The Weekly Reflektion 09/2022

A good risk assessment requires a comprehensive overview of the actual and potential hazards and threats, and scenarios on how these will manifest themselves. This means using the imagination.

The port engine of the Boeing 737 Southwest Airlines Flight 1380

Do you encourage the participants in risk assessment to use their imaginations?

Risk assessments are an important tool for ensuring the safe operation of our facilities. The risk assessments are used to determine what risk mitigation measures should be in place and whether the risk is acceptable or not. The methodology behind risk assessment includes identifying hazards and threats, estimating probabilities and consequences to determine the potential in these, evaluating the uncertainties including the strength and quality of knowledge and information, and identifying individual and collective measures that will mitigate the risk. The risks are then assessed against qualitative and/or quantitative acceptance criteria to determine their acceptability. The methodology often ends up as a linear process from hazard to acceptability and this process does not always encourage an imaginative approach.

Southwest Airlines Flight 1380 took off from New York LaGuardia airport on 17th April 2018 on a scheduled slight to Dallas. The aircraft was a Boeing 737-7H4. During the flight there was a ‘contained engine’ failure in the port engine. The cause of this failure was a fatigue crack in one of the fan blades that caused the blade to break off. The blade smashed through the engine cowling and debris from the impact hit the wing and the window at seat 14A causing an explosive depressurisation in the cabin. The unfortunate passenger in seat 14A was sucked out of the window only to be held in place by her seat belt. Despite efforts from the crew and other passengers her injuries were fatal. The crew recovered control of the aircraft, carried out an emergency decent and landed safely at Philadelphia International Airport.

The investigation found that the fatigue crack had probably started six years previously and had not been picked up during routine inspections. The inspection technique of dye penetration that was used was good enough. Inspections with ultrasound were subsequently made mandatory. The engine cowling is designed to tolerate fan and turbine blades that break off and should have contained the blade and preventeddamage to the aircraft. Further investigation revealed that the broken fan blade had hit the cowling where the three latches that hold the two sections of the cowling in place were located. This caused the cowling to fail and allowed other debris from the still spinning engine to hit the wing and fuselage. Essentially, here was a weak point in the cowling.

Should the people carrying out the risk assessment have identified this weakness? The linear approach to risk assessment does not necessarily encourage people to use their imagination. Sometimes we should ask the participants to construct a major failure and realistic scenarios for how it could happen. For example, instead of what is in place to prevent the fan blade penetrating the engine cowling, what can we do to make the fan blade penetrate the engine cowling. Once the scenarios are established, then the team can focus on what measures need to be in place to prevent these and how to make these reliable and robust. 

Do you have a risk process that encourages people to use their imaginations in identifying hazards/threats, constructing scenarios, and assessing risks?

Reflekt AS