The Air India Express B-737 800 aircraft on a flight from Dubai to Kozhikode on 7th Aug 2020 ended up reminding the aviation fraternity and the world about the fragility of life. The crew and the aircraft had everything going against them. Murphy’s law playing up to the hilt as the aircraft slid down the end of the runway to meet her fate. At this moment every possibility is open, even the most bizarre scenario.
Soon, there was a media frenzy and multiple theories were being whipped up. The blame game had already begun even before the ashes of the brave ones had cooled. I have been quite vocal about the lack of a generative safety culture and disregard for human factors, especially in investigations. This blog is only to make one think of other possible scenarios instead of the one obvious one.
Word of caution: The information is generic but may be important for discussion
Not speculating but presenting past accident facts to balance the perception
I am not speculating the cause of the accident but merely presenting a few scenarios so that those who can influence or participate in the investigation must also consider or rule out these possible scenarios. There is always a combination of factors to produce an accident. Fatigue, Weather, Unique Technical Situations etc are a few which may have prevailed on the day of the accident.
- Optical Illusions
- Thrust Reverser/Speed Brake deployment failure (technical)
- Human Factor. Fatigue, Stress & Cognitive Lockup
- Read the paper on Cognitive Lockup
Table top runways
A tabletop runway is a runway that is located on the top of a plateau or hill with one or both ends adjacent to a steep precipice which drops into a deep gorge. The landing distance available to the crew when flying an instrument approach (ILS) is 7886 feet. Enough has been covered in the media on the risk associated with a tabletop runway and the mitigating actions.
Sight, supported by other senses, allows a pilot to maintain orientation while flying. However, when visibility is restricted (i.e., no visual reference to the horizon or surface detected) the body’s supporting senses can conflict with what is seen. When this spatial disorientation occurs, sensory conflicts and optical illusions often make it difficult for a pilot to tell which way is up.
Some of these illusions can affect the pilot’s decisions or improper control inputs.
Heavy rain on the windshield magnifies the pilot’s vision and gives an optical illusion of being too close to the runway. There is a possibility of the pilot flaring high (pitching the nose up early) and thereby floating long or deep inside.
Stopping devices not engaging coupled with Human factor
The B-737 has 3 types of devices which are used in combination to reduce speed as soon the aircraft lands.
- Engine Reverse Thrust: The airplane‘s thrust reverser system is designed to help the airplane decelerate after landing by diverting the direction of the engine exhaust gas stream. The reverser system arms when the radio altimeter system senses an altitude of less than ten feet, or when the air/ground safety system transitions to ground mode. Movement of the reverse thrust levers is restricted until the forward thrust levers are in the idle position. Once the system is armed and the airplane is on the ground, aft movement of the reverse thrust levers actuates the reverser sleeves.
- Over wing speed brake :The airplane‘s automatic speed brake system consists of six panels on the upper surface of each wing that can be automatically or manually deployed at touchdown to disrupt the airflow over the wings, maximizing the weight on the landing gear and increasing the wheel brakes‘ effectiveness. Compression of the right main landing gear strut opens the ground spoiler shutoff valve, deploying the four ground spoilers. If wheel spin up is not detected, transition of the air/ground logic system from air mode to ground mode (resulting from compression of either main landing gear strut) causes the speed brake lever to move from the armed position to the ground deploy position.
- Wheel Brakes: Not the main focus
Inadequate braking can increase the landing roll substantially.
Runway OverrunAmerican Airlines Flight 2253Boeing 757-200, N668AAJackson Hole, WyomingDecember 29, 2010
On December 29, 2010, American Airlines flight 2253, a Boeing 757-200, ran off the departure end of runway 19 after landing at Jackson Hole Airport (JAC),2Jackson Hole, Wyoming.The airplane came to rest about 730 feet past the departure end of the runway in deep snow.
The thrust reverser did not deploy nor did the speed brake. The first officer (the pilot flying) reported that he tried to deploy the thrust reversers promptly after touchdown, but they did not initially deploy. After the first officer made several attempts to deploy the thrust reversers, the captain took over the thrust reverser controls and eventually succeeded in deploying the thrust reversers with about 2,100 feet of runway remaining.13Subsequently, the airplane continued off the departure end of the runway, coming to a stop in deep snow off the end of the paved surface. Both pilots stated that they were unaware until after the airplane came to a stop that the speed brakes, which they had armed for automatic deployment, had not automatically deployed after touchdown as they expected.
Technical analysis by National Transportation Safety Board, USA
The precise timing of the unloading of the main landing gear just after touchdown that coincided with the deployment of the thrust reversers resulted in a rare mechanical/hydraulic interaction in the thrust reverser system, and the thrust reversers were locked in transit instead of continuing to deploy. Further, an unrelated defect in the automatic speed brake mechanism prevented the speed brakes from automatically deploying. Although the pilots could have manually deployed the speed brakes at any time during the landing roll, neither pilot recognized that the speed brakes had not automatically deployed (as selected) because they were both distracted by, confused by, and trying to resolve the thrust reverser non-deployment.
The 757‘s air/ground sensing system provides air/ground status information to various airplane systems, including the automatic speed brake and thrust reverser control systems. Two proximity sensors on each of the airplane‘s two MLG assemblies provide ―ground‖ signals to the air/ground sensing system when both MLG assembly tilt angles reduce from about 9.6°rear-wheels-down when the gear is extended during flight to less than about 5.4°rear-wheels-down during touchdown. When all four proximity sensors sense that this has occurred, ―ground‖ signals are sent to pertinent airplane systems, allowing their activation. The air/ground signal will cycle from ―ground‖ mode to ―air‖ mode and back to ―ground‖ mode again if any one of the four proximity sensors on the MLG assemblies momentarily ―unloads‖ (fails to meet the specified tilt angle) after touchdown. The system was fully functional.
Thrust Reverser: During the incident landing, a momentary interruption in the ―ground‖ signal from the air/ground sensing system occurred almost immediately after the thrust reversers began to extend. Such interruptions in the ―ground‖ signal are not unusual (commonly occurring during bounced landings, for example). Under normal circumstances, such interruptions are benign and go undetected by pilots because the thrust reversers continue to deploy automatically when the air/ground ―ground‖ signal resumes with no further pilot action required. However, during the incident landing, the thrust reversers locked in transit and did not continue to deploy. The pilots made multiple attempts to deploy the thrust reversers after the air/ground sensing system returned to ―ground‖ mode; however, the thrust reversers did not deploy until about 18seconds after touchdown.
What prevented the Thrust Reverser to deploy?
A detailed review of the thrust reverser control system design identified one potential scenario in which the momentary change from ―ground‖ mode to ―air‖ mode could cause each engine‘s thrust reverser sync-lock mechanism to lock in transit. Such a lockout could only occur if a momentary change from the ―ground‖ mode to the ―air‖ mode occurs in the instant (1) immediately after the thrust reversers begin to extend after touchdown and(2) in the split second before the thrust reverser‘s auto re-stow system is activated. This lockout would prevent movement of the thrust reversers until about 5 seconds after a pilot moves the reverse thrust levers back to their stowed position, allowing the thrust reverser system to deactivate and begin deployment again when commanded.
Automatic Speed Brake: To deploy the speed brakes automatically, the pilots should move the speed brake lever to its ―armed‖ detent before touchdown. By design, when the speed brake lever is ―armed,‖ the speed brake actuator automatically drives the speed brake lever to its full aft position after the airplane touches down (indicated by the air/ground sensing system signal‘s transition from ―air‖mode to ―ground‖ mode). This normally results in the deployment of the speed brake panels to their fully deployed position.However, if the air/ground sensing system reverts back to ―air‖mode after the automatic speed brake actuator has begun to extend, the speed brake actuator will retract automatically and retract the speed brake lever.If the air/ground sensing system signal subsequently transitions back to ―ground‖ mode, the speed brake system is designed to again automatically drive the speed brake lever to extend the speed brakes.
Initial examination and testing of the incident airplane‘s automatic speed brake system and its components revealed no evidence of a malfunction that would have prevented normal operation during the incident landing.
What prevented the Speed Brake to deploy?
During the post-incident examination, the NTSB discovered that a bushing had not been installed in the automatic speed brake actuator‘s aft mounting attachment. This defect would not have prevented the automatic speed brake actuator from operating. This condition would only occur when the actuator was attempting to drive the speed brake lever towards the ―up‖ (speed brakes extended) position and would not occur when the actuator was retracting the speed brake lever. Further, it was noted that this defect only affected the speed brakes‘ automatic deployment function and would not have prevented the pilots from manually deploying the speed brakes.
Therefore in everyday flying, this defect would not have been noticed till this uniques situation arose due to smooth landing or a slight bounce and simultaneous deployment of thrust reversers.
B-737 800 Flight Crew Training Manual
Unless speed brakes are raised after touchdown, braking effectiveness may be reduced initially as much as 60%, since very little weight is on the wheels and brake application may cause rapid antiskid modulation. Normally, speed brakes are armed to extend automatically. Both pilots should
monitor speed brake extension after touchdown. In the event auto extension fails, the speed brake should be manually extended immediately. Pilot awareness of the position of the speed brake lever during the landing phase is important in the prevention of over-run.
Runway Overrun and CollisionSouthwest Airlines Flight 1248Boeing 737-7H4, N471WNChicago Midway International AirportChicago, Illinois December 8, 2005
On December 8, 2005, Southwest Airlines (SWA) flight 1248, a Boeing 737-7H4, ran off the departure end of runway 31C after landing at Chicago Midway International Airport, Chicago, Illinois. The airplane rolled through a blast fence, an airport perimeter fence, and onto an adjacent roadway.
The National Transportation Safety Board determined that the probable cause of this accident was the pilots’ failure to use available reverse thrust in a timely manner to safely slow or stop the airplane after landing, which resulted in a runway overrun.
On the accident flight, the reverse thrust was not selected until approximately 15 seconds after touchdown. The captain had stated that he attempted to deploy the reversers but was having difficulty. Following the touchdown, he further stated that he noticed a large amount of anti-skid system activity and sensed that the airplane had stopped decelerating. This prompted him to cease his attempts to deploy the reversers and to begin maximum effort manual braking. Fifteen seconds after touchdown, the first officer noticed that the reversers had not been deployed, and deployed the reversers himself. The reversers were fully deployed approximately 18 seconds after touchdown and approximately 500 feet from the departure end of the runway.
Cognitive Lockup (Human Factor)
Moray and Rotenberg (1989) have defined the term ‘cognitive lockup’ as the tendency of operators to deal with disturbances sequentially. Cognitive lockup, however, does not occur when people can perform all their tasks consecutively. Cognitive lockup can also be defined as holding on to a task or sticking to a problem. In terms of the task-switching paradigm, cognitive lockup can be considered as reluctance to switch to an alternative task or problem. (Meij, 2004).
Experiments on cognitive lockup
Research on fault management in process control (Moray, N., & Rotenberg, I. 1989), reveals the onset of “Cognitive lockup” when faults in the system are simulated. When multiple faults are triggered, the sequence of preferred fault management by operators of thermal-hydraulic systems is sequential. The result of the research was that operators liked to focus and complete one fault or if it can be replaced by stating one task at a time. There is a strong cognitive lockup, which restricts the operator’s information capability. The subsequent fault is noticed but no action is taken, till the handling of the first one is completed.
Factors influencing cognitive lockup
Sunk cost fallacy
Individuals commit the sunken cost fallacy when they continue a behavior or endeavor as a result of previously invested resources (time, money or effort) (Arkes & Blumer, 1985). This fallacy, which is related to status quo bias, can also be viewed as bias resulting from an ongoing commitment.
The project completion hypothesis—have shown that individuals become more willing to allocate resources to the invested option as goal attainment nears and goal completion becomes more important than economic concerns (Boehne & Paese, 2000). Garland and Conlon (Garland and Conlon,1998) stated: “as progress moves forward on a project, completion of the project itself takes increasing precedence over other goals that may have been salient at the time the decision was made to begin the project”. When task completion is high, the probability of cognitive lockup increases.
That means, in case people deal with a task, and another more urgent task is triggered, people tend to stick to the current task when they have almost completed this task. People have the tendency to stick to their current task when 90% or more of the total stages of a task have been completed (Boehne and Pease, 2000; Garland and Colon, 1998).
Time and task pressure
There are typically two types of pressure on pilots. Time pressure and task pressure. Since the aircraft is constantly moving, there is a finite amount of fuel, which relates to time. Nearing the destination the fuel remaining is sufficient to approach and land and additional fuel to divert if required, and hold for 30 minutes prior to landing at the alternate. The fuel remaining at approach is approximately 25% of the total fuel uplifted and the fuel required for approach approximately 85% of the total fuel required for approach and landing.
From the perspective of time, approximately 95% of the flight is completed and the two event amount to 70-80% of the remaining time. Time pressure is dependent on the number of tasks to perform at a given time. Time pressure is high when there is a perception that the time is scarce. According to a study on man-machine system design (Beevis, 1992), people experience time pressure when the time required to execute the task is more than 70% of the total time available to complete the task.
Study on the influence of time pressure.
Framing effect (Tversky & Kahneman, 1981) is a decision problem based on the decision maker’s perception of the problem, formulation of the problem and partly by norms, habits and personal characteristics. A problem can be framed and presented with a positive and a negative connotation, despite having the same end result. There is a tendency for the decision-maker to shift from risk aversion to risk-taker.
The pilots are trained and the policies are defined to indicate that the primary task is to fly from departure to destination and divert to alternate aerodrome if landing at the destination is not possible. The pilots flying the approach are under self-imposed task pressure to land at the destination and the diversion to alternate is taken in a negative connotation. However, if the policy is redefined to a word that the primary task of the pilot is to fly from departure to alternate aerodrome if landing at the destination is not possible, then the pilot’s task completion pressure is substantially reduced.
Pilots approaching the destination have invested a lot of time in their task and it is nearing completion. Task pressure of completing the flight and the framing of the policy with the primary task of landing at the destination increases the possibility and effect of cognitive lockup. As a result, the pilot will continue with the first task, that of landing at the destination, despite being unstable on approach or when performing a long landing. Carrying out a go-around can be inferred as task switching. This task will be carried out provided there is enough time to realize the consequences of persisting with the primary task. Since there is not enough time and the task completion is within sight, the pilots will continue and land.
Training has an effect of reducing cognitive lockup by increasing practicing task switching that of approach/flare followed by switching to the task of a go-around and reattempting a second time.
The policy, if framed to depict a go-around and a diversion in a positive light will reduce the pressure of task completion from the pilots and they would be more prone to switching the task to go-around with ease.
Cognitive lockout is the primary reason for the reluctance to go-around. If task switching practice is increased, as compared to other tasks, in the training, there will be a significant drop in the number of unstable approaches and long landings.
RUNWAY OVERRUN ON LANDING AMERICAN AIRLINES FLIGHT AA331 BOEING 737-823 KINGSTON, JAMAICA (MKJP)22 DECEMBER 2009
The aircraft exited the paved surface at 62 Kts and the impact was estimated to be between 10-12 G. There was no fire and the crew sustained minor injuries. The flight experienced heavy rain on finals. They landed late 4100 feet beyond the threshold with 14 kt tail wind, Speed brake and thrust Reversers were deployed.
The crew did not attempt to go-around. Note: The position of the throttles are full forward but Go around was not attempted as per the FDR. This could be due to the G load at impact when there is rapid forward movement of all objects.