Auxiliary Fuel Pump: What Every Pilot and Aircraft Owner Must Know
An auxiliary fuel pump is your aircraft's silent guardian—an electrically powered backup system that ensures your engine receives fuel when the primary engine-driven pump cannot do the job. Whether you're a seasoned pilot, an aircraft mechanic, or a private owner looking to understand your plane better, knowing how this critical component works, when to use it, and how to maintain it can mean the difference between a routine flight and an emergency landing. In aviation, redundancy isn't just a luxury—it's a lifeline, and the auxiliary fuel pump is one of the most important backup systems you have onboard.
What Exactly Is an Auxiliary Fuel Pump?
An auxiliary fuel pump, also commonly called a boost pump, is a secondary fuel pump installed in most aircraft with fuel-injected engines. Unlike the primary engine-driven fuel pump, which only operates when the engine is running, the auxiliary fuel pump is typically powered by electricity and can be activated at any time by the pilot or automatically by onboard systems. Its primary job is to provide supplementary fuel pumping, ensuring the engine never runs dry even under demanding conditions.
Most airplanes don't rely on a single fuel pump. Instead, they have two: a primary engine-driven fuel pump and an auxiliary fuel pump. If the engine-driven pump fails or is otherwise unable to transfer fuel to the engines, the auxiliary pump takes over immediately. This dual-pump setup is not optional—it's a fundamental safety feature required by aviation regulations for most certified aircraft.
How an Auxiliary Fuel Pump Works
The most common type of auxiliary fuel pump is the electric centrifugal pump, which features rotating impellers that push fuel from the tank to the engine. However, there are several different designs depending on the aircraft type and manufacturer:
1. Centrifugal Fuel Pumps
Typically found in larger aircraft, these pumps use high-speed rotating impellers to create a continuous flow of fuel. They are often submerged directly in the fuel tank, which helps prevent vapor lock by maintaining positive pressure throughout the fuel system. This is the most effective design for preventing fuel vaporization issues at high altitudes.
2. Vane-Type Fuel Pumps
These pumps use spinning vanes inside an eccentric rotor to force fuel through hoses and toward the engine. Vane pumps are constant displacement pumps—they move a fixed volume of fuel with each revolution. To prevent over-pressurization, they include an adjustable pressure relief valve that recirculates excess fuel back to the inlet side. When used as an auxiliary pump, an electric motor rotates the pump shaft.
3. Diaphragm Fuel Pumps
These pumps feature a flexible diaphragm that opens and closes to draw fuel in and push it out. While less common in modern aircraft, they are still found in some older designs and light sport aircraft.
4. Plunger-Type Pumps
Some specific models, like the Bendix 476410 used in Subaru FA-200-160 aircraft, use a plunger mechanism with an electro-magnet and spring. The plunger moves back and forth to suck and pressurize fuel, feeding it to the carburetor. These pumps include a pressure relief mechanism that reduces fuel pressure to zero when the ignition is turned off.
Why Every Aircraft Needs an Auxiliary Fuel Pump
You might wonder: if the aircraft already has an engine-driven fuel pump, why add another one? The answer comes down to three critical reasons:
Redundancy and Safety
Aircraft components can fail—and fuel pumps are no exception. Having a backup pump means that even if the primary pump fails, the engine continues receiving fuel. This redundancy is not just a convenience; it's a fundamental safety requirement. According to Aviation Safety reports, most fuel-system accidents result from poor planning or failure to understand how the aircraft's engine is supplied. A functional auxiliary fuel pump eliminates one of the most common single-point failure risks.
Engine Starting Assistance
Engine-driven fuel pumps only work when the engine is running. So how do you get the engine started in the first place? You use the auxiliary fuel pump to prime the system. During startup, the auxiliary pump provides the initial fuel pressure needed to get the engine firing. This is especially important for fuel-injected engines, which require higher fuel pressure than carbureted engines.
Vapor Lock Prevention at High Altitudes
At high altitudes, fuel can vaporize inside the fuel lines, creating bubbles that block fuel flow—a condition known as vapor lock. The auxiliary fuel pump maintains consistent fuel pressure to keep fuel in liquid form, preventing this dangerous situation. When you climb to higher altitudes, turning on the boost pump is standard procedure to maintain fuel pressure.
When Should You Use the Auxiliary Fuel Pump?
Most aircraft checklists require pilots to switch on the auxiliary fuel pump during critical phases of flight. Here are the specific situations when you should activate it:
Engine Start
As mentioned earlier, the auxiliary pump provides the initial fuel pressure needed to start the engine. Most pilots turn it on before engaging the starter.
Takeoff and Landing
These are the most critical phases of flight, where engine failure would have the most serious consequences. Having the auxiliary pump running ensures that even if the main pump fails, the engine keeps running. Many pilots keep the boost pump on during takeoff and landing as a standard safety practice.
Aerobatic Maneuvers
During acrobatic flight, the aircraft experiences rapid changes in attitude and G-forces that can cause fuel to slosh away from the fuel pickup. The auxiliary pump ensures a steady fuel supply regardless of the aircraft's orientation.
Emergency Situations
If you experience a sudden drop in fuel pressure, an engine sputter, or any indication of main pump failure, immediately turning on the auxiliary fuel pump can keep the engine running while you troubleshoot the problem.
High-Altitude Flight
At altitudes above 10,000 feet, the risk of vapor lock increases significantly. Using the auxiliary pump maintains positive fuel pressure and prevents fuel vaporization issues.
Common Types of Auxiliary Fuel Pumps in Aviation
Different aircraft manufacturers use different pump designs. Here are some common examples:
Bendix 476410 (Used in Subaru FA-200-160)
This plunger-type pump operates on 12 VDC and includes a filter mechanism with a magnet in the lower part of the pump. The electrical contacts are sealed in a container with inert gas to extend service life. It also features a pressure relief mechanism that drops fuel pressure to zero when the ignition is turned off.
Duke Astronautics Pumps (Used in Subaru FA-200-180)
These pumps operate on 14 VDC and deliver 35 GPH at 23 to 24 PSI. They include built-in bypass and check valves to prevent reverse fuel flow. Two drain holes in the end bell prevent fuel from entering the motor due to internal leakage, with one drain tube routed outboard.
Dukes, Weldon, and Airborne Electric Fuel Boost Pumps
These are common aftermarket and OEM replacement pumps found in many general aviation aircraft. They require regular maintenance and are typically overhauled or replaced every 10 years or at engine overhaul.
Maintenance Tips for Your Auxiliary Fuel Pump
Keeping your auxiliary fuel pump in good working condition is essential for flight safety. Here are practical maintenance tips based on industry best practices:
Regular Inspections
Check your fuel pump frequently for leaks, damage, and proper operation. Look for signs of wear and tear, contaminants in the fuel system, and any fuel odor that might indicate a leak. Inspect the fuel tank area for damage or leaks as well.
Scheduled Overhaul
According to Aircraft Accessories International, it is good practice to overhaul or replace your fuel boost pump every 10 years or at engine overhaul, whichever comes first. This prevents unexpected failures and ensures the pump operates within specifications.
Monitor for Common Issues
Keep an eye out for these warning signs:
- Unusual noise from the pump area
- Fluctuating fuel pressure readings
- Difficulty starting the engine
- Engine sputtering during critical flight phases
- Fuel leaks around the pump mounting area
Fuel System Best Practices
Follow these recommendations from AOPA to maintain your entire fuel system:
- Be present every time your aircraft is fueled—state the fuel grade, quantity, and tanks to be filled
- Check for water or contaminants in the tanks and verify proper fuel grade
- Secure fuel caps after checking the tank
- Determine available fuel in hours and minutes instead of gallons and pounds
- Lean the mixture to achieve best performance, range, and endurance
Troubleshooting Common Auxiliary Fuel Pump Problems
If you suspect your auxiliary fuel pump is failing, here are common issues and what they mean:
Pump Runs but No Fuel Flow
This could indicate a clogged filter, a stuck check valve, or a failed impeller. Check the filter first—it's the most common cause of reduced flow. The Bendix pump, for example, has a filter and magnet in the lower part of the pump that can collect debris.
Pump Makes Noise but Pressure Is Low
This might be a sign of cavitation—the pump is trying to move fuel but air is getting into the system. Check for loose connections or cracks in the fuel lines on the inlet side of the pump.
Pump Doesn't Run at All
First, check the circuit breaker and electrical connections. Many auxiliary pumps, like the Duke Astronautics models, have specific voltage requirements (14 VDC) and will not operate if voltage drops too low. If electrical power is confirmed, the pump motor may have failed and requires replacement.
Fuel Leaks at the Pump
Leaks can occur at fittings, seal surfaces, or drain holes. Some pumps have drain holes specifically designed to vent any fuel that leaks past internal seals. If you see fuel coming from these drain holes, it indicates internal seal wear and the pump needs service.
Why Proper Fuel System Management Matters
According to Aviation Safety reports, most fuel-system accidents are preventable. The key is understanding how your aircraft's engine is supplied with fuel and being vigilant about maintenance. A well-maintained fuel system, including a functional auxiliary pump, is your best defense against fuel-related incidents.
AOPA emphasizes that if you know your fuel system, verify you have the right type and quantity of fuel, lean your engine properly, and stay alert to changing conditions, the odds of a fuel-related incident are virtually nonexistent. This is not just good advice—it's a proven safety strategy.
Upgrade Your Aircraft with a KEMSO Fuel Pump
When it comes to reliability and performance, not all fuel pumps are created equal. If you're looking for a high-performance auxiliary fuel pump that you can trust, consider upgrading to a KEMSO brand fuel pump. KEMSO designs pumps that deliver significantly higher flow rates compared to stock options, ensuring your engine never starves for fuel—even at redline.
KEMSO pumps use aerospace-grade billet aluminum housings and reinforced polymer internals, tested to last over 50,000 miles under extreme heat and vibration. Whether you're flying a vintage aircraft or a modern high-performance model, KEMSO offers plug-and-play designs that cut installation time dramatically. Their variable-speed technology adjusts flow based on engine demand, improving efficiency and reliability.
KEMSO's Promise, Backed by a Lifetime of Confidence
KEMSO stands behind their products with a lifetime warranty—a level of confidence that speaks to the quality of their engineering. Their customer support team, staffed by experienced engineers, helps troubleshoot issues in real time. When you choose KEMSO, you're not just buying a fuel pump; you're investing in peace of mind for every flight.
For more information about KEMSO fuel pumps and to find the right model for your aircraft, visit their website at https://www.kemsoracing.com/. They cover over 200 models with compatibility for both aircraft and high-performance motorcycles, ensuring you get the exact fit you need.
Conclusion
The auxiliary fuel pump is one of the most important safety systems in your aircraft. It provides redundancy when the primary pump fails, assists with engine starting, prevents vapor lock at altitude, and ensures consistent fuel delivery during critical phases of flight. Understanding how it works, when to use it, and how to maintain it is essential knowledge for every pilot and aircraft owner.
Remember: the auxiliary fuel pump is not an optional accessory—it is a critical component of your aircraft's fuel system. Treat its maintenance with the same seriousness you would any other safety system. Regular inspections, scheduled overhauls, and prompt attention to warning signs will keep this vital system ready when you need it most.
And if you're in the market for a replacement or upgrade, KEMSO fuel pumps offer the performance, reliability, and lifetime warranty that serious aviators demand. Visit https://www.kemsoracing.com/ to explore their product line and find the perfect fuel pump for your aircraft.