Aircraft General Knowledge

Pressurized fluid transmits force efficiently to high-load users such as landing gear, brakes and flight controls. Actual users and backup arrangements depend on aircraft design.

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).

A hot start means the applicable starting temperature limit is exceeded. A hung start means combustion begins but the engine fails to accelerate to its normal idle speed. The problems can occur together, but they describe different observations: temperature versus acceleration. Seeing light-off is therefore only one milestone in a successful start. Engine speed, temperature and the other required indications must continue to be monitored against the type-specific limits and sequence; recognition and corrective actions come from that aircraft’s procedures.

Scope: Turbine engines; no universal numerical start limits or checklist actions are implied.

Sources

• FAA-H-8083-32B — Aviation Maintenance Technician Handbook, Powerplant, Chapter 10 — Printed p. 10-57, Turbine Engine Operation. Accessed 2026-10-03.

An air-cycle system removes heat and extracts energy from the air. In a typical arrangement, heat exchangers reject heat to an external airflow, and an air-cycle machine includes a compressor and an expansion turbine. After cooling at high pressure, the air drives the turbine and expands, reducing its temperature substantially. The cold air is then conditioned for cabin supply. The working air itself participates in this refrigeration process; exact pack arrangements and temperature-control methods vary between aircraft.

Scope: Air-cycle air conditioning; not all aircraft environmental systems use the same architecture.

Sources

• FAA-H-8083-31B — Aviation Maintenance Technician Handbook, Airframe, Chapter 16 — Printed pp. 16-38–16-39, Refrigeration Turbine Unit or Air Cycle Machine and Secondary Heat Exchanger. Accessed 2026-10-03.

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.

Voltage is not a complete measurement of how much charge a battery can deliver. Capacity is assessed by a specified discharge rate down to a specified cutoff voltage, and usable capacity varies with factors including discharge rate and temperature. A battery can therefore need a load or capacity assessment even when a voltage reading appears acceptable. For flight planning or abnormal operation, crews use the aircraft’s approved electrical procedures and endurance information; they should not invent a remaining-time estimate from voltage alone.

Scope: Aircraft battery capacity principles; chemistry, temperature and approved load profile affect available endurance.

Sources

• FAA-H-8083-31B — Aviation Maintenance Technician Handbook, Airframe, Chapter 9 — Printed p. 9-22, Capacity; Aircraft Battery Ratings by Specification; note on load check. Accessed 2026-10-03.

The engine needs a stable flame while air keeps flowing through it. A diffuser downstream of the compressor reduces air velocity and raises static pressure before the combustion section. If air crossed the flame region too quickly, it could extinguish the flame. Combustor design also promotes mixing and flame stability. This is why the air does not simply accelerate continuously from inlet to exhaust: different engine sections deliberately change velocity to suit compression, combustion or thrust production.

Scope: Gas-turbine engine airflow; a conceptual explanation rather than detailed combustor design.

Sources

• FAA-H-8083-32B — Aviation Maintenance Technician Handbook, Powerplant, Chapter 1 — Printed p. 1-43, Diffuser; p. 1-45, swirl vanes. Accessed 2026-10-03.

Not all air entering a combustor is used immediately to burn fuel. Primary air supports combustion near the fuel nozzles, while other air cools the liner and mixes with the combustion products farther downstream. That dilution reduces the temperature delivered to the turbine to a level its components can withstand. Holes and louvers in the liner distribute these flows and help keep the flame away from its walls. The division of air is therefore essential to engine durability and controlled combustion.

Scope: Conventional combustor architecture; proportions depend on engine and combustor design.

Sources

• FAA-H-8083-32B — Aviation Maintenance Technician Handbook, Powerplant, Chapter 1 — Printed pp. 1-43–1-46, Combustion Section; Fig. 1-57. Accessed 2026-10-03.

No. The compressor supplies the principal pressure increase before combustion. The burner primarily adds thermal energy, greatly raising gas temperature. In the ideal Brayton-cycle model, heat is added at constant pressure; in a real combustor, pressure decreases slightly because of losses. The hot gas can then expand through the turbine and nozzle. Confusing temperature rise with pressure rise obscures why the engine requires a compressor and why a combustor is not equivalent to a closed, exploding cylinder.

Scope: Gas turbines; ideal constant-pressure combustion distinguished from real pressure loss.

Sources

• NASA Glenn Research Center — Combustor: Burner — Concluding paragraph: temperature rise with slight pressure decrease. Accessed 2026-10-03.
• NASA Glenn Research Center — Turbine Engine Thermodynamic Cycle: Brayton Cycle — Station 2–3 compressor and station 3–4 combustion discussion. Accessed 2026-10-03.

The rotor rows transfer mechanical energy to the airflow. The following stator rows help diffuse that flow, turning part of its velocity into pressure, and redirect it for the next rotor row. Without suitable flow direction, the next row would receive air at an inefficient incidence. A rotor and its associated stator form a compressor stage. The stationary blades are therefore working aerodynamic components, not just supports: they help build pressure efficiently through successive stages.

Scope: Conventional axial-flow compressors; detailed stage geometry varies.

Sources

• FAA-H-8083-32B — Aviation Maintenance Technician Handbook, Powerplant, Chapter 1 — Printed p. 1-40, Axial-Flow Compressor. Accessed 2026-10-03.

A surge is an instability of the engine’s compression process. When compressor airflow breaks down, high-pressure air can escape forward, producing a sudden bang, vibration and sometimes visible flames at the inlet or exhaust. That event is physically different from flammable fluid burning outside the engine core in the nacelle. A surge may recover quickly or persist, so neither the noise nor a brief flame alone establishes the engine’s condition. Crews use the complete indications and the applicable abnormal procedure.

Scope: Transport turbofan malfunction recognition; no generic thrust-lever or takeoff decision procedure.

Sources

• FAA/industry — Turbofan Engine Malfunction Recognition and Response, Final Report — Printed pp. 42–43, Chapter 4: Compressor surge. Accessed 2026-10-03.

No. A nacelle fire can burn outside the engine core while the engine continues to produce apparently normal thrust and instrument readings. Leaking fuel, oil or hydraulic fluid can ignite on hot external surfaces without immediately changing the measured gas-path parameters. Airbus therefore warns crews not to delay the applicable engine-fire procedure because the engine display looks normal. A fire alert and normal engine indications are not mutually exclusive; the alert concerns a different physical area.

Scope: Transport turbine engines; Airbus nacelle-fire examples. Apply the aircraft-specific ECAM/EICAS/QRH procedure.

Sources

• Airbus Safety First — Do not Wait to Apply the Engine Fire Procedure — Event Analysis: Nominal engine parameter despite the fire; Two Types of Engine Fire: Engine fire (nacelle fire). Accessed 2026-10-03.

The percentage is referenced to a defined nominal rotor speed, not automatically to the maximum permitted speed. One hundred percent is a calibration reference for that engine’s indication. Its approved operating limit can be higher, as specified in the engine or aircraft documentation. Pilots therefore compare the indication with the applicable limits and alerts, rather than assuming every value above 100% is an overspeed. The same percentage on different engine models also need not represent the same physical revolutions per minute.

Scope: Turbine rotor-speed percentage indications; limits and reference speeds are engine-specific.

Sources

• FAA/industry — Turbofan Engine Malfunction Recognition and Response, Final Report — Printed p. 39, Chapter 3: Rotor RPM. Accessed 2026-10-03.

Light-off confirms that combustion has begun, but it does not guarantee that the turbine can yet drive the compressor without help. At low rotational speed, the engine may not produce enough power to overcome its compressor and friction loads while continuing to accelerate. The starter therefore continues assisting until the specified self-sustaining region is reached. Removing that assistance too early can leave the engine unable to reach idle. Exact starter cutout speeds and start logic are defined for each installation.

Scope: Turbine-engine starting principle; not a manual start sequence.

Sources

• FAA-H-8083-32B — Aviation Maintenance Technician Handbook, Powerplant, Chapter 5 — Printed p. 5-8, Gas Turbine Engine Starters. Accessed 2026-10-03.

No. The FAA explicitly distinguishes fire resistance from being fireproof. Phosphate-ester hydraulic fluids used in many transport aircraft are highly fire resistant, but can burn under certain conditions. Choosing such a fluid reduces the hazard; it does not remove the need to prevent leaks or protect hot and fire-prone areas. Correct fluid specification also matters because hydraulic seals, hoses and other materials must be compatible. A safety property of the approved fluid should never be treated as immunity to fire.

Scope: Phosphate-ester hydraulic fluids in compatible transport-aircraft systems; not all hydraulic fluids are the same.

Sources

• FAA-H-8083-31B — Aviation Maintenance Technician Handbook, Airframe, Chapter 12 — Printed p. 12-3, Phosphate Ester-Based Fluid; Compatibility with Aircraft Materials. Accessed 2026-10-03.

A capacitance probe uses fixed conductive surfaces separated by fuel, air or a mixture of both. Because fuel and air have different dielectric properties, the probe’s capacitance changes as the fuel level changes. Several probes can be combined to represent a large tank, and compensation accounts for properties that change with fuel temperature or type. The system converts the electrical measurement into the calibrated quantity indication. The absence of a moving float does not mean the measurement is independent of calibration or fuel properties.

Scope: Capacitance-type aircraft fuel-quantity systems; computation and compensation depend on installation.

Sources

• FAA-H-8083-31B — Aviation Maintenance Technician Handbook, Airframe, Chapter 14 — Printed pp. 14-38–14-39, capacitance-type fuel quantity indication; Figs. 14-71–14-74. Accessed 2026-10-03.

Heat transfers from the hotter oil to the cooler fuel across the heat exchanger’s internal surfaces. The fluids follow separate passages, so useful heat exchange does not require them to mix. This removes heat collected by the oil and raises the fuel temperature. A thermostatic arrangement can regulate how much oil passes through the cooler. The FAA describes fuel-cooled oil coolers as one option; some engines also use air-oil cooling, and the precise arrangement depends on the installation.

Scope: Turbine engines equipped with fuel-oil heat exchangers; not every engine has an identical cooling layout.

Sources

• FAA-H-8083-32B — Aviation Maintenance Technician Handbook, Powerplant, Chapter 6 — Printed pp. 6-27–6-28, Fuel-Oil Coolers; Fig. 6-43. Accessed 2026-10-03.

A flow-sensitive hydraulic fuse responds to flow above its calibrated limit and closes to limit fluid loss through the affected branch. Preserving fluid can allow the rest of the hydraulic system to remain useful, subject to its architecture and the failure location. This is different from a relief valve, which responds to excessive pressure and provides a pressure-relieving path. The word fuse describes an isolation function here; it is not an electrical wire melting. Other hydraulic-fuse designs operate differently, and installation and reset behavior vary by design.

Scope: Hydraulic systems fitted with flow-sensitive fuses; no claim that every leak is isolated.

Sources

• FAA-H-8083-31B — Aviation Maintenance Technician Handbook, Airframe, Chapter 12 — Printed pp. 12-26–12-27, Hydraulic Fuses; Relief Valves. Accessed 2026-10-03.
• Parker Hannifin, Stratoflex Products Division — Hydraulic Fuses — Printed p. 4, Introduction/Definition and General Application & Purpose: flow-sensitive Type III compared with quantity-sensitive Types I and II. Accessed 2026-10-04.

Yes. Hydraulic fluid expands as it warms. If a check valve or closed selector traps that fluid in a confined section, thermal expansion can produce excessive pressure without new pump delivery. A thermal relief valve protects such isolated sections by allowing excess fluid into a return path when its setting is reached. A main system relief valve alone may not protect fluid isolated from it. This is a pressure-control design issue, not evidence that the pump must still be operating.

Scope: Hydraulic circuits with trapped fluid and thermal-relief protection; component details vary.

Sources

• FAA-H-8083-31B — Aviation Maintenance Technician Handbook, Airframe, Chapter 12 — Printed p. 12-28, Relief Valves: Thermal relief valve. Accessed 2026-10-03.

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

• Airspeed Indicator (ASI) – calculates airspeed using both pitot (total) 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 senses total pressure. Total minus static pressure is impact pressure, which approximates dynamic pressure only at low Mach number
- The static ports supply ambient pressure used in altitude and vertical speed calculations

References: FAA Pilot Handbook chapters 5 to 8 and 10 to 12.

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.

A main-oil-filter bypass can maintain lubrication if the pressure difference across the filter becomes high enough. A clogged element is one possible cause, but not the only one: Honeywell describes very cold, viscous oil causing a momentary bypass opening on the TPE331. Some valve designs inhibit the bypass indicator at low oil temperature, so indications must be interpreted for the specific installation. The design trades filtration for continued oil supply to bearings. A cold transient must not automatically be treated as proof of a clogged filter. Required checks and actions depend on the engine documentation and operating conditions. Unfiltered flow does not make contamination harmless or establish that no damage has occurred.

Scope: Engines equipped with a main-oil-filter bypass; the cold-oil example is the Honeywell TPE331. Valve indications, thermal lockouts and required actions are installation-specific.

Sources

• FAA-H-8083-32B — Aviation Maintenance Technician Handbook, Powerplant, Chapter 6 — Printed p. 6-24, oil-filter bypass discussion; p. 6-25, Lubrication System Instrumentation. Accessed 2026-10-03.
• Honeywell — TPE331 Pilot Tips (manufacturer document in NTSB public docket) — Printed p. 31 (PDF page 33), Oil & Fuel Filter Bypass Valves – Check Indicators; printed pp. 1 and 5, AFM authority and installation-specific applicability. Accessed 2026-10-04.

Rapid compression can raise gas temperature sharply before the heat has time to dissipate. Airbus identified rapid pressurization followed by O-ring ignition as the most probable sequence in an oxygen-servicing fire. Oxygen supplies the oxidizer, while component material or contamination can provide fuel. This explains why oxygen handling includes controlled pressurization and strict cleanliness under approved procedures. The lesson concerns the physical ignition mechanism, not a substitute procedure for servicing oxygen equipment.

Scope: Aircraft high-pressure gaseous oxygen servicing; work is performed by qualified personnel under approved procedures.

Sources

• Airbus Safety First — Safe Oxygen Servicing — Case Study: Event Analysis; Importance of following oxygen servicing instructions; Prevent rapid oxygen pressure build-up. Accessed 2026-10-03.

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.

Yes. Cabin pressure need not equal outside pressure merely because the aircraft has landed or the engines are off. If outflow valves remain closed, pressure can be retained, and an external air-conditioning supply can also pressurize a closed aircraft on the ground. Opening a door against residual pressure can cause violent movement and injury. Airbus documents this hazard and associated warnings. Door operation must follow the applicable aircraft procedure and pressure checks, rather than relying on engine status.

Scope: Pressurized transport aircraft; Airbus examples. Door-warning logic varies by aircraft and modification.

Sources

• Airbus Safety First — Preventing Violent Door Opening Due to Residual Cabin Pressure — Introduction; Avoiding Unintentional Pressurization of the Aircraft on Ground: Event Description. Accessed 2026-10-03.

No. A reset can restore a function or clear an alert while an intermittent or deteriorating defect remains. Airbus describes repeated unauthorized resets masking a braking-system failure that later caused a serious landing event. A reset can also affect interconnected systems and trigger tests or unexpected movements. Crews should use only the reset authorized for the specific fault and conditions in their current procedures, and record the attempts and results so maintenance can identify recurring defects.

Scope: Airbus operational guidance; specific reset permissions depend on type, fault and flight phase.

Sources

• Airbus Safety First — System Reset: Use with Caution — Case study: Event Analysis; Inappropriate system resets can have serious consequences; Report any reset to maintenance. Accessed 2026-10-03.

A tailpipe fire burns inside the engine core, typically during starting or shutdown when residual fuel or oil can burn at very low rotational speed. Nacelle fire detectors monitor areas outside the core, so this internal fire can be outside their detection zone. Abnormal temperature behavior or a report from ground personnel may reveal it. It must be identified and handled using the aircraft-specific tailpipe-fire procedure; visible flames alone do not establish that it is a nacelle fire.

Scope: Transport turbine engines; detection layout and procedures remain aircraft-specific.

Sources

• Airbus Safety First — Do not Wait to Apply the Engine Fire Procedure — Two Types of Engine Fire: Tailpipe fire (internal fire). Accessed 2026-10-03.

No. Airflow can rotate a windmilling engine even when combustion has stopped. The FAA’s malfunction guidance notes that, in some flight conditions, N1 can be similar for a windmilling engine and one operating at flight idle. A successful restart therefore cannot be confirmed from N1 alone. Crews must assess the required primary parameters together, using the indications and criteria specified for their engine. Rotation, combustion and useful thrust are related observations, but they are not interchangeable proof of one another.

Scope: Turbofan in-flight restart recognition; no restart envelope or procedure supplied.

Sources

• FAA/industry — Turbofan Engine Malfunction Recognition and Response, Final Report — Printed p. 44, Chapter 4: Flame out, final paragraph. Accessed 2026-10-03.