Expectation bias has been described in research cited in a U.S. National Transportation Safety Board (NTSB) aircraft incident report as “a psychological concept associated with perception and decision making that can allow a mistaken assessment to persist.” (Bhattacherjee 2001) Expectation bias, and the related confirmation basis, can cause a person’s incorrect belief to persist despite available contradictory evidence.
In A Practical Guide for Improving Flight Path Monitoring, the Active Pilot Monitoring Working Group said that “individuals are vulnerable to thinking they see what they expect to see … . Inattention blindness, change blindness and expectation bias are not manifestations of laziness, but simply are part of the way everyone’s brain processes information.”
Expectation bias occurs when a pilot hears or sees something that he or she expects to hear or see rather than what actually may be occurring. That expectation often is driven by experience or repetition. For example, if a pilot is regularly cleared to cross a particular runway during operations at a familiar aerodrome, he/she may come to “expect” the clearance. This could cause a potentially dangerous situation if on a particular day, the pilot actually is instructed not to cross the runway in question due to another aircraft landing or taking off.
The same bias can extend to what the flight crew sees, or thinks it sees. For example, on 20 Aug. 2008, a misconfigured Spanair McDonnell Douglas MD-82 (MD82, Madrid Barajas Spain, 2008) crashed on takeoff from Madrid-Barajas Airport. It was established that the flight crew, with the aircraft commander as pilot flying, had unintentionally attempted the takeoff without setting the flaps/slats to the required and intended position.
Expectation bias likely played a role when the first officer called out a flap setting of 11 degrees while conducting both the takeoff briefing and the final check before takeoff. “There is a natural tendency for the brain to ‘see’ what it is used to seeing (look without seeing),” the final accident report said. “In this case, the first officer, accustomed to doing the final checks almost automatically, was highly vulnerable to this type of error. … The captain, for his part, should have been monitoring to ensure that the answers being read aloud by the first officer corresponded to the actual state of the controls.”
NTSB has investigated numerous accidents and incidents involving pilot errors resulting from expectation bias, particularly in night VMC when fewer clues were available to pilots to aid in airport and runway identification. For example, in January 2014, a Boeing 737 landed at the wrong airport in Branson, Missouri, in night Visual Meteorological Conditions (VMC). The flight crew expected that the visually identified airport and runway were the intended destination and did not reference cockpit displays to verify the airport and runway. As a result, the airplane landed on Runway 12 at M. Graham Clark Downtown Airport instead of Runway 14 at Branson Airport.
The U.S. Federal Aviation Administration (FAA) said in 2012 that an analysis of runway incursion data showed that expectation bias is one of the most common causal factors in occurrences involving pilot deviations.
The FAA Safety Team said that pilots need to understand that expectation bias often affects the verbal transmission of information. When issued instructions by air traffic control, pilots should “focus on listening and repeat to yourself exactly what is said in your head — and then apply that information actively. Does the clearance make sense? If something doesn’t make sense (incorrect call sign, runway assignment, altitude, etc.), then query the controller about it.”
NTSB, in its final report on an incident in which an Air Canada Airbus A320 mistakenly lined up with an occupied taxiway on approach to San Francisco International Airport in night VMC (A320 / B789 / A343, San Francisco CA USA, 2017) said expectation bias occurs automatically and can be difficult to overcome once established because of its inherent strength.
One way to overcome expectation bias is through training that stresses active questioning of observations and recognizing the presence of conflicting cues. In addition to active questioning to overcome expectation bias, which may be prone to human limitations such as fatigue impairment, cockpit systems to improve crew positional awareness could provide a conspicuous cue of a misalignment with an intended landing surface. “The NTSB concludes that flight safety would be enhanced if airplanes landing at primary airports within Class B and Class C airspace were equipped with a cockpit system that provided flight crews with positional awareness information that is independent of, and dissimilar from, the current ILS backup capability for navigating to a runway.”
Accident & Incidents
- MD82, Madrid Barajas Spain, 2008 (On 20 August 2008, a McDonnell Douglas MCDONNELL DOUGLAS MD-82 being operated by Spanair on a domestic scheduled passenger flight from Madrid Barajas to Gran Canaria crashed on departure immediately after becoming airborne from runway 36L in day Visual Meteorological Conditions (VMC) and impacted terrain to the east of the runway. The aircraft was destroyed by the impact and subsequent fire and 154 of the 172 occupants were killed with the remainder being seriously injured.)
- A320 / B789 / A343, San Francisco CA USA, 2017 (On 7 July 2017, an Airbus A320 (C-FKCK) being operated by Air Canada on a scheduled international passenger flight from Toronto to San Francisco as AC759 was cleared to make an approach and land on runway 28R at destination in night VMC but instead lined up with its parallel taxiway on which four aircraft were waiting to depart and descended to about 60 feet agl and overflew two of those aircraft before climbing away.)
- Lacagnina, Mark. "Lift Deficit." Flight Safety Foundation AeroSafety World December 2011-January 2012.
- NTSB. "Taxiway Overflight Air Canada Flight 759, Airbus A320-211, C-FKCK, San Francisco, California, July 7, 2017." Incident Report NTSB/AIR-18/01, 25 Sept. 2018.
- Active Pilot Working Group. A Practical Guide for Improving Flight Path Monitoring, Flight Safety Foundation, November 2014.