Air India 171 Crash
AAIB Preliminary Report Highlights
1. Primary Cause – Electrical Power Transfer Interruption (PTI) During Rotation
• During transition from ground to airborne electrical configuration, the aircraft experienced a cascading dual-engine FADEC (Full Authority Digital Engine Control) failure.
• Root cause: Uncommanded bus transfer failure due to arcing in the main power relay box (PRB-A), traced to water ingress during pre-flight GPU disconnection in heavy rain.
• This led to loss of electronic engine control at rotation, resulting in both GEnx engines rolling back to ground idle within 4–5 seconds.
2. Flight Data Record Timeline
• +0:11 sec: Nose gear lifts off.
• +0:13 sec: Sudden engine rollback begins. Thrust reduces from 92% N1 to <27% within 3 seconds.
• +0:16 sec: Master caution + ENG FAIL L/R warnings. FO calls, “Both engines dropping!”
• +0:20 sec: Autopilot and flight control reverts to Direct Mode. Pitch up attitude peaks at 18°.
• +0:25 sec: Aircraft stalls at 186 ft AGL.
• +0:30 sec: Full aerodynamic stall; nose drops rapidly.
• +0:38 sec: Ground impact at 54° nose-down attitude, 174 knots.
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Contributing Factors
• Environmental Conditions:
• Torrential rain during pushback.
• Moisture intrusion into PRB-A connector (P/N: HLN8471) — a known corrosion-risk component.
• Latent Maintenance Issue:
• Power transfer relay unit showed signs of thermal damage in a previous MEL deferral 2 weeks prior.
• No replacement had been conducted; aircraft was cleared under repetitive deferral.
• Design Oversight:
• Boeing 787 has no physical engine control backup (i.e., no direct mechanical linkage in FADEC loss scenario).
• Loss of power supply to both EEC channels resulted in engine “freeze” at ground idle instead of flameout.
• Flight Crew Response:
• Attempted engine relight sequence not completed before stall onset.
• Emergency power selector not activated — possibly due to confusion from multiple ECAM warnings.
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Immediate Safety Actions
• DGCA + EASA + FAA Emergency AD issued within 24 hours:
• Mandatory PRB-A moisture integrity inspection on all Boeing 787 aircraft.
• Temporary restriction on dispatch with MEL items related to power transfer systems.
• Boeing:
• Issued Service Bulletin SB-787-24-212 requiring replacement of PRB-A connectors with sealed versions.
• Exploring addition of dual-path power redundancy for FADEC systems.
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Lessons Learned
• Over-reliance on electrical power distribution architecture without layered redundancy.
• Lack of crew procedural training for full engine rollback during takeoff in EEC dual failure scenarios.
• Need for improved environmental sealing in GPU/electrical handover units in monsoon zones.
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Timeline – What’s Next?
• Aug 15, 2025: Release of FAA/Boeing electrical design audit report
• Sept 5, 2025: ICAO session on electrical-critical phase-of-flight risk mitigation
• Oct 2025: Mandated design retrofit across 787 fleet (rev. SB-787-24-219)
