Operating piston engines like a Pro, Part 1: Fuel Flow vs. Fuel Burn
Pilots who fly piston singles or multis have an enormous amount of decisions to make both before, and during a flight. We’re going to explore a few particularly poorly understood areas in this series. We’ll answer the question if flying slower saves money. You’ll be surprised at the answer.
What happens to the cost and duration of my flight when I..
- Increase fuel flow from 17gph to 23gph?
- Burn off fuel and decrease weight, how does it effect airspeed?
- Climb from 5000ft to 10,000ft?
- Increase RPM from 2200 to 2500?
- Increase TAS from 170kts to 180kts?
In this part, we’ll explore fuel flow vs fuel burn (1) and weight (2). We’ll use a Cessna 310R for the behind-the-scenes math, but these principles apply to all piston engines.
What happens if I increase fuel flow from 17gph to 23gph?
If your first thought was “I burn approximately 6 GPH more”, this is completely wrong. Remember, you’re flying faster, so you will spend less time in the air. But just how much time will you save, and how does this relate to fuel burn?
Take a look at the graph below. The blue line indicates the fuel flow you selected with the red mixture knob. The red line indicates how much fuel you’ll actually burn, taking into account the increase in air speed. The pilot flying 17 GPH fuel flow (FF) is actually going to burn 22 gallons to get to his destination, meanwhile, the pilot flying 23 GPH is going to burn about 23 gallons on that same flight. Just 1 gallon more, and they’ll arrive much sooner with less wear on engine, components, and passenger bladders!
The sweet spot is somewhere between 23gph up to 26gph. On the right side of the graph, notice the fuel burn accelerates, indicating an inefficiency, likely due to parasite drag, which is exponential.
A key takeaway is, avoid fuel flows in the area where the blue line is below the red line.
Above is a more detailed view of a hypothetical 167nm flight. We built this with the assumption we are cruising at 167 TAS, and we can tweak fuel flow to fly faster or slower. Let’s examine the 3 scenarios.
- Pilot ‘1’ is cruising at 22.8 GPH fuel flow. Effective fuel burn is 22.8 gallons, 167 TAS, 1:00 ETE.
- Pilot ‘2’ selects 17.3 GPH fuel flow. Effective fuel burn is 21.8 gallons, 133 TAS, 1:15 ETE. That’s right, almost 40 kts slower, at only 1 gallon saved!
- Pilot ‘3’ selects 25.2 GPH fuel flow. Effective fuel burn is 23.7 gallons, 177 TAS, 00:57 ETE. 10 kts faster for only 0.9 gallons more.
Study this chart more, the columns denote:
- Extra FF: Fuel flow in increase or decrease relative to the ‘default’ 167 TAS cruise configuration
- Extra Fuel: The actual fuel burned compared to the default configuration.
- FF vs Burn Cost: The ratio of fuel flow increase vs. actual fuel burn increase. It hovers around 30%-40%.
- Cost: Difference in fuel cost, assuming $5/gallon.
We’re not done quite yet. Flying longer incurs additional variable costs.
We need to overhaul accessories, engines, and the airframe depreciations. Rough estimate for a C310R is $1/minute for variable costs. Pilot 2 saved $5 on fuel by flying slow. But those 15 minutes will cost $15 in variable costs, for a net loss of $10. This doesn’t even account for lost time, bladder discomfort, and most importantly, the massive bragging rights of cruising along at nearly 190kts, a speed most light GA aircraft can only achieve in a steep descent.
What happens if we fly max cruise speed, 188kts?
The last row denotes the scenario of max speed. We arrive 7 minutes sooner than pilot 1, for $11.70 extra fuel cost. Subtracting out $7 in variable costs leaves us with $4.70 in effective increase to go max speed.
What if we go just a little slower, and throttle back to 183 kts? We arrive 5 minutes sooner for $7.50 extra fuel o $2.50 effective increase in cost.
These may seem like small number, and they are. Even with 250 annual flight hours, the difference between 188kts and 183kts is $2.20 * 250 = $550 effective cost.
Always Fly Above 10,500'
There’s an additional caveat. You can’t fly peak EGT above 65% BPH (read Mike Busch’s excellent Red Box / Red Fin article). Above 65%, you must fly ROP or LOP, one is quite fuel inefficient and wears out the engine, and the other is slow. My recommendation is always climb up over 10,500' (I’m happiest at 11.5–13.5) where the normally aspirated engine cannot produce more than 65% BPH (give or take, depending on atmospheric conditions).
Note that Cessna only permits peak EGT operation for the configurations with borders around them. This prohibits 59% BPH operation at 10,000'. If you follow the red fin advice, you don’t need to worry about this.
Finally, beware that on very cold winter days, 10,000' can easily produce 70% or more.
Memory item: Always cruise above 10,500' at peak EGT and prefer the fuel flow to achieve maximum cruise speed.
How does weight effect airspeed?
The table above illustrates how weight changes impact airspeed. We can convert 57 gallons to 2 hours of flight time.
Memory item: For a C310R, in typical cruise altitudes, expect to pick up 2 kts of TAS for every hour of cruise (or 1.1% extra speed/range) due to weight decrease as fuel burns off.
Closing Thoughts
While fuel is certainly not cheap, and it’s tempting to try to save money with lower fuel flow, I hope by now it’s evident that you’re not going to save any money this way. Climb up to > 10,000' and set that mixture for maximum cruise speed. If you’re still struggling to internalize this, print this chart and stick it to your panel. Remind yourself that when you select 26.7 GPH you’re really burning 24.3 GPH. Most definitely avoid any fuel flows < 22.8 GPH (at 10,000').
Memory item: A change in fuel flow results in only about 35% change in fuel burn, and variable costs are $1/minute!
