What aircraft was involved and where did it happen?

The accident on 2023-02-21 involved a AW169, registration G-UNIB, at Humberside Airport, GB.

What was the probable cause of the accident?

The severance of the hoist cable was likely caused by the inadvertent activation of the hoist cut button during out-of-sequence pre-flight checks, driven by procedural drift and a company checklist that did not adequately incorporate manufacturer warnings.

Hoist cable severed during pre-flight checks on AW169

A hoist cable on an AW169 helicopter was accidentally severed during out-of-sequence pre-flight procedures at Humberside Airport, resulting in the loss of the hoist hook.

What happened

On 21 February 2023, an AW169, registration G-UNIB, was undergoing pre-flight preparations at Humbersont Airport for scheduled offshore wind farm maintenance operations. The crew, consisting of a pilot, a pilot monitoring, and a hoist operator, were performing various checks before the scheduled 0800 UTC departure.

While the pilot was completing a walkaround of the aircraft, the pilot monitoring and the hoist operator began conducting hoist pre-start procedures. During this process, the crew deviated from the established sequence of the company checklist. The hoist power was applied before the completion of other necessary checks. As the crew proceeded to lift the guards on the hoist cut switches to verify cockpit caution messages, the hoist hook and approximately 20 cm of the hoist cable detached from the aircraft and fell to the ground.

No injuries were sustained by the 3 crew members or 4 passengers on board, and no damage was found to the aircraft itself, aside from the severed cable and hook.

The investigation

The AAIB examined the aircraft and the Goodrich hoist system. Detailed functional tests of the hoist, including the wiring connections and the control box, revealed no mechanical, electrical, or functional faults. The investigation focused on the sequence of the checklist execution and the potential for inadvertent activation of the cable-cutting mechanism.

Data from the aircraft's Data Transfer Device confirmed that several caution messages, including 'hoist cut arm' and 'hoist cable foul', appeared on the primary flight displays during the period when the guards were being manipulated. The investigation also noted that the company checklist did not fully align with the manufacturer's Rotorcraft Flight Manual (RFM) regarding the specific order of operations and the necessary warnings for the hoist system.

Findings

The crew experienced procedally drift, leading to a deviation from the approved aircraft start checklists.
The company checklist lacked specific warnings regarding the risks of activating the hoist power while certain caution messages were present.
The checklist did not adequately reflect the manufacturer's recommended start-up sequence for the external hoist.
Inadvertent activation of the hoist cut button during the out-of-sequence checks could not be ruled out as the cause of the cable severance.

Safety action

The operator has since amended its company checklists to include a more detailed hoist start-up sequence.
The updated procedures now incorporate specific warnings from the aircraft's Rotorcraft Flight Manual external hoist system supplement.
The operator has reinforced the necessity of strictly adhering to published aircraft start checklists with all flight crews.