MOGADORE, OH, USA
N4A
Loral Corp. GZ-22
The pilot of the fly-by-wire airship flew several traffic patterns and approaches from both the pilot and co-pilot stations. After he departed the traffic pattern and leveled the airship, it entered an uncommanded, descending left-hand turn. The control panel displayed no indication of a malfunction, and emergency and troubleshooting procedures failed to regain control of the airship. The airship landed in trees, and both the pilot and the technician on board escaped injury. Metallurgical examination of the actuators that convert electrical signals to mechanical flight control inputs revealed that the splined shafts on the output side of the starboard actuator were almost completely stripped, and provided no flight control input. The gear teeth of the splined shafts on the output side of the port actuator were worn to 25 percent of their original thickness, and did not provide full flight control input. Hardness testing revealed the output shafts of both the port and starboard actuators did not meet the manufacturer's specified hardness requirement. The manager of the airship fleet said the one-of-a-kind airship would not be rebuilt.
On October 28, 1999, at 1841 Eastern Daylight Time, a Lockheed/Martin GZ-22 airship, the Goodyear "Spirit of Akron", N4A, was substantially damaged during a collision with trees following an uncontrolled descent near Mogadore, Ohio. The certificated commercial pilot received minor injuries. The night sign technician was not injured. Visual meteorological conditions prevailed for the demonstration flight that originated at the Wingfoot Lake Airport (WFL), Akron, Ohio, about 1725. No flight plan was filed for the flight conducted under 14 CFR Part 91. In a written statement, the pilot said the purpose of the flight was to test the night sign and film the flight from the ground. The pilot reported that the airship was fueled prior to takeoff and that he performed weight, balance, and performance calculations. He said this was discussed with the airship's crew chief and the appropriate ballast adjustments were made prior to takeoff. In a telephone interview, the manager of airship operations stated N4A was a "one-of-a-kind, turbine-powered, fly-by-wire airship." He said the engine power and thrust vectoring controls were completely independent of the flight controls. The pilot reported that after takeoff, he flew several approaches at the videographer's request following both left and right-hand traffic patterns. He said that control authority was switched back and forth between the pilot control positions and "both control authority transfers were normal." The pilot said the filmed portion of the flight was completed and that he was released to turn the night sign on. He said: "Once released, I began a climb on a westerly heading to an altitude of 600 feet agl (above ground level). Coinciding with my level off, I noticed N4A was being blown slightly to the northeast so I placed in a little left correction to get back into the wind. The airship began a left turn. When I felt the airship was close to being into the wind, I placed into the controls right ruddervator input. At this time, the airship continued in a left-hand turn and the nose lowered as the ship went downwind. I immediately checked the master warning panel to see if I had ...[a] malfunction indication. There was no indication." The pilot reported that the airship continued in an uncontrolled, left-hand descent as he attempted several emergency and troubleshooting procedures. He said: "I asked [the technician] to go into the aft compartment and see if both sets of fins were moving. He returned with a response that only one actuator was operating and he said he heard 'a loud grinding noise.' Following this observation, I engaged the emergency flight control backup system and obtained no response from the controls. [The technician] went back again and reported the loud noise was still present." The pilot reported that only after repeated attempts to troubleshoot the system and regain control did malfunction indications appear on the warning panel. The airship continued its descent and came to rest in some trees. The airship gondola was removed from the site and examined at the Goodyear Facility on November 1 and 2, 1999, under the supervision of Federal Aviation Administration Aviation Safety inspectors. According to material provided by the flight control system manufacturer, the system "is a triple redundant primary flight control system (FCS) that provides full authority control of the airship's ruddervator control surfaces." The system sends a signal from the control stick through a series of mixing, processing, and amplifying electronics to a pair of Triplex Servo Actuators. These actuators convert the electronic control signal to a mechanical output shaft "which will interface directly with the existing drive capstan assembly." Examination of the starboard Triplex Servo Actuator revealed the final drive gear and the output shaft splines were stripped. Testing of the FCS revealed that all three systems performed as designed through the starboard actuator and provided output to the final drive gear. However, the stripped gear moved inside the stripped shaft and provided no output to the capstan assembly. The starboard actuator controlled the upper port and lower starboard control surfaces. In a telephone interview, the FAA inspector said, "With the shaft stripped out, the pilot lost approximately 50 percent of his control authority." A metallurgical examination of the final drive gear and the output shaft from both the starboard and port sides was performed at the Safety Board Materials Laboratory in Washington, DC. According to the metallurgist's report, examination of the starboard components revealed: "...more than one third of the gear teeth were completely destroyed down to the roots. The examination also showed that the other two thirds of the teeth were almost completely destroyed." Examination of the port side gears and shafts revealed "deep depressions" and "moderate wear" on the pressure sides of the gear teeth. According to the metallurgist, "Thickness of the teeth on the top lands was reduced by these depressions to approximately 25 percent of the original thickness." The metallurgist's report stated that both the port and starboard output shafts were made from a low allow steel with a specified hardness range of HRC 40 to HRC 44. Hardness testing for the port and starboard shafts revealed an HRC rating that averaged 38.2 HRC, and 37.8 HRC respectively. According to the report: "The hardness of both output shafts from the accident airship were below the hardness range specified for the shaft by the manufacturer." In a telephone interview, the manager for North American Airship Operations at Goodyear stated that there was no inspection interval for the actuator output shafts. Neither the manufacturer of the flight control system, nor the airship manufacturer prescribed an inspection interval. However, there was a requirement to inspect the actuators every 3,000 hours. According to the airship manager, the Goodyear policy was to inspect the output shafts during the actuator inspection, and that the inspection was performed every 3,000 hours. The airship manager said the actuators and the shafts were inspected April 29, 1999, at 11,690 aircraft hours. He said the airship accrued 942 hours between the inspection and the accident flight. The airship manager said because of the uniqueness and the high cost of this one-of-a-kind airship, there were no plans to rebuild the airship. The pilot reported 1,468 hours of flight experience, 374 hours of which were in this model airship. Winds reported at Akron, Ohio, at the time of the accident were from 190 degrees at 8 knots.
The failure of the flight control system manufacturer to meet design (hardness) specifications that resulted in stripped splines.
Source: NTSB Aviation Accident Database
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