Investigation Summary
The incident began during pushback when a tow bar failure damaged the left-side nose wheel tire of the DC-9. The aircraft returned to the gate area for a tire replacement. After the new tire was installed, a mechanic began servicing it using a nitrogen bottle pressurized to 4,000 psi. The target pressure for the tire was 150 psi. Investigation revealed that the mechanic likely confused the two scales on the regulator gauge: an inner scale for PSI and an outer scale for BAR. If the mechanic attempted to reach '150' on the BAR scale, he would have inadvertently introduced approximately 2,205 psi into the wheel assembly. Although the wheel rim was designed to withstand up to 580 psi, this massive over-pressurization caused the rim to fail catastrophically. The NTSB found that the airline had failed to make low-pressure regulators available to maintenance staff during normal work hours, as they were kept under lock and the key-holder had already departed for the day.
Final Conclusions
The National Transportation Safety Board determined the probable cause was a failed wheel rim following an inadvertent over-inflation by the mechanic. Contributing factors included the mechanic's failure to monitor the pressure being applied, his failure to follow established maintenance procedures, and the company's failure to provide accessible low-pressure regulators for maintenance personnel.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
Nose Wheel Rim
The structural component of the landing gear assembly designed to hold the tire; it failed due to internal pressure exceeding its 580 psi stress limit.
Nitrogen Service Bottle
A high-pressure vessel used to inflate aircraft tires, equipped with a regulator that was found to be properly calibrated but capable of delivering pressures far exceeding the tire's requirements.
Interesting Facts
- 01The aircraft was manufactured in 1968 (MSN 47172/263).
- 02The nitrogen bottle used for servicing had two scales: BAR on the outside and PSI on the inside.
- 03The nitrogen bottle was pressurized to 4,000 psi, while the tire required only 150 psi.
- 04Confusing 150 BAR for 150 PSI results in a pressure of approximately 2,205 PSI.
- 05The wheel rim was stressed to a limit of 580 psi according to FAA data.
- 06Post-accident examination of the fracture surface showed no manufacturing or material defects.
- 07Low-pressure regulators were kept under lock and key, and the responsible person had already gone home.
- 08The fracture was consistent with a tensile overload type of failure.
Safety Actions & Advisories
Regulator Accessibility Policy
Source ↗Following the investigation, it was noted that the airline's failure to provide accessible low-pressure regulators contributed to the accident. Safety recommendations emphasized that critical maintenance equipment, such as low-pressure regulators for tire servicing, must be available to maintenance personnel during all operational hours and not restricted by locked access controlled by off-duty personnel.
Location
APPROXIMATE POSITION · AIRPORT LEVEL · Coordinates are the estimate published in the ntsb final report. The ring shows the accuracy the record supports, not the point of impact.