Released 4 June 2018. Lack of contrast, poor visual references, attention alternation, high visual workload, narrowed visual attention, perceptual limitations, complex flight-path monitoring tasks, expectation bias, anticipation… Physiological and cognitive Human Factors at its best.
Some training and outdated regulations issues found as contributory factors.
Transportation Safety Board of Canada Aviation Investigation Report A17f0052. Risk of collision with terrain. West Jet Boeing 737-800, C-GWSV Princess Juliana International Airport, Saint Maarten 07 March 2017
History of the flight
On 07 March 2017, a West Jet Boeing 737-800 aircraft (registration C-GWSV, serial number 37158), operating as flight 2652 (WJA2652), was conducting a scheduled instrument flight rules (IFR) flight from Toronto/Lester B. Pearson International Airport (CYYZ), Ontario, to Princess Juliana International Airport (TNCM) in Saint Maarten, an autonomous country within the Kingdom of the Netherlands. There were 158 passengers, 2 flight crew members, and 4 cabin crew members on board.
Before departure, West Jet dispatch issued a flight release package to the crew containing all of the information pertinent to the flight, including current and forecasted weather and winds-aloft data, notices to airmen (NOTAMs), and airport and runway analysis data. The forecasted conditions for TNCM indicated wind from 070° true (T) at 16 knots, visibility greater than 6 statute miles-sm (One statute mile is equivalent to 5280 feet) in light rain showers, few clouds at 1800 feet, scattered cloud at 2200 feet, and another scattered cloud layer at 3000 feet.
The aircraft departed from CYYZ at 1137 (All times are Atlantic Standard Time (Coordinated Universal Time minus 4 hours), which is the local time in Saint Maarten). The planned duration of the flight was 4 hours and 24 minutes at flight level (FL) 350. The captain occupied the left seat and was the pilot monitoring (PM), while the first officer occupied the right seat and was the pilot flying (PF).
While approaching their destination, the flight crew listened to the automatic terminal information system (ATIS) information Mike, which was issued at 1501 and reported winds from 060º magnetic (M) at 18 knots, unlimited visibility, few clouds at 1400 feet, and no weather conditions of significance in the vicinity of TNCM. In addition to preparing for a visual approach to Runway 10, the flight crew loaded the RNAV (GNSS) approach to Runway 10 (Appendix A) in the flight management system (FMS) and conducted the briefing as a precaution in the event of a change in weather conditions.
At 1518:14, the TNCM air traffic controller (controller) instructed the flight crew to report when they were over the SLUGO waypoint (Figure 1) and to expect descent instructions at that time, and advised them that the current ATIS information was Mike and the altimeter was 30.09 inches of mercury.
At 1523:12, the crew was instructed to descend to 4000 feet, and, at 1523:12, they reported that they were over the SLUGO waypoint.
At 1524:36, the controller informed the flight crew of another aircraft that was on approach to TNCM, ahead of WJA2652, that rain showers were approaching the airport.
At 1524:48, the flight crew of WJA2652 was instructed to fly direct to the AVAKI initial approach waypoint (Figure 1) once they had descended through 8000 feet above sea level (ASL). One minute later, when the aircraft was approximately 25 nautical miles (nm) northwest of TNCM and descending through 9800 feet, the controller advised the flight crew that there were moderate to heavy rain showers at the airport. The crew did not acknowledge this information.
At 1526, ATIS information November was issued, indicating that visibility was 2000 m. The crew did not receive this updated visibility information, but they observed clouds and rain showers around the airport and decided to switch from the visual approach to the RNAV (GNSS) Rwy 10 approach. A minimum visibility of 3600 m (1.94 nm) is required for WJA2652’s aircraft category to conduct the RNAV (GNSS) approach.
At 1527:02, the controller instructed the flight crew a second time to fly direct to the AVAKI initial approach waypoint. One minute later, when the aircraft was 15 nm northwest of the airport and descending through 6700 feet, the controller directed them to descend to 2600 feet.
At 1528:56, when the aircraft was 13 nm west of the airport and descending through 4900 feet, the flight crew was cleared to fly the RNAV approach to Runway 10. Approximately 1 minute later, the controller informed the crew a second time that there were moderate to heavy rain showers at the airport. The flight crew acknowledged the information when they were 12 nm west of the airport and descending through 3700 feet.
At 1530:32, the flight crew of an aircraft that had just landed at the airport reported that there had been steady winds and reduced visibility during the approach, but that they had visually acquired the runway while over the MAPON missed approach point (MAP)(Figure 1). The WJA2652 flight crew acknowledged the information when they were 10 nm from the airport, on final approach and descending through 2100 feet.
At an undetermined point during the approach, because of the moderate to heavy rain showers, the controller illuminated the runway lights using an automatic setting for night operations, that sets the runway lights at 3% intensity and the precision approach path indicator (PAPI) lights at 10% intensity. The controller did not tell the crew about the change in lighting intensity and was not required by regulation to do so.
At 1532, when the aircraft was 4.5 nm from the runway and descending through 1600 feet, the flight crew were cleared to land and informed that the winds were from 060°M at 17 knots. The aircraft’s rate of descent varied between 700 and 800 feet per minute (fpm), and the aircraft was established on a 3° angle of descent. About 0.5 nm before crossing MAPON, the flight crew noticed a rain shower ahead and to their left; however, given that they had the shoreline in sight and expected to see the runway shortly afterwards, they decided to continue their approach visually. The PF disconnected the autopilot and reduced the pitch from 0.5° nose up to 1.2° nose down. Three seconds later, the engine thrust decreased from 62% to 52% N1. Shortly afterwards, the rate of descent increased to 1150 fpm, and the aircraft began to deviate below the 3° descent angle of the standard approach path. Approximately 2 seconds after the aircraft’s descent rate was increased, the crew cycled the flight directors, in accordance with WestJet’s approach procedures for landing at TNCM. The autothrottle changed from speed mode to ARM mode14 when the flight directors were cycled, and thereafter did not provide automatic thrust control.
At 1533:30, the aircraft crossed MAPON at approximately 700 feet above ground level (AGL). The PF indicated that he had the runway in sight and began to roll the aircraft to the left, deviating to a point approximately 250 feet left of the inbound final approach course. The flight crew saw neither the runway lights nor the PAPI lights during the approach, and did not request that the intensity of the lights be increased. After crossing MAPON, the aircraft entered the rain shower, which had moved toward the final approach path, reducing the visibility significantly. Eleven seconds later, when the aircraft was 1.5 nm from the runway on final approach and descending through 500 feet, the flight crew were advised that the wind was 060°M at 14 knots, gusting to 25 knots Approximately 1 nm from the runway, the aircraft exited the shower; the visibility sharply improved, and the crew realized that they had been tracking toward an incorrect visual reference, which was a hotel situated to the left of the runway. At this point, the aircraft was 190 feet AGL, descending at 940 fpm, rather than 320 feet AGL on a standard 3º angle of descent. Now able to see the actual runway, the crew recognized that the aircraft had deviated laterally to the left of the inbound final approach course, but they were not immediately able to assess their height above water. The PF advanced the throttles from 52% to 75% N1 and began to correct the lateral deviation, but the aircraft continued to descend at about 860 fpm.
At 1534:03, when the aircraft was 63 feet above the water, the aircraft’s enhanced ground proximity warning system (EGPWS) issued an aural alert of “TOO LOW, TERRAIN” and the PF increased the pitch to 4° nose up. The aircraft continued to descend, and a second aural alert of “TOO LOW, TERRAIN” sounded as it passed from 54 feet to 49 feet AGL (Figure 2).
At 1534:12, when the aircraft was 40 feet above the water and 0.3 nm from the runway threshold, the crew initiated a go-around. The lowest altitude recorded by the EGPWS during the descent had been 39 feet AGL.
After the go-around, the controller instructed the crew to conduct a holding pattern. Because the visibility was below the level required to conduct an approach (3600 m), the controller then closed Runway 10 for departures and instructed several other aircraft on approach to conduct holding patterns.
About 45 minutes later, the visibility at TNCM increased and WJA2652 was cleared for the RNAV (GNSS) Rwy 10 approach. The aircraft landed safely at 1618:19.
Injuries to persons
There were no reported injuries.
Damage to aircraft
There was no damage to the aircraft.
There was no damage to property or objects.
Aids to navigation
Navigational aids at TNCM include a non-directional beacon and a VOR (VHF omnidirectional range) with associated distance measuring equipment. Runway 10 is served by an RNAV (GNSS) approach and a VOR approach.
The aircraft was equipped with the appropriate navigational aids to conduct an RNAV (GNSS) approach, and there were no reported outages involving these aids at the time of the aircraft’s approach to TNCM.
TNCM has 1 asphalt runway (Runway 10/28), which is 7546 feet in length and 148 feet wide. Runway 10 is oriented 096°M; its threshold is displaced by 98 feet and its touchdown zone elevation is 12 feet ASL.
Runway 10/28 is equipped with a medium-intensity runway lighting system, which includes green threshold lights, red runway end lights, and white runway edge lights.
Runway 10 is not serviced by approach lights, but is equipped with a PAPI. At TNCM, the PAPI indicates a 3° angle of descent, and its lights are situated on both sides of the runway.
2. Aerodrome lighting intensity
In its Procedures for Air Navigation Services—Air Traffic Management (PANS-ATM), the International Civil Aviation Organization (ICAO) specifies the following regarding aerodrome lighting:
At aerodromes equipped with lights of variable intensity a table of intensity settings, based on conditions of visibility and ambient light, should be provided for the guidance of air traffic controllers in effecting adjustment of these lights to suit the prevailing conditions. When so requested by an aircraft, further adjustment of intensity shall be made whenever possible.
However, the Princess Juliana International Airport Air Traffic Services (ATS) Standard Operating Procedures Manual does not provide guidance to TNCM controllers on adjusting the intensity of the runway lights or PAPI lights. The operation of the lighting system at TNCM, including light intensity control, is at the controller’s discretion. By comparison, air traffic controllers in Canada are provided with direction regarding aerodrome lighting operation and when to employ specific intensity settings based on reported visibility.
The aircraft is equipped with a digital flight data recorder (DFDR) and a cockpit voice recorder (CVR). Because the occurrence was originally assessed by WestJet as a non-reportable event, it was not reported directly to the TSB. The CVR and the DFDR data were overwritten and were not available to the investigation.
The quick access recorder (QAR) data file was sent to the TSB laboratory for analysis. QAR data is recorded by the aircraft’s digital flight data acquisition and management unit, which stores the data on a Personal Computer Memory Card International Association (PCMCIA) card for flight data monitoring (FDM) purposes. The data on the card is an exact duplicate of that collected by the DFDR; however, the DFDR is crash-protected whereas the QAR is not, and the latter typically holds many more hours of data.
Tests and research
1. Simulator session
On 26 July 2017, the TSB conducted a session in one of WestJet’s 3 B737-700 flight simulators in Calgary, Alberta. The simulator, a full-flight, level D unit certified by Transport Canada (TC), is used to train WestJet B737-800 crews. The purpose of the session was to assess WJA2652’s approach under similar virtual weather conditions, while familiarizing TSB investigators with procedural workflow in the B737; with the operation of its autopilot, flight director, and navigation systems; and with the user interface of each of these components.
The simulator did not allow for an exact replication of all of the visual conditions at the time of the occurrence, such as the effect of the rain on the windshield. However, the exercise demonstrated a significant reduction in the visual discernibility of the runway environment when visibility diminished to 2000 m. It also highlighted the necessity for runway and PAPI lights to be illuminated at high intensity to clearly demarcate the runway edges under conditions of low visibility.
Through a series of approaches conducted in the simulator, the visual cues available to the flight crew were assessed, beginning from approximately 1000 feet AGL until the height at which the aircraft conducted its go-around, i.e. 40 feet AGL. The assessments indicated that, although the shape of a hotel to the left of the runway (Figure 3) differed from that of the actual runway, its discernible geometric features changed (as with most visual references) according to the approach angle and distance of the aircraft. From a distance, the hotel appeared wider at its base and narrower on top, similar in aspect to a runway. As the aircraft approached, however, its shape became more apparent as that of a building.
2. TSB laboratory reports
The TSB completed the following laboratory reports in support of this investigation:
- LP074/2017 − EGPWS Download
- LP124/2017 − Analysis of Maintenance Records
- LP054/2017 − QAR Data Analysis
3. Route and aerodrome qualification
WestJet produces a document known as a route and aerodrome qualification for each aerodrome at which it operates. The document provides flight crews with general information pertinent to their destination. WestJet’s Route & Aerodrome Qualification for TNCM provides crews with aerodrome-specific guidance that includes cautions, weather-related approach and departure minima, and procedures pertaining to enroute flight, approach and landing, and departure. The arrival procedure described therein states, in part:
RNAV 10 MAP located 2NM prior to threshold – ensure the autopilot is disengage [sic] and the FD cycled or disengaged prior to this point or aircraft will turn in LNAV [lateral navigation] at ONBED and thrust will increase. Be prepared for the thrust to annunciate “ARM” at this point and closely monitor speed. Manual manipulation of thrust or selecting “speed” is required.
1. Instrument approaches at Princess Juliana International Airport
There are 2 approaches at TNCM: VOR Rwy 10 and RNAV (GNSS) Rwy 10. During the occurrence, the crew was conducting the RNAV (GNSS) Rwy 10 approach.
The RNAV approach provides flight crews with lateral navigation information (LNAV) for the approach, starting at an initial approach waypoint fix and ending at the MAP. The vertical flight path management for this approach is assured by the crew.
In this occurrence, the FMS provided the crew with vertical guidance to the MAP during the inbound final approach course. Because there is a mountain close to the runway, the MAP is situated 2 nm (3704 m) before the runway threshold, and the MDA is at 700 feet ASL (688 feet AGL), to meet the PANS-OPS criteria for obstacle clearance in the event of a go-around (International Civil Aviation Organization (ICAO), Doc 8168, Procedures for Air Navigation Services: Aircraft Operations (PANS-OPS), Volume II, Construction of Visual and Instrument Flight Procedures, Sixth edition – 2014). Therefore, there is a long visual flight segment following the MAP where the crew is required to manage the descent to the runway threshold in order to complete the landing (Figure 4). It is not common for WestJet pilots to fly long visual segments of an IFR approach such as that of the RNAV (GNSS) Rwy 10 at TNCM. Even less common are long visual segments over water and with the type of weather encountered during the occurrence approach.
1.1. Minimum visibility to conduct an approach to Princess Juliana International Airport
When operating abroad, Canadian air operators must follow the laws, regulations, and procedures of both Canada and the foreign state in which they are operating. Under the Saint Maarten Civil Aviation Regulations, the minimum visibility requirements to conduct an approach are more restrictive than those in the CARs. The Saint Maarten Civil Aviation Regulations stipulate that an aircraft may not continue with an approach past the FAF unless visibility is “equal to or more than the minimums prescribed for that procedure.”
For WJA2652’s aircraft category, the minimum visibility for the RNAV (GNSS) Rwy 10 approach to TNCM is 3600 metres. Visibility must, therefore, be at least 3600 m for an aircraft to continue an approach beyond the FAF (LESOR).
The aircraft was certified, equipped, and maintained in accordance with existing regulations and approved procedures, and no mechanical defects that could have contributed to the occurrence were found. The flight crew were certified and qualified for the flight in accordance with existing regulations, and there were no indications that their performance was in any way degraded as a result of physiological factors, such as fatigue.
In an effort to understand why the aircraft descended too low on the visual approach after MAPON missed approach point (MAP) before conducting a go-around, this analysis will focus on the weather information available and visibility, visual references, aircraft handling, airport lighting systems, and human factors.
On 15 September 2017, the island of St. Martin was severely damaged by Hurricane Irma and communication with the Sint Maarten Civil Aviation Authority (SMCAA) was lost. As a result, some local air traffic control information was not available to the investigation.
Before WestJet flight 2652 (WJA2652) began its descent toward Princess Juliana International Airport (TNCM), the visibility in the vicinity was reported to be unlimited. However, when the aircraft was approximately 15 nautical miles (nm) from the threshold of the runway, the visibility at the airport deteriorated significantly, becoming 2000 m, with moderate to heavy rain showers. Automatic terminal information system (ATIS) November had been issued approximately 3 minutes before the crew were cleared for the approach. During this phase of flight, crews are not aware when there is a change of ATIS unless advised of it by the controller.
The minimum visibility required to continue an approach to TNCM beyond the final approach fix is 3600 m. The air traffic controller cleared the flight crew for the RNAV Runway 10 approach when the aircraft was approximately 13 nm from the runway. Just after issuing the approach clearance, the controller advised the crew of the presence of moderate to heavy rain showers at the airport but did not inform them of the updated visibility. Unaware that the visibility was below that required to conduct the approach, the crew continued the approach toward the runway.
Significant changes in visibility were not communicated to the crew, which allowed them to continue the approach when the visibility was below the minimum required to do so.
Deviation from approach profile
On final approach, the aircraft was stabilized on a 3° angle of descent and configured for landing. Approximately 0.5 nm before the MAP, the flight crew decided that, given that they had the shoreline in sight and expected to see the runway shortly afterwards, they would continue the approach visually. At that point, the aircraft was descending at approximately 820 feet per minute (fpm) and at 159 knots indicated airspeed, with an N1 of approximately 62%.
The pilot flying (PF) then disconnected the autopilot as per WestJet’s approach procedures for landing at TNCM. Shortly afterwards, the PF reduced the pitch from 0.5° nose up to 1.2° nose down, which initiated an increase in airspeed. In response to the airspeed increase, the autothrottle command reduced the engine thrust from 62% to 52% N1 to maintain the 160-knot speed previously set in the flight management computer (FMC). Following the reduction in thrust, the aircraft began to deviate below the 3° angle of descent, at a descent rate of between 1000 and 1150 fpm. Shortly after, the PF cycled the flight directors and started to manually manipulate the thrust as per WestJet’s approach procedures for landing at TNCM.
The reduction in the pitch attitude led to an increase in airspeed, which resulted in a reduction in engine thrust and a higher rate of descent than that required by the 3° angle of descent.
Acquiring visual references
During the approach phase of flight, pilots may be prone to visual errors as they switch from scanning the instrument panel within the cockpit to scanning outside the aircraft to acquire visual references. The alternation of attention from one to the other increases their cognitive workload, the demand on their perceptual faculties, and the complexity of their flight-path monitoring tasks, particularly in conditions of reduced visibility. Conditions such as expectation bias and anticipation may also contribute to visual errors.
In this occurrence, the crew of an aircraft that had landed just ahead of WJA2652 had reported seeing the runway upon reaching minima. The crew of WJA2652 were expecting to see the runway shortly after crossing MAPON. The occurrence of a moderate to heavy rain shower, after the aircraft crossed MAPON, led to a significant reduction in visibility. The low-intensity setting of the runway lights and precision approach path indicator (PAPI) lights limited the visual references that were available to the crew to properly identify the runway.
Among the visual references that remained available, the features of a hotel located to the left of the runway, such as its colour, shape, and location, made it more conspicuous than the runway environment and led the crew to misidentify it as the runway. As the crew crossed MAPON, the PF advised that he had the runway in sight. He began to roll the aircraft to the left to align it with what he thought was the runway but what was actually the hotel.
The hotel located to the left of the runway appeared from a distance to be wider at its base and narrower on top than it actually was, causing it to appear similar to a runway. However, as the aircraft approached, it became more apparent that the shape was in fact a building. Those changing geometrics would have differed from what the pilot expected of an actual runway’s appearance on approach. Further, rain may have distorted visual references such as the hotel and made the changing geometric shape more difficult to interpret.
The reduced visibility and conspicuity of the runway environment diminished the crew’s ability to detect that they had misidentified the runway.
1. Airport lighting management
On the day of the occurrence, the runway lights at TNCM were off and the PAPI lights were at 30%. Realizing that visibility was declining due to moderate to heavy rain showers, the controller turned the runway lights on to an automatic setting for night use. That setting illuminated the runway edge lights to 3% but reduced the intensity of the PAPI lights to 10%. The low intensity of the runway lights and PAPI lights reduced their effectiveness as a visual reference and limited the likelihood that they would capture the attention of the flight crew in the crew’s visual scan.
According to the International Civil Aviation Organization (ICAO) Procedures for Air Navigation Services: Air Traffic Management (PANS-ATM), air traffic controllers should be provided with guidance on the adjustment of airport lighting settings to suit prevailing conditions. At TNCM, no guidance is provided for the operation of lighting systems, and the control of light intensity for all conditions is therefore at the controller’s discretion.
If the ICAO PANS-ATM are not implemented in the management of aerodrome light intensity, there is a risk that the optimal light intensity settings for prevailing weather conditions will not be selected.
Flight path monitoring
A high visual workload can lead pilots to narrow their visual attention and focus only on those stimuli that they perceive to be most important. This narrowing of attention may influence the way they visually scan their flight instruments, such that critical items may be dropped from their scan. Pilots may also intensify their focus on a specific area where they anticipate a change, which prevents them from fully monitoring all relevant flight instruments and degrades their situational awareness.
The Flight Safety Foundation (FSF) has found inadequate flight path monitoring to be a frequent underlying causal factor in approach-and-landing occurrences and provides 20 recommendations for improving flight-path monitoring performance in its 2014 publication, A Practical Guide for Improving Flight Path Monitoring. The guide states, however, that “regardless of any action taken by any operator, […] elevating the monitoring role on the flight deck is a significant and worthwhile operational challenge.”
WestJet provides its crews with guidance on flight path monitoring during all phases of flight and in adverse weather. However, a review of WestJet training and operational procedures indicated that only some of the FSF’s recommendations are in place.
In this occurrence, when the aircraft was on final approach prior to MAPON, a moderate to heavy rain shower ahead and to their left obscured the flight crew’s view of the airport environment and reduced their ability to identify the runway. After crossing MAPON, the crew encountered a greater reduction in forward visibility than they had anticipated when the aircraft entered the shower. The resulting increase in the crew’s visual workload led them to focus their attention on monitoring for external visual references and prevented them from adequately monitoring the aircraft’s altitude.
An increase in visual workload led to inadequate altitude monitoring, which reduced the crew’s situational awareness. As a result, the crew did not notice that the aircraft had descended below the normal 3° angle of descent to the runway threshold. The lack of visual texture and other visual cues available over water contributed to the crew’s inability to detect the aircraft’s reduced height above the water.
Threat and error management
The practice of threat and error management (TEM) includes preparing and adapting crew action plans following the identification of current threats, in order to reduce the risks associated with those threats.
During the visual segment of the aircraft’s final approach over the water, the rain reduced the visibility by a greater degree than the crew had anticipated, given that the prior segments of the approach had been conducted in daylight and under conditions of good visibility. The low intensity of the runway edge lights and PAPI lights and the lack of visual cues over water were not identified as threats. Consequently, the crew did not consider the consequences of such threats or take action to mitigate them.
If crews do not identify and manage threats, there is an increased risk of crew errors, which could lead to undesired aircraft states.
1. Threat and error management training
As detailed in a 2011 TSB investigation report, Canadian Aviation Regulations (CARs) Subpart 705 operators are currently required to conduct crew resource management (CRM) training, but the regulations have not kept pace with advances in CRM theory and application, and are now outdated.
Since the Board’s issuance of TSB Recommendation A09-02, which calls for the provision of contemporary CRM training, and it’s 2011 safety concern regarding the necessity for a comprehensive and integrated approach to CRM by TC and aviation operators, Transport Canada (TC) has taken steps to address the gaps in the existing regulations. A new Commercial Air Service Standard (CASS), set to replace the current CASS subsection 725.124(39) in January 2019, will require operators under subparts 703 (AirTaxi), 704 (Commuter), and 705 (Airline) to provide contemporary CRM training, which includes training in TEM.
The enhanced ground proximity warning system
Enhanced ground proximity warning systems (EGPWSs) are designed to improve safety by providing alerts to flight crews when the aircraft is in a dangerous situation and corrective action is required. Some alerts require a change in aircraft configuration, while others require a change of flight path.
1. Alert response procedures
The EGPWS alert response procedures of both the aircraft manufacturer and the operator instruct pilots to ensure positive visual verification in the event of an EGPWS alert of “TOO LOW, TERRAIN” during flight under daylight visual meteorological conditions (VMC). This step is intended to limit the number of unnecessary go-arounds resulting from nuisance alerts. However, current EGPWS technology has reduced the incidence of nuisance alerts such that they are now rare and almost always predictable. As a result, the positive visual verification step within the response procedure may no longer be necessary. Further, it is not consistent with the EGPWS manufacturer’s procedures, which state that climbing is the only recommended response when receiving an EGPWS alert.
2. Alert recovery reaction time
At 63 feet above ground level (AGL), the flight crew unexpectedly received an EGPWS aural alert of “TOO LOW, TERRAIN,” which caused them to readjust their degraded situational awareness. On receipt of the aural alert, the crew carried out a “positive visual verification that no obstacle or terrain hazard exists” (as per both the aircraft manufacturer’s and operator’s recommendations for EGPWS alert response in daylight VMC), before deciding on a course of action. The PF increased the pitch to 4° nose up; however, as the aircraft continued descending, the crew received a second EGPWS alert when the aircraft was between 54 and 49 feet AGL.
While carrying out the positive visual verification, the crew’s ability to evaluate their height above the water was made more challenging by a lack of texture and other visual cues in the external environment, and it took them several seconds to understand that they were indeed too low. They initiated a go-around 9 seconds after the first EGPWS alert, by which time the aircraft had descended to 40 feet above the water. The alert response procedure recommended by the aircraft manufacturer and the operator led to a delayed response to the first EGPWS alert and resulted in the aircraft’s descent from 63 to 40 feet AGL before corrective action was taken.
Findings as to causes and contributing factors
- Significant changes in visibility were not communicated to the crew, which allowed them to continue the approach when the visibility was below the minimum required to do so.
- The reduction in the pitch attitude led to an increase in airspeed, which resulted in a reduction in engine thrust and a higher rate of descent than that required by the 3° angle of descent.
- The occurrence of a moderate to heavy rain shower, after the aircraft crossed the missed approach point, led to a significant reduction in visibility. The low-intensity setting of the runway lights and precision approach path indicator lights limited the visual references that were available to the crew to properly identify the runway.
- The features of a hotel located to the left of the runway, such as its colour, shape, and location, made it more conspicuous than the runway environment and led the crew to misidentify it as the runway.
- The reduced visibility and conspicuity of the runway environment diminished the crew’s ability to detect that they had misidentified the runway.
- The lack of visual texture and other visual cues available over water contributed to the crew’s inability to detect the aircraft’s height above the water.
- An increase in visual workload led to inadequate altitude monitoring, which reduced the crew’s situational awareness. As a result, the crew did not notice that the aircraft had descended below the normal 3° angle of descent to the runway threshold.
Findings as to risk
- If the International Civil Aviation Organization Procedures for Air Navigation Services: Air Traffic Management are not implemented in the management of aerodrome light intensity, there is a risk that the optimal light intensity settings for prevailing weather conditions will not be selected.
- If crews do not identify and manage threats, there is an increased risk of crew errors, which could lead to undesired aircraft states.
- Because the occurrence was originally assessed by WestJet as a non-reportable event, the cockpit voice recorder and the digital flight data recorder data were overwritten and were not available to the investigation.
- The enhanced ground proximity warning system (EGPWS) alert response procedures of the aircraft manufacturer and the operator differ from those in the guidance material of the EGPWS manufacturer.
- The alert response procedure recommended by the aircraft manufacturer and the operator led to a delayed response to the first EGPWS alert and resulted in the aircraft’s descent from 63 to 40 feet above ground level before corrective action was taken.
Transportation Safety Board of Canada. AVIATION INVESTIGATION REPORT A17F0052. Risk of collision with terrain. WestJet Boeing 737-800, C-GWSV Princess Juliana International Airport, Sint Maarten 07 March 2017.
- Multitasking in Complex Operations, a real danger
- When the error comes from an expert: The Limits of Expertise
By Laura Victoria Duque Arrubla, a medical doctor with postgraduate studies in Aviation Medicine, Human Factors and Aviation Safety. In the aviation field since 1988, Human Factors instructor since 1994. Follow me on facebook Living Safely with Human Error and twitter@SafelyWith. Human Factors information almost every day