What happened
On May 31, 2013, a Boeing 737-800 was performing an instrument approach to runway 21 at Eindhoven Airport under instrument meteorological conditions. While flying at an altitude of approximately 1,060 feet, roughly two kilometers from the runway, the aircraft encountered a false glide slope. As the aircraft crossed a 9-degree glide slope, the autopilot responded to a signal reversal, triggering a sudden and violent pitch-up upset. This maneuver caused the airspeed to drop significantly, activating the stick shaker warning. The flight crew initiated a go-around and, despite a second brief activation of the stick shaker during the recovery, successfully regained control and landed safely.
The investigation
The Dutch Safety Board investigated the incident and discovered that the event was not isolated, identifying several similar occurrences at various European airports involving different aircraft and operators. To understand the underlying cause, the Board conducted flight tests and signal measurements at Eindhoven Airport and Woensdrecht Air Base, as well as in the United States.
Using a research aircraft, the investigation examined the characteristics of the Instrument Landing System (ILS) signals. The testing focused on the area above the standard 3-degree glide slope. The Board specifically looked at how the autopilot and autothrottle interact with the glide slope signals at higher angles, such as the 6-degree and 9-degree marks.
Findings
Testing revealed that the existing understanding of ILS false glide slopes was incorrect. Contrary to the belief that false glide slopes simply guide aircraft at a steeper descent rate, the investigation found that signal reversal occurs at the 9-degree glide slope. This reversal changes a 'fly down' command into a 'fly up' command, which the aircraft's autopilot captures, leading to an unexpected pitch-up.
Furthermore, the investigation established that:
- At the 6-degree glide slope, a small area of 'fly up' indication may exist, which can trigger the glide slope capture logic.
- There is a lack of regulatory requirement for ILS flight inspections to cover angles above 5.25 degrees, meaning aircraft flying at higher angles are operating outside the verified reliability envelope.
- The cockpit instruments provided no automatic warnings to the crew as they crossed these false glide slope boundaries.
Safety action
Following the discovery of these unknown signal characteristics, the Dutch Safety Board issued a global aviation Safety Alert in November 2013. The Board also issued recommendations to major regulatory bodies, including the EASA and the FAA, to ensure that information regarding false glide slope consequences is updated in aviation manuals and pilot training programs.