Answers - Induction and Engine Airflow Systems | Aircraft Systems

Answers - Induction and Engine Airflow Systems

1. a
Induction system ice forms in float and pressure carburetors when water condenses out of the air in a carburetor's venturi and freezes. Therefore, the easiest way to prevent icing is to heat the intake air to the point where ice cannot form.

2. a
Normally aspirated engines using float-type carburetors are most susceptible to icing when operated in temperatures between 30 and 40 degrees Fahrenheit. The reason for this is that any time the air being brought into the carburetor is near freezing, the additional temperature drop created by the carburetor venture can readily cause water vapor to condense and freeze. However, it is important to note that carburetor icing can occur when the outside temperature is as high as 70 degrees Fahrenheit.

3. b
In addition to a carburetor heat system, some large reciprocating engines utilize an alcohol deicing system. This system allows the pilot to spray alcohol into the inlet of the carburetor to remove ice and assist the warm air in keeping the carburetor free of ice.

4. c
On engines equipped with constant speed propellers, the engine's power output is indicated on the manifold pressure gauge. Therefore, induction system icing is readily detected by a reduction in manifold pressure and no change in rpm.


5. c
When the air temperature is above freezing and there is no visible moisture, a carburetor will be the first part of an aircraft to accumulate ice. The reason for this is that as fuel vaporizes and the air pressure drops in the venturi, the air temperature typically drops enough to cause water vapor to condense and freeze.

6. c
Heating the air in the inlet duct and spraying alcohol in the carburetor inlet are the two primary methods of eliminating carburetor ice.

7. b
Because fuel injection systems inject fuel directly into an engine's intake manifold, there is no need for a carburetor. Therefore, no carburetor air heater is required.

8. a
On engines equipped with constant speed propellers, engine power output is indicated on the manifold pressure gauge. Therefore, the onset of induction system icing is indicated by a reduction in manifold pressure. Another indication of the presence of carburetor ice is when manifold pressure increases after carburetor heat is applied.

9. c
In the induction system of an unsupercharged engine, the air pressure from the venturi to the intake valve is always less than atmospheric pressure when the engine is running. Therefore, of the options given, the highest pressure exists in the carburetor air scoop where ram air enters the induction system.

10. c
When carburetor heat is used, warm air is routed into the carburetor. Since warm air is not as dense as cool air, the application of carburetor heat when it is not needed results in a slight decrease in power and an increase in cylinder head temperature which could lead to detonation.

11. c
The pressure of a gas and its density are directly proportional to each other. In other words, as the pressure of a gas increases, its density also increases. Therefore, as an engine's manifold pressure increases, the density of the fuel/air charge going to the cylinders increases proportionally.

12. c
The volumetric efficiency of an engine is the ratio of the volume of fuel/air charge drawn into the cylinders to the engine's total piston displacement. If an engine draws in a fuel/air charge that equals the piston's displacement, volumetric efficiency is 100 percent. All unsupercharged engines have volumetric efficiencies less than 100 percent. However, by using a supercharger, it is possible to force a fuel/air charge with a greater volume than that of the cylinder displacement into a cylinder and achieve a volumetric efficiency greater than 100 percent.

13. b
Bootstrapping occurs when a turbocharger system senses small changes in temperature or rpm and continually changes the turbocharger output in an attempt to establish an equilibrium. Bootstrapping typically occurs during part-throttle operation and is characterized by a continual drift or transient increase in manifold pressure.


14. a
When carburetor heat is used, warm, less dense air is let into the engine and a drop in engine power output results. In addition, the warmer intake air causes cylinder head temperatures to increase, which can lead to detonation, especially during high-power operations. Therefore, if the carburetor heat valve is improperly adjusted and warm air is allowed to enter the engine during takeoff, less than full power will be developed.

15. c
The waste gate in a turbocharging system controls the amount of exhaust gas that is routed to the turbocharger, which ultimately dictates the amount of air that is forced into the engine. When the waste gate is closed, all the exhaust gas is routed to the turbocharger. However, if the waste gate is completely open, no exhaust gases flow to the turbocharger.

16. c
A boosted manifold pressure is any pressure that is higher than atmospheric pressure or 29.92 inches of mercury. For practical purposes, this is rounded to 30 inches of mercury.

17. b
A density controller contains a nitrogen-filled bellows that responds to changes in pressure and temperature to control the position of the waste gate and prevent an overboost condition. Therefore, the density controller in a turbocharger system limits the manifold pressure produced by the turbocharger at full throttle.

18. b
The rate-of-change controller in a turbocharger system controls the rate at which the turbocharger discharge pressure increases. If discharge pressure increases too rapidly, the controller unseats and bleeds oil pressure from the waste gate actuator, opening the waste gate and decreasing manifold pressure.

19. b
The speed of a turbocharger is most directly affected by the amount of exhaust gas entering the turbocharger. The component that controls the amount of exhaust gas that is allowed to flow through the turbine is the waste gate.

20. c
As an aircraft gains altitude, the decrease in air density causes a decrease in engine power output. One way to help maintain sea level pressure within an engine is to use a turbocharger to compress the air before it enters the engine, thereby maintaining sea level air density and manifold pressure up to the turbocharger's critical altitude.

21. b
A typical sea-level boosted turbocharger system is automatically regulated by an exhaust bypass valve assembly, a density controller, and a differential pressure controller.

22. a
A differential pressure controller senses air pressure upstream and downstream of the throttle valve and repositions the waste gate to smooth out pressure fluctuations, or bootstrapping.


23. a
In some aircraft, air for cabin pressurization is provided by the turbocharger. However, before it reaches the pressurization system it must first pass through a sonic venturi, which acts as a flow limiter. A sonic venture accelerates air to the speed of sound, thereby creating a shock wave that limits the amount of airflow into the cabin.

24. b

25. b
A bellmouth inlet has smooth, rounded surfaces that create very little resistance to air flow. Because of this, bellmouth inlets are extremely efficient.

26. c
Some aircraft utilize a variety of screens, inertia separators, or particle separators to remove foreign objects such as sand. One type of particle separator relies on sharp directional changes in airflow to keep foreign particles from entering the engine. The exact type of filter used depends on the airflow of the engine and the type of installation.

27. c
Vortex dissipators, sometimes called blow-away jets, destroy the low pressure vortex which forms between the ground and the engine inlet by blowing a stream of bleed air ahead of the engine during ground operations. By destroying this low pressure vortex, the engine is less likely to suck up and ingest foreign objects that can cause compressor blade damage.

28. a
Vortex dissipator systems, or blow-away jets, destroy the low pressure vortex which forms between the ground and the engine inlet by blowing a stream of bleed air ahead of the engine during ground operations. These systems are typically activated by a landing gear switch.

29. b
When a gas turbine engine with a divergent type inlet is operated in place on the ground, negative, or low pressure develops within its inlet because the engine inlet acts like a venturi and as the airflow accelerates through a venturi, air pressure decreases.

30. c
When an anti-ice system utilizing engine bleed air is turned on, an indicator light will illuminate in the cockpit and the EGT will rise slightly. Furthermore, both the engine pressure ratio (EPR) and compressor rpm will shift due to the momentary change in compression delivered to the combustor.

31. b
Anti-ice systems are used to prevent ice from forming, while de-ice systems remove ice that has already formed. Therefore, an engine/inlet anti-ice system is used to prevent ice formation in the engine and inlet areas. Using a turbine engine anti-ice system to de-ice the inlet could result in ice ingestion and compressor damage.

32. a
When carburetor heat is turned on, warm, less dense air is drawn into the carburetor. Therefore, if the mixture is not adjusted, the same amount of fuel continues to mix with the air. The combination of less dense air with no change in the amount of fuel supplied produces a mixture that is richer than it was before carburetor heat was applied.

33. b

34. a
When carburetor heat is turned on, warm, less dense air is drawn into the engine. Since air density is directly proportional to the weight of a given volume of air, less dense air decreases the weight of the fuel/air charge.

35. c
When carburetor heat is turned on, warm, less dense air is drawn into the carburetor. Therefore, if the mixture is not adjusted, the same amount of fuel continues to mix with the air. The combination of less dense air with no change in the amount of fuel supplied produces a mixture that is richer than it was before carburetor heat was applied.

36. b
When carburetor heat is turned on, warm, less dense air is drawn into the carburetor. However, if the mixture is not adjusted, the same amount of fuel continues to mix with the air. The combination of less dense air with no change in the amount of fuel supplied produces a mixture that is richer than it was before carburetor heat was applied. However, on carburetors equipped with an automatic mixture control (AMC) the mixture will only remain rich until the AMC can compensate.

37. a
Silicon glazing occurs when sand or dust enters an engine's induction system and proceeds to the cylinders where the sand forms a glaze on the nose of spark plugs.

38. a
There are actually two correct answers to this question. The alternate air system on most aircraft is the carburetor heat box. With this type of system, if the main air duct becomes clogged, airflow to the engine will be slowed or cut off unless carburetor heat or alternate air is selected (answer B). However, some aircraft are equipped with an alternate air system that automatically opens when the main air filter becomes sufficiently blocked (answer A). Both systems allow warm, unfiltered air from inside the cowling to be drawn into the carburetor or fuel injection unit. In our opinion, the question is referring to an alternate air system that opens automatically which, therefore, makes (A) the correct answer.

39. b
If a fire breaks out in a reciprocating engine's induction system during a start attempt, you should continue to crank the engine to try and draw the fire into the engine. If the fire does not go out, the fuel selector valve and ignition should be shut off, the mixture placed in the idle cutoff position, and a fire extinguisher used to put out the fire.

40. b
Many horizontally opposed engines have their carburetors mounted on the oil sump so the induction pipes pass through the sump and allow the fuel/air mixture to be heated to aid fuel vaporization.

41. c
In addition to supplying air to the carburetor, a typical air scoop is positioned to increase intake air pressure by utilizing the ram effect supplied by the slipstream.

42. a
The use of carburetor heat results in less dense air entering the engine. This less dense air causes a decrease in engine power and leads to increased cylinder head temperatures that can cause detonation. Therefore, carburetor heat should only be used on takeoff when absolutely necessary.


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