The following report was sent to the Balloon Repair and Maintenance Association by Steve Trieber of Do-Little Lazy-S Loft in Arizona. It is reprinted here by permission of BRMA. Although this specific report involves a hot air airship the circumstances surrounding the incident can apply to other hot air balloons. Editor.
Catastrophic envelope failure: Colt Thermal Airship, Model Colt AS105, Serial Number 490, certificated June 1983.
Total time on aircraft, 89.1 hours. Total time since envelope rebuilt (excluding tail fins), 36.3 hours. The original tail fins were installed on the new envelope. Envelope material HTN 90, tailfins ripstop nylon. Maximum recorded temperatures: forward telltale 220 F, aft telltale 230 F.
The aircraft was inspected March 1995 in accordance with the Colt AS105 maintenance manual. All fabric pull tests were 66 pounds or greater on both the envelope and tail fins. All fabric pull tests were satisfactory.
The airship was assembled and preflighted for its third flight since annual inspection. The outside air temperature was 63 F at 4:45 p.m.; there were two passengers for a total of 315 pounds. The inflation and pre-takeoff checklists were accomplished without incident. The ship was heated until it reached equilibrium; envelope pressure was monitored. Envelope temperature at this time was 201 F, envelope pressure was 14mm. The propulsion engine power was gradually increased while temperature and pressure gauges were monitored to assure a slow and easy climb out. At an altitude of approximately ten feet, a loud "POP" was heard and the ship lurched forward. Ground speed was approximately six miles per hours. I turned my attention to the tail of the ship only to see the tail rapidly deflating, and at a 30 angle off the starboard side. At the same instant, the ship fell to the ground, landing on the forward wheels first. Just after impact, I shut down all engines, electrical, and fuel systems. The envelope was continuing its rapid deflation. I returned the aster switch and fuel valve to the "on" position in an attempt to cool the burners before the envelope fell on them. Due to the length of fuel line from the pilot light switch to the burner, the pilot light was still ignited and the port side of the envelope was scorched.
From the time I heard the POP until the ship was on the ground was only a matter of seconds. Once the ship was on the ground it was only a couple of minutes before it was almost completely deflated.
On initial inspection of the envelope we found the upper vertical fin split apart at the seams. Further inspection revealed that the envelope had split at the top of the ship underneath the fin, from the forward/leading edge of the fin all the way aft to the tail of the ship. The split started at the forward fin inflation port and ran forward to the aftmost circumferential load tape on the envelope, which is located at the forward edge of the fins. There are no other load tapes aft of this location, consequently the split was allowed to continue aft all the way to the load ring at the tail. The tear was about 20 feet long. It appears that the fin split at the seams due to the envelope splitting and pulling it apart. The HTN 90 envelope fabric under the fin showed signs of degradation. The fabric was pull-tested at 15.6 pounds in the area of the forward fin inflation port. I believe the extreme fabric degradation of the envelope fabric under the fin is due to there being no outside air boundary layer available to those panels. That, in conjunction with the original fin, which was found to be slightly porous, being re-installed on the envelope, may have caused a chimney effect, furthering the degradation.
The inspection was performed in accordance with the manufacturer’s checklist. I had even gone beyond what was required by the checklist in order to be as thorough as possible. It never occurred to me to check the envelope fabric under the fins. This is an area of the envelope you cannot see during a gore-by-gore inspection. In order to test the fabric in this area, you would either have to turn the envelope inside out, or cold- inflate it and go inside.
The inspection checklist did not address this area. I believe it is critical to inspect these inflation/pressurizing ports along with curtains, catenaries, formers, or any other fabric which has no outside air boundary layer. These areas are found not only in airships. but also in many hot air balloons in rotating vents and special shapes. I also recall that the other manufacturers manuals and checklists do not address these areas.
Fortunately, no one was hurt. Hopefully we can make repair persons and manufacturers aware of this problem to prevent a serious accident from happening.