taxi and takeoff or (2) figures from a fuel flow meter in the cockpit for what an O-300 is burning per hour when idling at 800 rpm, taxiing at normal speeds and during a runup at 1600 or 1700 RPM?

Reason for the question: I was brought up flying with navigation logs such as the Air Force Form 70, accounting for fuel burn leg-by-leg during a flight, with a standardized allowance for start/taxi/takeoff. In the C-17 that standard value is 1200 pounds. I'm guessing that the process from startup to liftoff takes about 1/2 gallon in my O-300 equipped 170B with my leisurely pace of checklists and slow taxiing, but that is entirely a WAG. I'd like to really know a better number based on real fuel flows. If it's really 1/2 gallon, then I'll base my future planning on 36.5 usable when spinning the winds on DUATS, etc.

It's a persnickity thing, I know. If I ever cut it so close that the last 1/2 gallon makes all the difference, I'm a fool. And taxi times and throttle techniques vary. It takes me about 6-7 minutes to get off the ground, with about 30 seconds spent at runup RPM, and about 15 more seconds at full throttle prior to liftoff. I just kinda want to nail down the number to keep my fuel planning habit patterns the same.

Scotty

I'm not trying to be snide when I say this but.......If you're really worried about this just have an accurate fuel flow meter installed instead of asking for opinions. I'm thinking about putting one in my plane when I have a pile of cash laying around, and I used to think they were a joke, but if you really want to know there are just to many variables involved to say .3 gallons. I start the timer when the engine lights and just assume my hourly rate of fuel burn starts there, anything left over is insurance.

Jr.CubBuilder wrote:... there are just to many variables involved to say .3 gallons. I start the timer when the engine lights and just assume my hourly rate of fuel burn starts there, anything left over is insurance.

I use the hour/odometer in my tachometer. Since it runs slower during slow engine speed (when fuel burn is less) and faster at high rpm (when fuel burn is greater) ... I feel it's more accurate to determine average fuel flows using the tach's hour-recording than an actual clock. (It's biased in concert with power settings.) 8 gph is the figure I use for safety purposes.

gahorn wrote: I use the hour/odometer in my tachometer. Since it runs slower during slow engine speed (when fuel burn is less) and faster at high rpm (when fuel burn is greater) ... I feel it's more accurate to determine average fuel flows using the tach's hour-recording than an actual clock. (It's biased in concert with power settings.) 8 gph is the figure I use for safety purposes.

This is the method I use as well and I find the fuel figures to be amazingly consistent.

Thanks all, even the curmudgeons who think I don't have any fun, don't have enough stuff in the panel, have too much in the panel, and/or don't have the sense to stop flying before I run out of gas. Love this owner's group for all the suggestions, curmudgeonly or otherwise. I don't know why I didn't think of looking at the tachometer hours x cruise fuel flow. Great idea and the tools are already installed!

After 9 months as a new owner playing the mental game of "How much gas do I think I've burned vs. how much did it take to top off?" I was getting pretty impressed with myself about being able to estimate fuel burn to within 2-2.5 gallons on 2.5 hour round trips and 1.5 hour local flights.

However, on a recent long cross country I ran a tank dry sooner than expected and had about 5 gallons less in the tanks than I expected when I topped off. This was on a 3.5 hour leg, starting full to 1 inch below the top of the filler neck, leaning until RPM stopped increasing at cruise, and being pretty close on all my expected times at checkpoints. Don't know exactly where that missing 5 gallons went, and it wasn't a comfortable feeling to realize I had 30 minutes to flameout when I thought I had almost an hour. It think it would be smart to get a fuel flow meter in future for these longer trips, as one poster suggested, even though it would spoil the nice "round dial" panel I've got going at present. I may start with an EGT/CHT for better leaning at first.

Thanks again all!
Scotty

Scotty,

A couple of things from my experience. First: I've been using a CHT/EGT gauge for a couple of years on my O-300 powered 170B, and while I think it's useful for monitoring engine temperatures and diagnosing problems like a bad spark plug or ignition harness, I don't think it gives me better leaning. I've tried to use it as an aid to leaning, but I find it's really better to go by Cessna's recommended method of leaning to maximum RPM and then enriching until the RPM begins to drop. The problem is that these instruments are designed with fuel-injected engines in mind, and a carbureted O-300 just doesn't give you the nice, even fuel distribution and consistent EGTs that make the instrument useful for precise leaning.

Second: I had an interesting experience recently when I bought a Fuelhawk universal fuel gauge (http://www.mypilotstore.com/mypilotstore/sep/1177) and tried to calibrate it for the 170. (I had not read, or had forgotten about, George's suggestion of using the 172 Fuelhawk with the bottom of the tube cut off at the zero mark, which you'll find elsewhere in this forum.) I calibrated my tube by measuring out fuel a couple of gallons at a time in the hangar, and for the last couple of gallons, topping the tank off with a 1-quart jar. That seemed very accurate to me, and it gave a nice smooth curve. I was surprised to discover that in the real world at the gas pump, each tank seemed to have somewhere between 1 and 1-1/2 gallon more fuel in it than the Fuelhawk said it did. So now I'm in the process of recalibrating the gauge at the pump, and ignoring the nice clean numbers I got in the hangar. (I'm getting results closer to the modified 172 gauge. On the "universal" Fuelhawk gauge, that's one gallon for each mark on the tube. In practice, that seems to be optimistic by about 1/2 gallon.)

I believe the reason for the difference is that in the still air of the hangar, and milking the last drops of fuel into the tank, I was able to fill the tank to a higher level than is possible at the pump, given ordinary human patience. Due to dihedral and the position of the filler neck relative to the forward part of the tank (which may be higher in a taildragger), there's always air in the tank, even when the fuel level is up into the filler neck. I think the figure of 2-1/2 gallons unuseable per tank is in part to allow for this. Depending on how good a job you've done of squeezing the air out when you filled, and squeezing the tank dry as you fly, you can probably get more than 18.5 gallons useable out of it -- but nothing beyond 18.5 is reliable in general.

This has been long-winded, I know. But my point is: there is unuseable space both at the top and at the bottom of the tank, and just because a particular fill-up took more than you expected, doesn't mean necessarily that you burned more fuel than you thought. You might have simply gotten a better fill than at other times. I do think a fuel flow gauge is a very useful thing, probably more useful than the 6-cylinder CHT/EGT gauge if you're flying extended X-C in remote areas.

Best Regards,

John

Scotty,

1. One inch below the filler neck is not "Full". In the filler neck is.

2. If you're cruising above about 2000 ft. MSL or higher and at less than 2500 RPM, you're going to be pulling less than 70% power. At this power, you will not be able to damage the engine by over leaning. As a result, under these conditions it is most advantageous to lean to where the engine starts to "stumble" and then enrichen just to smooth operation. A stock tachometer is too difficult to read close enough to use it for leaning. The RPM differences are too small.
At around 65% power you should be down around 7 GPH. Figure about .44 lbs/HP/Hr.