HISTORY OF FLIGHTOn June 15, 2014, about 2044 Pacific daylight time, a Lockheed SP-2H, N4692A, was substantially damaged when the nose wheel landing gear collapsed during landing roll at the Fresno Yosemite International Airport (FAT), Fresno, California. The airplane was registered to Minden Air Corporation, Minden, Nevada, and operated as Tanker 48 by the United States Department of Agriculture (USDA), Forestry Service, under the provisions of Title 14 Code of Federal Regulations Part 137. The airline transport pilot (ATP) rated captain and the ATP rated first officer were not injured. Visual meteorological conditions prevailed, and a company flight plan was filed for the local firefighting flight. The flight originated from Porterville Municipal Airport (PTV), Porterville, California, at 1934. The captain reported that following an uneventful aerial drop, the flight was returning to PTV. During the descent check, he noticed that the hydraulic pressure indicated 0 and that the first officer subsequently verified that the sight gauge for the main hydraulic fluid reservoir was empty. The first officer opened the jet engine doors successfully as the captain selected gear down with no response. The captain notified base personnel at PTV of the situation, and informed them that they would be orbiting to the east of the airport to troubleshoot. The captain and first officer performed the emergency checklist, and verified that the nose wheel landing gear was extended. The captain stated that the first officer then installed the pin in the nose wheel landing gear as part of the emergency checklist. The flight diverted to FAT due to a longer runway and emergency resources as both pilots briefed the no-flap landing procedure, airspeeds, and approach profile. As the flight continued toward FAT, the flight crew informed Fresno Approach Control of the hydraulic system failure, and continued to perform the emergency gear extension checklist. The first officer extended the main landing gear using the emergency gear release, which resulted in three down and locked landing gear indications in the cockpit. As the flight neared FAT, the first officer added 2 gallons of hydraulic fluid to the main hydraulic reservoir while the captain attempted to extend the flaps unsuccessfully. Subsequently, the flight landed normally on runway 26R. During the landing roll, the nose wheel landing gear collapsed, and the airplane came to rest nose low. The first officer reported that following completion of the emergency nose gear extension checklist, he physically inserted the nose gear pin in place prior to the landing at FAT. Examination of the airplane by representatives from the Forest Service revealed that the forward portion of the fuselage was structurally damaged. The airplane was recovered to a secure location for further examination. AIRCRAFT INFORMATIONThe accident airplane was manufactured in 1959 as an SP-2H Neptune. On April 18, 2002 it was issued a Special Airworthiness Certificate in the Restricted Category, for Forest and Wildlife Conservation. At the time of the accident, the airframe total time was 10,431.9 hours, of which 2,617.7 hours were as an air tanker. The airplane's main hydraulic system is 3,000 psi system powered by two engine driven hydraulic pumps. A 1,500 psi emergency hydraulic power system for extending the nose landing gear, and operating the brakes and varicam is provided by an electrical hydraulic pump. As part of the retardant tank modification, an additional electric auxiliary pump was installed, which was capable of pressurizing the main hydraulic system to 3,000 psi. The landing gear emergency extension checklist states the following items should be performed: 1. Normal landing gear circuit breaker pull 2. Airspeed below 155 KTS 3. Gear handle down 4. Emergency nose gear switch down 5. Nose gear pin install 6. Emergency nose gear switch off 7. MLG emergency release pulled The nose gear pin is a steel pin, which has a retaining bearing at the end of the pin. The top of the pin is pressed, which allows for the bearing to become disengaged, and allows removal of the pin. In addition, the button portion of the pin has a ring with a red flag attached. AIRPORT INFORMATIONThe accident airplane was manufactured in 1959 as an SP-2H Neptune. On April 18, 2002 it was issued a Special Airworthiness Certificate in the Restricted Category, for Forest and Wildlife Conservation. At the time of the accident, the airframe total time was 10,431.9 hours, of which 2,617.7 hours were as an air tanker. The airplane's main hydraulic system is 3,000 psi system powered by two engine driven hydraulic pumps. A 1,500 psi emergency hydraulic power system for extending the nose landing gear, and operating the brakes and varicam is provided by an electrical hydraulic pump. As part of the retardant tank modification, an additional electric auxiliary pump was installed, which was capable of pressurizing the main hydraulic system to 3,000 psi. The landing gear emergency extension checklist states the following items should be performed: 1. Normal landing gear circuit breaker pull 2. Airspeed below 155 KTS 3. Gear handle down 4. Emergency nose gear switch down 5. Nose gear pin install 6. Emergency nose gear switch off 7. MLG emergency release pulled The nose gear pin is a steel pin, which has a retaining bearing at the end of the pin. The top of the pin is pressed, which allows for the bearing to become disengaged, and allows removal of the pin. In addition, the button portion of the pin has a ring with a red flag attached. TESTS AND RESEARCHExamination of the airplane by US Forest Service personnel revealed evidence of hydraulic fluid leakage from the right engine cowling drain. Examination of the right engine accessory pad revealed that two of the eight bolts attaching the hydraulic pump were separated. Hydraulic fluid was observed puddled in the lower cowling, and residual fluid was found on components in the vicinity to the hydraulic pump. The system was pressurized with the auxiliary pump, and no apparent leaking lines or fittings were noted. The engine was motored utilizing the starter, and fluid was observed dripping off the bottom of the hydraulic pump. The nose gear pin was not located during the postaccident examination of the airplane. It could not be determined how or when the nose gear pin became disengaged from the nose gear jury strut. According to US Forest Service personnel, the installation of the nose gear pin is a critical step to be completed for a landing with a main hydraulic system failure. If not performed, the nose landing gear will collapse on touchdown. The installation of the pin requires a pilot to climb down into the nose landing gear wheel well that is located below the cockpit flight deck. While space is limited and it is difficult to maneuver within this area, the pin is installed while the nose landing gear door is already open and the gear is extended. The hydraulic pump was a Stratopower Variable Delivery pump manufactured by The New York Air Brake Company, part number 66YD300, serial number DO-39. It was overhauled by Thunder Airmotive, Inc., on May 3, 2010, and installed on the accident airplane February 20, 2013. The hydraulic pump accumulated a total of 362.5 hours since overhaul at the time of the accident. The most recent inspection performed in the accessory area of the right engine was a 50 hour inspection, completed on May 23, 2014, about 30.8 hours prior to the accident. Examination of the hydraulic pump was conducted at the facilities of Mark Air Motive, Moses Lake, Washington, under the supervision of the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on December 18, 2014. Examination of the pump revealed that the outer portion of the case was cracked. The hydraulic pump drive shaft rotated by hand. The pump was disassembled, and all internal components were intact and unremarkable. The backup ring was found partially extruded from the pump housing, which the technician stated was consistent with an over pressurization event of the hydraulic system.