On January 21, 2018, at 1304 eastern standard time, a Cessna 172RG airplane, N9415B, sustained substantial damage while landing at the Eastern West Virginia Regional Airport (MRB), Martinsburg, West Virginia. The flight instructor and the private pilot were not injured. The airplane was registered to Dulles Aviation, Manassas, Virginia, and operated by Av-Ed Flight School, Leesburg, Virginia, as a Title 14 Code of Federal Regulations Part 91 instructional flight. Visual meteorological conditions prevailed and no flight plan was filed for the flight that originated from Leesburg Executive Airport (JYO), Leesburg, Virginia, about 1130. The flight instructor stated that the purpose of the flight was to practice commercial pilot maneuvers and landings. The flight was normal, and they had completed about six short and soft-field takeoffs and landings without incident. On the seventh landing, after the private pilot extended the landing gear, the gear down-and-locked light did not illuminate. A visual check revealed that the nose gear was extended but the main gear was trailing and not fully extended. The flight instructor said they used the manual emergency gear handle to try and pump the main gear down, but there was not enough hydraulic pressure in the system to extend the gear. The flight instructor then landed the airplane with the nose wheel still extended. He said he was able to keep the airplane straight for about 600 ft, but the airplane's left wing dropped resulting in substantial damage to the wing and elevator. After exiting the airplane, hydraulic fluid was observed pooling under the airplane and along the side of the empennage. The hydraulic reservoir was empty. A postaccident examination of the airplane revealed that an O-ring had failed internally inside the right gear actuator and it was leaking hydraulic fluid. Since the landing gear is extended/retracted by hydraulic pressure, the leak prevented the system from having adequate pressure for the electric pump and the manual gear handle to extend the gear. At 1306, the weather conditions reported at MRB were calm wind, visibility 10 statute miles, scattered clouds at 3,900 ft, overcast clouds at 7,500 ft, temperature 13° C, dewpoint 5° C, and an altimeter setting of 30.12 inches of mercury.