Dangerous autopilot response due to false glide slope
The 3-degree glide slope is prescribed to ensure a stable and safe landing. The instrument landing system will guide the aircraft to the runway along this route. However, if the aircraft approaches from a higher than usual altitude and aims to capture the 3-degree glide slope from above, it risks intercepting a ‘false’ glide slope. This term is used in aviation to denote the non-prescribed 6 and 9-degree glide slopes. The ILS may send a reversed signal to the aircraft when it crosses a false glide slope.
False glide slopes and incidents
False glide slopes are system characteristics which are based on radio waves. Indeed, false glide slopes were already known to exist. However, until recently it was believed that false glide slopes also guide the aircraft to the runway, albeit at a steeper angle of descent and a higher descent rate. During the investigation of an incident with a landing aircraft, the Dutch Safety Board discovered that the characteristics of the instrument landing system did not match common theories about false glide slopes. Further investigation revealed that incidents of this type had occurred before: four incidents are known to have occurred in Europe, while nineteen incidents were found in a US database. These incidents took place at different airports and involved different air operators.
Further investigation into the instrument landing system revealed that various types of – internationally used – ILS glide slope antennas reverse the signal on false glide slopes. This sometimes occurs at the 6-degree glide slope and always occurs at the 9-degree glide slope. In such a case, instead of the required ‘fly down’ signal, the antennas send out a ‘fly up’ signal (pitch-up upset). The nose will then suddenly pitch up, causing the aircraft to lose speed. In its reports the Dutch Safety Board has made a number of technical and operational recommendations the aviation community. Overall, the essence of the recommendations is that pilots and other professionals in the aviation sector should be aware of the existence of reversed signals at false glide slopes and of the response to such signals by the autopilot, and that they should prevent the aircraft from capturing a false glide slope. Should they inadvertently capture a false glide slope, they should take measures to prevent the response of the autopilot to the reversed signals.
Focus on automation
In addition to discussing the autopilot’s response to false glide slopes, the Dutch Safety Board also looked at the high degree of cockpit automation that has become standard in virtually all commercial aircraft. While improving aviation safety, in the course of time these automated systems have also come to replace ‘manual’ flying. This has resulted in reduced proficiency of pilots’ aviation skills. The availability of automated systems encourages people to adopt a natural tendency to follow the choice of least cognitive effort. When faced with making decisions during the flight, pilots will rely on these automated aids as a replacement for vigilance and for actively seeking information and processing it. The Dutch Safety Board is concerned about this development.
The Dutch Safety Board launched an investigation into an incident at Eindhoven Airport. During this incident, the flight crew of a Boeing 737 were confronted with an automated warning – a ‘stick shaker’ – indicating an approach to stall condition. The flight crew made a successful go-around, after which the aircraft landed safely. The existence of false glide slope reversal emerged during this investigation, and was further analysed. In November 2013 a safety alert was issued to the international aviation sector to warn pilots and other professionals for autopilot behaviour in response to reversed ILS signals when capturing a false glide slope. Two reports have been published on the basis of the investigations: ‘Stick Shaker Warning on ILS Final’ and ‘Pitch-Up Upsets due to ILS False Glide Slope’.