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Topic question bank: Aircraft General Knowledge

Questions and Answers

General AGK

What is the function of the Ground Proximity Warning System (GPWS)?

The Ground Proximity Warning System (GPWS) is designed to enhance flight safety by issuing aural and visual warnings to the pilots when there is a risk of controlled flight into terrain (CFIT) or unsafe flight configurations.

It uses radio altitude (i.e. the distance between the aircraft and the terrain directly below) to monitor for hazards such as:

  • Excessive descent rate
  • Excessive terrain closure rate
  • Altitude loss after takeoff or go-around
  • Unsafe terrain clearance with gear or flaps not in landing configuration
  • Excessive deviation below glideslope

Note: GPWS operates based on terrain directly below the aircraft, and is most effective in the vicinity of airports or known terrain environments. For improved forward-looking terrain awareness, modern aircraft are equipped with EGPWS (Enhanced GPWS).

Explain the pressurization system of a jet aircraft.

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How is inflight anti-ice protection typically provided on modern jet aircraft?

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Define reverse thrust and explain its purpose.

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Define APU and state its primary uses.

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Define spiraling slipstream.

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Describe the function of the Flight Management Computer (FMC).

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Describe the function of a radio altimeter.

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Describe the purpose of the TCAS system.

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What should a pilot do if a TCAS resolution advisory conflicts with an ATC instruction?

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What is the function of the Enhanced Ground Proximity Warning System (EGPWS)?

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Aircraft Systems

Identify the type of fuel commonly used by jet aircraft.

Jet aircraft typically use kerosene-based fuel:

  • In Europe, the standard is Jet A-1, which has a fuel freezing point of -47 °C
  • In the United States, the standard is Jet A, with a fuel freezing point of -40 °C

Note: The main difference between Jet A and Jet A-1 is the freezing temperature, with Jet A-1 offering slightly better performance in cold environments.

Why is fuel stored in the wings of an aircraft?

Fuel is stored in the wings to reduce structural stress during flight.

As lift is generated, the wings tend to flex upward. By distributing fuel in the wings, the weight of the fuel provides a downward force, which counteracts this upward bending.

This reduces bending moments at the wing root and improves the structural efficiency and fatigue life of the wings.
Additionally, it helps optimize the aircraft's center of gravity and frees up space in the fuselage for passengers or cargo.

What types of electrical power are typically used on commercial aircraft?

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What are the primary benefits of using AC power instead of DC power on jet aircraft?

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Explain why batteries are installed on aircraft.

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List the typical sources of AC and DC electrical power on a jet aircraft.

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How do you calculate the fuel weight uplifted from litres and specific gravity?

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Describe the function of a crossfeed valve in an aircraft fuel system.

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How to balance fuel between two main tanks when one shows more than the other?

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Why are jet aircraft hydraulically powered?

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What fundamental physical principle allows aircraft hydraulic systems to operate?

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Instrumentation

List the flight instruments that depend on the pitot-static system.

The following flight instruments operate using data from the pitot-static system:

  • Airspeed Indicator (ASI) – calculates airspeed using both pitot (dynamic) and static pressure
  • Machmeter – determines Mach number using pitot and static pressure
  • Altimeter – measures altitude based solely on static pressure
  • Vertical Speed Indicator (VSI) – indicates rate of climb or descent based on changes in static pressure

Note:
- The pitot tube provides dynamic pressure, essential for speed-related instruments
- The static ports supply ambient pressure used in altitude and vertical speed calculations

List and explain the typical errors affecting Indicated Airspeed (IAS).

Indicated Airspeed (IAS) is the airspeed shown on the Airspeed Indicator (ASI) of aircraft without an Air Data Computer. It is derived from the dynamic pressure of the airflow using the formula:
q = ½ ρ V^{2}, where V = TAS and ρ is air density.

IAS is affected by several types of errors:

  • Instrument Error: Caused by mechanical imperfections or calibration issues in the ASI system.
  • Position Error: Results from disturbed airflow around the pitot-static system.
  • Compressibility Error: At higher speeds and altitudes, the compressibility of air affects pressure readings. This error becomes significant above approximately 200 kt and is always negative, meaning it causes CAS > EAS.
  • Density Error: Due to variations in air density with altitude and temperature, this error affects the relationship between Equivalent Airspeed (EAS) and True Airspeed (TAS).

Airspeed Correction Chain:

  • CAS (Calibrated Airspeed) = IAS corrected for position and instrument errors.
  • EAS (Equivalent Airspeed) = CAS corrected for compressibility error.
  • TAS (True Airspeed) = EAS corrected for density error.

This correction chain is important because only TAS represents the aircraft’s actual speed relative to the surrounding air mass, which is used for navigation and performance calculations.

Note: On modern aircraft, the **Air Data Computer

Explain how altitude is calculated by the aircraft instruments.

Altitude is determined by comparing the static pressure sensed at the aircraft’s current altitude with a reference pressure set on the altimeter sub-scale (e.g. QNH for altitude above MSL or STD for flight levels).

  • The static ports measure the ambient air pressure around the aircraft.
  • The altimeter uses the difference between the sensed static pressure and the reference pressure to compute altitude.
  • This calculation assumes a standard pressure lapse rate of approximately 30 ft per hPa.

Example:
If the static pressure at the aircraft’s current altitude is 713 hPa, and the reference pressure set on the altimeter is 1013 hPa, the pressure difference is 300 hPa.
Using the standard lapse rate:
300 × 30 ft = 9000 ft
So, the altimeter will display an altitude of 9000 ft.

Note: This method assumes ISA conditions. In reality, temperature deviations from ISA can cause altimeter errors.

Explain how Indicated Airspeed (IAS) is determined.

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Describe how a sudden increase in headwind during approach affects Indicated Airspeed (IAS).

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Explain how a blocked pitot probe affects the airspeed indication.

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Describe the effect of a blocked static port on the airspeed indicator.

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Clarify whether a blocked pitot probe affects the altimeter.

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Explain the effect of a blocked static port on the altimeter.

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Explain what happens to the VSI if the pitot probe becomes blocked.

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Describe the effect of a blocked static port on the VSI.

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Explain the difference between True Airspeed (TAS) and Groundspeed (GS).

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Explain how True Airspeed (TAS) changes during a climb at constant Mach number.

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How does True Airspeed (TAS) vary during a descent at constant Mach number?

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How does Mach number vary during a climb at constant True Airspeed (TAS)?

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How does Mach number vary during a descent at constant True Airspeed (TAS)?

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How does Indicated Airspeed (IAS) change during a climb at constant True Airspeed (TAS)?

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How does Indicated Airspeed (IAS) change during a climb at constant True Airspeed (TAS)?

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Explain how Mach number is calculated.

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Describe how vertical speed is measured on an aircraft.

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Explain how IAS changes during a climb at constant Mach number.

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Describe how IAS changes during a descent at constant Mach number.

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Explain how TAS changes during a climb at constant IAS.

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Explain how TAS changes during a descent at constant IAS.

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Explain how Mach number changes during a climb at constant IAS.

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Explain how Mach number changes during a descent at constant IAS.

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If one aircraft is flying at FL360 and another aircraft is flying at FL320, and both are maintaining Mach 0.78, which aircraft has the higher TAS?

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Can aircraft weather radar detect clouds?

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What are the effects of vacuum system failure on gyroscopic instruments?

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What causes drift in a heading indicator and how is it corrected?

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What is a laser gyroscope (gyrolaser) and how does it work?

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Powerplant

Define high bypass ratio turbofan engines and explain their advantages.

High bypass ratio turbofan engines are jet engines in which a large portion of air is directed around the engine core rather than through it. The bypass ratio refers to the ratio of the mass of air that bypasses the core to the mass of air that passes through the core. For example, a bypass ratio of 12:1 means that 12 times more air flows around the core than through it.

Advantages of high bypass ratio engines:

  • Improved fuel efficiency
    A larger volume of air bypasses the core and contributes to thrust without undergoing combustion.
    This results in more thrust per unit of fuel, reducing thrust specific fuel consumption (TSFC).

  • Lower noise emissions
    Because the bypass airflow moves slower than the hot core exhaust, it reduces the exhaust jet velocity.
    This slower exhaust flow mixes with the faster core exhaust, dampening engine noise significantly.

Additional info:

  • The A320neo features engines with bypass ratios of 11:1 (CFM LEAP-1A) and 12.5:1 (PW1100G).
  • The B737 MAX is powered by CFM LEAP-1B engines with a bypass ratio of 9:1.
Describe the working principle of a jet engine.

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Explain how a four-stroke piston engine operates.

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Define twin-spool turbofan engine.

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