20111218029699I

The instructor pilot involved reported having performed 4 prior take-off and landings, bringing the gear up & down all 4 sessions successfully. During the 5th session the nose landing gear failed to extend during the downwind approach to land. Several attempts were made to extend the nose gear, to no avail. Manual gear extension was also attempted in accordance with emergency check list, to no avail. An emergency was declared and the aircraft landed with the nose gear up. When the aircraft nose was lifted up off the runway, the owner/operator/mechanic of the aircraft was able to pull the nose gear part way down from the uplocked position by hand. The nose gear then continued to the down & locked position under its own weight. The owner/operator/mechanic reported performing a gear swing while on jacks and was not able to duplicate the nose gear sticking in the up position. The owner/operator/mechanic also reported he suggested the cowling may have interfered with the nose gear doors and subsequently jammed the gear in the up position. Several successful gear swing cycles were observed while on the jacks, including an emergency hand pump extension without failure. A full gear rigging check has not yet been performed. The landing gear hydraulic power pack reservoir was observed to be approximately 1 centimeter below the max fill line.a review of the service difficulty reporting (SDR) website shows three similar incidents occurred on other Cessna 177RG aircraft; on two separate occasions no single cause could be identified, on another occasion the submitter suspected the nose gear mechanical uplock failed to release the nose gear due to being out of adjustment. The owner/operator/mechanic was encouraged to report this incident as a malfunction or defect report via the SDR website. Note: this report will be amended if additional details become available as to the cause of the incident.