NTSB Identification: LAX01FA071
14 CFR Part 91: General Aviation
Accident occurred Tuesday, January 09, 2001 in Oakland, CA
Probable Cause Approval Date: 01/24/2005
Aircraft: Worldwide Aeros 40B, registration: N819AC
Injuries: 2 Minor.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
During an instructional flight in rain showers, the airship became uncontrollable due to an out-of-balance (trim/pressure) envelope condition and collided with the ground and multiple obstacles during a landing attempt. While inbound to the airport for landing in high winds and moderate rain, the crew decided they could not do a “normal ‘weight-off’” to determine the weight and trim because they were heavy with rain, had no ballast to drop, and could only estimate their trim by visually checking the ballonet volume. The certified flight instructor (CFI) then got out of his seat to read the ballonet numbers and found that they had “6-7 in the rear and the front was flat.” The crew then adjusted the levels by dumping from the front ballonet and pumping air into the aft ballonet and then noted that they were even around “3 ½ each.” The CFI then suggested that they leave the aft pump on and lock off the front valves to hold the trim condition. While setting up for the approach, the nose dropped. The CFI attributed this to gusty weather conditions that prevailed at the time. He simultaneously noted that the hull pressure indicator (HPI) was low and switched the fan blower to the ON position to add air to the front ballonet. He indicated that the rear ballonet was in the AUTO position. The nose recovered and then dropped again. The CFI again noted that the HPI reading was low. He recovered using the same procedure as before, and the nose dropped a third time. The CFI checked the air pressure system and saw that the rear ballonet valve was open but stated that the indicator light on the annunciator panel indicated that the valve should have been closed. (Subsequent investigation determined that the annunciator light was wired through a valve position sensor switch and that the air valves and annunciator lights functioned properly in the open and closed positions.) Both pilots visually confirmed that a least one of the aft valves was open and would not respond to air valve control inputs. The CFI attempted to manually close the valve, with no response. Observations of the ground crew confirmed that the aft ballonet valves were open. On the first attempt to land, the airship was too high and came in too fast. The CFI aborted the landing then set up for a second approach. Due to the nose-low condition, he added more power and placed the joystick (flight control system) to the full aft position to raise the nose. The CFI stated that he needed full aft on the joystick to keep the nose up, and any movement forward resulted in an immediate drop of the nose. He concluded that the flight controls were malfunctioning, but because of low altitude, high airspeed, deteriorating weather, and the need to get the airship on the ground, he did not have time to accomplish a complete emergency procedure for a flight control malfunction. The airship landed very hard at a fast forward airspeed and with a very heavy nose. The landing gear collapsed and the gondola dug into the ground. The airship then skidded across an adjacent taxiway and struck a parked airplane. Both pilots jumped out on opposite sides of the gondola and pulled the emergency envelope deflation ripcords, but the deflation panels did not open because of the advertising banner that was attached to the airship. The airship took off, unmanned, and reached a peak altitude of 1,600 feet above ground level. The airship traveled about 4 miles northeast and struck a marina where the envelope draped over sailboats and a restaurant.
The investigation reviewed the Federal Aviation Administration-approved flight manual. In the emergency procedures section, under “pressure-related emergencies,” the manual stated that, with a high pressure indication, the pilot should check that the helium release valves and air valves are in the UNLOCKED position. The flight manual did not address added weight to the airship caused by environmental conditions (rain). With the aft ballonet valves in the OPEN position and the fan for the forward ballonet in the ON position, the forward ballonet became fully inflated, which caused the out-of-trim/unequal hull pressure condition. A review of the airship design certification indicated that it did not address the aerodynamic effects of advertising banners being draped over the rapid envelope deflation emergency ripcords or the effects of rain on the airship. The emergency ripcord deflation system was never tested on the airship in various environmental conditions, only on a mock-up in a hangar. Because of structural damage sustained in the impact sequence, the airship’s systems could not be tested as installed on the airship. However, each individual system was functionally tested, and no anomalies were found. There was no minimum equipment list for the airship. If a component was inoperative, the airship was considered to be in an unairworthy condition. The dual ballonet level cockpit indicator had been taped over and marked “In-Op” before the flight.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: One or both of the rear ballonet air relief valves remained in an open position for undetermined reasons, which caused an out-of-balance trim condition; also causal was the flight crew’s decision to the fly the airship with a known deficiency (the inoperative ballonet indicator) and the pilot’s failure to follow proper emergency procedures. Full narrative available
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