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 speed displayed on the airspeed indicator or flight display. It exists on both conventional aircraft and aircraft equipped with an Air Data Computer.

The airspeed system responds to the difference between total pressure and static pressure:

$q_c = p_t - p_s$

This is the impact pressure. At low Mach numbers, it can be approximated by the incompressible dynamic-pressure relationship:

$q \approx \frac{1}{2}\rho V^2$

At higher Mach numbers, air compressibility must also be considered.

Main sources of error or correction

• Instrument error: Caused by imperfections, calibration tolerances or sensor errors in the indication system.
• Position error: Caused mainly by the static source sensing a pressure that differs from the undisturbed ambient pressure. It varies with aircraft configuration, angle of attack and speed.
• Compressibility effect: As Mach number and altitude increase in subsonic flight, compressibility causes CAS progressively to over-read relative to EAS. The correction from CAS to EAS therefore normally reduces the indicated value.
• Density effect: This is not an instrument error. It explains why true airspeed differs from equivalent airspeed when air density differs from standard sea-level density.

Airspeed correction chain

• CAS — Calibrated Airspeed: IAS corrected for instrument and position errors.
• EAS — Equivalent Airspeed: CAS corrected for compressibility.
• TAS — True Airspeed: EAS corrected for air density.

In simplified form:

IAS → instrument and position corrections → CAS → compressibility correction → EAS → density correction → TAS

IAS or CAS is primarily used for aircraft handling and operational speed limits, EAS is useful for aerodynamic loads and performance, and TAS represents the aircraft’s speed relative to the surrounding air mass.

On modern aircraft, the Air Data Computer uses pitot-static information, temperature inputs and stored calibration data to calculate the required airspeed parameters. The exact parameter displayed and the corrections applied remain aircraft-specific.

Pitot or static blockages, leaks and invalid sensor data are system failures rather than normal corrections in the IAS–CAS–EAS–TAS chain.

Reference: FAA Instrument Flying Handbook — airspeed definitions.

A pressure altimeter does not measure geometric altitude directly. It measures static pressure and converts that pressure into an altitude using the pressure–altitude relationship of the ICAO Standard Atmosphere.

• The aircraft’s static ports sense ambient static pressure.
• In a mechanical altimeter, sealed aneroid capsules expand as pressure decreases and contract as pressure increases.
• In a modern system, an Air Data Computer performs the pressure-to-altitude conversion electronically.
• The pressure selected on the altimeter sub-scale determines the reference datum.

Common pressure settings

• QNH: The altimeter indicates altitude relative to mean sea level and should indicate aerodrome elevation when on the ground, subject to normal system tolerances.
• STD — 1013.25 hPa: The instrument indicates pressure altitude, used to express flight levels.
• QFE: The altimeter indicates height above the QFE reference datum.

The pressure-to-altitude relationship is not linear. The commonly used value of approximately 30 ft per hPa is an ATPL rule of thumb, not the exact calculation performed by the instrument.

Example

With:

• Reference pressure: 1013.25 hPa
• Sensed static pressure: 713 hPa

The rough calculation gives:

$(1013 - 713) \times 30 \approx 9,000\ \text{ft}$

This is acceptable as a simplified mental estimate in an ATPL-style exercise. However, applying the ICAO standard-atmosphere pressure relationship gives a pressure altitude of approximately 9,400 ft.

The difference demonstrates why multiplying a large pressure difference by 30 ft/hPa must not be presented as the exact altimeter calculation.

Temperature note: A non-standard temperature does not change the altimeter’s internal pressure-altitude conversion. It changes the relationship between indicated altitude and true geometric altitude. In colder-than-standard air, true altitude is lower than indicated altitude.

References: ICAO Doc 7488, Manual of the ICAO Standard Atmosphere; EASA ATPL altimetry learning objectives; FAA Pilot’s Handbook of Aeronautical Knowledge — altimeter operation.

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.