A320 Single-Engine Taxi: Decoding Pilot's Choice

Imagine standing at a bustling UK airport, gazing at a magnificent Airbus A320, a workhorse of the skies, gracefully manoeuvring across the tarmac. But wait – is it just me, or is only one of its powerful engines running? For the keen observer, this isn't a malfunction; it's a deliberate and highly efficient operational procedure known as single-engine taxi. This practice, common across the aviation industry, is a testament to modern aircraft design and the meticulous planning of flight crews. It's a fascinating insight into how airlines balance performance, fuel efficiency, and environmental responsibility. But if pilots choose to taxi with a single engine, which one do they start first? Do they fire up both engines only to shut one down? And is there a specific engine preferred for shutdown or operation? Let's delve into the intricacies of A320 single-engine taxi procedures, from pushback to take-off and beyond.

The Strategic Imperative: Why Taxi on One Engine?

At first glance, it might seem counter-intuitive for a twin-engine aircraft to operate on just one. However, the decision to perform a single-engine taxi is rooted in significant economic and environmental advantages. Airlines are constantly seeking ways to optimise operations, and this procedure offers tangible benefits.

• Fuel Efficiency: The most obvious advantage is the substantial saving in jet fuel. Taxiing, especially at large airports with long taxi routes, can consume a considerable amount of fuel. By operating with only one engine, airlines can reduce their fuel burn, leading to significant cost savings over thousands of flights annually. This also translates to a smaller carbon footprint, aligning with growing environmental concerns.
• Reduced Brake Wear: When an aircraft taxis with both engines running, even at idle thrust, it generates more forward momentum, often requiring pilots to use the brakes more frequently to control speed. With a single engine, less thrust is produced, reducing the need for continuous braking. This minimises brake wear, extending the lifespan of these critical components and leading to lower maintenance costs for the airline.
• Noise Reduction: While perhaps a secondary benefit, operating with one less engine contributes to a quieter airport environment, particularly for ground personnel and communities located near busy flight paths.

Navigating the Tarmac: Critical Considerations and Safety Protocols

While single-engine taxi offers clear advantages, it's not a 'one-size-fits-all' solution. Pilots must adhere to strict safety protocols and consider various operational factors before deciding to proceed with this procedure. Safety remains paramount in all aviation operations.

When Single-Engine Taxi is Not Advised:

There are specific scenarios where the risks outweigh the benefits, and pilots will opt to taxi with both engines running:

• Uphill Slopes: Ascending an incline requires more thrust, which a single engine might struggle to provide efficiently, potentially straining the operating engine and delaying taxi.
• High Aircraft Gross Weights: A heavier aircraft demands more power to move and maintain control. In such cases, the additional thrust from both engines is necessary for safe and efficient taxiing.
• Slippery Taxiways: On icy, wet, or otherwise slippery surfaces, maximum control and thrust availability are crucial. Operating with both engines provides better traction and responsiveness, reducing the risk of skidding or loss of control.

Operational Challenges and Pilot Awareness:

Even when conditions are suitable, pilots must be acutely aware of certain operational nuances:

• Higher Thrust Requirements: The single operating engine may need to be advanced to a higher thrust setting than normal to achieve and maintain taxi speed. This increases the risk of jet-blast, which can be hazardous to ground personnel, equipment, and other aircraft. Pilots must exercise caution and be mindful of their surroundings.
• Foreign Object Damage (FOD): Higher thrust settings mean increased engine suction, raising the risk of ingesting foreign objects from the tarmac, such as debris, stones, or even small tools. This can cause significant and costly damage to the engine.
• Manoeuvrability Limitations: At high gross weights, slow or tight turns, especially in the direction of the operating engine, may be more challenging or even impossible. This is because the operating engine's thrust might counteract the turning force, requiring more precise handling from the pilot.
• Engine Warm-up Time: A critical consideration for taxi-out is the warm-up time required for the second engine before take-off. Different engine types have specific requirements: CFM engines typically need 2 minutes of warm-up, while IAE and NEO engines require 5 minutes. Pilots must factor this into their taxi plan to ensure the engine is ready for full power when needed.

The Departure Sequence: Which Engine Takes the Lead?

When an A320 is preparing for departure, the decision of which engine to start first for a single-engine taxi is not arbitrary. It's dictated by the aircraft's hydraulic systems and the need to ensure critical functions are immediately available. For taxi-out, the preference is overwhelmingly to start Engine 1 first.

Why Engine 1? The Green Hydraulic System:

Engine 1 is designed to pressurise the green hydraulic system. This system is paramount for normal braking, extending and retracting the landing gear, and powering other essential flight controls. By starting Engine 1, pilots ensure that normal braking capability is immediately available, which is crucial for safe taxiing and stopping the aircraft if needed. This provides a fundamental layer of safety and control from the moment the aircraft begins to move.

Step-by-Step Departure Procedures:

The sequence for a single-engine taxi-out is a carefully orchestrated ballet of checks and actions:

1. Brake Accumulator Pressure: Before moving, pilots ensure sufficient brake accumulator pressure. If necessary, the Yellow Electric Pump is used to pressurise the brake accumulator, ensuring immediate braking power.
2. Engine 1 Start: Engine 1 is started. Once stable, it begins pressurising the green hydraulic system, providing normal braking.
3. Cross-Bleed Valve Open: The cross-bleed valve is opened to allow bleed air from Engine 1 to supply both air conditioning packs, ensuring comfortable cabin conditions for passengers.
4. APU Management: The Auxiliary Power Unit (APU) is kept running, but its bleed air is typically switched OFF. This is to prevent the ingestion of engine exhaust gases into the air conditioning system. However, the APU generator continues to provide electrical power for the engine fire extinguisher system, prevents electrical transients, and keeps the galley and In-Flight Entertainment (IFE) systems operational.
5. Yellow Electric Pump On: The Yellow Electric Pump is switched ON. This pressurises the yellow hydraulic system, providing nosewheel steering without needing to use the Power Transfer Unit (PTU), which can be noisy and is generally avoided unless necessary.
6. Delayed Checks: Certain checks, such as the ECAM (Electronic Centralised Aircraft Monitoring) STATUS check and wing anti-ice setting, are delayed until all engines are started.
7. Autobrake Arming: After the flight controls check, the autobrake system is armed, providing an additional layer of safety for landing.

During the taxi, the flight crew meticulously monitors the aircraft's speed, direction, and systems, ensuring smooth and safe progress towards the runway threshold.

Bringing the Second Engine to Life: Pre-Takeoff Synchronisation

While single-engine taxi is efficient, an A320 cannot take off on just one engine. Therefore, the second engine, Engine 2, must be started before the aircraft enters the runway for departure. The timing of this is crucial, factoring in the required engine warm-up period.

Timing is Everything:

Pilots must initiate the start of Engine 2 early enough to ensure that the applicable warm-up time is met before take-off thrust is applied. As mentioned, this is typically 2 minutes for CFM engines and 5 minutes for IAE and NEO engines. This warm-up period is vital for the thermal stabilisation of the hot section of the engine, preventing premature wear and ensuring optimal performance for take-off.

Specific Steps for Engine 2 Start:

1. Parking Brake Set: For the Engine 2 start, the parking brake must be set to prevent any inadvertent movement of the aircraft during the engine start sequence.
2. Yellow Electric Pump OFF: The Yellow Electric Pump must be switched OFF. This is a critical step that enables the automatic test of the PTU (Power Transfer Unit) during the Engine 2 start, ensuring its functionality.
3. APU Bleed On: The APU bleed is typically switched ON again to provide the necessary air pressure for the Engine 2 start.
4. 10-Second Delay: After setting the APU bleed, pilots wait for approximately 10 seconds before initiating the Engine 2 start. This delay ensures that the bleed system valves are no longer in transit, preventing a potential Engine 1 stall due to sudden pressure changes.
5. Engine 2 Start: Once the conditions are met, Engine 2 is started.

After Engine 2 is started and stabilised, the APU may be shut down if no longer required, and the cross-bleed valve is set to AUTO. The flight crew then completes the remaining 'AFTER START' normal procedures, including the ECAM Status Check, selection of engine and wing anti-ice as required, and the final flight controls check and autobrake setting before lining up for take-off.

The Arrival Protocol: Efficient Shutdown on the Tarmac

Just as there's a specific order for starting engines for taxi-out, there's also a preferred sequence for shutting them down upon arrival. This procedure, also aimed at efficiency and system longevity, typically involves shutting down Engine 2 first.

Why Engine 2 First? Avoiding the PTU:

Upon arrival, after the aircraft has landed and is taxiing to the gate, pilots will typically shut down Engine 2 first. The primary reason for this is to avoid the unnecessary running of the Power Transfer Unit (PTU). The PTU is a hydraulic component that can transfer hydraulic power between the green and yellow hydraulic systems. It can be quite noisy when operating, and by shutting down Engine 2 (which is linked to the yellow hydraulic system) and managing the Yellow Electric Pump, pilots can prevent the PTU from cycling, thus reducing noise and wear.

The Role of the APU on Arrival:

Before shutting down any engine, the APU is started and allowed to reach 'AVAIL' status. This is crucial for several reasons:

• Continuous Power Supply: The APU provides electrical power to the aircraft, preventing electrical transients (brief, undesirable surges or drops in voltage) that could affect sensitive avionics during engine shutdown.
• Engine Fire Extinguisher: The APU ensures that the engine fire extinguisher system remains powered and available even after the main engines are shut down.
• Cabin Comfort: It allows the galley and In-Flight Entertainment (IFE) systems to continue operating, maintaining passenger comfort until the aircraft is fully powered down at the gate.

Step-by-Step Arrival Procedures:

1. APU Start and Availability: As the aircraft taxis in, the APU is started. Once the APU indicates 'AVAIL' (ready for use) and the aircraft is taxiing in a straight line, the shutdown sequence can begin.
2. Engine Cooling Time: Before shutting down, engines should be operated below 40% thrust / 1.02 EPR (Engine Pressure Ratio) for a minimum of 3 minutes. This crucial cooling period allows the hot section of the engine to thermally stabilise, preventing thermal shock and prolonging engine life.
3. Engine 2 Shutdown: Engine 2 is shut down first. During this process, the Yellow Electric Pump is activated (turned ON). This ensures that the yellow hydraulic system remains pressurised, preventing the PTU from running due to a pressure differential between the green and yellow systems.
4. At Parking: Once the aircraft reaches the parking stand, the Yellow Electric Pump is switched OFF, and then Engine 1 is shut down.

Comparative Overview: Departure vs. Arrival Engine Preferences

To summarise the distinct approaches for single-engine taxi at different phases of flight, here's a comparative table:

Frequently Asked Questions (FAQs)

Q: Is single-engine taxi safe?

A: Yes, single-engine taxi is a safe and widely accepted procedure. It is performed under strict airline and manufacturer guidelines, with pilots receiving specific training for these operations. The aircraft's design accounts for such operations, and critical systems like hydraulics and brakes remain fully functional.

Q: Does every flight use single-engine taxi?

A: No, it's not used for every flight. The decision rests with the flight crew, who assess various factors such as aircraft weight, taxiway conditions (slopes, slipperiness), taxi route length, and airline policies. If conditions are not optimal, or if there's any doubt, pilots will opt for a two-engine taxi.

Q: How much fuel does single-engine taxi save?

A: The exact amount varies depending on the aircraft type, taxi duration, and engine efficiency. However, for a major airline operating thousands of flights annually, the cumulative fuel efficiency savings can be substantial, contributing significantly to operational cost reductions and environmental benefits.

Q: What is the PTU (Power Transfer Unit)?

A: The PTU is a component in the Airbus A320's hydraulic system. It's a motor-pump that can transfer hydraulic power between the green and yellow hydraulic systems without transferring hydraulic fluid. It's primarily used to ensure that the yellow hydraulic system can be pressurised from the green system (or vice-versa) during certain operations, like landing gear retraction if a pump fails. It's known for its distinctive 'barking dog' sound when operating.

Q: Can an A320 take off with one engine?

A: No, an Airbus A320 is certified and designed for take-off with both engines operating. Single-engine operations are strictly limited to taxiing. While an aircraft can safely fly and land with one engine inoperative in an emergency, it cannot commence a take-off with only one engine.

Conclusion

The sight of an Airbus A320 taxiing on a single engine is a quiet testament to the sophisticated engineering and meticulous operational flexibility that defines modern aviation. Far from being a sign of trouble, it represents a deliberate choice by pilots and airlines to enhance fuel efficiency, reduce brake wear, and minimise environmental impact, all while upholding the highest standards of safety protocols. The precise sequence of engine starts and shutdowns, driven by the critical requirements of the aircraft's hydraulic systems and operational considerations, underscores the incredible expertise of flight crews. So, the next time you're at a UK airport, perhaps sipping a cuppa and watching the giants of the sky, you'll have a deeper appreciation for the intricate dance of power and precision that unfolds on the tarmac, long before the aircraft ever takes to the skies.

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