It takes two to tango, Jet Airways runway excursion analysis

 

Synopsis

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.

Final resting position
DGCA Final report

Cause

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.

Ground trajectory
Airbus material

Event Analysis

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.

Two step stabilisation
Two step stabilisation
Airbus images

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.

GoAir passengers survived a wrong engine shutdown, where British Midland and TransAsia couldn’t

Go Air wrong engine shut down
 

Synopsis

The GoAir flight 338, an A320 was accelerating for takeoff on the runway on 21st Jun 2017 when the aircraft suffered a bird hit on Engine 2. The crew continued the takeoff but noticed abnormal sounds and vibrations. After takeoff the crew did not follow the documented procedure as a result identified the wrong engine and shut down Engine 1. The co-pilot read out N1 as ENG number 1, thereby giving incorrect input to the Captain. The aircraft climbed to 2500feet and soon the crew realized their mistake and attempted starting Engine 1. They failed to start in the first attempt and by the time the engine was eventually started, the aircraft had descended to 1800 feet and lost speed to a near stall when the Airbus stall protection feature, the alpha floor activated to prevent the impending stall. The aircraft landed safely, thereafter.

Go Air investigation report

Errors

Errors typically are defined as deviations from a criterion of accuracy. However, no clear standard of “correctness” may exist in naturalistic contexts. The “best” decision may not be well defined, as it often is in a highly structured laboratory task. Second, there is a loose coupling of event outcome and decision process, so that outcomes cannot be used as reliable indicators of the quality of the decision. Redundancies in the system can “save” a poor decision or error. Conversely, even the best decision may be overwhelmed by events over which the decision maker has no control, resulting in an undesirable outcome

Decision making

Intuitive decisions taken based on the famous concept of Thin Slicing (Gladwell,2005) help in faster decisions based on limited exposure. Sometimes it refers to using only a small slice of the available information for decision-making and ignoring the rest.At other times it implies compressing a great deal of information into a simple underlying pattern to be used in snap decision-making.The term is also used to refer to the simple underlying pattern itself and sometimes to the thin slice of time in which rapid cognition occurs. Thin slicing works well in day to day life when the risk levels are low, however when the risk is high, normative decisions and slowing down thinking (Kahnemann,2011) helps in cognitive ease and accuracy of actions.

Read More

Study reveals serious deficiencies in instrument scan by Pilot Monitoring

 

scan-your-flight-instruments-n.jpg

Definition of Pilot Monitoring

Monitoring is an overarching process requiring knowledge, skills and attitudes that enables flight crews to perform safely, effectively and efficiently. Monitoring includes the process of observing and creating a mental model, by seeking out available information to compare actual and expected aircraft state (IATA Guidance material on improving Flight Crew monitoring).Monitoring requires a combination of cognitive resource allocation such as attention, and a link to previously acquired knowledges (scripts and scenarios), which allow a pilot to detect, understand, project into the future, and then take the right decision/action.

Study by BEA on approach and go-round

Towards the end of 2000, the BEA (French authority for safety investigators) initiated a study after they observed a number of accident or serious incidents caused by a problem relating to airplane state awareness during go-around.

Read the BEA study: Aeroplane State Awareness during Go-Around

These events seemed to have some common features, such as startle effect, the phenomenon of excessive preoccupation by at least one member of the crew, poor communication between crew members and difficulties in managing the automatic systems.

Full flight simulator with eye tracking devices were used to carry out the study. Following are the results.

Screen Shot 2018-12-18 at 4.35.47 AM

Result of the Simulator session

No flight crew applied strictly the go-around specified go-around. Deviations were noted with regards to:

  • Who (PF or PNF/PM) performed the actions in the gear and flap retraction sequence;The PF’s callout of the go-around;Reading the FMA modes (incomplete or non-existent);Monitoring the flight path;Cross-checks;Reducing the thrust (deliberate disengagement of the auto thrust);Technical callouts;Teamwork (significantly disrupted).

Key finding relating to Pilot MonitoringThe survey indicates that many pilot monitoring (PM’s) do not know where and when to look during a go-around. Pilots are looking for a way that would help them maximize performance of all the actions required while maintaining a high level of monitoring. Without training on the visual scan to use, it is difficult to imagine that a pilot knows how to organize it for a procedure that is rarely undertaken and which requires a lot of actions.

Screen Shot 2018-12-18 at 4.35.58 AM Zone of interest

The figure below shows the average time, as a percentage, that the PF and PNF/PM spent viewing each Zone of Interest.

Screen Shot 2018-12-18 at 4.39.52 AM.png

Summary

To cope with the task overload, the flight crew had to prioritise their actions. When questioned on this subject during debriefing, the PFs were almost unanimous regarding their priorities: “pitch, thrust or management of the flight path”. The replies from the PNF/PMs were more complex and varied. They referred to the “callouts” to be made and to the various “monitoring tasks” such as “positive (rate of) climb, ensure that the aeroplane is climbing, flight path, pitch attitude and thrust”.The crews that experienced difficulties made adaptations to the procedure. Some adaptations had positive effects (approach to interception altitude); others led to deviations from the expected result (flight path, for example).Assessment on the role of PMThe performance of the monitoring function is essential but insufficient during the go-around. During performance of the go-around studied, the PM’s attention was focused on the actions to take and not on their monitoring. It is therefore necessary to focus on this issue in particular during initial training in MCC and then assess the results during ongoing and recurrent training.

Recommendation on role of CRM

The study highlighted the difficulties of maintaining a good level of CRM throughout a go-around. The priorities of the PF and PM are different. Their respective workloads limit their interaction and mutual monitoring of actions. Although fundamental, current CRM alone cannot constitute a reliable safety barrier in the case of disruptive elements. In general, whatever the type of recent accident, investigative findings often point to shortcomings in CRM.

mindFly analysis human factor

My observation is that the instrument scan sequence is no longer taught in the training as was done in the non-EFIS aircrafts. The “T” scan sequence was an integral part of the training and critical for instrument flying. With modern EFIS aircrafts, the ‘T” scan is lost due over reliance on automation and low arousal effect. The pilot monitoring scan has deteriorated even further thereby eroding an important barrier.The OEM and training departments need to review and advise the pilots on then new and effective means of instrument scan for various phases of flight.

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