Brasilia EMB-120 All Series
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented
Aircraft; Human Factors
At about 2 miles from the final approach fix; with the autopilot coupled to the approach system and a normal; on-glide indication on my side; the Captain noticed that his side was showing approximately one dot low. We referenced the approach plate and verified that; on our present track; we would cross the final approach fix below the minimum altitude. It was at this point that we realized that the glideslope indicator on my side was stuck in the middle position. The Captain announced 'my controls;' disconnected the autopilot; and climbed the aircraft to rejoin the glideslope. During the entire time; we had the airport and the surrounding terrain in sight. For this reason; we elected to continue the approach and make a normal landing. After arriving at the gate; we contacted Maintenance and wrote up the discrepancy. The glideslope indicator on my side had failed; showing an 'on glideslope' indication that would have been normal during a coupled ILS approach. The autopilot performed normally but was receiving incorrect inputs from the navigation system; causing the aircraft to descend below the actual glideslope. Contributing to the event was the fact that; since we had the airport in sight; I started focusing more of my attention outside of the cockpit to the runway environment. In retrospect; I should have continued cross-referencing step-down altitudes to verify that we were still on-course. Even during an instrument approach with the runway or airport in sight or a visual approach that is supplemented with a navigation aid; it is still necessary to reference the approach plate and verify that the aircraft is on the correct course and at the correct altitude. That way; should an event like this one happen; the fault in the equipment can be detected; Maintenance contacted; and the issue resolved.
EMB-120 First Officer discovers during a coupled ILS approach that his glideslope indicator is frozen in the centered position and the aircraft is actually one dot low on the Captain's indicator. The Captain takes control and continues for visual landing.