A Cessna 172 is highly versatile and can carry different combinations of payload and fuel, resulting in mission-specific range capabilities.
Assuming zero winds, the Cessna 172 could travel 600 nautical miles. When fully fueled and launching at Maximum Take Off Weight on a standard day, its range is influenced by the power setting at cruise, its CG location, and the winds aloft.
As a CFI and a corporate pilot, I teach my students to always calculate the effective range their aircraft can achieve for each flight, keeping in mind the need for VFR or IFR reserves.
Range Versus Endurance
To understand the range of an aircraft, our first order of business is to distinguish between the range of an aircraft and its endurance, as I have heard new pilots constantly say one and mean the other.
Endurance is how long you can stay airborne. Range is the distance you can fly.
Think of it this way, if you fire up the plane and leave it sitting on the tarmac running idle, it will eventually burn up all its fuel after a few hours. That’s endurance. It is the time you can have an engine run at a particular power and mixture setting with the amount of usable fuel it has in its tanks.
Once you power up and take off, all the variables change. You are now running at a higher RPM than you did on the tarmac, and you’ve probably leaned it out. In this configuration, the engine is burning approximately 9 gallons per hour (gph.)
A Cessna 172 has a 40-gallon capacity. Burning 9gph, your endurance – the time you can stay in the air will be (40 gallons/9 gph) approximately 4 hours and 30 minutes. That’s just how long you can stay in the air. It says nothing about VFR or IFR reserves. It also does not measure tailwinds or a headwind.
If you have no wind at altitude (which is unlikely) then your true airspeed is going equal to your ground speed. Looking at the chart, flying at six thousand feet burning 9 gallons per hour like the example above is going to give you a True Air Speed (TAS) of 122 knots.
With zero wind as the assumption, your ground speed will also be 122 knots. Being able to fly for 4 hours and 30 minutes (your endurance) means you will cover 549 nautical miles (122 ktas x 4 hours and 30 minutes.)
While range and endurance are not the same thing, they are related.
Factors to Consider When Extending Endurance
Now that we understand the difference between range and endurance, first focus on its endurance, then you can determine its range.
Endurance is dependent on a number of factors, the most important of which is burn rate, or fuel flow rate. Most high-powered aircraft have fuel flow gauges to give pilots a real-time understanding of how much fuel they are burning.
Fuel flow rate is dependent on factors you can control and factors you can’t. You can control your throttle and mixture setting. But you can’t control the air density in the atmosphere.
At higher altitudes or on hotter days, there are less molecules of air per given area. No matter how much fuel you give it, it will only burn to the extent of air molecules available.
You should also consider the condition of your engine. If it's close to its scheduled overhaul, there is a possibility that its burn rate is higher than you are used to.
Fuel flow is usually disregarded by most GA pilots who are looking to get somewhere as fast as they can carrying the most they can. And there is nothing wrong with that.
To increase endurance, and therefore range, the goal is to reduce fuel flow. Refer to your POH for the best power or best economy settings.
There are two things to think about when you adjust the mixture. Are you trying to extend how far you fly? Or, are you trying to get there quicker? The answer will determine if you want Best Power settings or Best Economy.
Best Power will not give you the best consumption but will get you somewhere faster, while Best Economy will reduce fuel flow and increase range.
In the hot days of summer, density altitude plays a major role in fuel consumption. Take that into account when planning a flight.
Density altitude is pressure altitude corrected for temperature. Without getting too deep into it, just remember that the higher the altitude, the less dense the air.
Horsepower is generated when fuel is burned. Fuel burns when it is mixed with air and oxygen and ignited. A pound of fuel needs exactly 14.7 pounds of air to burn. The higher you climb the lesser air molecules there are available.
At the full rich setting, the cylinder gets more fuel than there is air to burn, resulting in only partial combustion and reduced power. Yet, your consumption is still high. So even though your throttle is open to disperse 9 gallons per hour, only seven gallons per hour is getting burned. But your endurance is still going to be the same four-and-a-half hours.
But at least you have cooler temperatures to compensate. It gets worse when the temperatures are significantly above normal. Higher temperatures reduce air density, causing less air to mix with the fuel that is going into the cylinder.
There is also the effect of humidity. Lower pressure and warm air can be made worse when high humidity is injected into the mix. Humidity displaces air, and now there is even less air to mix with the fuel. This increases consumption and reduces range.
CG Effect on Range
The location of the CG not not only determines stability, it also has an effect on fuel consumption.
If you want to squeeze the fuel effectiveness of your aircraft, one way to do it would be to be mindful of how you load your aircraft.
An aircraft with full forward CG will require more tail down force to keep the necessary pitch, requiring the wings to generate more lift to compensate for the downward force of the tail. This results in higher induced drag. Higher induced drag requires more thrust and increased fuel consumption.
A full aft CG, on the other hand, reduces the necessary tail-down force the plane needs to generate and therefore reduces the lift needed to be generated by the weight. This reduced induced drag.
So to squeeze out the best performance of the aircraft, load it as far aft as possible without exceeding its envelope. And remember that while aft CG position increases performance, it reduces the effectiveness of the rudder and vertical stabilizer.
Weight Effect on Endurance
Most pilots are quick to understand that the higher the weight carried, the more fuel it is going to take to get there. It is the main reason when I fly, I don’t always top off my tanks. On a clear day, even though I am on an IFR flight plan, I take no more than 90 minutes' worth of reserves.
With higher weight, comes the need for a greater lift to carry that weight. Greater lift means greater drag. More drag needs more thrust to overcome it, and that increases my fuel consumption.
Whenever possible, reduce the weight to what is necessary and take on full fuel to increase endurance. If the distance is the concern, sacrifice payload for fuel.
Flight Altitude and Endurance
When pressing for range, altitude selection can leverage your endurance. With a full weather briefing for your route of flight, you can find the altitude that gives you the highest tailwind component or the lowest headwind component.
The higher you ascend, the winds are generally faster. If the winds are against you, fly lower. If they favor you, fly higher.
Carburetor Heat Effect on Endurance
The colder the air, the better the aircraft’s performance. But this is not necessarily better for endurance and maximizing range. This is the case when flying in the winter in northern latitudes.
While the aircraft performs better, mechanically and aerodynamically, low-altitude flying will slightly increase your endurance. Climb as high as possible if the winds favor your direction of flight.
However, to leverage your cold weather condition, apply carburetor heat at altitude. This thins out the air and creates a lower full requirement thereby consuming less fuel and raising endurance. With a tailwind, you will increase your range.
At an altitude of twelve thousand feet, you could get your fuel flow to about 4 gph resulting in almost 8 hours of endurance flying at 96 kias. With a twenty-four-knot tailwind for instance, that increases the range to an impressive (and somewhat staggering) 960 nautical miles!
About THE AUTHOR
After spending years watching every video I could find about flying, I finally scratched the itch and got my pilots license. Now I fly every chance I get, and share the information I learn, here.Read More About Joe Haygood