When Green is for Stop: Poor Design HF
On 28 April 2017 Cessna 310R VH‑COQ was on approach to Tindal Airport, NT. Tindal is used by the Royal Australian Air Force (RAAF) as well as being a civil airport. The airfield is one of around 2000 world wide fitted with a Runway Arrestor Gear (RAG) or Aircraft Arrestor System (AAS) for use by military fast jets. This uses cables across the runway that can be raised 100 mm off the runway surface to allow a fast jet hook the cable and be decelerated. When lowered the cable is held just off the surface by rubber ‘donuts’.
The air traffic control tower had opened for a scheduled military jet departure and was therefore active when VH-COQ made its approach to land.
During the tower opening checklist procedure, the tower controller annotated the ‘cables’ check was completed.
About 21 minutes after the tower opened, VH-COQ requested a clearance…. The tower controller scanned the control console, noted that both hookcable pushbutton lights were green, and cleared VH-COQ to land…
When… on short final approach to land…the pilot noticed the approach end hookcable was raised. The pilot adjusted their aim point beyond the hookcable and landed without incident. The pilot of COQ reported the position of the hookcable to the tower controller, who then rectified the situation.
This ATSB say this incident “highlights the risks of expectation bias” with a strong anticipation of a particular outcome:
The tower controller observed two green lights on the control console, but did not recognise they were the UP indicators. There are four pushbutton selection/indicator lights for each hookcable. Two separate green UP and green DOWN pushbuttons are used to select, and then indicate, the desired position for each hookcable.
The ATSB say:
…the design of the indicators, where green lights can have two different meanings, removes the usefulness of the colour of the lights in determining whether the hookcable is up or down.
The pilot detected the problem in time to avoid trampling the hookcable during the landing. However, pilots should take note that the hookcables will automatically raise in the event of a power failure.
The air traffic service provider is taking the following safety actions:
- The possibility of changing the colour of the UP lights will be investigated through an engineering process to better differentiate between the UP and DOWN positions (this will be for all our sites that have arrestor systems).
- Furthermore, due to the relative low number of civilian aircraft operating at Tindal Airport, Tindal air traffic control will be advising the position of the cable with every landing and take-off clearance given to civilian aircraft. This will help force the controller to verify the position of the cable in addition to the conduct of the instrument scan.
The Tindal RAG has been troublesome before. Eight months before on 9 August 2016 Beechcraft King Air B200 air ambulance VH‑ZCJ was ready to depart from Tindal to transfer a patient to Darwin, NT. On take off, according to the ATSB, the nose wheel struck a raise cable at about 86 knots, fortunately without incurring damage.
Previously the RAG/AAS was used to stop a military fast jet…
The barrier crew then entered the runway and performed the servicing and reset of the cable. This task required the control of the AAS to be passed from the tower to the runway site control.
[In these circumstances]…the amber maintenance pushbutton will illuminate to indicate the runway site have control of the AAS. The red fail pushbutton will also flash, and be accompanied by an audible clicking. This is a warning to the tower controllers that they do not have control of the AAS. The warning is cancelled by manually depressing the fail pushbutton.
In maintenance control [mode], the tower console UP and DOWN pushbuttons are indicator lights only, which will respond to cable position changes made at the runway location. The runway site control has a physical switch, which is selected to either the UP or DOWN position. When control of the cable is passed from the tower to the runway site control, the hookcable control module will command the cable into the position selected on the runway site control switch.
The servicing and reset [task] also required the cable to be cycled between the UP and DOWN positions. On completion of the reset the barrier crew passed AAS control back to the tower with the runway site control switch selected in the UP position. This deviated from the barrier crew’s normal procedures, which required them to lower the cable into the DOWN position before passing control back to the tower.
Tindal ATC did not notice the discrepancy with the handover.
For Further RAG/AAS Background
Aerossurance will be presenting at the Royal Aeronautical Society (RAeS) Human Factors in Aircraft Maintenance conference on 2 November 2017 in London. Our topic is: Helicopter Flying Control Maintenance HF Accidents: A Human Centred Design Opportunity
Aerossurance is also pleased to be supporting the annual Chartered Institute of Ergonomics & Human Factors’ (CIEHF) Human Factors in Aviation Safety Conference for the third year running. We will be presenting for the second year running too, this time on the subject of the FSF‘s Maintenance Observation Programme concept.
This year the conference takes place 13 to 14 November 2017 at the Hilton London Gatwick Airport, UK with the theme: How do we improve human performance in today’s aviation business?