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Slips – Executing Trained Behaviour Patterns

When the Helios crew boarded the aircraft, they completed the checklists associated with preparing the flight deck. This included scanning the various panels on the flight deck to check the position of switches and controls. Neither pilot detected that the pressurisation controller was in the wrong position. Scanning arrays and controls and conducting routine checks are a fundamental part of the job and represent classic examples of skill-based behaviour. It is the high level of automaticity associated with well-rehearsed skills that gives rise to slips.

Consider, for a moment, the way pilots move their hands around the cockpit space. With increasing familiarity, a pilot can reach for an object in the cockpit without necessarily moving their eyes to track the position of their hands and to locate the target object. Kinaesthetic feedback from the position of their limbs allows them to roughly position themselves within the workspace and the sense of touch facilitates the final, accurate location of the control. In October 2004, a pilot knocked off the conditioning pack switches of an MD-83 when he attempted to put the tech log back on the glare-shield after writing up some items. In this case, the degree of muscle motor control, the execution of a skill, was inadequate and the pilot exhibited ‘clumsiness’. The control of our limbs in space is controlled by structures in the cerebellum. This region of the brain is larger in Highland Gorillas that any other primate (including humans), reflecting the fact that these large animals negotiate complex arboreal habitats that require considerable planning of movement and thus require additional computing power.

In information processing terms, the cue represented by the pressurisation controller in the wrong position on the Helios aircraft lacked salience: it was not distinctive enough to draw the crew’s attention to the condition. Having missed the configuration of the controller on first boarding the aircraft, the pilots had four further opportunities to check the system according to the aircraft checklist: before engine start, cleared for start, after start and after take-off. There were two cues that should have triggered the correct behaviour in relation to the cabin pressurisation system: the green caption indicating that the pressurisation controller was in manual mode and the position of the selector switch. When the Helios pilots did their visual scan, for whatever reason, these were missed. The design of the working environment can reduce the effectiveness of cues. The intensity of the annunciators on the aircraft can be set to two levels, bright and dim. The light switch on the Helios aircraft was found to be in the bright position. In March 2004, the crew of a 737-300 were doing their pre-flight checks for an early morning start. With the rising sun in their eyes and the lights set to ‘dim’, the pilots reported that the manual annunciator was invisible. Switches and annunciators are not equally visible from both seats and are often embedded in a complex array of other controls and indicators. The ergonomics of the cockpit can impede detection of a miss-set item, especially if inadequate attention is paid to the task.

Signal detection can be shaped by our expectations. In January 2003, the crew of a B-737-300 also got airborne with the controller in manual mode. The pilot had done a bleeds-off take-off and, having selected the bleeds ON (which would have no effect with the controller set to manual), he verified that the cabin was pressurising. In fact, the cabin was climbing at the same rate as the aircraft, but the captain reported his expected visual cue rather than the actual cue.

Prior learning can interfere with new skills. For example, in February 2006, an Embraer 170 landed with the parking brake applied, causing its tyres to burst (SIBF, 2006). The captain had only recently completed his type conversion course. Because the aircraft was too fast on the approach, the captain had selected what he thought was the speed brake. However, he was using a behavioural routine ingrained from flying his previous type of aircraft. On the new aircraft, the parking brake is in the same location as the speed brake was on his former aircraft. The captain joked about his error and then set the brake to OFF. Unfortunately, a brake circuit failure resulted in the brake remaining partially applied. The Embraer does have a warning light for inadvertent brake selection in flight, but its inconspicuous design means that it is easily overlooked. The crew did not notice that the park brake did not appear to have been released. (The same error was made by the crew of an EMB 145 on landing at Bristol Airport, UK on 22 December 2017. The report (AAIB 1/2019) refers to two previous events involving the ERJ 135/140/145 family of aircraft. The first was in August 2000 and the second was in August 2008. The 170 is a derivative of this type). Stress, as well as recent experience, can result in the reversion to prior learned habits.

Finally, in September 2002, the pilot of a Boeing 737-204 demonstrated a bleeds- off take-off to an inexperienced FO. After take-off, the FO turned the bleeds on and then inadvertently turned the air conditioning packs off. The demonstration was an interruption of the normal sequence but the outcome was a classic example of how slips occur. The action of moving one set of switches was associated in the mind of the FO with a similar action involving a second set of switches. However, in this case, the air conditioning packs had already been turned on and the two linked behavioural responses resulted in the switches -already in the correct position - being moved back to the off position.

Slips, then, are the outcome of control actions that go awry. Novice pilots are less likely to make skill-based errors than expert pilots simply because they are concentrating more on the control task than an expert would. It is typically the reduced allocation of attention to a well-rehearsed task that increases the risk of inadequate control. Sequences of skill-based behaviour are linked into event chains, each sequence triggering the subsequent actions in the chain. Elements can fail to be triggered, or steps can be triggered out of sequence or in reverse. The dynamic interplay between the environment and the individual is worked out in terms of the quality of the final performance.

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