Crew Confusion in Firefighting 737 Terrain Impact

Posted by on Nov 17, 2024 in Accidents & Incidents, Fixed Wing, Human Factors / Performance, Safety Management, Special Mission Aircraft | 0 comments

Crew Confusion in Firefighting Boeing 737 Terrain Impact (Coulson, N619SW)

On 6 February 2023 Coulson Aviation Boeing 737-3H4 N619SW, callsign Bomber 139, on an aerial firefighting task in Fitzgerald River National Park, Western Australia (WA), impacted a ridgeline during a retardant drop.

Coulson Boeing 737-300 N619SW Firefighting Accident Site – Note: bulldozer tracks post-accident (Credit: ATSB)

The aircraft was destroyed but the two pilots miraculously only suffered minor injuries.

The Australian Transport Safety Bureau (ATSB) issued their safety investigation report into on 6 November 2024.  While undoubtedly a “controlled flight into terrain”, under the  CAST/ICAO Common Taxonomy Team definition this was not a CFIT but a ‘LALT’ accident due to it being during low altitude operation.

That Day’s Flying & the Accident Flight

The aircraft was fitted with a Coulson Aerial Firefighter Tanker Modification (retardant aerial delivery system [RADS]).

RADS incorporated two tanks on with a total capacity of 36,000 lbs of retardant.   There were  drop switches on both pilot’s controls:

When the flight crew activates either switch, the primary user interface software will produce a door position command based on the predefined Coverage Level (CL) requirement, drop quantity, tank levels, ground speed and aircraft height above the ground.

RADS also includes an emergency dump switch to dump the full load in less than 2 seconds. if a performance problem developed before all the retardant was released.

To avoid nuisance audio alerts during a retardant drop, the RADS modification incorporated an audio inhibit switch, which inhibited the landing gear configuration, ground proximity warning system and traffic collision avoidance system for 5 minutes.

The aircraft commander (8,233 hours of flying experience, with 1,399 hours on type and c 5,500 hours aerial firefighting) was the pilot flying (PF),  The co-pilot (5,852 hours, with 128 hours on type and c 500 hours aerial firefighting experience) had been employed by the operator for less than a year and was the pilot monitoring (PM).  Both had a a US Dept of Agriculture Forest Service (USFS) Air Tanker qualification.  Their tour of duty had started on 20 January 2023. They had flown 7.3 hours and 13 drops on this tour prior to the day of the accident.

Bomber 139 conducted 3 flights on that day, which was the last day of the flight crew’s tour of duty. The first 2 flights each comprised a single full load drop at the fire ground with a return to Busselton.

The retardant drop is a manually flown contour flight manoeuvre in potentially rough air and over undulating terrain across a drop zone that has not been formally surveyed for obstacle clearance and gradient. Therefore, airspeed and height deviations could be expected.

Coulson Boeing 737-300 N619SW Firefighting Retardant Drops (Credit: Department of Fire and Emergency Services, annotated by ATSB)

The second flight was intended to ‘tag and extend’ the retardant dropped during the first flight.

Coulson Boeing 737-300 N619SW First drop (left) starting at road and second drop (right) tag and extend (Credit: DFES, annotated by ATSB)

ATSB note that:

The aircraft captain completed the operator’s flight risk assessment tool (FRAT) at 1015. The FRAT score was 14, which was an acceptable level of risk with no mitigation or escalation required.

If the FRAT had been reviewed between each flight, the SIGMET issued at 1400 would have added 8 points, resulting in a FRAT score of 22 for the accident flight that departed from Busselton at about 1530.

A score >21 required mitigation or escalation for approval.

The weather conditions, including the thunderstorm activity advised in the SIGMET, were not considered to be a factor in this accident.

On the first two flights Bomber 139 was guided by another aircraft, callsign Birddog 125.  For the third flight of the day Birddog 123 (BD123) took over that role.

The BD123 pilot reported having prior experience working in terrain with more extensive vertical relief in Canada and considered the accident flight target and exit to be relatively flat.

Coulson Boeing 737-300 N619SW Flight Path (Credit: Google, annotated by ATSB)

In preparation for the drop, BD123 briefed Bomber 139 that the drop zone was clear of ground crew, there were no hazards, and that it was a downhill drop. The captain declined BD123’s offer of a Show-Me profile, consistent with the second flight.

A Show-Me profile where the Air Tanker follows the path of the Birddog is more common during the first drops at a site.

Bomber 139 then followed BD123 on a ‘wide’ right-hand circuit pattern with a final approach and descent to the target down the right side of the smoke.

Coulson had adopted 150 ft as their target drop height.

ATSB found that the US Interagency Airtanker Board (IAB) had investigated and determined that 150 ft was a LAT minimum safe drop height for their airtanker evaluation process. This was the reference drop height in the US airtanker and aerial supervision standards for a LAT.

However, a lower drop height and speed would improve the accuracy of a drop at a low coverage level and the drop system was set for low coverage.

The aircraft captain confirmed that the decision height setting on the radio altimeter was not used because it did not provide an aural alert and because the pilot flying is predominantly ‘eyes-out’ during the drop and therefore not monitoring the decision height light on the attitude indicator. The co-pilot did not use the decision height on the radio altimeter and did not believe it was a SOP to use it because it could cycle on-and-off during a retardant drop over undulating terrain.

Additionally:

The aircraft captain reported the co-pilot’s duty was to scan inside and out and call deviations, such as airspeed, if different from what was briefed, and confirmed the 150 ft drop height published by the USFS was a minimum height.

However:

The co-pilot…believed there was no minimum drop height and indicated that they had interpreted the 150 ft standard as a maximum drop height to mitigate the potential adverse effects of the wind on the fall of the retardant…but also reported having limited industry knowledge.

The co-pilot reported the airspeed indicator was one of their primary flight instruments they were scanning during the drop, and that they believed it was indicating their target drop speed of 118 kt throughout the run.

ATSB explain thatL

Recorded data indicated the airspeed went below, and remained below, the lower deviation limit about 2 seconds before the start of the drop after fluctuating intermittently above and below the lower limit during the approach. However, the co-pilot did not detect and announce the low and decreasing airspeed.

Furthermore:

A colleague of the co-pilot confirmed the operator’s CRM philosophy was not to overcommunicate – ‘brief first, then only communicate if there is a deviation from the brief’.

ATSB note that:

The Coulson Aviation fixed-wing flight operations manual did not include detail of the setup and conduct of retardant drops and the Bomber 139 captain referred the ATSB to the US National Wildfire Coordinating Group (NWCG) aerial supervision standards for how they expected a Lead Plane (Birddog) would conduct operations in the US.

The Birddog operator’s standard operating procedures (SOPs) did match the NWCG standards and…

…also stated that drops ‘should be downhill’, that they were to provide ‘the final heading for the run and the altitude for the drop start point’, the drop height ‘minimum is 150 ft above the top of the vegetation for heavy tankers’ and that ‘The airtanker pilot is responsible for maintaining safe drop heights.’

Meanwhile:

The captain, who preferred a closer circuit pattern to maintain visual contact with the drop zone and exit during the approach, had trouble identifying the start of the drop (a road crossing) on the first approach. The captain’s difficulty identifying the road was likely due to the wide right-hand circuit pattern and resulted in the captain’s attention becoming focussed on sighting it for the start of the drop.

During the drop the captain detected that they were not on the correct line to keep the retardant drop clear of the burnt area. Therefore, they stopped the drop and conducted a go-around from the high ground with the fire in the depression burning across their path.

ATSB explain that:

The elevation of the terrain beneath at the time of that go-around was higher than the elevation of the terrain at the accident site, which was on the far side of the depression. Therefore, the accident ridgeline remained below the horizon for the crew on the first drop, potentially obscured by smoke from the fire in the depression and likely indistinguishable from the surrounding terrain due to the relatively consistent vegetation coverage.

The captain’s decline of the Show-Me profile, wide right-hand circuit, Birddog brief of a downhill drop and subsequent go-around from the high ground, meant that the captain did not expect, or detect, rising terrain in the exit path prior to attempting the second drop.

[On] the 2 previous flights that day, Bomber 139 recorded minimum drop height parameters of 46 ft and 69 ft with the engines at 60‍–‍70% N1.

When the thrust levers were advanced at the end of each drop, there was an immediate positive response in N1, airspeed and vertical speed. The minimum parameters on the first (partial) drop of the accident flight were 78 ft and 124 kt at about 70% N1.

According to the ATSB, this previous performance response and an expectation of a downhill drop with no hazards “likely contributed to the captain’s confidence that the aircraft could be safely flown and recovered from a low drop height and speed.”  However:

N1 was a key parameter for a successful go-around and no performance problems were evident when N1 was in the region of 60‍–‍70% at the end of the drop.

Following this initial partial drop, and a wider than intended circuit, the crew conducted a second drop, which released all the remaining retardant.  During the circuit the fire-line had extended further east and Bomber 139 approach though smoke that obscured the ridgeline

Coulson Boeing 737-300 N619SW: Accident flight first and second drops (Credit: Google, annotated by ATSB)

During the second drop, Bomber 139 descended to a minimum height of 57 ft at 110 kt and about 30% N1… The aircraft recovered to a height of 81 ft at 107 kt at the end of the drop at which stage the captain had started to advance the thrust levers as the rate of descent peaked at about 1,800 ft/min.

The engines did not immediately respond to the movement of the thrust levers and the captain started to pitch the nose up, which resulted in a reversal of the rate of descent and a decay of the airspeed.

The captain then announced ’fly airplane’ followed immediately, at about 1614, by the activation of the stick shaker and an abrupt vertical acceleration associated with the aircraft impacting a ridgeline…

(Figure 5).

After contacting the ridgeline, the aircraft became airborne for about 69 m, shedding engine, wing, and fuselage debris before impacting a second time in a slight nose down attitude on a heading of about 140º.

The aircraft came to rest about 176 m from the ridgeline and yawed left to the direction of travel onto a heading of about 080º. The fuselage had a main fracture near the tail and the left engine had separated from the left pylon and was resting adjacent to the forward fuselage.

Coulson Boeing 737-300 N619SW: Accident sequence distances and elevations (Credit: ATSB)

ATSB Analysis

Review of cockpit voice recorder (CVR) data indicated to investigators that the crew were…

….working cooperatively with a division of duties during the set-up for the first drop. Between the first and second drop, they were again working cooperatively to track the position of BD123, update their checks and set their aircraft up for the second drop.

However,…the co-pilot was silent in the cockpit for about the last 70 seconds of the flight, during which Bomber 139 was flown below the operator’s standard target drop height and speed deviation limits…with the engines at idle.

This required a ‘call-out’ from the pilot monitoring, the co-pilot, that did not occur.

The operator’s expectations of pilot monitoring duties were presented in their CRM training but their controlled flight into terrain prevention training package did not present their CRM and SOPs as prevention strategies.

Retardant drops are conducted to unsurveyed areas, which may result in lower safety margins than an approach to, or departure from, an airport. Hence the application of CRM and SOPs to managing the energy state of the aircraft should be as important for a retardant drop as for a departure or an arrival.

The pilot monitoring call-outs in the operator’s retardant drop procedure were reactive to exceeding the target drop height or drop speed deviation limits. They did not include proactive ‘approaching target height/speed’ or ‘on height/speed’ call-outs.

ATSB concluded that the reactive nature of the pilot monitoring duties in their retardant drop procedure increased the risk of their aircraft entering an unrecoverable energy state and therefore have assessed it as a safety issue for Coulson Aviation.

ATSB’s Contributing Factors 

  • During the retardant drop downhill, the aircraft descended significantly below the operator’s standard target drop height and airspeed and entered a high rate of descent with the engines at idle. While the engines were starting to accelerate at completion of the drop, the airspeed and thrust were insufficient to climb above a ridgeline in the exit path, which resulted in the collision with terrain.
  • Prior to the retardant drop, the aircraft captain (pilot flying) did not detect there was rising terrain in the exit from the drop, which likely contributed to the captain allowing the aircraft to enter a low energy state during the drop.
  • After arrival at the fireground, the aircraft captain (pilot flying) declined a ‘Show-Me’ run and was briefed by the Birddog pilot that it would be a downhill drop. Bomber 139 then conducted a go-around from the high ground after the first drop and was led to the target through the smoke on the second drop. These factors likely contributed to the captain not expecting or detecting the rising terrain in the exit path.
  • The co-pilot (pilot monitoring) did not identify and announce any deviations during the retardant drop, which could have alerted the aircraft captain (pilot flying) to the low-energy state of the aircraft when it descended below the target drop height with the engines at idle.
    The flight crew did not brief a target retardant drop height and, contrary to published standard operating procedures, did not set it as a decision height reference on the radio altimeter. Subsequently, the co-pilot (pilot monitoring), who did not believe there was a minimum drop height, did not alert the aircraft captain (pilot flying) to the low-energy state of the aircraft.
  • Coulson Aviation and the relevant Western Australian Government Departments had not published a minimum retardant drop height in their respective operating procedures for large air tankers. Consequently, the co-pilot (pilot monitoring), who did not believe there was a minimum drop height, did not alert the aircraft captain (pilot flying) to a drop height deviation prior to the collision. (Safety issue).

Mong the other factors identified by ATSB was:

  • The Coulson Aviation crew resource management practice of limiting the pilot monitoring (PM) announcements to deviations outside the target retardant drop parameter tolerances increased the risk of the aircraft entering an unrecoverable state before the PM would alert the pilot flying. (Safety issue).

Previous Accident

Though hardly mentioned in the ATSB report, Coulson had a previous fatal accident with Lockheed EC-130Q Hercules N134CH in New South Wales on 23 January 2020.  In that case ATSB commented that:

  • Aircraft likely stalled following a retardant drop when flying in hazardous conditions that included windshear and an increasing tailwind;
  • Crew very likely did not know that other smaller firefighting aircraft had ceased flying in the area, and the assigned birddog aircraft had turned down the tasking, due to the hazardous conditions;
  • Aerial firefighting operations necessarily take place in a high-risk environment, which requires a continued focus on risk mitigation, a responsibility that is shared between the tasking agency and the aircraft operator.

They noted that:

Coulson Aviation’s safety risk management processes did not adequately manage the risks associated with large air tanker operations. There were no operational risk assessments conducted or a risk register maintained. Further, as safety incident reports submitted were mainly related to maintenance issues, operational risks were less likely to be considered or monitored. Overall, this limited their ability to identify and implement mitigations to manage the risks associated with their aerial firefighting operations.

They raised a safety recommendation on risk/safety management.

Our Observations

At the time of the 2020 NSW EC-130Q accident Coulson Aviation did not provide a pre-flight risk assessment tool for their crews.  In September 2021, they advised the ATSB that they had introduced such a tool, to be completed, at a minimum, prior to the first tasking of the day.  Although ATSB do not elaborated on the ‘FRAT’ form in use, on the day of the 737 accident it seemingly contributed nothing to managing the risk that day.  This is not uncommon with these forms that do nothing for the crew that a through pre-flight briefing with proper attention to Threat and Error Management (TEM) techniques.  FRAT proponents would argue that seeking management input for flights with a higher risk rating is useful, but we would argue that many accidents seem to occur when scores are low as most of these tools are poorly calibrated for determining a realistic assessment of risk.   

It is not clear in the ATSB report what pre-flight and pre-drop briefings the aircraft commander actually conducted on the day of the accident but it is evident there was a dangerous difference in understanding on drop height between Pilot Flying and Pilot Monitoring that undermined the value of multi crew operation.

ATSB have taken the view that it is, at least partly, a customer responsibility to define the minimum drop height, presumably simply echoing US practice.  When standardisation is necessary across multiple contractors for safe coordination and operation that would be valid.  In this case, when determining how to safely operate an aircraft, we would tend to believe that is for a competent air operator and their SMS, not a non-aviation end customer, to ensure an appropriate procedure is developed.

Safety Resources

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