Aircraft Details
Type: Bombardier Global 5500
Location: UK, parked on ramp
MEL Status: Category C – 5 days remaining
Total Downtime: 3 days
A Bombardier Global 5500 returned to the UK with an active right-hand windscreen heat fault already logged under MEL Cat C, with five days remaining before mandatory rectification. Diagnostics confirmed internal failure of the windscreen’s heating element, prompting an AOG windscreen replacement. Despite a short delay in part delivery, the aircraft was returned to service within three days of the initial inspection.
Initial Assessment
The aircraft arrived with a pre-existing “R WINDOW HEAT” message posted on EICAS. With the aircraft operating under MEL for this defect, the task was to determine whether it could safely continue operations or if rectification would be required onsite.
Given the role of the windscreen anti-ice system in long-range dispatch and low-temperature operations, a full inspection was initiated that morning.
Diagnostic Approach
The investigation began with resistance and continuity checks between the right-hand window heat controller and the associated windscreen heater terminals. No wiring or controller faults were found.
At this stage, the team considered transposing the window heat controllers to compare behaviour and isolate the issue. However, continuity checks across the circuit—along with targeted resistance testing—clearly indicated the fault lay within the windscreen itself. This approach eliminated the need to disturb the opposite-side controller, which would have required substantial labour to access. The diagnostic path saved both time and resources.
Subsequent checks of the windscreen’s temperature sensing terminals confirmed readings within expected limits. However, resistance across the heating element terminals was far outside tolerance, and visual inspection revealed thermal damage embedded in the laminate. The evidence confirmed an internal failure of the heating element, necessitating full windscreen replacement.
Communication and Planning
Within three hours of initiating the inspection, the Maintenance Control Centre (MCC) was informed of the findings and advised that windscreen replacement was required. While a return-to-base option was briefly discussed—since the aircraft remained compliant under MEL—MCC opted to resolve the issue onsite to minimise risk and avoid additional downtime later in the MEL cycle.
Logistics and Installation
An AOG order was raised and a new side window started moving from Frankfurt. However, the windscreen missed its initial cargo flight and was rerouted onto the next available shipment, delaying delivery by approximately 12 hours.
Once on site, the damaged windscreen was removed and replaced with the newly delivered unit using OEM sealants and procedures. The required 24-hour curing period was observed before further testing.
Testing and Return to Service
Following sealant cure, a full pressurisation check was completed without issue. A final operational test of the right-hand window anti-ice system confirmed the system was functioning normally.
All maintenance records and regulatory paperwork were finalised, and the aircraft was released back to service. The total time from first inspection to return to operation was three days, comfortably within the MEL deferral period.
Technical Reflection
This event reinforced the value of targeted electrical diagnostics in isolating component failures. By confirming the heater element fault without resorting to transposing controllers, the team avoided unnecessary disassembly and saved both time and labour.
Clear and early communication with MCC allowed operational decision-making to be aligned with technical realities, ensuring minimal disruption to the operator's schedule.
Overall, the case illustrated how precise fault isolation, effective logistics coordination, and well-timed decisions lead to efficient AOG recovery with minimal impact.
