Aircraft General Knowledge

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).

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.

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.

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

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 V², 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

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.

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.