On 24 August 2016, an ATR 72-600, (EI-FCY) being operated by Stobart Air on a scheduled international passenger flight from Birmingham to Cork for Aer Lingus Regional in day VMC experienced indications of two simultaneous electrical faults followed by another indicating electrical Smoke which was followed by the sight and smell of electrical smoke in the flight deck. A MAYDAY was declared and the smoke / smell cleared on completion of the electrical smoke checklist but the flight had to be completed without any AC electrical power available but was otherwise without further events.
An Investigation was carried out by the Irish AAIU. The 39 year-old Captain, who was PF for the flight, had a total of 6,850 flying hours on type and a total of 4,230 hours in command but her total flying hours on all types and her total time in command of the ATR72 were not recorded. The 33 year-old First Officer was recorded as having a total of 1,892 flying hours on type but their time on all types was not recorded.
It was established that just prior to beginning descent to destination, a Master Caution appeared highlighting faults affecting AC Bus 2 and Static Inverter 2 followed almost immediately by a Master Warning accompanied by an ‘ELEC SMK’ annunciation. The flight crew quickly became aware of both the smell of electrical smoke and sight of it and both pilots donned their oxygen masks. Both pilots subsequently stated that they had seen smoke coming from the electrical panel behind their colleague’s seat. A MAYDAY was declared and the ‘SMOKE’ checklist memory actions were completed. The QRH was then used to confirm the correct completion of these actions and continue with the remaining checklist items. The first of these items invited them to determine the source of the smoke and since they were aware of this, led them to the ‘ELECTRICAL SMOKE’ checklist. This required that both AC generators be turned off and the separate checklist for this situation be also run as well as advising landing as soon as possible. Given the location of the aircraft, the only option in respect of landing was to continue to the planned destination.
As the ‘ELECTRICAL SMOKE’ checklist was being run, the corresponding ‘ELEC SMK’ annunciation ceased as did the smell of smoke and the sight of it and the Captain determined that both pilots could safely remove their oxygen masks. As they did so, the cabin attendant stationed at the front of the cabin called on the interphone and informed them that she had briefly been aware of “a funny smell” in the forward cabin just aft of the flight deck access door. The Captain explained the situation they had just experienced in the flight deck and that the intention was to continue and complete the flight as planned.
Once the loss of AC generators checklist had been completed, the Captain passed control of the aircraft to the First Officer and, having realised that one of the consequences of an absence of AC power was the disabling of both main hydraulic pumps and the consequent gravity-assisted extension of the landing gear, she called Cork ATC to advise them that an extended final approach would be required on that account. She noted with no AC power, much of the aircraft Ice and Rain Protection would no longer be available although this was not a problem in the prevailing flight conditions. She also reported having (incorrectly) concluded that since the flaps would now need to be extended using the auxiliary hydraulic system, their deployment would be delayed as this system would not be available until the gear was down whereas according to the checklist for loss of all AC power, the auxiliary hydraulic system should be functional once the landing gear lever has been moved to the down position (although this action does not have any effect on the gear). She then called the senior cabin crew to the flight deck for a NITS briefing which included that a normal landing should be expected. The rest of the flight was without further event and a landing was made in benign weather conditions approximately 25 minutes after the smoke and fumes event had begun.
The airworthiness issues and their operational implications
The aircraft manufacturer stated that the number of reported static inverter failures affecting ATR 42/72 aircraft with or without consequential smoke in the flight deck they had recorded since 1989 was as follows:
- 68 events where the ELEC SMK warning was triggered (all causes)
- 21 events where the ELEC SMK warning was triggered and the cause was failure of a static inverter
- 28 events where smoke was reported in the flight deck and the cause was failure of a static inverter
The source of the smoke and fumes in the case under investigation was found to have been the no. 2 static inverter. The failed inverter was a MOD LVL ‘B’ one whereas a 2016 SB had recommended upgrading these units to MOD LVL ‘E’ on the (usually non mandatory and therefore chargeable) basis of “product improvement”. This upgrade included the specific upgrade of the inverter output capacitor C311 to increase reliability. However, when the failed unit in this event was returned to the OEM it was found that the cause of inverter failure was malfunction of the C602 capacitor.
The Investigation was told by the aircraft manufacturer that the event being investigated was the first case where a C600 series capacitor had failed without any evidence that C311 capacitor had also failed. It also advised that although the failure under investigation had been the first case of a C600 series capacitor failure leading to smoke in the flight deck, there had since been a number of other cases and that the OEM had issued an SB in May 2017 which had facilitated upgrading of static inverters from MOD LVL ‘E’ to MOD LVL ‘F’ by replacing five C600 series capacitors including C602 with ones which have had ‘burn in’ (the application of a known electrical load prior to system assembly from components).
The aircraft manufacturer stated that although there had been an increase in reported smoke events associated with static inverters, the rate of these occurrences as of April 2017 was still within CS25 type certification requirements. Nevertheless, in June 2017, they had raised an Airworthiness Review Sheet (ARS) with EASA about the potentially unsafe condition. EASA had subsequently concurred with the steps being taken to address the matter and after it became clear that the measures being taken by the OEM and the aircraft manufacturer had been effective, the ARS was closed. EASA advised the Investigation that this action was in accordance with their failure condition classification of a static inverter failure and the demonstrated probability of it occurring per flight hour so that mandating retrofit of later modification status inverters was not justified.
The Investigation considered the implications of procedures which required both AC generators to be switched off when the electrical fault was in a single unit malfunction which in reality affected only one half the AC electrical system. Amongst the systems lost with no AC power available apart from both normal and alternate landing gear operation were:
- L & R Angle of Attack heating
- L & R Pitot Probe heating
- L & R TAT Probe heating
- L & R Windshield Heating
- Anti Icing of the ailerons, elevators and rudder balance horns
- L & R propeller anti icing
- All landing lights and all strobe lights
- Flight deck overhead panel and instrument lighting
- Taxi and Takeoff lights
Although in the case of a daylight flight clear of icing conditions none of the unavailable systems had added much to the level of operational risk caused by the equipment failure whereas at night and/or in icing conditions, loss of all AC power could have resulted in significant risk in its own right. Some of the published work on the expected frequency of icing conditions in the area and altitude band where the event under investigation took place and the relevance of these conditions to turboprop aircraft was considered by the Investigation and it was noted that there was a relatively high probability of encountering icing conditions.
It was found that although the ELEC SMK checklist made reference to the option of “restoring unaffected equipment” it was found that no guidance was provided on how to go about this.
The formally documented Conclusions of the Investigation included but were not limited to the following:
- There is no guidance available to flight crew on the reinstating of electrical power in order to regain the use of lost electrical equipment.
- The OEM has instigated a production change involving a change of capacitor supplier and the ‘burn-in’ of capacitors used in the assembly of the subject static inverters.
- The Aircraft Manufacturer has commenced a program for replacement of the subject static inverters on the world fleet of ATR 42/72 aircraft. Currently, this program is not mandatory.
- EASA has reviewed the occurrence rate of static inverter failure including emission of smoke, its consequences, including additional flight crew workload, and stated that the number of events is commensurate with the safety objective.
The Probable Cause of the Serious Incident was documented as “the failure of the C602 (1-001-0306-0136) capacitor within the number two static inverter”.
One Safety Recommendation was made as a result of the Investigation as follows:
- that Avions de Transport Régional should consider consulting with operators of the ATR 42/72, as to the possibility of providing specific guidance to flight crew on re-instating electrical supply to unaffected equipment for essential services following use of the Electrical Smoke checklist, subsequent to a Static Inverter failure. (IRLD2018011)
The Final Report was published on 27 December 2018.