The Boeing B-737-800 aircraft, operating a scheduled flight from Goa to Mumbai was involved in an accident at Goa.
The aircraft was configured for a flaps 5 departure. The calculated takeoff
speeds for 64.6 tons were V1 as 135, VR as 141 and V2 as 146. The crew
completed the before take-off checklist and started pushing TLA to increase the thrust. As per the statement of crew, after taking TLA ahead of 40%, PIC pressed TOGA for takeoff.
As soon as TOGA was pressed, the aircraft started drifting towards right.
Within 10 to 12 seconds of pressing TOGA, the aircraft went into unpaved
surface. As per the crew, they tried to apply brakes, rudder and use NWS to steer the aircraft, but due to heavy bumps could not apply control effectively. The aircraft went out of control and continued into unpaved surface.
The asymmetry in the thrust setting prior to TOGA application caused
the number one engine to increase thrust at a faster rate than the number 2 engine.
Airbus incident as published in Airbus repository
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. During this event, the ground speed did not exceed 31 kt.
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.
Takeoff thrust assemetry
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 that could be difficult to counteract with nose wheel steering only, due to limited effectivity of the rudder at low speed.
Need for two step thrust application
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. The N1/EPR/THR stabilization value is defined during flight test campaign for every engine type with collaboration from engine manufacturers.
Cross wind and tail wind takeoff
In tailwind and significant crosswind conditions, the airflow entering into the engines is modified. 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.
mindFly human factor analysis
The crowded airports and airspace has introduced higher stress for the operating crew. The workload is maximum during takeoff and landing phases. There is tight coupling of actions, communication and checklists. In the process, crew tends to act in haste or do not spend adequate time to analyse the situation in hand.
In order to break the error chain, there is a need to pause and slow down the action flow during these critical phases. A slight pause and scan of the visuals outside as well as the cockpit will break and error chain. The crew will be in a better position to dedicate their attention to the SOP required tasks rather than focus on getting airborne ASAP. Being mindful in the situation will save the day.