“To my horror… I unintentionally shut down the number two engine as well….”

Muscle memory
NOTE: The plane on the photo is not related with the incidents reported here

Muscle memory is an interesting physiological phenomenon involving our muscles and their interaction with the brain. The more often we perform a given physical action, the more likely we are to do it as needed, when needed, without having to think about the specific combination of movements involved. These habits thus become an unconscious process that occurs when triggered by a given circumstance or set of cues.

Practicing a procedure until the process is automatic develops muscle memory that can be crucial when an immediate action emergency (such as an engine failure at V1) occurs. However, as in the incident reports below, muscle memory can be a problem when the cues are right, but the circumstances are wrong. That is when the brain has to be “conscious” enough to stop the automatic response of well-trained muscles.

The following ASRS reports recount a sequence of ground incidents in which muscle memory took over at the wrong time.


Photo: http://www.privatejetcharter.co.uk/aircraft-type/boeing/boeing-737

Faced with a distraction and a familiar set of circumstances, a B737 Captain let muscle memory take over just long enough to create an embarrassing situation.
■ Inoperative APU; second flight of the day; started the number one engine at the gate…; asked for taxi to a remote area for cross-bleed start of the number two engine. Stopping at the designated location, the aircraft began to shimmy slightly under braking. I stopped braking then applied brakes again. The shimmy did not happen again so I set the parking brake. I then grabbed the number one engine start lever and began to shut the number one engine down. Realizing what I was doing, I quickly returned it to the previous position, but the engine had already shut down. We were now on battery power. I told the Flight Attendants to remain seated, then told ATC we would need a tow back to the gate and we had one radio and would need to go off frequency to coordinate with Company Operations. We turned IRS 1 and 2 off and tried to explain to the passengers what had happened. We were back at the gate in approximately 10 minutes. We started the engine and did the procedure properly the second time. The remainder of the flight was uneventful.

I guess I would say it was muscle memory, the same motion as arriving at a gate, number two engine shut down, parking brake set. I should be more deliberate in all of my actions, but it happened so fast that the First Officer did not even have time to react. The brake shimmy was a distraction, but that does not excuse me from my action.

This B737 Captain’s method of checking the start lever position was problem enough, but then muscle memory kicked in and made the situation worse.
■ It was my leg. Preflight activities had been normal and we were not rushed at all.… We had been instructed to hold short of [the runway] and were almost stopped. I had already called for the Before Takeoff Checklist and the First Officer challenged me with “Start Levers” at the next to last step in that checklist. I reached down to confirm “Idle.” My practice has been to hold the start levers with my thumb and forefinger, confirm the idle detent position with a slight nudge forward and a slight nudge rearward, then to respond, “Idle.” However this time with the slight nudge to the rear, the number one start lever felt like it was not quite fully down in the idle detent. It came up over the edge and I unintentionally shut down the number one engine. I was surprised and stunned.

I announced the situation to the First Officer and set the parking brake. Then instinctively I reached down again to confirm the start lever positions. At that point muscle memory kicked in and I must have “matched” the start lever heights. To my horror, when I nudged the levers rearward again, I unintentionally shut down the number two engine as well. I started the APU and put electrical power back on the aircraft. We told ATC that we had a problem and that it would be a few minutes before we could move. Feeling completely inept and embarrassed, I told the First Officer that we would start over and re-accomplish everything beginning with the Before Start Checklist. The First Officer agreed.

I made a short and embarrassing announcement to the Passengers and apologized for the delay while we dealt with a cockpit issue. We then flew an otherwise uneventful flight. Several suggestions come to mind in order to prevent this from happening again. Primarily, I have changed the way that I check the start levers in the idle detent. No longer will I hold them with my thumb and forefinger. And no longer will I nudge them rearward, but only forward and down.

A B737-800 Captain’s prescription for inhibiting muscle memory involves slowing down and thinking before a particular situation triggers your internal automation and results in a dose of humility.
■ We were told to line up and wait. I brought the aircraft to a stop and, for some strange reason, I reached over and shut down both engines instead of setting the parking brake. We told Tower that we had an “issue” and would be in place for a minute or two and then we would have to taxi clear. We started the right engine and taxied clear of the runway so we could redo checklists and regroup. When the Tower later asked what our issue was, I think we told them that we had to look at a light. Actually, lots of lights.

With the start levers being right next to the parking brake, I guess that once my hand was on the start levers, positioned right next to the parking brake, muscle memory took over and moved them to off. I need to slow down and think about what I am doing before moving any switch or lever. This was definitely the healthiest dose of humility ever in my many years of flying.


Automatic Response

Automaticity is one of the by-products of practice. As procedures become automatic, less attention is required to carry them out, so it is possible to do other things simultaneously, or at least do other things more comfortably. (FAA-H8083-9A)

The first mental process of safe flight is AUTOMATIC REACTION. Automatic reaction is used to maintain ongoing control of the aircraft, such as stabilizing heading and altitude by making small, automatic adjustments to the controls. It may also be used in certain emergency situations where specific, prompt action is required.

Pilots learn to do many things automatically, simultaneously, and without thinking about each individual act. They have learned skills and procedures that are now more or less automatic reactions for them. When they first practiced them, however, they had to devote a great deal of their attention to them and concentrate in order to perform successfully. Gradually, with more practice, there was a decline and eventual elimination (possibly without their realization) of their need to “think about” what they were doing as these skills became truly automatic reactions. (DOT/FAA/PM-86/45)

When skills are learned to the point of automaticity, the load on working memory typically is reduced by 90%. Typically, skills learned to the point of automaticity are also difficult to describe.

During Skill Acquisition

After Skill Automaticity
fMRIs – Differences in Brain Activation
Transition from Working Memory & Attentional Control to Sensory & Motor Processing

*Schneider, W. (2003), “Automaticity in complex cognition,” Center for Cognitive Brain Imaging at Carnegie Mellon University.

Tasks learned to the point of “automaticity” are not easily changed by conscious control.



minime2By Laura Victoria Duque Arrubla, a medical doctor with postgraduate studies in Aviation Medicine, Human Factors and Aviation Safety. In the aviation field since 1988, Human Factors instructor since 1994. Follow me on facebook Living Safely with Human Error and twitter@dralaurita. Human Factors information almost every day 


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