An A320 equipped with IAE engines was lined up for a static takeoff using Flex Thrust. It was 10:40 pm local time, the runway was dry, the wind negligible and the outside air temperature had reached 33°C. There was a slight difference in the position of left and right thrust levers when the aircraft  lined up on the runway that resulted in the following Engine Pressure Ratios (EPR)  and N2 values:

• ENG 1: 1.01 EPR     61% N2
• ENG 2: 1.03 EPR     75% N2

The Pilot Flying (PF) moved both thrust levers forward and paused for around 3 seconds near to the CLB detent where, the EPR and N2 increased to the the following values:

• ENG 1: 1.03 EPR     78% N2
• ENG 2: 1.24 EPR     91% N2

Eventually, the PF released the brakes and moved the thrust levers forward to the FLX detent. Engine 2 accelerated more rapidly than engine 1 and the resulting thrust asymmetry caused the aircraft to veer to the left. The PF tried to recover the trajectory by applying right rudder input and retarding the thrust levers to reduce thrust on both engines. Consequently, this caused the aircraft to sharply veer to the right. The PF applied differential thrust combined with left rudder pedals and tiller inputs. This caused the aircraft to veer sharply to its left while continuing to accelerate. The PF reacted again to apply full right rudder input combined with asymmetric braking and applied maximum thrust reversers in an attempt to stop the aircraft. The aircraft eventually came to rest to the left of the runway at 300 meters from the threshold (fig.1). During this event, the ground speed did not exceed 31 kt.

(fig.1) View from above of the aircraft trajectory 

The root cause of this event was the initial difficulty to control the aircraft laterally due to the rapid asymmetric thrust increase at low speed. We will analyse this phenomenon in the following paragraphs and explain how the pilots can ensure a symmetric thrust increases to ease the lateral control of the aircraft in the early takeoff roll.

On all jet engines, but particularly on high bypass ratio engines, the engine acceleration profile is not linear (fig.2). It follows the engine control law that is defined to optimize the acceleration in a way that the risk of engine stall is reduced. It also takes into account the influence of the position of the engine installed on the aircraft and the effect on the airflow at the engine’s inlet due to its proximity to the ground and the surrounding aircraft structure. 

Every engine has its own performance level due to manufacturing tolerances. In addition, engine performance evolves with time due to wear and ageing. As a consequence, the acceleration profiles may slightly differ from one engine to another on an aircraft (fig.3), even if fitted with new engines. 

Similarly, the idle thrust can slightly differ from one engine to the other, moving the acceleration profile to the left on the graph (fig.4).

Taking into consideration both of these parameters, if the flight crew applies the takeoff thrust directly from idle thrust, without doing any stabilization step, the difference in engine acceleration performance could cause a strong asymmetric thrust condition (fig.5) that could be difficult to counteract with nose wheel steering only, due to limited effectivity of the rudder at low speed. 

To avoid this potential strong thrust asymmetry, the FCOM SOP for takeoff provides a procedure that requests pilots to apply takeoff thrust in two distinct steps (fig.6) with some additional guidance for certain aircraft operating in the case of tailwind or significant crosswind conditions. 

(fig.6) Example of the standard thrust setting procedure at takeoff for an A320 aircraft depending on the engine type.

The stabilization step ensures that all engines reach a rotation speed value from where the increase of engine thrust will be almost identical to each other (fig.7). The N1/EPR/THR stabilization value is defined during flight test campaign for every engine type with collaboration from engine manufacturers.

Specific use-case when differential thrust is used for the aircraft line-up

In some cases, differential thrust is used to line-up the aircraft on the runway. If the pilot commands takeoff power without first doing the engine thrust stabilization step, the resulting asymmetric thrust condition may be significant due the engines accelerating from an already very different rotation speed.

This is why a thrust stabilization step is important after using differential thrust to line-up the aircraft, to avoid causing a strong thrust asymmetry condition during the early stages of the takeoff roll.

Static takeoff or rolling takeoff?

The FCOM describes procedures for a static takeoff, but a rolling takeoff is also permitted. 

During static takeoff, the brakes are released at ts (fig.7) once all engines have reached the stabilization step value, therefore, the aircraft is not affected by any potential thrust asymmetry that can happen between t₀ and t_s.

If a rolling takeoff is performed, the flight crew must also respect the stabilization step. The pilot uses opposite rudder pedal inputs as the aircraft is rolling to counteract any thrust asymmetry experienced during the stabilization phase between t₀ and t_s. With the engines at a low rotation speed the potential thrust asymmetry remains limited up to the engine stabilization step value.

In tailwind and significant crosswind conditions, the airflow entering into the engines is modified (fig.8). Some perturbations may appear downstream of the leading edge of the engine inlet and potentially cause an engine stall if the perturbed airflow enters the core of the engine.

The FCOM thrust setting procedure in the case of tailwind or significant crosswind is in two steps:

• Step one is to ensure engines increase their thrust symmetrically by using the stabilization step.
• Step two is acceleration of the aircraft with the pilot progressively increasing thrust from the stabilization step value to reach takeoff thrust. As the aircraft accelerates the relative wind resulting from the forward momentum counters the disturbed airflow conditions caused by crosswind or tailwind, reducing the risk of engine stall and the risk of experiencing the associated thrust asymmetry.

In the case of an asymmetric thrust event at takeoff, the flight crew should reject the takeoff if the veering moment cannot be counteracted using nose wheel steering.

The technique described in the FCTM “engine failure at low speed” should be applied:

• Immediately reduce all thrust levers to IDLE
• Select all reversers
• Use rudder pedals for directional control, supplemented by symmetrical or differential braking if needed

During the takeoff roll, a key role of the Pilot Monitoring (PM) is to monitor engine thrust. The PM must monitor the N1/EPR below 80kt, and announce “THRUST SET” when the takeoff thrust is reached for all engines. Should any thrust asymmetry be observed, the PM must immediately inform the PF and may call for a rejected takeoff if needed.

The event described at the beginning of this article is also the opportunity to remind flight crews with some recommendations to facilitate the lateral control of the aircraft. 

(Except on A380) - The FCOM also requests pilots apply a half-forward side stick input, or full forward input if there is tailwind or significant crosswind, up to 80kt IAS to counter the nose up effect of the thrust application and then release this input gradually to reach the side stick neutral position at 100kt. This will increase the load on the nose wheel to aid in directional control of the aircraft.

Since the flight crew can only rely on the nose wheel steering and to the last extend differential braking to maintain the centerline during the low speed part of the takeoff roll, they must make sure that they are properly seated. The pilot should be able to move the rudder pedals to their maximum deflection and apply maximum manual braking at the same time, should a rejected takeoff be initiated. 

An incorrect pedal adjustment can make it difficult for the pilot to apply differential braking when needed.  It can even lead to an opposite differential braking if the pedals are too far from the pilot. 

Refer to the FCTM and to the "Are you properly seated" Safety first article published in January 2018 for more information on adjustments to achieve optimal pilot’s seating position.