What happened
On 19 October 2018, a China Airlines B747-400F, registration B-18719, arrived at Singapore Changi Airport from Taipei. During a scheduled transit, maintenance personnel conducted a walk-around inspection and identified low pressure in the No. 11 tyre (right body landing gear rear inboard).
While a technician was inflating the tyre using nitrogen, the tyre suddenly burst. The force of the explosion propelled a manual tyre pressure gauge against the aircraft, causing cracks in the wing body fairing and underlying composite and support structures. A nearby technician was struck by a blast of cold air but sustained no injuries after being evaluated at a hospital.
The investigation
The TSIB investigation focused on the decision-making process regarding tyre temperature and pressure calculations. The investigation established that the aircraft had landed less than two hours prior to the incident. Although the manufacturer recommends a two-hour cooling period for a Cold Tyre Pressure Check, the engineer opted for this method after assessing the tyre by touch and noting a low brake temperature reading on the cockpit's EICAS display.
Furthermore, the investigation examined the mathematical approach used to determine the required inflation level. The engineer calculated the pressure deviation based on the minimum nominal inflation pressure rather than the operator's established service pressure. Additionally, the investigation looked into the physical state of the tyre following the burst.
Findings
- The primary cause of the burst was a pre-existing ply cord deterioration within the tyre wall, which had likely been weakened by periods of operating under significantly low pressure.
- The engineer misinterpreted the Aircraft Maintenance Manual (AMM) by using the minimum nominal inflation pressure of 185 psi as a reference point for calculating pressure loss, instead of using the operator's set service pressure of 200 psi.
- Had the correct reference pressure been used, the pressure deviation would have been identified as 15%, necessitating the immediate replacement of the wheel and tyre assembly.
- The method of using touch to assess tyre temperature is subjective and potentially unreliable for determining whether a Hot or Cold Tyre Pressure Check is required.
- Conflicting terminology within the AMM—using various terms like "specified pressure" and "necessary tyre pressure" to refer to the same value—contributed to the misinterpretation of maintenance procedures.