The FAA has issued AD 2023-03-06, prohibiting autopilot and autothrottle operation below 400 feet when near 5G C-band interference in Bombardier Global Express, XRS, 5000, 5500, 6000, and 6500 jets. The AD also prohibits dispatch with certain weight-on-wheels calibrated airspeed messages.
Taking effect on March 13, the AD specifies that the autopilot/autothrottle restrictions apply only “when in the presence of 5G C-band interference as identified by notams.” Transport Canada issued a similar AD on Nov. 4, 2022.
Both the FAA and Transport Canada had published earlier ADs that limited operations that require radar altimeter data during low-visibility conditions. But Transport Canada continued examining 5G effects on Bombardier jets and, according to the FAA, “Bombardier has determined that 5G C-band interference can result in unavailable or misleading radio altimeter information” that could cause performance problems with the automatic flight control system.
These problems include erroneous radio altimeter information, which has the potential to cause incorrect gains on approach, flight guidance oscillation, and crew over-correction. The flight director uses the glideslope to linearize the angular deviation and if the radio altimeter erroneously changes to an incorrect value, the resulting pitch command may be inadequate, resulting in flight path oscillations.
Also included is misleading radio altimeter information that can adversely impact the autothrottle function, resulting in early or late activation of the retard mode, leading to an inappropriate level of thrust. This may result in a low energy state or longer landing distance. This malfunction will increase pilot workload as the crew disconnects the autothrottle and overrides the throttle levers.
In the event of a weight-on-wheels signal failure in combination with a related master minimum equipment list dispatch, interference may result in the radio altimeter deploying the two pairs of ground spoilers at heights above 7 feet agl.
“These effects may lead to increased flight crew workload and adversely affect the safe operation of the airplane during takeoff, approach, and landing below 400 feet agl,” the FAA said.
According to a Bombardier spokesman, “Bombardier is currently exploring and developing solutions for all models of in-service aircraft, as well as in-production aircraft. Solutions are to be implemented in 2023.”