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The right main landing gear down-light did not illuminate because the down-lock hook had not completely engaged and actuated the microswitch. Post-accident extension of the landing gear indicated that stiffness of the down-lock assembly prevented complete engagement. The initial inspection indicated that failure of the right landing gear down-lock to properly engage was the result of inadequate lubrication. This was consistent with the circumstances involved in four Chieftain occurrences in the preceding 12 months that each involved failure of a main landing gear down-lock assembly to properly engage.

Had the main landing gear down-lock assemblies been lubricated in accordance with Part I of Service Letter 755C, it is likely that the right main gear down-lock assembly would have fully engaged. Although the service letter referenced in the operator's 50-hour maintenance schedule indicated that the down-lock assembly should have been lubricated, that task was not specifically stated in the schedule. Incorporation of a specific requirement for down-lock assembly lubrication at 50-hour intervals in Chieftain maintenance schedules will reduce the risk of incomplete down-lock engagement and gear-up landings.

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