1 The Accident as a Situational example
You are the captain of a twin-jet on a 35-minute flight leaving in the late afternoon. It is the crew’s fourth and last leg of a journey that started in mid-morning. The automatic terminal information service (ATIS) at the destination indicates scattered clouds at 5,000 ft, fair visibility in haze, wind 150 at 8 kt. Air traffic control (ATC) has cleared you to cruise altitude and to expect Runway 18R at the destination.
You observe two scattered thunderstorm cells on your weather radar, one south and the second east of the airport. After coordinating with ATC, the controller clears you for descent with a northerly heading to turn around a cell.
Soon afterward, you are cleared to 6,000 ft and thereafter to contact approach control, which requests you to maintain 4,000 ft for Runway 18R.
You acknowledge, then brief the approach with your first officer, the pilot flying, who has visual contact with the airport but is flying the ILS (instrument landing system) approach as a backup. ATC then issues a clearance to turn right 10 degrees, descend and maintain 2,300 ft for a visual approach to Runway 18R.
At just that time, the supervisor in the tower says it is “raining like hell” at the south end of the airport. ATC amends your clearance to maintain 3,000 ft and mentions “some rain” just south of the field that might move north.
Do you suspect something?
Minutes later, the controller vectors you to turn right to 170 degrees, 4 nm (7 km) from the outer marker for the Runway 18R ILS, to cross it at or above 3,000 ft.
As you are maneuvering the airplane from base leg of the visual approach to final, you and the first officer have visual contact with the airport. The tower clears you to land on Runway 18R and to follow a similar-size airplane on short final, who reports a smooth ride just as the previous one had.
At just about that time, a special weather observation is recorded: ceiling 4,500 ft broken, 6 mi visibility, thunderstorm, light rain, haze, wind 110 at 16 kt. This new ATIS information is not broadcast until 15 minutes later, so you never receive it.
Three minutes later, the tower says that the wind is 100 at 19, then a short time later at 21. The tower then issues a wind shear warning alert northeast, with wind 190 at 14.
What is your immediate reaction?
On final, the airplane enters the area of rainfall, and the first officer comments that he is adding 10 kt. You command him to go around and turn right. The go-around begins. The tower instructs you to fly runway heading, climb and maintain 3,000 ft.
Do you retract flaps and the gear?
The first officer initially rotates the airplane to the proper 15-degree nose-up attitude during the go-around. However, thrust is set below the standard go-around engine pressure ratio (EPR) limit, and pitch attitude is reduced to 5 degrees nose-down before both of you recognize the situation. While the flaps are moving from 40 to 15 degrees, the airplane encounters wind shear. Although you are equipped with an on-board wind shear warning system, it does not activate. The airplane stalls and hits the ground.
2 Data, Discussion and Human Factors
Approach-and-landing accidents account for 55 percent of hull losses and 50 percent of fatalities (Flight Safety Foundation, Flight Safety Digest, “Killers in Aviation,” 1999).
The following factors were involved in this accident:
- The crew avoided precipitation cells in the terminal area and prepared for an anticipated visual approach.
While entering the terminal area, the captain obtained the latest ATIS information and was concerned about heavy precipitation ahead, as shown on the airplane’s weather radar.
The crew coordinated several heading changes with ATC to avoid buildups and to ensure a smoother ride for passengers. ATC informed the pilots that they would be turning before reaching the radar-indicated precipitation.
This probably developed an expectation that the flight would indeed encounter better weather at the destination. While conducting the descent checklist and approach briefing, the crew prepared for a visual approach with instruments set for an ILS.
- Lacking significant information, the flight crew continued the approach into an area of heavy precipitation and wind shear.
Even though the approach controller noticed heavy precipitation cells popping up on his radar display, he did not tell the crew, which continued the approach unaware of the magnitude of the weather threat. At some point, the captain referred to a right turn that he was planning in the event of a missed approach to avoid the worst of the weather.
When the airplane was about 4.5 nm (8.3 km) from the runway, less than two minutes’ flying time, the airport’s low level wind shear alert system (LLWAS) alerted controllers to wind shear due to a thunderstorm. Once again, the controller failed to warn the crew.
The crew lacked specific information that the wind shear was rapidly becoming severe. A microburst of 30 fps was developing within the cell as the flight reached the area.
The crew continued in spite of ambiguous data about the feasibility of safely completing the approach. That was logical, as continuous landings and takeoffs throughout the approach may have contributed to some form of continuation bias with no timely decision making to go around or even to being prepared to go around.
- The captain commanded, and the first officer initiated, a go-around.
As soon as the crew noticed the difference between the airplane’s target speed and the actual airspeed, they understood they were encountering increased headwind, the performance-increasing portion of a microburst. The captain selected maximum go-around power, and the first officer initiated a go-around and turned right. The airplane was then barely 200 ft above the ground. However, power remained 9 percent below maximum thrust, an error likely caused by workload and stress. The captain retracted the flaps but not the landing gear.
For the pilots to be able to recognize that they needed to change from a normal go-around procedure, which emphasized speed, to the wind shear recovery procedure, which aimed to maintain pitch attitude, would imply that they had been monitoring, had noted and had interpreted the additional cues when the airplane’s performance began to deteriorate, with decreasing airspeed and poor climb capability. This is really difficult to achieve when under heavy workload, under time pressure and under stress. The flap system could also have prevented the wind shear warning system from functioning during retraction, as by design.
From an organizational point of view, the airline’s training syllabus did not adequately cover some of these issues. Further, even though the pilots grasped the implications of the situation, they never entirely understood that they had penetrated the performance-decreasing part of the microburst.
- The captain gave the command, and the first officer executed, a pitch-down during the wind shear encounter.
After he had pulled the airplane up to the 15-degree nose-up go-around attitude, and as the airplane went into the downdraft and increasing-tailwind part of the microburst, the captain ordered a reduction in pitch attitude, which the copilot promptly executed. That put the airplane into a steep descent.
Clearly defined cues about wind shear can pose problems for tactical decision making because swift action is difficult under conditions of high workload, stress, time pressure and uncertainty. Moreover, the difficulty of switching to the wind shear procedure is that it is the exception to maintaining safe target airspeed in all flight conditions.
- The crew struggled against the high sink rate but was unable to recover before impact.
A ground-proximity warning system (GPWS) warning was generated when the airplane was passing through 330 ft and continued until impact. The captain ordered firewall power consistent with wind shear recovery and terrain escape maneuvers. Recovery inputs included pulling back hard on the yoke, but it was countered by the stick pusher, thus reducing its effectiveness in the short time of 13 seconds. The initial wind component encountered during short final was between 10 and 20 kt. But a headwind of 35 kt had transitioned to a 26-kt tailwind (a 61-kt shift) within 14 seconds, causing the airplane to crash seconds later.
3 Prevention Strategies and Lines of Defense
Wind shear encounters are rare, but such occurrences require reviewing existing countermeasures.
- Make accurate risk assessments and tactical decision making based on:
- Weather and winds;
- Approach type; and,
- Risk of specific threats such as workload.
- Whenever there is a weather factor during approach, crews must focus more on being prepared for a go-around, fight the trap of continuation bias and be able to switch from a missed approach to the wind shear escape procedure.
- ATC controllers must constantly inform pilots of weather changes, and crews must maintain situational awareness, apply critical thinking and keep approach criteria on a par with wind shear criteria.
- Proper procedures for wind shear identification, avoidance and escape must be coupled with line-oriented flight training (LOFT) scenarios involving heavy workload, time pressure and stress to expose crews to these threats.
4 Key Points
This accident occurred, with loss of life, because the crew continued the ILS approach until they encountered wind shear, which they failed to recognize promptly and then to recover from effectively. However, the crew had received inadequate weather information, and their actions were comprehensible in both operational and cognitive terms.
- Insufficient clarity about weather threats led the crew to continue the approach until encountering a severe microburst.
- Operational behavior was consistent with routine concerns for reliability and practices of working around weather until proven unsafe.
- The go-around procedure was initiated before penetrating the dangerous part of the wind shear, thus making the switch to the wind shear escape maneuver difficult to decide on, the more so after slow identification of its effect.
- This continuation bias may have been reinforced by the silence of the on-board reactive wind shear warning system and by the absence of turbulence.
- Factors that shaped crew performance were also linked with typical human cognitive tendencies under high workload, time pressure and stress with tunnel vision, attention narrowing and difficulty in integrating ambiguous and multiple cues.
5 Associated OGHFA Material
6 Additional Reading Material
- International Civil Aviation Organization. Manual on Low-level Wind Shear and Turbulence, Doc 9817.
U.S. Federal Aviation Administration (FAA). Advisory Circulars (AC)
Safety Notice: Missed Approaches in Response to Onboard Windshear Alerts, 04 June 2013
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