Understanding Jet Engine Power during Taxiing: Idle or Low Thrust

Jets typically run at very low power or idle when taxiing. This crucial phase of flight involves operating the engines under minimal thrust to manage fuel consumption and wear, while still maintaining control of the aircraft. Let’s explore the intricacies of jet engine power during this phase.

Jet Engine Power Settings during Taxiing

During taxi maneuvers, the engines are set to a low-thrust setting, which is typically around 65% of the redline RPM. This moderate power output is sufficient to move the aircraft across the runway at taxi speeds, which are generally much lower compared to takeoff or cruising. Pilots must balance fuel conservation, engine health, and the need to maneuver the aircraft safely on the ground.

The Role of Economics and Safety

Conserving fuel and reducing engine wear are primary reasons for operating jet engines at low power during taxiing. While these considerations are important, safety also plays a crucial role. Pilots need to maintain control of the aircraft as the ground speed is typically much lower, making precise maneuvering more critical.

Understanding the Physics of Thrust

The power required to taxi a jet-powered aircraft can be broken down into several components, including overcoming acceleration forces, conquering drag forces, handling friction in bearings and tires, and compensating for any headwind. Additionally, the direction of the wind relative to the aircraft and gravitational forces must be taken into account. The fact that the ground is not always level further complicates the equation.

The Effect of Ground Conditions

The amount of power required to taxi can vary based on the ground conditions. On a smooth runway, it takes less thrust to get the aircraft moving. Conversely, if the runway is rough, more power might be needed. The idea is to achieve some forward momentum, which can then be carried along as the aircraft moves. Jet-powered aircraft can still taxi even with engines at idle, as the N1 indication, the measure of engine speed, typically remains around 20%.

Tuning Power During Taxiing

Managing power during taxi involves careful modulation. As the aircraft begins to move from a stationary position, a moderate amount of thrust is needed to overcome the friction between the tires and the ground. This thrust is primarily used to achieve momentum, which the aircraft can then continue to move with. If the pilots need to brake during taxi, the aircraft will lose momentum, and additional thrust may be required to regain it.

Speed Limits during Taxi

The maximum taxi speed for jet aircraft is typically limited to ensure safety. Most jet aircraft should not exceed 30 knots in a straight line, and the speed should never exceed 20 knots in a turn. These limits are set by the operator to prevent excessive wear and tear on the engines and the aircraft itself.

Turboprop Engine Taxiing

While jet engines operate at a low-thrust setting during taxiing, turboprop engines require constant manipulation of power levers. This is because the pilot directly controls the propeller blade angle. During taxi, the propeller angle can be varied between 0 to about 20 degrees. This represents a range above true idle power and is often referred to as ground beta range, where the propeller angle is controlled by the power levers rather than by the Propeller Control Unit (PCU).

Understanding the nuances of jet engine power during taxiing is crucial for both pilots and maintenance personnel. This knowledge ensures that aircraft can efficiently manage fuel, avoid unnecessary wear, and maintain safety throughout the taxi process.