Aircraft Electrical Systems: A320/A321 Briefing

 

Cold Weather Operations Briefing

Executive Summary

This document provides a comprehensive synthesis of procedures and considerations for Airbus aircraft operations in cold weather, addressing aircraft contamination, performance on compromised runways, fuel freezing, and altimeter corrections. Safe operations are contingent upon a thorough understanding of these interconnected factors.

Key takeaways include:

• Aircraft Contamination: The "clean aircraft concept" is paramount; any contamination from ice, snow, or frost on critical surfaces must be removed before takeoff. In-flight icing conditions typically occur between slightly positive temperatures down to -40°C, with stratiform clouds posing a significant threat. Ice protection systems (NAI, WAI) are to be used according to FCOM procedures, which are not superseded by advisory ice detection systems.
• Ground De-icing/Anti-icing: A two-stage process is often required. De-icing removes existing contaminants, while anti-icing provides a protective fluid film for a limited duration known as the holdover time. Holdover time is highly variable, depending on fluid type (I, II, or IV), concentration, and weather conditions (precipitation type, OAT). The aircraft Commander holds the final responsibility for accepting the aircraft's condition for dispatch.
• Contaminated Runway Performance: Runway contaminants significantly degrade performance by reducing the friction coefficient (µ), creating precipitation drag, and potentially causing aquaplaning. There is a critical distinction between fluid contaminants (water, slush), which affect both acceleration and braking, and hard contaminants (compacted snow, ice), which primarily affect braking. Thrust reversers are indispensable on contaminated runways. Performance optimization involves selecting higher flap settings, lower takeoff speeds, and using derated (not flexible) thrust.
• Fuel Freezing Limitations: The minimum allowed fuel temperature is determined by either the fuel's freezing point (plus a required engine-specific margin) or the engine's fuel heat management system limit. Using JET A fuel (max freeze point -40°C) can be operationally restrictive on long-haul routes over cold regions. Knowledge of the fuel's actual freezing point, which is often significantly lower than the specification, can provide a large operational benefit.
• Altimeter Corrections: In temperatures below International Standard Atmosphere (ISA) conditions, an aircraft's true altitude is lower than its indicated altitude, creating a potential terrain clearance hazard. Corrections must be applied to published minimum altitudes (e.g., MSA, step-down altitudes, FAF altitude) to ensure safety. Takeoff chart acceleration altitudes are calculated for the lowest temperature on the chart; if the OAT is colder, this altitude must be increased.

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1.0 Aircraft Contamination in Flight

Enhanced pilot awareness of icing is a key factor in mitigating the threat. While icing conditions are frequent, effective ice accretion is less common. Understanding the principles of atmospheric physics, certification standards, and on-board protection systems is critical for safe flight.

1.1 Icing Principles

• Atmospheric Conditions: Icing is caused by supercooled liquid water droplets, which can remain liquid at temperatures below 0°C but freeze instantly on impact with an object.
  • Temperature Range: Icing conditions generally occur from slightly positive OAT down to -40°C. Severe icing rarely occurs below -12°C.
  • Altitude: At mid-latitudes, severe icing is most likely around FL 100 down to the ground.
  • Cloud Types: High accretion rates are not exclusive to Cumulonimbus clouds; stratiform clouds can also cause severe icing ("ice like hell"). Convective activity within stratus layers can generate freezing drizzle. Flying under the anvil of a Cumulonimbus should be avoided as it can generate freezing rain or drizzle.
• Ice Accretion: The actual ice that forms is a result of incoming supercooled water versus the outgoing amount from erosion, evaporation, and sublimation.
  • Factors Influencing Accretion: The process is influenced by numerous non-additive parameters, including OAT, TAT, aircraft speed, aircraft size, cloud type, liquid water content, and drop size.
  • Effect of Speed: An increase in speed generally decreases the amount of ice accreted due to kinetic heating (ram effect). At 250 kt, kinetic heating is approximately +10°C, which helps protect leading edges.
• Operational Recommendations:
  • A moderate change of altitude is typically sufficient to decrease or stop rapid ice accretion in a stratiform cloud. ATC is obligated to accept such a pilot request immediately.
  • If icing conditions are present on approach, speed should be kept as high as permitted and flap extension delayed as long as possible. Flaps should not be retracted after landing.

1.2 Icing Certification

Aircraft performance is certified based on a clean, uncontaminated structure. Any ice accretion degrades lifting performance by disrupting smooth airflow over the wing, causing the boundary layer to thicken and separate at a lower angle of attack, thereby increasing stall speed.

• Certification Standard (FAR 25 Appendix C): This standard defines an "envelope case" for certification, requiring aircraft to demonstrate they can sustain a significant (3-inch) ice accretion on unprotected parts, derived from a 45-minute exposure. This standard has proven adequate for large jets, which are less susceptible to icing than smaller aircraft due to:
  • Faster flying speeds and greater kinetic heating.
  • Larger leading-edge radii, which pick up less ice.
  • The boundary-layer-energizing effect of slotted slats.
• Certification Limitations: Aircraft are not certified for sustained flight in icing conditions with slats and flaps extended. The assumption is that an aircraft will not remain in this configuration long enough to accrete an amount of ice that would significantly alter performance.

1.3 In-Flight Ice Protection Systems

Airbus aircraft utilize several methods to protect critical surfaces from ice accretion.

• Hot Bleed Air: Used for wing and nacelle anti-ice systems.
  • Wing Anti-Ice (WAI): Heats the leading edges of the outboard slats to a temperature high enough to evaporate accreting moisture. It is inhibited on the ground to prevent overheating. The tailplane and fin are not de-iced, as they have sufficient efficiency margins to meet certification targets with ice shapes.
  • Nacelle Anti-Ice (NAI): Heats the entire engine intake leading edge to protect the fan. It is never inhibited. Procedures call for greater use of NAI than WAI because temperature can drop inside the intake, allowing for icing at slightly positive OATs.
• Electrical Heating: Used on smaller surfaces like pitot tubes, static ports, AoA probes, TAT probes, and cockpit windows. Overheating on the ground is prevented by an automatic air/ground logic.
• FCOM Procedures:
  • Icing Conditions Definition: OAT (ground) or TAT (flight) is at or below 10°C with visible moisture present.
  • NAI Activation: Must be activated immediately when entering icing conditions (unless SAT is below -40°C, except in cumulonimbus clouds).
  • WAI Activation: Required whenever there is an indication of airframe ice accumulation (e.g., on windshield wipers or the visual ice indicator).

1.4 Ice Detection Systems

• General: Optional ice detectors are advisory systems and do not replace FCOM/AFM procedures. Airbus is not convinced of the benefits of a primary ice detection system.
• Dual Advisory Ice Detection System (DAIDS): This system is standard on A330/A340 and optional on the A320 family.
  • Function: It supports the pilot in identifying meteorological conditions leading to ice accumulation. It is operational in flight only (Altitude ≥ 1500 ft and TAT < 8°C).
  • Components: Two magnetostrictive probes installed on the forward fuselage vibrate at a resonant frequency (approx. 40 kHz). Ice accretion adds mass, decreasing the frequency.
  • Alerting Levels:
    1. ICE DETECTED: Generated when ~0.5 mm of ice accretes on a probe. Triggers a MASTER CAUTION and advises the crew to activate NAI.
    2. SEVERE ICE DETECTED: Generated after 7 detection cycles, corresponding to ~5 mm of ice on the protected wing surface. Triggers a MASTER CAUTION and advises the crew to activate WAI.
  • Crew Action: The flight crew retains responsibility for activating anti-ice systems based on FCOM criteria. The DAIDS provides additional, more precise indication of actual ice build-up.

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2.0 Aircraft De-icing / Anti-icing on the Ground

The clean aircraft concept is fundamental to safety. Any contamination of critical flight surfaces can lead to performance degradation, handling difficulties, and mechanical damage.

2.1 Principles and Responsibilities

• Clean Aircraft Concept: The aircraft must not have ice, snow, slush, or frost adhering to wings, stabilizers, or control surfaces before takeoff.
  • Exceptions: A frost layer under 3mm on the wing underside in fuel tank areas (due to cold-soaked fuel) is acceptable. A thin layer of rime or light powdery snow on the fuselage upper surface is also acceptable.
• Clear Ice: A significant hazard is clear ice, which forms when precipitation falls on a cold-soaked wing structure (fuel temperature below 0°C), even at positive ambient temperatures. It is very difficult to detect visually. A tactile check (by hand) is the most reliable detection method.
• Responsibility:
  • Ground Crew: The person technically releasing the aircraft is responsible for the performance and verification of the de-/anti-icing treatment.
  • Commander: The aircraft Commander is ultimately responsible for accepting the performed treatment and for the aircraft's condition at takeoff. The transfer of responsibility occurs when the aircraft begins to move under its own power.

2.2 Procedures: De-icing and Anti-icing

• De-icing: The procedure to remove existing contaminants. This can be done with heated fluids (Type I, II, or IV) or mechanical methods.
• Anti-icing: A precautionary procedure providing a protective fluid film against new accumulation for a limited period.
• Process Selection:
  • One-Step: De-icing and anti-icing are performed simultaneously with a heated anti-icing fluid. Holdover time begins at the start of the application.
  • Two-Step: De-icing is performed first (e.g., with a hot Type I fluid/water mix), followed by a separate application of anti-icing fluid (e.g., undiluted Type II or IV fluid). Holdover time begins at the start of the second step. This method is recommended when a long holdover time is needed.

2.3 Fluid Characteristics and Holdover Time

• Fluid Types:
  • Type I: Newtonian fluid (viscosity depends on temperature). Primarily used for de-icing. Provides very limited holdover time.
  • Type II / IV: Non-Newtonian (pseudo-plastic) fluids containing a thickener. Viscosity decreases under shear force (e.g., airflow during takeoff), allowing it to flow off the wing. They form a thicker film, providing significantly longer holdover times.
• Holdover Time (HOT): The estimated time the anti-icing fluid will prevent ice/snow from forming on protected surfaces.
  • Influencing Factors: HOT is critically dependent on OAT, precipitation type and intensity, wind, and jet blast.
  • Communication: Ground crew must provide the flight crew with an anti-icing code specifying the fluid type, concentration, and start time of the final application. This allows the flight crew to estimate the available holdover time using published guidelines.
• Holdover Time Guideline Tables: The following tables are for departure planning only and must be used in conjunction with pre-takeoff check procedures. Times can be significantly shortened by severe weather conditions.

OAT	Frost (*)	Freezing Fog	Snow	Freezing Drizzle (**)	Light Freezing Rain	Rain on Cold Soaked Wing	Others (1)
°C	°F	(hours: minutes)
above 0	above 32	0:45	0:12-0:30	0:06-0:15	0:05-0:08	0:02-0:05	0:02-0:05
0 to -10	32 to 14	0:45	0:06-0:15	0:06-0:15	0:05-0:08	0:02-0:05	CAUTION:
below -10	below 14	0:45	0:06-0:15	0:06-0:15			Clear ice may require touch for confirmation

OAT	SAE Type II Fluid Concentration (Vol. %/Vol. %)	Frost (*)	Freezing Fog	Snow	Freezing Drizzle (***)	Light Freezing Rain	Rain on Cold Soaked Wing	Others (1)
°C	°F		(hours: minutes)
above 0	above 32	100/0	12:00	1:05-2:15	0:20-1:00	0:30-1:00	0:15-0:30	0:05-0:40
		75/25	6:00	0:50-1:45	0:15-0:40	0:20-0:45	0:10-0:25	0:05-0:25
		50/50	4:00	0:15-0:35	0:05-0:15	0:05-0:20	0:05-0:10
0 to -3	32 to 27	100/0	8:00	0:35-1:30	0:20-0:45	0:30-1:00	0:15-0:30	CAUTION: Clear ice may require touch for confirmation
		75/25	5:00	0:25-1:00	0:15-0:30	0:20-0:45	0:10-0:25
		50/50	3:00	0:15-0:35	0:05-0:15	0:05-0:20	0:05-0:10
below -3 to -14	below 27 to 7	100/0	8:00	0:30-1:05	0:15-0:35	0:15-0:45 (**)	0:10-0:30 (**)
		75/25	5:00	0:20-0:55	0:15-0:25	0:15-0:30 (**)	0:10-0:20 (**)
below -14 to -25	below 7 to -13	100/0	8:00	0:15-0:20	0:15-0:30

OAT	SAE Type IV Fluid Concentration (Vol. %/Vol. %)	Frost (*)	Freezing Fog	Snow	Freezing Drizzle (***)	Light Freezing Rain	Rain on Cold Soaked Wing	Others (1)
°C	°F		(hours: minutes)
above 0	above 32	100/0	18:00	1:05-2:15	0:35-1:05	0:40-1:00	0:25-0:40	0:10-0:50
		75/25	6:00	1:05-1:45	0:20-0:40	0:30-1:00	0:15-0:30	0:05-0:35
		50/50	4:00	0:20-0:35	0:05-0:20	0:10-0:20	0:05-0:10
0 to -3	32 to 27	100/0	12:00	1:05-2:15	0:30-0:55	0:40-1:00	0:25-0:40	CAUTION: Clear ice may require touch for confirmation
		75/25	5:00	1:05-1:45	0:20-0:35	0:30-1:00	0:15-0:30
		50/50	3:00	0:20-0:35	0:05-0:15	0:10-0:20	0:05-0:10
below -3 to -14	below 27 to 7	100/0	12:00	0:40-1:30	0:20-0:40	0:20-0:55 (**)	0:10-0:30 (**)
		75/25	5:00	0:25-1:00	0:15-0:25	0:20-0:55 (**)	0:10-0:30 (**)
below -14 to -25	below 7 to -13	100/0	12:00	0:20-0:40	0:15-0:30

Notes for Tables 1-3: () Active Frost: Frost is actively forming.() No HOT guidelines exist for this condition below -10°C.() Use light freezing rain HOT if freezing drizzle cannot be positively identified. (1) No HOT guidelines exist for snow pellets, snow grains, ice pellets, moderate/heavy freezing rain, or hail. Takeoff in these conditions is not recommended by Airbus.

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3.0 Performance on Contaminated Runways

A runway is contaminated when more than 25% of the surface is covered by more than 3mm of water/slush/loose snow, or by any amount of compacted snow or ice. This affects braking, acceleration, and directional control. FLEX takeoff is not allowed from a contaminated runway.

3.1 Aircraft Braking Means

Three systems contribute to deceleration:

1. Wheel Brakes (Primary): Efficiency depends on the load on the wheels and the slip ratio. The anti-skid system optimizes braking by maintaining the slip ratio at its most effective point (around 12%).
2. Ground Spoilers: Increase aerodynamic drag and, more importantly, dump lift to increase the load on the wheels, thereby improving brake efficiency.
3. Thrust Reversers: Provide a decelerating force independent of runway condition. Their effect is proportionally more significant on slippery runways and their use is indispensable.

3.2 Impact of Contaminants on Performance

• Hard Contaminants (Compacted Snow, Ice): Affect performance primarily by reducing the friction coefficient (µ) between the tires and the runway.
• Fluid Contaminants (Water, Slush, Loose Snow): Affect performance in three ways:
  1. Reduced Friction: A film of fluid reduces the direct contact area between the tire and runway.
  2. Precipitation Drag: An additional drag force is created by the displacement of the contaminant by the tires and the impingement of spray on the airframe. This affects both braking and acceleration, leading to takeoff limitations based on contaminant depth.
  3. Aquaplaning (Hydroplaning): At high speeds, tires can be separated from the runway by a thin fluid film, dropping tire traction to near-negligible values and rendering braking and steering ineffective.

3.3 Friction Coefficient (µ) and Directional Control

• Reported vs. Effective µ: Airport authorities provide a "reported µ" measured by ground vehicles. This often correlates poorly with the "effective µ" experienced by an aircraft, which depends on speed, weight, tire pressure, and anti-skid efficiency. Therefore, Airbus performance data is based on contaminant type and depth, not reported µ.
• Directional Control: The total friction force on a yawed wheel is split between a braking force and a "cornering force" (side-friction).
  • Maximum cornering force (steering) is achieved with zero braking.
  • Maximum braking force is achieved with zero cornering capability.
  • On a slippery runway, the total available friction force is severely reduced. In critical crosswind conditions, a pilot may need to choose between braking and controlling the aircraft by releasing the brakes to regain cornering force.

3.4 Crosswind Recommendations

The AFM provides a "demonstrated crosswind" value, which is the maximum experienced during certification on dry/wet runways and is not a limitation. For contaminated runways, Airbus provides conservative recommendations based on estimated braking action.

Runway Condition	Recommended Max Crosswind (kt)
GOOD	38
MEDIUM TO GOOD	25
MEDIUM	20
MEDIUM TO POOR	15
POOR	10
UNRELIABLE	Not recommended

3.5 Performance Optimization

To minimize payload loss on contaminated runways:

• Flap Setting: Use a higher flap setting (e.g., CONF 3) to reduce accelerate-go and accelerate-stop distances. An obstacle in the flight path may still require a lower flap setting for better climb performance.
• Takeoff Speeds: Use lower takeoff speeds (V1, VR, V2). Performance programs automatically optimize speeds to be as low as possible while respecting margins to VMCG, VMCA, and stall speed. Regulations permit reducing screen height from 35 ft to 15 ft on contaminated runways, which facilitates lower speeds.
• Derated Thrust: Flexible thrust is not permitted. Derated thrust is allowed and offers two benefits:
  1. It reduces engine wear.
  2. It reduces the engine-out yawing moment, which lowers VMCG. This allows for lower takeoff speeds and, on VMCG-limited runways (typically short and contaminated), can lead to a higher allowable takeoff weight.

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4.0 Fuel Freezing Limitations

Operations on routes with very low temperatures require careful management of fuel temperature to prevent fuel waxing and potential engine issues.

4.1 Minimum Allowed Fuel Temperature

The operational limit is the more restrictive of two factors:

1. Fuel Freezing Point: To prevent wax crystals from blocking fuel lines and filters, the fuel temperature must be kept above its freezing point plus a margin required by the engine manufacturer. This margin varies:
   • GE: 3°C
   • CFM (A340): 5°C
   • CFM (A320 family) / IAE: 4°C
   • PW / RR: 0°C
2. Fuel Heat Management System: A fixed temperature limit based on the engine's ability to warm water-saturated fuel to prevent ice crystals from blocking the fuel filter. This is often outside the normal flight envelope.

4.2 Fuel Types and Operational Impact

• JET A1: Standard international fuel with a maximum freezing point of -47°C.
• JET A: Primarily used in the USA, with a maximum freezing point of -40°C.
• Operational Considerations: The use of JET A can be a limiting factor on long-haul routes over cold regions (e.g., Trans-Siberian routes), where TAT can fall below -34°C, leading to fuel temperatures approaching the limit for JET A.
• Actual Freezing Point ("Give-Away"): Fuel suppliers typically produce fuel with an actual freezing point significantly lower than the specification maximum. A 1998 survey showed the average freezing point of JET A was -44.5°C. Knowing the actual freezing point of the uplifted fuel can provide a significant operational advantage.
• Mixing Fuels: Predicting the freezing point of a mixture of JET A and JET A1 is not possible with a simple linear relationship; both elevation and depression of the freezing point can occur. A pragmatic operator approach is to consider a mixture with less than 10% JET A as JET A1 for operational purposes.

4.3 Low Temperature Fuel Behavior

• Pumpability Limit: Fuel does not freeze solid at a single temperature. The pumpability limit, where flow is impeded, typically occurs 4°C to 16°C below the official "freezing point." This provides a significant inherent safety margin.
• Mitigation Techniques: If fuel temperature approaches the limit, increasing speed or descending to a lower altitude (warmer SAT) are potential actions, though they increase fuel consumption.

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5.0 Low Temperature Effect on Altimeter Indication

Barometric altimeters are calibrated for ISA conditions. Deviations from ISA, particularly cold temperatures, introduce significant errors.

5.1 The Altimetry Error Principle

• Effect of Cold Temperature: When the air temperature is colder than ISA, the air column is denser and contracts. This causes isobaric surfaces to be closer together.
• Resulting Error: The aircraft's true altitude will be lower than its indicated altitude. This creates a critical risk of reduced terrain clearance.

5.2 Required Corrections

When temperatures are below ISA, corrections must be applied to published altitudes to ensure safety. The correction must be applied to the height above the elevation of the altimeter setting source (e.g., the airport).

• Low Altitude Corrections: For approach procedures, the following published altitudes must be increased when OAT is low:
  • Minimum Safe Altitude (MSA)
  • Final Approach Fix (FAF) altitude
  • Step-down altitudes and MDA/H
  • Outer Marker (OM) altitude for an ILS
• Application: The correction can be calculated or taken from tables provided in the FCOM. The pilot must either increase the target indicated altitude or mentally increase the height of obstacles.
• VNAV Approaches: For non-precision approaches flown in FINAL APPR mode, a minimum OAT must be established for the procedure's use, as the coded altitude constraints cannot be modified by the pilot. Below this temperature, selected vertical navigation must be used.

5.3 Takeoff Performance Considerations

• Acceleration Altitude: The minimum acceleration altitude on Airbus takeoff charts is calculated to ensure obstacle clearance at the lowest temperature shown on the chart. Therefore, if the actual OAT is within the chart's range, no further correction is needed.
• OAT Below Chart Minimum: If the actual OAT is colder than the lowest temperature on the applicable takeoff chart, the published minimum acceleration altitude/height must be increased using the FCOM correction tables.
• Takeoff Margins: Flying at a colder OAT with a given FLEX temperature does not negatively affect performance. The engine thrust remains the same, but the lower air density results in a lower ground speed for a given IAS. This decreases takeoff distances and increases margins for tire speed and brake energy.