Calculating fuel burn

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UngaWunga

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UngaWunga
What do y'all do to calculate your actual fuel burn without an engine monitor with a fuel monitor?
I try to calculate it using the book values for climb, decent, and cruise, and it isn't always terribly accurate.
 
I stick the tanks post-flight and do the math. We have years of data now in our aircraft book and our spreadsheet.
 
UngaWunga said:
What do y'all do to calculate your actual fuel burn without an engine monitor with a fuel monitor?
I try to calculate it using the book values for climb, decent, and cruise, and it isn't always terribly accurate.
Click to expand...

"Book values" are derived from new planes with new engines flown by factory test pilots. Don't expect to achieve them in real life.

Bob Gardner
 
bobmrg said:
"Book values" are derived from new planes with new engines flown by factory test pilots. Don't expect to achieve them in real life.
Click to expand...

Understood. So how to do you calculate it? I also dip tanks before/after. I'm trying to figure out how I can more accurately plan for longer legs.
 
UngaWunga said:
Understood. So how to do you calculate it? I also dip tanks before/after. I'm trying to figure out how I can more accurately plan for longer legs.
Click to expand...
I pick the worst estimate, in a cherokee 180 that's 10 gph, and never fly more than 3.5 hrs at a time. 3.5 is my personal deadline because that's all my body can handle comfortably. Even west bound against the winds, I've nver had a problem in 15 years. Over that time, the real fuel flow is about 8-9 gph but plan for 10.
 
UngaWunga said:
Understood. So how to do you calculate it? I also dip tanks before/after. I'm trying to figure out how I can more accurately plan for longer legs.
Click to expand...

Honestly if you don't have a lot of data, use book numbers and choose a safety margin above book that you're comfortable with. Pretend the airplane burns more than the book until you know it doesn't. Or does.

With the LR tanks in our 182, it'll fly longer than my bladder feels like flying anyway. Full it'll go over five and a half hours with legal VFR reserves at 13 GPH, and we know from lots of data it doesn't burn 13 at altitude.

But I plan for 13 anyway and plan to be on the ground with an hour in the tanks. Which means it's rarely tight.

The one time it got tight, my co-owner and I got stuck beneath a benign stratus layer at 2500 for hours. With our "plan for 13 and an hour" rule we landed with VFR minimums on board. We learned a lesson about flying at lower altitudes on long XC legs that day. Neither of us was real happy when we sticked the tanks in Iowa City.
 
I always used the most pessimistic on time based on my preflight planning and what the gauges say. Calibrated my expectations based on how much fuel it took to fill up (or the semi-calibrated paint stirrer I had). With the fuel flow installed, I now use the most pessimistic based on all three.
 
UngaWunga said:
Understood. So how to do you calculate it? I also dip tanks before/after. I'm trying to figure out how I can more accurately plan for longer legs.
Click to expand...

I don't. I follow the practice of others in this thread and carry more than is necessary.

Bob
 
When planning round fuel consumption and time enroute up.

My actual practice... my fuel burn is 10gal/hr. I have 48 gallons usable fuel split between 2 24gallon tanks. So 2.4hrs/wing and 4.8hours total. In 3 hours I'm about ready to take a break and find a bathroom. I try to limit individual legs to about 3 hours.
 
Just after purchase, I spent a few days (in calm winds) sticking my tanks, flying fixed times at constant power settings, and then seeing how much fuel I had to add to bring it back to the same level it was when I measured it with the dipstick before the flight. Over the course of several days, I tried just about every combination of cruise altitude and cruise settings. Made me a nice little updated fuel burn chart.

Not sure I'd recommend that though, since now, I just carry twice as much gas as legally required. I figure between tankering the extra gas, and my body's 2-hr sit-still limit, I'm highly unlikely to run out of gas. (Me or my plane.)
 
Rykymus said:
Just after purchase, I spent a few days (in calm winds) sticking my tanks, flying fixed times at constant power settings, and then seeing how much fuel I had to add to bring it back to the same level it was when I measured it with the dipstick before the flight. Over the course of several days, I tried just about every combination of cruise altitude and cruise settings. Made me a nice little updated fuel burn chart.
Click to expand...

As Rykymus states, after a few flights he has a 'more accurate' fuel burn chart than what's in the manual, which as Bob says is new conditions and test pilots. Really about the only way to have an accurate knowledge of fuel consumption.
 
I do it just like you would in your car. Start with a full tank, when you get back fill it, and calculate the consumption.
 
Always overestimate in your pre flight planning. As you get older you will have no problem ,as the kidneys take over.
 
I burn and cruise dang near book, I input all the numbers from the book into fltplan, it knows the wind, fuel at cruise altitude, climb, decent, taxi fuel numbers, and it's pretty close to what my tanks show after landing, I still plan on 1hr min reserve and round up.

This is also in a plane that I never top the tanks in, cuz it holds a chit ton of fuel and also goes into some odd landing areas where I don't need the extra weight.

No matter what, always stick your tanks before and after flight, and after fueling.



Here's the initial setup

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Once you get your plane all configured, here's what a sample flight plan looks like

image.png
 
lancie00 said:
I use the worst estimate and still plan for 1 hour reserve. You can only have too much gas if you're on fire.
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I thought I was on fire once but it was a skydive plane so no chance I had too much fuel.
 
Cessna POH fuel burn numbers are accurate in my experience, which is close enough. I plan what it burns, and don't pad it. I'd rather know the reality. I'm good with taking it down to legal reserves, if it fits the flight, but usually that's not what I need to do.
 
everyones advice in this thread is very good. personally in my bird i notice i get a better GPH and TAS than the book says I will, this is something I've noticed over the 5 years I've had my plane. As a result of this i just plan for what the book says because I know I'll actually be a little better than that.

However I prefer an alternative method of just having your airplane converted to be able to do inflight refueling.
 
Fill the tanks, flip the fuel selector to the left tank then start the engine. Taxi around for a couple hours, make a note of the time then refuel while also noting the quantity. Restart the engine and make a note of how long you taxied before you took the runway. Climb to altitude then switch to the right tank after you are in cruise configuration. Fly around a while, note the time then pull the mixture to cut-off and dead-stick it to an airport with fuel. Fill 'er up noting the quantity.

Now you know the fuel burns for taxi, climb and cruise. If you need to know fuel burn for descent then you'll need to make another flight or you can just keep doing dead-stick landings.
 
Garmin pilot flightplan tool now uses performance tables and calculates burn down to the 10th automatically. You just tell altitude, MP, and RPM....pretty accurate but I've never landed with less than 2 hours of fuel, 5 hours is my leg limit.
 
In the history of flight, no one has ever accurately and correctly predicted future fuel burn.
All that sticking and math accomplish is to show what the burn was on that day, under those conditions.
But, it does, kind of, sort of provide an approximate baseline for what might happen.
The expression, "Your mileage may vary." is never more true than when flying.
 
Another reason for trying to get accurate numbers for fuel burn is to make sure I'm operating the engine correctly. Leaning correctly for the rpms and altitude. With cars and bikes, fuel burn also helps show the condition of the engine or other potential problems. Some of the reasons why I'm trying to sort this out...
 
One time I filled both tanks. When I got to cruise altitude, I switched tanks and flew for about an hour,then landed.
On the ground, I refueled both tanks and recorded how much each took. The one tank told me how much fuel I used to climb to cruise altitude.
The other tank told me how much fuel I used to fly for an hour at cruise altitude while leaned.

Both values were so close to "book value" that I just round that up a little and make sure I have plenty for the trip with at least an hour reserve.
 
coloradobluesky said:
Keep a log of fuel and engine time. Dont push it.
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This is what I do. Start the clock when the engine starts and stop it at shutdown. Record the time of each flight and record how much fuel you put in. Most of my flights are short around the local area so when I do the math I get about 7.5 gal/hr. That's what I plan for but on long flights (like to OSH) I calculated I got 6.8 gal/hr. I still plan for 7.5 though.
 
I have checked our Cessna multiple times on longer flights. Checking against book values, I am VERY close. Based on that, I feel comfortable with max book range (be sure to lean!), but like others, I stop at about 2.5 hours for a break. It makes the trips a lot more enjoyable to get out and walk around a little, use the facilities, etc. I never push fuel. I think that is one of the dumbest things we can do as pilots.
 
bobmrg said:
"Book values" are derived from new planes with new engines flown by factory test pilots. Don't expect to achieve them in real life.

Bob Gardner
Click to expand...

IF I meet the book assumptions (which may include a specific fuel flow during climb, and usually includes a specific airspeed and configuration), I find I can pretty reliably match the book numbers. I vary, it's different. For instance, in a 182, you climb at 90 knots indicated at 23/2400 at 16 GPH (at sea level) with the cowl flaps hanging all the way open.

If I can't, it's time to call the airplane's condition into question. Even if it's 40 years old.
 
UngaWunga said:
What do y'all do to calculate your actual fuel burn without an engine monitor with a fuel monitor?
I try to calculate it using the book values for climb, decent, and cruise, and it isn't always terribly accurate.
Click to expand...

I over estimate - the Arrow I fly for rescue holds 48 gallons useable - I usually take what the book or gauge says and add 1GPH, then subtract extra for climb and taxi - I never, EVER want to put a plane down because of fuel exhaustion.
 
I was flying a club plane before I bought my first plane, probably had like 150hrs at that time, the plane had one of those "fuel hawk" fuel straws with the gallon markers on the side, I check the fuel, cool, hit my watch and launch, come back and land, should have had about a hour of fuel, pretty crappy bouncing Cessna fuel gauges bobbing all over the place, whatever I have my watch going and know my fuel flow... Not so much, I landed on fumes, turns out it was the wrong fuel hawk and was for a different plane, some muppet put the wrong gauge in the plane when they flew it, "oops", reason #1427 I own and don't rent.

I take my tank to empty, add 5 gal at a time and mark my fuel stick, after that I take a dremel to it.

It's like that old saying, garbage in garbage out, if you don't have a solid number on your fuel to start with its all for nothing.


MAKG1 said:
IF I meet the book assumptions (which may include a specific fuel flow during climb, and usually includes a specific airspeed and configuration), I find I can pretty reliably match the book numbers. I vary, it's different. For instance, in a 182, you climb at 90 knots indicated at 23/2400 at 16 GPH (at sea level) with the cowl flaps hanging all the way open.

If I can't, it's time to call the airplane's condition into question. Even if it's 40 years old.
Click to expand...

SL climb on a 182 is only 16GPH? That's like 200hp? Thought they were over 200
 
James331 said:
SL climb on a 182 is only 16GPH? That's like 200hp? Thought they were over 200
Click to expand...

230 HP. And that's a nearly perfect 80% power. You never see max rated power on an aircraft engine. 200 real horsepower is about right for a normal climb. Maybe a tad high.

Normal climbs are not done at full power. They are done at 23/2400/16 GPH. Full power would be more like 29 inches and full rich.
 
Wasn't thread about not having a fuel flow indication? And in my experience if there is no fuel flow indication it's probably fixed prop, so the MP/Tach examples don't help the OP either.
 
MAKG1 said:
230 HP. And that's a nearly perfect 80% power. You never see max rated power on an aircraft engine. 200 real horsepower is about right for a normal climb. Maybe a tad high.

Normal climbs are not done at full power. They are done at 23/2400/16 GPH. Full power would be more like 29 inches and full rich.
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Thought they were 235hp? Maybe the one I flew back in the late 70s was. Probably a 70 something model, I don't recall. And yes, didn't climb at full power either.
 
mscard88 said:
Thought they were 235hp? Maybe the one I flew back in the late 70s was. Probably a 70 something model, I don't recall. And yes, didn't climb at full power either.
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The retractible and turbocharged 182s are 235 HP. But that's the full rated power, not what we climb with.
 
MAKG1 said:
The retractible and turbocharged 182s are 235 HP. But that's the full rated power, not what we climb with.
Click to expand...

This one was neither, been awhile, but that's what I recall as far as the HP. :confused:
 
My plane holds 60gal with 56 gal usable. Most of my local flights are under 2 hours. My normal fuel load is 40 gal. I know over the years flying my plane the burn is around 8gph. I use the stick method and fill the tanks fly and fill when I return. For flight planning I use 10gph. So I fudge on the concretive side, I also plan a 1 1/2 hour minimum fuel reserve.
 
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