NTSB specified:
"Before the SIC reached the steps to cycle the landing gear, the flight crew was distracted by a hydraulic overheat condition and diverted to the Hydraulic System Overheat checklist to address that condition. When the SIC returned to the Landing Gear Down Lock Indication Failure checklist, he could not cycle the landing gear per the checklist instructions, because the hydraulic pressure was low. He then began the Emergency Landing Gear Extension checklist.
The emergency extension resulted in all three landing gear remaining extended, but only the nosegear locked, and no further pertinent information remained in the checklist. The flight crew then performed an emergency landing at an airport with a longer runway. During the landing, the right main landing gear collapsed, and the airplane came to rest on the runway. "
"During postaccident examination of the airplane, the landing gear selector handle was found 1/8- to 1/4-inch from the full down position. Subsequent ground testing revealed that when the landing gear selector handle was positioned full up, followed by full down, the landing gear cycled successfully, indicating that, if the flight crew had placed the handle in the full down position, the landing gear would likely have operated normally.
When the landing gear selector handle was positioned where it was found, the landing gear extended, but did not lock. A hydraulic bypass also occurred, with a resulting increase in hydraulic fluid temperature and decrease in hydraulic fluid pressure. The hydraulic bypass was most likely the reason that the landing gear did not lock when the emergency gear extension procedure (blow down) was followed during the incident flight. Although, the rigging of the landing gear selector valve arm was found to be 2 degrees beyond specifications, the fact that the landing gear was successfully cycled numerous times with this discrepancy indicates that it was not a contributing factor to this incident.
After the incident, the airplane manufacturer revised several checklists by replacing the terms "normal" and "low" with actual numerical values.
Additionally, the Landing Gear Down Lock Indication Failure and Emergency Landing Gear Extension checklists were revised to include more guidance on ensuring that the landing gear handle was positioned full down. Lastly, the Emergency Landing Gear Extension checklist was expanded to include a situation where the blow-down procedure failed to extend and lock all three landing gear.
The National Transportation Safety Board determines the probable cause(s) of this incident as follows:
The flight crew did not ensure that the landing gear selector handle was in the full down (extend) position. Contributing to the incident was inadequate checklist information.
http://www.ntsb.gov/aviationquery/br...11IA316&akey=1