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Accident Case Study: Risk Stacking
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Accident Case Study: Risk Stacking

Introduction and Weather Conditions

This section introduces the setting of the story at Salt Lake City International Airport in Utah. The pilot is preparing to depart for Burley, Idaho, but is aware of the unpredictable spring weather in the area.

Weather Conditions and Decision Making

  • The pilot is departing from Salt Lake City International Airport in a Cessna 208 Caravan.
  • The forecast for Burley calls for VFR (Visual Flight Rules) conditions, but the pilot knows that spring weather in Idaho can be unpredictable.
  • The pilot had attempted the same run the day before but was forced to divert due to unfavorable conditions.
  • After receiving a full weather briefing and consulting with other staff pilots, the pilot decides that conditions are suitable for departure.

Flight Plan and Approach

In this section, we learn about the flight plan and approach chosen by the pilot.

Flight Plan and Approach Decision

  • The pilot's IFR (Instrument Flight Rules) flight plan will take her from Salt Lake City to Burley Municipal Airport using the GPS approach to Runway 20 at Burley.
  • The majority of the flight proceeds without any issues.
  • At 7:53am, about 22 nautical miles from Burley, the pilot contacts Twin Falls Approach.
  • Two minutes later, she reports being 10 miles from waypoint MALTT, and Twin Falls Approach clears her for RNAV runway 20 approach via MALTT transition at Burley Municipal Airport.

First Approach and Missed Approach Procedure

This section focuses on the first approach made by the pilot and what happens during a missed approach procedure.

First Approach and Missed Approach

  • The pilot begins her descent and follows the approach procedure with precise course tracking and adherence to step-down altitudes.
  • She descends at a rate of about 1,000 feet per minute from the final approach fix to the minimum descent altitude.
  • The pilot maintains a speed of 120 knots on approach in icing conditions until visual contact with the runway is established.
  • Visibility drops to 1 mile with mist and light snow, making it challenging for the pilot.
  • At the minimum descent altitude of 4,560 feet MSL (about 400 feet AGL), the pilot executes the missed approach procedure by continuing directly over the runway before making a climbing left turn.

Second Approach and Accident

This section covers the second approach made by the pilot and details about the accident.

Second Approach and Accident

  • After executing the missed approach procedure, the Caravan climbs to 7,000 feet and heads back to the initial approach fix for another attempt.
  • The pilot flies outbound from the initial approach fix on a procedure turn before turning inbound to intercept the final approach course.
  • Upon crossing waypoint IREME, she checks in at 8:26am.
  • The pilot begins a controlled descent but is slower this time, approximately 20 knots slower than during her first approach.
  • Half a mile from Runway 20, as she emerges below clouds in a wings-level descent, she flies into a plume of steam produced by stacks at a potato processing plant located directly on final approach to the runway.
  • The Caravan strikes one of these stacks and crashes onto the roof of the plant. Unfortunately, there are no survivors.

Investigation Questions

This section raises questions about why this accident occurred and what factors may have contributed to it.

Possible Factors Contributing to the Accident

  • The NTSB investigation aims to determine why the flight unraveled.
  • Small clues may offer insight into what happened.
  • Culture and mission pressure could have played a role, as the pilot had diverted on the same route the day before.
  • Visibility conditions may have been a factor, with visibility dropping significantly during the approach.
  • ADS-B speeds are being considered as potential clues in the investigation.

Conclusion

This section concludes by reflecting on the challenges faced by freight pilots and how various factors might have contributed to this tragic accident.

Challenges and Reflection

  • Delivering freight on time under tough conditions has been a proud heritage for freight pilots.
  • The pilot may not have fully recognized how low visibility had dropped at Burley before starting her first approach.
  • The METAR reports indicate that visibility dropped from 6 statute miles to only 1 mile during her approach.
  • The NTSB investigation will provide further insights into what led to this accident.

The Impact of Icing Conditions on the Approach

This section discusses how icing conditions can pose a significant risk during aircraft approaches, particularly in the case of the Caravan aircraft. The tail of the Caravan is prone to icing first, which can lead to loss of control.

Icing Conditions and Tail Icing

  • In icing conditions, the Caravan's tail is susceptible to icing first.
  • Tail icing can be treacherous and insidious, potentially causing loss of control.

Distractions and Causal Factors at Burley Airport

This section explores distractions and causal factors at Burley Airport that may have contributed to the accident. Specifically, it mentions steam stacks as potential distractions and highlights their usual insignificance but increased danger under specific weather conditions.

Steam Stacks as Distractions

  • The steam stacks at Burley Airport were typically considered a nuisance rather than treacherous.
  • However, on this particular day, weather conditions made them more dangerous.
  • Approaching the steam stacks directly downwind from their plumes could have been substantial due to cold temperatures.

FAA Guidance on Flying through Exhaust Plumes

  • The FAA's Aeronautical Information Manual warns against flying through exhaust plumes.
  • While the manual specifically mentions "smokestacks," it refers to heated steam emissions rather than smoke.

Potential Effects of Flying through Plumes

  • Flying through exhaust plumes can result in turbulence, reduced visibility, engine problems, icing, and flight control deficiencies.
  • These ill effects could have occurred at Burley Airport during poor weather conditions close to the ground.
  • Additionally, flying through a plume of heated air in a colder environment could mimic high density altitude effects and cause a perilous loss of lift.

Challenges of the RNAV 20 Approach at Burley Airport

This section focuses on the challenges posed by the RNAV 20 approach at Burley Airport, particularly due to the presence of steam stacks close to the runway threshold.

Vertical Descent Angle and Obstacle Clearance

  • The RNAV 20 approach at Burley Airport has a steep, 3.75-degree vertical descent angle.
  • This steep descent angle is necessary due to the steam stacks located directly on the runway centerline near the threshold.
  • Pilots must make an aggressive descent from the minimum descent altitude at the missed approach point to reach the runway.

Factors Influencing Decision-Making during Approaches

This section raises questions about how previous missed approaches and factors such as proximity to steam stacks may have influenced decision-making during subsequent approaches.

Influence of Previous Missed Approach

  • Did the first missed approach and steep descent angle influence the pilot's decision to descend low early on the second approach?
  • If so, did she consider the close proximity of steam stacks?

Indications on Approach Chart

  • The approach chart does not specifically mention steam stacks but shows obstructions at certain altitudes.
  • A light gray line (stipple) on RNAV approaches indicates a clear path from minimum descent altitude to the runway surface.
  • However, for Runway 20 at Burley, there is no gray stipple on the glide slope, suggesting obstacles are present that pilots need to see and avoid.

FAA Study and Safety Concerns at Burley Airport

This section discusses an FAA study conducted on steam stacks at Burley Airport and highlights safety concerns associated with airport design deficiencies.

FAA Study on Steam Stacks

  • In 2017, the FAA conducted an aeronautical study on the steam stacks at Burley Airport.
  • The study found that some stacks penetrated the Runway 20 approach surface area by up to 61 feet.
  • Despite this, the FAA determined there was no hazard to air navigation.

Airport Design Deficiencies and Funding Withdrawal

  • However, due to numerous other deficiencies in airport design standards at Burley, the FAA withdrew federal funding from the airport in 2018.
  • Safety concerns associated with these issues have led to discussions about relocating the airport for expansion and improvements to meet FAA standards.

The Perils of Risk-Stacking

This section emphasizes the dangers of risk-stacking, where multiple risk factors accumulate and create a negative synergy that even skilled pilots may struggle to overcome.

Accumulation of Risk Factors

  • Each individual risk element may not be dangerous on its own.
  • However, when multiple risk factors are stacked together (such as single-pilot operation, winter conditions, demanding approach, nonstandard obstacle clearance, nonstandard vertical descent angle, proximity to steam stacks emitting high temperature steam), they can lead to a negative accumulation of risks.
  • Even talented and motivated pilots may find it challenging to overcome such accumulated risks.
Video description

NTSB Final Report - https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/104938/pdf NOTE: At the time of this video’s release, the National Transportation Safety Board had not completed the final report on this accident. This video represents the AOPA Air Safety Institute’s understanding of events, compiling multiple sources, at the time of release. It’s a cold spring morning on April 13, 2022, when a cargo pilot in a Cessna 208 Caravan departs Salt Lake City International Airport in Utah. Her IFR flight plan takes her 133 nautical miles to the northwest, where she plans to fly the RNAV Runway 20 approach into Burley, Idaho. Join the AOPA Air Safety Institute as we examine the circumstances that led to a tragic outcome during the GPS approach at Burley Municipal Airport. We look at several risk factors - including weather conditions and a factory's steam stacks located below the flight path, close to the runway threshold - that turned out lethal during the final descent. ASI’s Accident Case Studies seek to share critical lessons to make us all safer pilots. Apply credit to your ASI transcript for watching this video: https://bit.ly/ACSRiskStackingCert NOTE: The video states that a new NOTAM has been issued for the Burley airport. Technically, it is a “Letter to Airmen” that was issued through the FAA’s NOTAM system. This LTA may not be found through all means of accessing NOTAMs, such as ForeFlight. We apologize for the discrepancy. The Aircraft Owners and Pilots Association (AOPA) is the largest community of pilots in the world, providing aviation advocacy, education & inspiration. AOPA has represented the freedom to fly for all pilots since 1939. To learn more about becoming a member visit https://www.aopa.org/jointoday. Check out the Air Safety Institute Website: https://www.aopa.org/training-and-safety/air-safety-institute Follow AOPA on Twitter: https://twitter.com/AOPA Follow AOPA on Instagram: https://www.instagram.com/flywithaopa/ Follow AOPA on Facebook: https://www.facebook.com/AOPApilots #aopa #flywithaopa #aviation #pilot #flying