Knowledge-based mistakes are naturally covered within the decision-making chapter (Chapter 9), and therefore this chapter focuses mainly on skill based errors and violations.
Slips
Once a skill is learned to the point of being autonomous, it is reliable and robust. However it is also vulnerable to situational and contextual change. Skills literature (rather than error literature) uses terms such as ‘skill transfer’ or ‘transfer of training’ to denote a
phenomenon whereby skills learned in one situation are applied to a different situation (rightly or wrongly). Usually this is positive; otherwise a pilot would have to learn each aircraft as if a completely new vehicle requiring a completely new skill set. However, where situations are quite similar, negative transfer can take place. This is where the skill for one situation is deployed in another with negative effects, “when a new response is needed for an old stimulus” (Magill 1989). A classic example of the risk is associated with a change of aircraft type. The autonomous skill has developed or been recently practiced on one aircraft and the ‘new’ aircraft is similar enough to lead to the ‘old’ skill being used in error. An example would be a pilot changing from a Cessna 172 light aircraft to a Robin DR400. The C172 has a plunger-type throttle control in the same location as a plunger-type
mixture control of the Robin. If the pilot is concentrating on the approach, he/she could easily set some flap, return their hand to the throttle autonomously and retard it, only to find the engine cutting because the hand had located the mixture control of the Robin. Flare height could present the same problem in small and large aircraft, as can general handling and even stall recovery, particularly in aerobatic aircraft. Aircraft with common type ratings are particularly vulnerable particularly if the pilot has flown one type many times continuously and then suddenly flies the other.
I had just moved the throttle to taxi forward when the seat suddenly slid backwards and the aircraft lurched forward out of control. I grabbed and pulled on the handbrake between the seats………unfortunately my brain had decided I was in a car and the ‘handbrake’ was actually the flap lever.
Error literature would refer to such errors as slips, and specifically ‘interference errors’ or ‘habit capture errors’. For the practical purposes of aviation professionals, it can be considered that the same phenomena are being referred to, just using different
terminology. Skills and errors are closely related and their respective literature has many parallels but uses different terminology. This is not unusual for two sub-disciplines but it can be confusing for the CRM trainer trying to increase their knowledge across a number of areas. There are currently no resources that can easily answer whether a phenomenon found in one discipline is the same phenomenon described by another but under a
different label. Very often, since the phenomena are hypothetical even the scientists involved are not sure!
Just as the use of an old skill can be prompted by a new aircraft or situation, so subtly different tasks and controls within the same aircraft can also produce these kinds of errors. The following anecdote (from a commercial helicopter operation) shows a classic skill based error caused by two similar controls. It also shows how confusing the aftermath can be for a crew.
The aircraft parking brake and nose wheel lock are identical ‘T handles’ situated
adjacent to each other. As the aircraft came to a stop to disembark the passengers the pilot monitoring reached out and applied what he believed was the parking brake. Having disembarked the passengers the crew ran through the pre-taxi checklist in order to reposition to a suitable parking spot for shutdown. As the checks were carried out the pilot flying highlighted that the parking brake was already off. The monitoring pilot felt confused as he was certain that he had applied the brake, however, as there was another aircraft waiting for them to move he didn’t question the discrepancy. As they taxied they soon realised it was impossible to turn the aircraft. An engaged and now jammed nose wheel-locking pin was diagnosed which required the aircraft to be shut down in its current position.
Although in the described event the error caused inconvenience, the same issue can lead to more hazardous outcomes (see reference AAIB 2008).
One particularly modern area of concern in automated aircraft is that of crew errors when programming the flight management computer (FMC). Serious incidents and accidents have been caused by simple but highly consequential errors, including slips, transposition errors and misreading data.
Lapses
A further common error type in aviation is the lapse. Human memory is not well suited to remembering all the tasks and order of steps required in all situations, or even in several common situations. The use of checklists has proved highly effective over many years in terms of preventing and mitigating these problems, and the error types discussed
previously. Occasionally however, checklists are vulnerable to the way human skills work. The repeatable nature of checklist tasks means that the brain can turn checklist tasks into skills in the same way as any other tasks. The reason the brain does this is in order to save attention, which is the opposite of the intention. This can lead to pilots using checklists in the way that skills are performed; i.e. without paying attention to the items being checked. In CRM courses this issue is often taught through a concept labelled ‘expectation bias’. This describes that people will see what they expect to see.
There are many examples where aircraft have attempted to take off without flaps correctly configured. In most cases the take-off configuration alert did not operate (the holes in the cheese lined up). In most cases the aircraft took off and the situation was resolved, but in
CAP 737 Section A, Part 1, Chapter 5: Human error, skill, reliability, and error management
a number of well-known accidents the aircraft did not take off, or crashed shortly afterwards.
In 1989 a Delta Boeing 727 attempted to take off from Dallas Fort Worth but crashed with loss of life. The cockpit voice recorder captured the moment when the first officer replied to the challenge checklist item “flaps?” with “fifteen, fifteen, green light”. This reply would be the correct reply to the appropriate take-off flap setting, but the flaps were not set. One of the stated causes was that the flight deck environment was not sterile; the pilots were distracted by non-operation conversation.
More recently, in August 2008, the same accident occurred to a Spanair MD82 taking off from Madrid. During the ‘Final Items’ checklist, the first officer stated “… Eleven, Aligned, Eleven…” which would be the normal response to seeing flaps correctly set at eleven degrees. The flaps were not set.
In almost all cases where checklist items are bypassed or performed wrongly, such errors will be trapped by the individual at the time or immediately afterwards, or be trapped by the other crew-member. Distractions and parallel tasks in the flight deck are apparent in many cases where a checklist error is made and not trapped. These might be general
distractors, or other tasks impinging or interrupting the checklist task.
Lapses can also occur when a situation that normally prompts an action does not occur, and so the crew do not take the action. The following anecdote is an extreme example from a commercial rotary wing crew.
It was a weekend; the crew had experienced multiple delays and were now tight against a departure time in order to beat forecast deteriorating conditions at their destination. It was normal practice to start engines, taxi to the gate, then complete the pre-take-off checks as the passengers boarded. On this occasion the task was to carry freight so there was no requirement to taxi to the gate. The freight seemed to take forever to load and the commander was very conscious of the need to make the departure time. The commander was aware they were rushing as they started
engines, ran through the checks, and requested taxi. ATC were very co-operative and cleared them for an immediate departure. Shortly after take-off, ATC requested they re-cycle their Squawk. The pilot monitoring looked at the transponder and thought, “that's funny it's still set to standby”. ATC then provided a direct routing to their first en- route waypoint. The pilot monitoring looked at the FMS, it was blank. Then the penny dropped, they had omitted the pre-take off checks in their entirety!