History of Flight
On October 10, 2014, at approximately 1300 UTC, a Boeing 787-824 (registration N26906) operated by United Airlines as a scheduled international flight from London Heathrow Airport (LHR) to Houston Intercontinental Airport (IAH) was struck by lightning about six minutes after takeoff during climb. The event caused three of five primary display units to go blank, a status message related to a power distribution channel to appear, and the captain's forward windshield heat to become inoperative. The standby instruments and heads-up displays were not affected.
The flight crew found no checklist for a partial loss of displays and therefore used the checklist for "Loss of All Displays." This procedure did not restore any of the three failed displays. The crew also accomplished the abnormal checklist for window heat, but it did not restore function. The two remaining displays were reconfigured, and the crew landed the airplane uneventfully back at LHR. No injuries to crew or passengers were reported.
Injuries and Damage
No injuries occurred. After landing, LHR maintenance performed a lightning inspection and found external nonstructural damage at five locations along the nose, which was addressed per the structural repair manual (SRM) allowable damage criteria. A power reset per aircraft maintenance manual procedures restored the displays and other affected systems to normal function. The aircraft was approved for dispatch and continued the flight to IAH with no further faults.
An additional examination in March 2015 inspected the interior of the nose area near the lightning strike points and visually inspected cockpit windows for delamination; no notable additional damage was found. Ground bonding checks on display units and electrical panels were satisfactory.
Aircraft Information
The Boeing 787-824, serial number 34829, was delivered new in October 2012 and had accumulated 6,582 hours and 996 cycles. The left inboard and outboard displays were original; the right outboard display was installed in December 2012 to replace a unit removed due to erroneous display switching. None of the affected display units had been returned to the manufacturer since delivery.
Meteorological Information
The crew reported flying through a "moderate non-CB rain shower" with cloud tops of 10,000 feet at the time of the lightning strike.
Flight Recorders
The aircraft was equipped with an Enhanced Airborne Flight Recorder, and data was successfully retrieved and sent to the NTSB for analysis.
Test and Research
A grounding/bonding test in March 2015 on the event aircraft showed bonding measurements consistent with an exemplar aircraft. Visual inspection confirmed grounding straps were attached and secured.
Component-level testing was conducted on display units, the Secondary Power Distribution Unit microprocessor card, Graphics Generator Module, and Window Heat Protection Unit. Fault history data showed that the blanked display units recorded an "LCD Supply Voltage Fault" at the time of the lightning strike. The right inboard and outboard units passed acceptance and return-to-service tests. The left outboard unit was used for additional testing; the left inboard was quarantined as a control.
Exemplar unit testing with radiated field and cable coupling, under conditions exceeding specifications, did not produce the same fault messages as the event.
Near-strike lightning testing at the aircraft manufacturer's laboratory exposed an exemplar unit and the event airplane's left outboard unit to near lightning strikes. One exemplar test caused a blanking event with a "Grayscale voltage fault". The left outboard unit blanked during three of four tests, each logging an "LCD Supply Voltage Fault".
Flat plate testing, performed on exemplar and event units, showed that all four units experienced blanking at various voltages, recording both "Grayscale voltage fault" and "LCD Supply Voltage Fault".
Additional Information
Boeing conducted an electrical field analysis using lightning waveforms from ARP 5412 revision B. The analysis confirmed that under some dynamic lightning conditions, a spark gap device in the Window Heat Protection Unit could fire, affecting transient electrical field characteristics. Comparing display blanking thresholds from testing, the lightning model results exceeded the lower bound estimate of the blanking threshold in every case where a fast spark gap fired or waveform H was applied; the upper threshold was never exceeded.
The Boeing 787-8 was certified under 14 CFR Part 25, including lightning protection requirements per §25.1316. The current industry standard lightning tests do not include conditions simulating rapid transient radiated electric fields.
After the event, the aircraft manufacturer notified fleet operators and implemented a software change for the 787-10 (entry into service estimated 2018) that automatically resets a display unit if it blanks due to certain internal faults. Equivalent software changes for 787-8 and 787-9 models are expected in 2019.