After a PC-12 firm landing; the nose strut compressed upon touch down allowing the propeller blades to strike the runway. The aircraft had recently been in maintenance. It was later determined the left angle of attach (AOA) system was miscalibrated by 5 units.
Synopsis
After a PC-12 firm landing; the nose strut compressed upon touch down allowing the propeller blades to strike the runway. The aircraft had recently been in maintenance. It was later determined the left angle of attach (AOA) system was miscalibrated by 5 units.
Narrative
Normal approach profile to 3500' X 50' asphalt runway. No services. Aircraft AOA guidance utilized on approach; call-outs by SIC were 'Blue Diamond to Donut;' which I was happy with. Wind direction called 130 @ 13-19; gusty. Tree line to left side of runway; several small hangars to right side. I added 40 degrees flaps on final and yaw damper was disengaged on short final. Pleased with profile and looked good for touchdown point. Just prior to touchdown power was reduced full; holding off for mains to make contact; sudden drop of aircraft (SIC noticed sudden drop of AOA) whereupon mains hit and then hard onto nose. At this point blades struck asphalt (approximately 550' down runway). Finish landing roll; turned aircraft around on runway for taxi back to off-load passengers. It was on post flight inspection that the damage was realized. The landing did not seem unusually hard; and passengers were not aware.I will now have more of an appreciation for possible wind-shear conditions due to gusty winds combined with local ground features. When I observe these conditions in the future I may elect to proceed to an alternate airport with better wind to runway alignment or simply a larger landing surface where ground friction possibilities can be minimized.
NASA callback
The reporter stated that the aircraft had extensive engine inspections and propeller blades repaired after this event. His maintenance department determined that the landing would not be considered hard by inspection standards. What was determined after an extensive inspection was that the left Angle of Attack sensor calibration was off by 5 units. That deviation meant that the aircraft would not get a stall warning 5 KTS above stall and that the stick pusher would not exert the 65 pounds of forward yoke pressure to assist in stall recovery. The reporter stated that the AOA calibration process is quite extensive and if any step is missed or the test equipment is not calibrated the AOA indicator will not be accurate.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.