What happened
On January 19, 2010, a Brit Air Bombardier CRJ700, registration F-GRZK, was performing a series of approaches to runway 08R at Paris Charles de Gaulle under Low Visibility Procedures (LVP). During the first two CAT III A approaches, the crew observed intermittent operation of radioaltimeter 1. Discrepancies between the two radioaltimeters triggered APCH WARN messages on the Head-Up Guidance System (HGS) and PFD, leading the crew to abort both approaches at approximately 800 ft AMSL.
For the third approach, the crew opted for a CAT I procedure. However, the malfunction persisted. Between 700 ft and 340 ft AMSL, radioaltimeter 1 failed to provide height data. During this period, the aircraft's longitudinal attitude began to oscillate. The Flight Director (FD) used radioaltimeter data to weight its guidance gains; because the system incorrectly believed the aircraft was at an altitude greater than 2,500 ft, the FD applied excessive corrections to the glide slope. This resulted in an increasing oscillation of the aircraft's pitch, which eventually reached a 7-degree nose-down attitude at 100 ft AGL. The pilot flying disconnected the autopilot at 120 ft AGL to manually recover the glide path and landed without further incident.
The investigation
The investigation focused on the cause of the intermittent radioaltimeter 1 signal and why the flight guidance system reacted so violently to the erroneous data. Maintenance inspections by the operator revealed a short-circuit in the coaxial cable between the receiving antenna and the radioaltimeter 1 unit. Specifically, investigators found a short between the core and the shielding, as well as insulation failure in the connector caused by the presence of greasy fluid.
Findings
- The primary cause of the signal intermittency was a faulty coaxial cable resulting from a short-circuit and fluid contamination.
- The Flight Director (FD) guidance laws use radioaltimeter data to adjust gain for glide slope corrections. Because the system processed the erroneous height as being above 2,500 ft, it commanded over-corrections that led to the unstable pitch oscillations.
- The crew was unaware of the specific impact that erroneous radioaltimeter data would have on the flight guidance system's behavior.
- The pilot flying maintained the autopilot connection until 120 ft AGL, which was 40 ft above the maximum authorized altitude for use during this specific approach type.
Safety action
- FRAN-2012-008: Recommends that aircraft manufacturers examine and correct the adverse effects of erroneous altitude values provided by radioaltimeters on onboard systems.
- FRAN-2012-009: Recommends that manufacturers update aircraft operating manuals to clarify how onboard systems utilize radioaltimeter data and provide specific procedures for crews when discrepancies are detected.
- FRAN-2012-010: Recommends that manufacturers modify maintenance procedures to prevent the accidental degradation of radioaltimeter systems during servicing.