Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 240; Flight Crew Total 22930; Flight Crew Type 13410
Human-Machine Interface
Aircraft Equipment Problem Critical
Flight Crew Regained Aircraft Control; General Maintenance Action
Human Factors; Procedure; Aircraft
With clear weather at SEA we decided to do a practice auto-land. We briefed; setup; and advised both Approach and Tower of our intention. Just after my 150 foot call the autopilot and autothrottle disengaged. I took control manually. The landing resulted in a bounce with a subsequent touchdown. I entered the unsuccessful auto-land in the maintenance logbook. The landing was not good from a pilot's perspective but was not hard by Maintenance definition. There was no aircraft generated advisory message or visible indications of a hard landing. The Maintenance Technician confirmed that oil marks on the landing gear struts only indicated partial compression. Other than some light hearted remarks there were no complaints of injury by passengers or crew. This event has completely changed my opinion of our current auto-land procedure and the need for our pending change. The maintenance BITE revealed that the automation disconnected because the Captain's Radar Altimeter failed at 75 FT. I had not yet said that I had the aircraft but it was so close to that point that the First Officer thought that I did. Thankfully and to his credit he verbalized that I was now the pilot flying. Fifty feet is not the place for a transfer of control especially when it is from automation to First Officer to Captain.
Air Carrier Captain experiences autopilot disengagement at 75 FT during an auto land approach in VMC. A firm manual landing ensues after a small bounce. Maintenance determines that the Captain's radar altimeter failed at 75 FT causing the autopilot disengagement.