On 25 September 2001 one of the two Allison AE 3007/A1/1 engines on an Embraer EMB-145EP (G-RJXG) on a scheduled domestic passenger flight from Aberdeen to Manchester in day Instrument Meteorological Conditions (IMC) and descending through FL70 flamed out immediately following a lightning strike. A MAYDAY was declared and the aircraft subsequently completed an uneventful single engine approach and landing at its destination.
An Investigation was carried out by the UK AAIB supported by FDR data. It was noted that the 42 year-old aircraft Captain, who was PF for the incident flight, had 905 hours flying experience on the aircraft type out of 8919 hours in total.
It was found that the lightning strike had occurred when there were only "weak returns of cumulonimbus cloud activity" which had been avoided by manoeuvring the aircraft proactively and primarily visually. The context and likely nature of the incident lightning strike was assessed using remote sensing evidence of strikes in the general area and it was concluded that it was most likely to have been of relatively low power and of cloud to ground origin.
At the time of the strike, the aircraft was in receipt of radar vectors to position the aircraft downwind and descending, briefly in IMC, with a low engine thrust setting. It was reported that the left engine had begun to show over temperature within a few seconds of the strike and had then indicated an un-commanded run down without any of the usual warnings or cautions being seen. A MAYDAY call had been made and Quick Reference Handbook (QRH) drills for both engine failure and single engine approach had been completed whilst continuing as intended with the approach to destination.
Superficial damage attributable to the strike was evident on the left side of the fuselage, extending "from just aft of the flight-deck windshields to a point above the junction of the wing trailing edge and the fuselage". It "took the form of marks on longitudinal skin joints and a row of rivet burns, initially low down on the fuselage side, then continuing aft at a higher level above the wing" with considerable damage evident "on the composite right wing tip fairing". None of this was considered to have constituted an "identifiable hazard".
Examination of the left engine by borescope found no damage and both FADEC units for this engine were removed, bench tested serviceable without any rectification being required and successfully returned to service with no subsequent abnormalities reported. It was noted that a reduction of N2 below 53.5% (below which recovery is not possible) results in the active FADEC Unit initiating a shutdown sequence and found that this was the cause of the run down in this instance.
It was concluded that the un-commanded reduction in both the N1 and N2 of the left engine which culminated in automatic engine shut-down 11 seconds after the strike "was consistent with a compressor stall” initiated “by the loss of mass flow through the engine”, caused directly by the aero-thermal effects of the lightning strike at the engine intake.
It was found that FADEC logic of the engine involved "had, by design, no surge recovery features and the surge prevention logic was unable to re-establish stable running conditions during the length of time which passed between the lightning strike and the eventual auto-shutdown of the left engine". It was also considered that the distance which the aircraft had moved through the air during that 5 or 6 seconds - estimated as around 500 metres - "should have carried it clear of any air directly affected by the lightning strike and thus restored the availability of acceptable intake conditions". It was concluded that the fact it had not done so implied that either the poor airflow conditions created by the strike had persisted within the intake, the engine itself or both, or that "the scheduled fuel control inputs made were optimised to restoring demanded power rather than ensuring that stable running conditions were obtained before restoring set power". The latter explanation would have been consistent with the effects on the FADEC of either "very hot, low density plasma passing into the engine" or "the large static charge associated with lightning discharges" especially on engine temperature sensing.
It was considered that whilst engines with non-FADEC fuel scheduling "are more likely to suffer transient over-temperature conditions as an indirect result of a lightning strike" than engines with FADEC fuel scheduling, there may well be a higher chance that such engines would continue to run thereafter since the pilot retains more control over intentional shutdown.
On the subject of engine restart below 10,000 feet following a FADEC-commanded shutdown, it was noted that although the windmill start requirement of N2 rotation below 10% would have been reached within 20 seconds, "an APU Bleed Air assisted start is required if the airspeed is below 220 knots" and requires that the APU be started first. It was considered that sufficient time to start the APU and then one engine may not necessarily be available and that certainly diving the aircraft to enter the unassisted re-start envelope would not be "a viable option". The fact that the APU was allowed to be inoperative for despatch under the MEL was considered pertinent, especially since FADEC shut down action is not communicated to the FADECs on the opposite engine and it is therefore possible for this type of FADEC to command shutdowns independently and simultaneously. However, it was also noted that a serviceable APU could be vulnerable to air disturbance-induced flame out from a lightning strike which had this effect on an engine.
In respect of the risk that both engines might be shut down simultaneously as a consequence of the same lightning strike event, expert opinion sought during the Investigation suggested that all aircraft with fuselage-mounted or otherwise closely-spaced engines were theoretically vulnerable due to the possibility that propagation of such a disturbance could occur longitudinally down both sides of the fuselage. In the complete absence of hard evidence on the subject, it was suggested that this risk appeared to be "most prevalent on aircraft with narrow fuselages".
Three Safety Recommendations were made as a result of the Investigation as follows:
- that EASA, FAA and the Centro Tecnico Aeroespacial (CTA) of Brazil in conjunction with aircraft and engine manufacturers, should, in order to minimise the risk of uncommanded shut-downs, review and, if necessary, initiate appropriate research into the aero-thermal disruption of intake flow and other effects of lightning strikes on fuselage-mounted turbine engines in order to establish whether there is a safety of flight issue that should be addressed by appropriate future rulemaking. They should also consider the application of any proposed rules to types currently in service. (2005-094)
- that Rolls-Royce Corporation should, with advances in the technology which become available to them, continue to explore the potential to make modifications to the FADEC logic to enable the re-establishment of stable running conditions, after detection of a surge condition, before the FADEC attempts to restore selected engine power. (2005-095)
- that Embraer should give consideration to amending the EMB 145 operating procedures and minimum equipment list to ensure that, in the event of an engine flame-out and continued flight in a zone with a high probability of lightning strikes, the supply of APU air for main engine starting remains available. (2005-096)
The Final Report of the Investigation was published in November 2005.