French investigators have determined that a short-circuit in a Bombardier CRJ700's radio altimeter antenna cable caused the autopilot to over-correct, and the jet to oscillate near the ground, as it tried to maintain the vertical glideslope profile.

The Brit Air aircraft had been attempting a low-visibility Category IIIa approach to Paris Charles de Gaulle's Runway 08R, and the crew aborted the landing twice.

As the jet descended below 1,700ft, the crew noticed the radio altimeter was behaving intermittently. The pilots executed a missed approach at 800ft but during the second landing attempt, a similar situation occurred, prompting a second go-around.

Having opted for a third attempt - this time under Category I conditions - the pilots found the radio altimeter was again behaving intermittently. Just 160ft above the decision height, the altimeter stopped giving information, which effectively led the aircraft's flight-guidance systems to conclude that the jet was operating above 2,500ft.

Because of this incorrect height assumption, the aircraft's systems misinterpreted the scale of its vertical deviation from the glideslope. The autopilot over-corrected, generating pitch oscillations with increasing amplitude as the aircraft attempted to fly the correct profile.

French investigation authority BEA said the pilot was "surprised" by this behaviour and had to react "at very low height" to regain the glideslope. At 120ft above ground, the pilot disconnected the autopilot and, having visually acquired the approach lights, landed the aircraft safely.

Examination identified a short-circuit between the core and shielding of the co-axial cable linking the captain's radio altimeter and its antenna. BEA said there had been several queries from operators over radio altimeter reliability and noted that stress, moisture and other factors could degrade the cable over time.

In its report into the 19 January 2010 incident, the BEA pointed out that flawed data from the captain's faulty radio altimeter, and the response of aircraft systems, had led to the Turkish Airlines Boeing 737-800 crash at Amsterdam in 2009.

Source: Flight International