The Boeing 737-800 can be flown either manually or automatically. This also applies to the management of the engines. The autothrottle regulates the thrust of the engines. The aircraft is fitted with two radio altimeter systems, one on the left and one on the right. In principle, the autothrottle uses the altitude measurements provided by the left radio altimeter system. Only if there is an error in the left system that is recognised as such by the system, the autothrottle will use the right-hand radio altimeter system.
In a worst-case scenario, the left-side altimeter tells the autothrottle that the aircraft is lower than it is while the right-side autopilot is flying the plane, but the left altimeter thinks its data is good. Fly low enough in this condition, such as a final approach, and data from the left altimeter can roll the throttles back to idle while the right-side autopilot, relying on accurate data from its altimeter, will keep pushing the nose up to maintain the glideslope.
This [design] a relic from the Boeing 737, certificated long ago, which in the original design prioritised the provision of information to the left pilot (captain). This original design has now been superseded by both technical facilities and a democratisation and reallocation of pilot duties in the cockpit. It is noticeable that this subject cannot be found in any of the Boeing 737 manuals or training documents for pilots. Pilots therefore do not have the correct knowledge about links between the control systems and data input for their own aircraft. The result of this is an incomplete or even incorrect ‘mental model’ of the automated flight control.
FAA's AD is based on Boeing service bulletin issued in November. The directive will require autothrottle computer replacements or modifications on affected 737NGs, which FAA says number 497 in the U.S. alone.