Answers - Engine Fuel Systems | Aircraft Systems

Answers - Engine Fuel Systems

1. b
When an aircraft's boost pump pressure exceeds that of the primary fuel pressure pump, a bypass valve in the pressure pump opens and allows fuel to flow directly to the engine. This occurs during start, when the pressure pump is not operating, and any time the engine driven fuel pump becomes clogged or fails.

2. c
A positive displacement pump provides a fixed quantity of fuel per pump revolution. With a centrifugal-type fuel boost pump, once the pressure builds to a predetermined pressure, fuel bypasses the impeller and remains in the fuel tank. Therefore, the pump does not continually displace fuel and is not a positive displacement pump.

3. a
According to the 14 CFR 23.995, the engine fuel shutoff valve may not be located on the engine side of the firewall.

4. c

5. b
A fuel transfer ejector helps transfer fuel from the main tank to the boost pump sump by creating a low pressure area at the fuel pick-up point. By pumping fuel past the venturi in the injector, a low pressure area is created that drains fuel into the line that feeds the boost pump sump.

6. b
When an aircraft's boost pump pressure is greater than that of the main pressure pump, a bypass valve in the pressure pump opens and allows fuel to flow directly to the engine. This prevents a damaged or inoperative pump from blocking fuel flow to the engine.

7. a
The purpose of the engine-driven fuel pump is to deliver a continuous supply of fuel at the proper pressure at all times during engine operation. Therefore, a positive displacement pump must be used. One type of positive displacement pump that is widely used is the rotary-vane-type fuel pump.

8. c
One of the many purposes of a boost pump is to supply fuel under pressure for priming prior to starting an engine.

9. b
Engine-driven fuel pumps typically discharge more fuel than an engine requires. Therefore, fuel systems must incorporate a relief valve to prevent the build up of excessive fuel pressures at the carburetor. A typical fuel pump relief valve is spring-loaded and, when opened, allows excess fuel to flow back to the inlet side of the fuel pump.

10. c

11. c
In a compensated vane-type fuel pump, the fuel pressure delivered to the carburetor inlet varies with altitude and atmospheric pressure. This js done by allowing spring tension and either atmospheric or carburetor inlet air pressure to act on a diaphragm which controls the pump's relief valve. As the amount of pressure acting on the diaphragm varies, the pressure at which the relief valve bypasses fuel back to the pump's inlet varies.

12. b
There are over 100 different varieties of microorganisms which can live in the free water which accumulates in the sumps of aircraft fuel tanks. Because they thrive in water, the best way to prevent their growth is to eliminate the water through proper fuel handling procedures.

13. a
To reduce the possibility of vapor lock, fuel lines are kept away from heat sources, are sometimes wrapped with insulation, and are installed in a smooth, flowing manner with no sharp bends.

14. b
According to 14 CFR 23.1189, aircraft that are certified in the normal category must have a positive means of shutting off the fuel to all engines.

15. b
The main fuel strainer in an aircraft's fuel system is installed so that fuel flows through it before reaching the engine-driven pump. Furthermore, the strainer is typically located at the lowest point in the fuel system so that water and other debris can collect in the strainer, where they can be drained from the system.

16. a
Every effort should be made to physically separate electric wiring and lines carrying flammable fluids. However, when separation is impractical, electrical wire should be located above the flammable fluid line and both the electrical line and fluid line should be securely clamped to structure.

17. a
The swirling action of a centrifugal boost pump impeller separates air and vapor from fuel before it enters the fuel line to the carburetor. By removing air and vapor, the possibility of vapor lock greatly decreases.

18. c
According to 14 CFR 23.955, the fuel flow rate of a gravity-fed system must be at least 150 percent of the takeoff fuel consumption of the engine.

19. a
The primary function of a fuel boost pump is to supply a positive flow of fuel to the engine-driven fuel pump. Secondary functions include supplying positive fuel flow to the engine when the engine-driven pump fails, and transferring fuel.

20. b
During normal operation, the fuel pump delivers more fuel than the engine needs and, therefore, a fuel pump relief valve must be used to prevent excessive pressures from reaching the carburetor inlet. The relief valve is held closed by spring tension and, once opened by excessive fuel pressure, directs fuel back to the pump inlet. If a relief valve sticks closed when an engine accelerates, excessive fuel pressure will build until the relief valve opens. Therefore, a sticking fuel pump relief valve will cause fluctuating fuel pressure readings and, if the relief valve sticks enough, excessive pressure readings.

21. b
According to FAR 23.997, there must be a fuel strainer or filter between the fuel tank outlet and the engine-driven pump or fuel metering device.

22. c
Fuel pump relief valves designed to compensate for atmospheric pressure variations operate on the principle that, within the valve, a balance between fuel pressure and atmospheric or carburetor inlet air pressure is maintained. Thus, these valves are often referred to as balanced-type relief valves.

23. b

24. c

25. c
When a carburetor's throttle valve is opened quickly, a large volume of air rushes into the carburetor. To prevent an excessively lean mixture from developing when this occurs, float-type carburetors employ an accelerator pump that injects a momentary burst of fuel into the engine to maintain the proper fuel/air mixture. If an accelerator pump does not operate properly, an excessively lean mixture that can lead to backfiring and missing will result when the throttle is advanced rapidly.

26. b
Since an engine-driven vane-type fuel pump discharges more fuel than an engine requires, there must be some way of routing excess fuel away from the carburetor inlet line to prevent excess pressure from building. This is accomplished by using a spring loaded relief valve that is adjusted to deliver fuel at the recommended pressure.

27. a
Rotary-vane pumps used as main fuel supply pumps provide a fixed quantity of fuel per pump revolution. Therefore, rotary-vane pumps are positive displacement pumps.

28. b

29. b
Aviation gasoline (100LL) has a heat energy of 112,320 BTUs per gallon while kerosene (Jet A fuel) has a heat energy of 123,541 BTUs per gallon. In addition, kerosene's high viscosity allows it to act as a lubricant in pumps and fuel control units.

30. b

31. a
Because of the large changes in airflow associated with changes in power settings, turbine engines do not respond well to rapid power changes. For example, too rapid an acceleration or deceleration could cause a compressor stall, which could lead to a rich blowout or lean dieout.

32. a
Of the filters listed, the micron filter has the greatest filtering action. Micron filters can be made of cloth or paper and remove foreign matter measuring from 10 to 25 microns which equates to particles between .000,010 and .000,025 inch in size.

33. b
The flow divider in a duplex fuel nozzle divides the fuel supply into a primary and secondary flow that discharge through separate, concentric spray tips. Primary fuel flows at all power settings, while secondary fuel flows only when fuel pressure builds enough to unseat the flow divider.

34. a
The flow divider in a turbine engine duplex fuel nozzle opens when the fuel pressure reaches approximately 90 psig. Once open, fuel is directed into the secondary chamber in the fuel nozzle and then discharges through the secondary tip into the combustion liner.

35. a
Although float carburetors typically have no required overhaul period, good operating practice dictates that a carburetor be completely overhauled when the engine is overhauled. Operating beyond this point can result in poor fuel metering, which could lead to detonation and subsequent damage to a freshly overhauled engine.

36. c
A carburetor manufacturer's recommendations are FAA approved and represent the final authority for maintaining a carburetor. Only those alterations or repairs that are in the manufacturer's maintenance materials should be made.

37. b
Excessively rich or lean idle mixtures typically result in incomplete combustion. For example, with an excessively rich mixture carbon deposits form on the spark plugs and cause subsequent plug fouling. Excessively lean mixtures, on the other hand, can burn so slowly that combustion can carry into the intake stroke and cause backfiring.

38. a
Throttle and mixture controls must operate freely throughout their full range of travel, and the stops on the carburetor must be contacted before the cockpit control reaches its stop. Springback in the control system ensures that the carburetor control is fully actuated.

39. c
The use of thread lubricant on a float bowl plug helps prevent thread damage and provide a better seal. However, it is important that no thread lubricant be allowed to enter the carburetor bowl since the lubricant is insoluble and, therefore, can plug the fuel jets. When applying a thread lubricant, begin by screwing the plug one turn into the float bowl and applying lubricant to the second thread. This helps ensure that no lubricant enters the carburetor interior.

40. b