Rajan Wilcox
The Cessna 175 is a popular aircraft, but some pilots have raised concerns about its fuel capacity and the amount of usable fuel in its tanks. The issue of unusable fuel in the Cessna 175 tanks is a complex one, with some models certified with up to 10 gallons of unusable fuel. This has led to discussions on how to maximise the usable fuel in these tanks. Some pilots have suggested running one tank dry and then switching to the other, while others have explored options for modifying the fuel tanks to increase their capacity. Digital fuel level sensing and calibration of fuel sender units are also mentioned as potential ways to improve fuel management and accuracy. While there are no clear solutions, the topic of maximising usable fuel in Cessna 175 tanks is an important one for pilots and owners of these aircraft.
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What You'll Learn
Calibrate fuel sensors to ensure accuracy
Calibration of fuel sensors is essential to ensure accurate fuel readings in your Cessna 175. Here are some detailed steps to help you calibrate your fuel sensors:
First, it is important to understand that the "unuseable" fuel in the tank refers to the residual fuel that cannot be drained by normal means. This unuseable fuel amount must be considered when calibrating your sensors.
To begin the calibration process, find level ground to park your Cessna 175. This ensures an accurate baseline for your measurements. Next, drain the fuel tanks using the lowest sump drain, leaving the unusable fuel in the tank as per standard procedures.
Now, you will need to add fuel in a controlled manner to calibrate the sensors. Start by adding 1.5 gallons of fuel to represent the unusable amount, as recommended by some Cessna 172 pilots. Then, continue adding fuel in precise increments, such as one gallon at a time, while simultaneously taking measurements using a dipstick or a clear tube-type fuel gauge. Ensure that your dipstick is calibrated correctly by first emptying one tank and then filling it with known amounts of fuel to verify the accuracy of the dipstick readings.
During this process, you can also use a FuelHawk brand clear measuring tube, which has been found to provide accurate readings for Cessna aircraft. Cut off the lower gallon indices below the zero-mark to modify the tube for your Cessna 175. Place the tube at the aft end of the fuel-cap opening, with the bottom of the FuelHawk resting against the baffle at the mid-tank line.
Additionally, you can consider replacing the old cork floats in your fuel sender units with new Rochester Nitrile rubber floats, which are available at a relatively low cost. This can help improve the accuracy of your fuel readings.
By carefully following these steps and cross-referencing your measurements with the fuel gauge readings, you can effectively calibrate the fuel sensors in your Cessna 175, ensuring accuracy in your fuel quantity measurements.
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Use digital fuel level sensing
Upgrading to digital fuel level sensing can help you get more usable fuel from your Cessna 175 tanks. This is a particularly good option if you are experiencing issues with your fuel gauge, such as erratic behaviour or inaccurate readings.
CiES offers a digital fuel quantity system that can provide highly accurate fuel measurements down to less than 10ths of a gallon of AVGAS or Jet A. This system uses a patented sensor that does not come into contact with the fuel, ensuring a lifetime of operation and safety. The initial STC allows the CiES system to be installed on various Cessna models, including the 175.
The conversion to digital fuel level sensing involves replacing the mechanical gauge in the end of the tank with a digital sender. CiES offers physical bolt-in replacements for Cessna Aircraft fuel tank quantity, which are applicable to most Cessna models. These digital senders are constructed with hard anodized aluminum and have a Teflon coating, ensuring durability and accuracy.
To ensure accuracy during the conversion process, it is important to carefully calibrate the digital senders and gauges according to the provided instructions. This may involve flying the fuel down to a low level, draining the rest, and ensuring your airplane is on level ground during calibration. Additionally, you will need to connect a power wire to the digital sender to operate the complex sender circuitry.
By upgrading to digital fuel level sensing, you can benefit from accurate and reliable fuel level readings, which can help you maximize the usable fuel in your Cessna 175 tanks.
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Install long-range tanks
Installing long-range tanks is a viable option to increase the usable fuel capacity of your Cessna 175. This modification typically involves replacing the standard fuel tanks with larger ones or adding auxiliary tanks to increase the overall fuel capacity. Here are some key considerations and steps to help you through the process:
Selecting the Right Long-Range Tanks
Before starting the installation process, it is crucial to choose the appropriate long-range tanks for your Cessna 175. There are several options available, including:
- Del-Air Long-Range Tanks: These tanks are a popular choice among Cessna owners. They typically add 7 gallons to each tank, resulting in a total fuel capacity of 56 gallons (28 gallons per tank). Del-Air also offers a larger modification that may provide up to 60 gallons total.
- Monarch Auxiliary Tanks: Monarch offers auxiliary tanks that are installed by removing the wings and inserting hard plastic tanks. This modification is known to be robust and well-liked, but it may result in a significant loss of capacity in the main tanks.
- Flint Slide-In Tanks: Flint offers slide-in tanks that are simpler to install compared to Monarch tanks. These tanks can provide up to 23 gallons of extra fuel and can be left empty when not needed. However, they do require pumps for their operation.
- Tip Tanks: Tip tanks are often mentioned as an option to increase fuel capacity. However, they may not be suitable for all aircraft configurations and can impact the aircraft's performance and aerodynamics.
Installation Process
Once you have selected the appropriate long-range tanks, the next step is to install them in your Cessna 175. Here's a general overview of the installation process:
- Find a Qualified Installer: It is essential to engage a qualified and experienced technician or mechanic to perform the long-range tank installation. Ensure they have specific experience with Cessna aircraft and long-range tank installations.
- Obtain Necessary Approvals: Before proceeding with any modifications, ensure that you obtain the necessary approvals and certifications. This may include consulting with aviation authorities and ensuring that the modifications comply with relevant regulations.
- Modify the Aircraft Structure: Installing long-range tanks typically involves modifying the wings and fuel system of your Cessna 175. This process can be complex and may require cutting into the wings, reinforcing structures, and reconfiguring fuel lines and gauges.
- Test and Calibrate: After the installation, thoroughly test and calibrate the new fuel system. Ensure that all fuel gauges are accurate and that the aircraft's weight and balance calculations are updated to reflect the increased fuel capacity.
- Document the Modification: Properly document the long-range tank installation in the aircraft's maintenance records. This documentation is essential for future maintenance, inspections, and resale purposes.
Benefits and Considerations:
Installing long-range tanks in your Cessna 175 can provide several benefits, including:
- Increased Fuel Capacity: The primary advantage is the ability to carry more fuel, extending the aircraft's range and endurance, making it better suited for longer cross-country flights.
- Reduced Fuel Stops: With a higher fuel capacity, you may be able to reduce the number of fuel stops during your journeys, saving time and improving the efficiency of your flights.
However, there are also some considerations to keep in mind:
- Weight and Balance: The added weight of the long-range tanks and fuel can affect the aircraft's weight and balance characteristics. Ensure that you carefully consider the impact on performance, climb rates, and take-off and landing distances.
- Cost: Installing long-range tanks can be a significant expense, with parts and installation costs potentially running into thousands of dollars. It is important to carefully consider the costs involved before proceeding.
- Aircraft Configuration: Long-range tanks may not be suitable for all Cessna 175 configurations. Some installations may require removing seats or compromising baggage space to accommodate the larger tanks.
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Use the both setting to draw fuel from both tanks
The Cessna 172S has two fuel tanks, with one tank mounted in each wing. Each tank holds 28 gallons, for a total fuel capacity of 56 gallons, though only 53 gallons of fuel is usable. The lines that draw fuel to the engine are slightly above the bottom of the tank to avoid picking up contaminants, making the fuel below the lines unusable. The Cessna 172's fuel selector valve can be placed in a "both" position, in addition to the "Left" and "Right" settings. This allows fuel to flow from both the left and right tanks simultaneously.
The fuel selector valve should be in the "BOTH" position for take-off, climb, landing, and maneuvers that involve prolonged slips or skids. During cruising flight, the "BOTH" setting may result in unequal fuel flow from each tank if the wings are not maintained exactly level. This can be corrected by gradually turning the selector valve handle to the "heavy" wing, or by switching the selector to the opposite side to balance the draw and avoid draining one tank.
Uneven fuel flow can also be caused by blocked or mispositioned vent tubes, which pressurize the tank and prevent proper venting. It is recommended to perform the air bubble in the water" test to ensure the vent line is open and air can flow both ways. If the vent tubes are not in the dead air zone behind the strut, the tank will pressurize and fuel will flow out unevenly.
Digital fuel level sensing can help ensure accurate fuel readings. Calibration is important to ensure that gauges are accurate throughout their entire range.
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Avoid running on one tank when low on fuel
It is advisable to avoid running on one tank when low on fuel in a Cessna 175. This is because, when the same amount of remaining fuel is in either a 170 or 175 tank, it is spread out over a larger area and is not as "deep" in the 175 tank. This makes the fuel less reliably available to the tank outlets, which can result in a sputtering and dying engine.
The Cessna 172S, for example, has two fuel tanks, with one tank mounted in each wing. Each tank holds 28 gallons, for a total fuel capacity of 56 gallons. However, only 53 gallons of fuel is usable. This is because the lines that draw fuel to the engine are slightly above the bottom of the tank to avoid picking up contaminants, making the fuel below it unusable.
In addition, uneven fuel levels can occur in a Cessna 172 due to several factors. One possible reason is pilot error, where the fuel selector is mispositioned, resulting in restricted fuel flow to one tank. Another factor could be the position of the fuel vent, which should be set according to the specific measurements in the Cessna Service manual.
To address uneven fuel levels and maximize usable fuel, it is recommended to switch to the fuller tank briefly during flight. This can be done by monitoring the fuel gauges and ensuring they are accurate. Digital fuel level sensing systems, such as those offered by CiES, can provide more accurate readings than analog gauges.
Furthermore, the Cessna 172's fuel selector valve can be placed in a ""both" position, allowing fuel to flow from both tanks simultaneously. This can help ensure that fuel is evenly distributed and maximize the usable fuel in the tanks.
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