AINsight: Explaining the Unexplainable Pilot ‘Action Slips’

On June 28, 2024, the first officer of a Boeing 777-200 unexpectedly responded to an automated V1 callout by retarding the thrust levers instead of moving his hands to the control column. He then briefly advanced the thrust levers before finally rejecting the takeoff.

Under normal conditions, V1 is the maximum speed during takeoff at which a pilot must initiate the first action to safely abort the takeoff within the accelerate-stop distance. After this point in the takeoff roll, the pilot is committed to flying.

At the time of the event, the aircraft weighed more than 546,000 pounds (40,000 pounds less than its mtow) with 13 crew and 334 passengers. According to flight data, the thrust levers were retarded first at 160 knots (V1) and again at 162 knots—the maximum speed during the takeoff roll was 167 knots. The aircraft came to a stop on the runway with 2,300 feet remaining. After a brake fire on the right side of the aircraft was extinguished, the aircraft was then towed to the gate. There were no injuries.

UK AAIB investigators found no obvious reason for the first officer’s actions. The “cognitive task analysis” of the report described the morning of the incident as “unremarkable” for the crew with no obvious distractions or workload issues.

In addition, the first officer was highly experienced on the Boeing 777. He had accumulated 6,156 hours total flight time and 2,700 hours on the aircraft type. Following the incident, the first officer stated that he was “well rested and feeling fine” and had “expressed surprise in himself over the inadvertent thrust reduction and could not identify a reason for it.”

Action Slips

Investigators concluded that the first officer’s incorrect actions were characterized as an “action slip,” which is defined as “an action which is not performed as intended and arises in routine or highly learned motor action sequences.” An action slip is an unintentional error in executing a planned action often occurring during routine or automatic tasks due to lapses in attention or habitual behavior.

Psychologist Donald Norman popularized the term in his 1988 book, “The Design of Everyday Things,” describing action slips as errors that occur while carrying out planned action sequences. These errors are not caused by a lack of knowledge or skill but by a temporary failure in attentional control or cognitive monitoring.

In aviation, the consequences of inadvertently executing the wrong action can lead to a serious incident or accident. In the case of the Boeing 777 inadvertent rejected takeoff, the report stated that given “the somewhat benign performance conditions,” the aircraft had stopped “some distance before the end of the runway surface.” A fast response from airport crash, fire, and rescue services quickly extinguished the resultant brake fire with no injuries reported.

Triggers and the Inappropriate Response: ‘Positive rate, Flaps Up?’

In another case, on July 16, 2018, the first officer of a Bombardier Dash 8-400 inadvertently raised the flaps instead of the landing gear after takeoff. Fortunately, the crew used a “normal takeoff power” for departure rather than a reduced power setting, and the performance of the aircraft masked the effects of the early flap retraction.

Investigators classified the event as human error, noting it was not unique to this operator. They suggested the flaps may have been retracted instead of the landing gear because both actions are expected during this phase of flight.

On Aug. 4, 2003, a Bombardier Learjet 35A crashed during a circling approach in Groton, Connecticut. Both pilots were killed. Flight data indicated that the aircraft was maneuvered at high bank angles (ranging from 60 to 90 degrees of bank) at altitudes below 500 feet agl. During its final turn, the first officer inadvertently retracted the flaps, causing the aircraft to stall and crash one-quarter mile from the airport.

NTSB determined that the probable cause of the accident was "the first officer's inadvertent retraction of the flaps during the low altitude maneuvering, which resulted in the inadvertent stall and subsequent in-flight collision with a residential home. Factors in the accident were the captain's decision to perform a low-altitude maneuver using excessive bank angle, the flight crew’s inadequate coordination, and low clouds surrounding the airport."

Psychologists have identified three types of action slips:

• Capture errors: A more frequent or habitual action overrides a less familiar intended action. This occurs when the mind is preoccupied.  

• Loss of activation: Forgetting to carry out a planned action entirely, such as driving home without stopping at the store as intended.

• Mode errors: Performing an action in one context but inappropriate in another, often seen in complex systems or interface use.

Before the inadvertent Boeing 777 rejected takeoff, the operator had already reviewed its standard operating procedures relating to movement of critical flight controls and found “that absence of cognitive thought and speed of execution [were] commonly featured during action slips.” Four days before the B777 event, the operator issued an “Operational Safety Notice” that stated, “Pause before execution and cognitively consider what the action is” before methodically executing the action and confirming it is correct.

Investigators concluded that preventing action slips remains an ongoing challenge for both operators and flight crews. The report noted that the B777 operator had issued guidance encouraging deliberate and methodical control selections, and had emphasized human factors in training and briefing materials. It also recommended that experienced pilots mentally rehearse the takeoff roll and other routine procedures.