Free CPL Aircraft General Knowledge Questions and Answers 1 — Questions and Answers
Question 1: The valves in a four-stroke engine open and close:
- precisely on TDC and BDC
- exactly 55° after TDC and BDC
- exactly 15° after TDC and BDC
- none of these (Correct answer)
Correct answer: none of these
Explanation: <br> The exhaust closes around 15° after TDC. <br> The exhaust opens around 55° before BDC. <br> The inlet opens around 15° before TDC. <br> The inlet closes around 60° after BDC.
Question 2: Detonation in AVGAS is reduced by fuel companies adding certain compounds. Fuel then has an octane rating. Typical blue AVGAS is approximately:
- 130
- 120
- 110
- 100 (Correct answer)
Correct answer: 100
Explanation: <br> The main difference between 100LL (100 octane) and 100/130 relates to the minimum octane rating prescribed and different fuel mixtures (100 lean and 130 rich). This is the most simple explanation as to why they need different colors. Blue will always be 100LL, whereas 100/130 is green.
Question 3: When fuel enters the combustion chamber from the carby it is:
- dribbling in as a fluid
- flowing in as a gas (Correct answer)
- running as a fluid
- in a mixture of mainly liquid and some gas
Correct answer: flowing in as a gas
Explanation: <br> The fuel has been vaporized by fast-moving air and is suspended in the air as a gas.
Question 4: During a glide descent carb heat should be applied in the following manner.
- Applied prior to commencing glide
- Applied prior to commencing glide and removed just prior to re-applying power (Correct answer)
- Applied prior to commencing glide and removed after re-applying power
- Applied after commencing glide and removed just after re-applying power
Correct answer: Applied prior to commencing glide and removed just prior to re-applying power
Explanation: <br> When it comes to carb ice prevention is better than cure. Applying carb heat before closing the throttle helps prevent formation. Removing before re-application of power ensures when you apply power, you have full power available. As we know from our carb heat test during our run-ups, we are 100-150 rpm lower with carb heat applied.
Question 5: The strength or power that acts on a piston during the power stroke is dependent on:
- the air intake only
- the air and fuel intake only
- the amount of fuel intake only
- the air and fuel intake and the temperature it reaches after ignition (Correct answer)
Correct answer: the air and fuel intake and the temperature it reaches after ignition
Explanation: <br> This question sounds confusing at first but think about the orientation of the engine in the engine bay. The fuel/air mixture from the carburetor enters the engine at the top. This means that the piston will move down first (to draw in the fuel) then up (to compress the mixture), down (to get the power after ignition), and up again for the exhaust stroke.
Question 6: Detonation in aircraft engines is caused by:
- fuel below the minimum recommended octane (Correct answer)
- using the wrong viscosity oil
- fuel above the maximum recommended octane
- fuel above the minimum recommended octane
Correct answer: fuel below the minimum recommended octane
Explanation: <br> High octane fuel burns slower with a more controlled and precise burn, which produces more power. Low-octane fuel burns rapidly and can explode under pressure. Fuel with lower than specified octane or old fuel that has lost octane can cause detonation on the compression stroke which may cause engine failure and/or serious damage.
Question 7: Gas-filled pipes routed around abnormal heat sources using linear pneumatic sensing systems are:
- heat loops for detecting fire in normal locations
- heat loops for detecting heat ranges in normal locations
- fire loops for detecting fires or heat near fuel sources
- fire loops for detecting fires or heat in abnormal locations (Correct answer)
Correct answer: fire loops for detecting fires or heat in abnormal locations
Explanation: <br> Detection uses a linear pneumatic sensing system, known as fire loops. These are gas-filled pipes routed around abnormal heat sources. If the temperature in the vicinity of the sensor pipes rises, the electrical resistance in the core material decreases. This decrease in electrical resistance is triggered by the warning system. They use duplicated loops to allow for continued detection if a single loop is faulty or has been activated. Damage to a loop will cause a false warning. Always check them before the flight.
Question 8: A typical piston aircraft engine would have a thermal efficiency of approximately:
- 30% (Correct answer)
- 10%
- 75%
- 50%
Correct answer: 30%
Explanation: <br> This means 70% of all energy produced is lost as heat, friction, or exhaust gasses.
Question 9: The volumetric efficiency of an engine is best when:
- cold ambient temp, high altitude, full throttle
- cold ambient temp, low altitude, half throttle
- cold ambient temp, low altitude, full throttle (Correct answer)
- hot ambient temp, high altitude, half throttle
Correct answer: cold ambient temp, low altitude, full throttle
Explanation: <br> Volumetric efficiency is essentially the amount of air that enters the cylinder before it fills up or the valve closes. This is compared to the maximum, to determine how efficient the engine is operating. At low altitudes, the air is more dense so you will be using up less space in the cylinders for a greater chunk of air. The same if for cold air. At full throttle the valves are open for a shorter period however the throttle butterfly valve allows more fuel/air mixture to pass into the cylinder. Superchargers and turbochargers are another way to increase volumetric efficiency.
Question 10: In an aircraft with CSU, applying carb heat with ice present will:
- a drop in rpm
- initially increases, then a drop in manifold pressure and fluctuating rpm
- initially increases, then a drop in manifold pressure at a constant rpm
- initially drop, then an increase in manifold pressure at a constant rpm (Correct answer)
Correct answer: initially drop, then an increase in manifold pressure at a constant rpm
Explanation: <br> On the ground with the prop fully fine, the RPM will drop for as long as carby heat is applied. With the prop fully fine, it acts the same way as a fixed pitch. The propeller cannot go any more fine once it reaches the pitch stops resulting in the drop in RPM.
Question 11: The application of carb heat should instantly make the engine run smoother.
- True
- False (Correct answer)
Correct answer: False
Applying carburetor heat initially introduces warm, less dense air which can actually cause the engine to run rougher or drop RPM momentarily—not smoother. The engine will only run smoother after any accumulated carburetor ice has melted and normal airflow is restored, which takes time. This is why the statement is false; the immediate effect is often a slight performance decrease.
Free CPL Aircraft General Knowledge Questions and Answers 1
The valves in a four-stroke engine open and close: