Interim Report, published May 2017. Bundesstelle für Flugunfalluntersuchung. BFU – German Federal Bureau of Aircraft Accident Investigation
Type of Occurrence: Accident. Date: 7 January 2017. Location: Enroute, above the Arabian Sea. Manufacturer / Model: 1) Bombardier / CL-600-2B16 (604 Variant) 2) Airbus / A380-861. Injuries to Persons: 1) Two severely injured passengers, two passengers and one flight attendant suffered minor injuries 2) None. Damage: 1) Aircraft severely damaged 2) None.
During cruise flight above the Arabian Sea, the Indian Ocean, approximately one minute after it had been passed overhead by an Airbus A380 on opposite course, the CL604 was subject to temporary loss of control.
After it had lost approximately 9,000 ft of altitude the pilots regained control of the aircraft and subsequently landed at an alternate aerodrome at Muscat Airport, Oman.
The accident occurred over international waters. Thus the BFU as representative of the State of Registry of the accident aircraft is responsible for the conduct of the investigation. In accordance with international regulations, the air accident investigation authorities of Oman, India, the United Arab Emirates, Canada, USA, and France will assist the BFU in this investigation.
History of the Flight
At 1152 hrs -0652 UTC (All times local, unless otherwise stated) the CL604 had taken off from runway 36 at Malé, Maldive Islands, for a flight to Al-Bateen, United Arab Emirates. Three crew members and six passengers were on board the airplane.
The Flight Data Recorder (FDR) recordings show that the CL604 autopilot had been engaged approximately one minute after take-off. At 0720 UTC the airplane reached cruise level FL340. At 0729 UTC the aircraft entered Indian airspace (Mumbai FIR) at the reporting point BIBGO and had received the clearance to fly to reporting point KITAL via route L894. At approximately 0818 UTC the co-pilot radioed reaching reporting point GOLEM.
At 0655 UTC an Airbus A380-861 (A380) had taken off at Dubai Airport, United Arab Emirates, for a flight to Sydney, Australia. The aircraft flew at FL350 with a southern heading.
The analysis of the flight data of both aircraft showed that at 0838:07 UTC the A380 had passed the CL604 overhead with a vertical distance of 1,000 ft.
At 0838:54 UTC the CL604, with engaged autopilot, began to slightly roll right. At the same time, a counter-rotating aileron deflection was recorded and fluctuation of the vertical acceleration began. In the subsequent approximately 10 seconds the airplane had a right bank angle of 4° to 6°. At 0839:03 UTC the right bank angle began to increase. Within one second the bank angle increased to 42° to the right. At the same time, the aileron deflection to the left increased to 20° and the vertical acceleration increased to 1.6 G. In the following second, vertical acceleration changed to -3.2 G.
At 0839:04 UTC a lateral acceleration of 0.45 G to the right was recorded. The pitch angle changed from about 3° to about 1°, then within one second increased to 9° and decreased again in the following second to -20°. At the same time, the FDR recorded a rudder deflection to the left reaching 11.2° after about two seconds whereas the bank angle changed from 42° right to 31° left.
Between 0839:05 UTC and 0839:10 UTC Indicated Airspeed (in knots) changed from approximately 277 KIAS to 248 KIAS. The N1 of the left engine of 95% began to decrease.
At 0839:07 UTC the validity of IRS parameter is lost, the lateral acceleration reached 0.94 g left, the autopilot disengaged, and a master warning, lasting seven seconds, was recorded.
Between 0839:09 UTC and 0839:41 UTC the FDR recorded a loss of altitude of approximately 8,700 ft. Large control surface deflections and acceleration were recorded. The speed increased and at 0839:31 UTC reached approximately 330 KIAS. At 0839:30 UTC the spoilers extended and 13 seconds later were retracted again. The N1 of the left engine had decreased to approximately 40% when the Interstage Turbine Temperature (ITT) began to increase and nine seconds later had reached 850°. The left engine was shut off.
At about 0856 UTC the Pilot in Command (PIC) informed the air traffic controller in Mumbai of the occurrence, declared the emergency and reported their position, altitude and their intention to fly via KITAL to Oman.
At about 0915 UTC the crew restarted the left engine. Subsequently, the airplane climbed to FL250. At about 0956 UTC the autopilot was re-engaged.
At 1105 UTC the CL604 landed at Muscat Airport.
The A380 continued the flight to Sydney and landed there at 1958 UTC.
The recordings of the Omani air traffic control services show that at about 0920 UTC the neighbouring Indian regional air traffic control Mumbai informed them that the CL604 was at FL230 and would probably pass the reporting point KITAL at 0937 UTC. Mumbai also informed ATC that via a relay station the information had been received that the airplane would divert to Oman. Initially, the reason for the low altitude was given by Mumbai ATC as being due to engine failure. At 0957:50 UTC the airplane was depicted on the Omani ATC radar. At 1014:14 UTC the CL604 reached reporting point KITAL.
Statements of the CL604 Pilots
According to the statement of the CL604 pilots, the PIC was Pilot Flying (PF) and the co-pilot Pilot Non Flying (PNF). The PIC stated that TCAS had drawn his attention to the opposite traffic. He then recognised the aircraft type A380, the airline, and informed the co-pilot. The PIC also stated that the A380 had passed them in opposite direction, slightly to the left and according to TCAS 1,000 ft above. He further stated that a short time later the airplane had been hit by the wake turbulence of the A380. The airplane had shook briefly, then rolled heavily to the left and the autopilot disengaged. Both pilots had actuated the aileron to the right in order to stop the rolling motion. But the airplane had continued to roll to the left thereby completing several rotations. Subsequently both Inertial Reference Systems (IRS), the Flight Management System (FMS), and the attitude indication failed. According to the pilots’ statements at the time of the accident both pilots had fastened their lap belts and in addition the co-pilot had worn his shoulder belts. According to the PIC he had lost his headset during the rolling motion of the airplane. The Quick Reference Handbook (QRH) had flown around the cockpit and was damaged. As a result individual pages had been scattered around the cockpit. The PIC explained since the sky had been blue and the ocean’s surface almost the same colour he had been able to recognize the aircraft’s flight attitude with the help of the clouds. Later both pilots had been able to recover the airplane at FL240 using control inputs on the aileron and later the rudder and slight elevator deflection. Regarding the left engine the PIC stated that he had observed that N1 and N2 had “run apart”. N1 had decreased severely. ITT had increased, reached more than 1,000°C, and the indication flashed red. Subsequently the engine was shut off. Based on the memory items the pilots were able to reactivate the IRS in attitude mode and fly the airplane again towards reporting point KITAL. Then the pilots used the cross bleed of the right engine to restart the left. After the second IRS had been reactivated and position and heading been entered manually into the FMS the autopilot was engaged again. After they had assessed the situation the flight crew decided to fly to Muscat.
Statements of the CL604 Flight Attendant
The flight attendant stated in an interview conducted by the BFU that during take-off and climb she had been seated in the jump seat with the seat belt fastened. She had opened the seat belt while they were passing FL100. At the time of the accident, she had been standing in the middle of the cabin preparing the service. Four of the six passengers had also not been seated. In her recollection, the airplane had turned three times around its longitudinal axis, during which the occupants had been thrown against the ceiling and the seats. Several of the passengers suffered injuries, some of which were bleeding. She herself suffered minor injuries. Using the on-board first aid kit she had attended to the passengers. In the further course of the flight she informed the pilots of the situation in the cabin and reassured the passengers.
Reconstruction of the encounter of the two airplanes
Images: Interim Report Bundesstelle für Flugunfalluntersuchung. BFU – German Federal Bureau of Aircraft Accident Investigation
Pilot in Command CL604
The 39-year-old PIC held an Air Transport Pilot’s License (ATPL(A)) of the European Union issued in accordance with Part-FCL. It was first issued by the Luftfahrt-Bundesamt (LBA) and valid until 6 June 2014. The licence listed the ratings as PIC for CL604/605 and the Instrument Rating (IR) valid until 31 March 2017, and for single engine piston land (SEP).
His class 1 medical certificate was last issued on 26 September 2016 and valid until 8 October 2017.
His total flying experience was about 5,334 hours, about 4,564 hours of which were on type.
He had been employed by the operator as a pilot since October 2012.
On the day of the accident, the entire crew had begun their shift at 0500 UTC.
The 41-year-old co-pilot held an Commercial Pilot’s License (CPL(A)) of the European Union issued in accordance with Part-FCL. It was first issued by the LBA on 31 October 2013. The licence listed the ratings as co-pilot for CL604/605 and the Instrument Rating (IR), valid until 31 October 2017, and for single engine piston land (SEP) and Touring Motor Glider (TMG).
His class 1 medical certificate was last issued on 8 March 2016 and valid until 8 April 2017.
The co-pilot had a total flying experience of about 1,554 hours; of which 912 hours were on type.
Since November 2015 the co-pilot had been employed by the operator.
Pre-flight Meteorological Preparation CL604
The BFU was provided with the pre-flight preparation documentation of the CL604
flight crew including the weather data of 6 January 2017 at 2336 UTC.
According to the forecast tropopause was at approximately FL525 at a temperature
For cruise level FL340 wind with 20 kt from north-west and a temperature of -42°C
The Significant Weather Fixed Time Prognostic Chart for the planned flight did not
contain any warnings of Clear Air Turbulence (CAT) for the area of the Arabian Sea.
Weather at the Time of the Accident
At the time of the accident it was daylight. According to the CL604 pilots’ statements very good Visual Meteorological Conditions (VMC) with blue skies prevailed. The ocean’s surface had been visible. In an estimated altitude of 3,000 to 4,000 ft AMSL the cloud cover had been 1/8 to 2/8. Condensation trails had not been visible.
No significant meteorological information (SIGMET) had been issued for the flight information region Mumbai (VABF).
According to the Digital Access Recorder (DAR) of the A380 the wind at their cruise level at FL350 came from about 315° with about 23 kt. The Static Air Temperature (SAT) was -44°C.
Wreckage and Impact Information
The accident occurred above international waters, the Arabian Sea, approximately 500 NM from any land.
The aircraft manufacturer determined that the airframe structure could not be restored to an airworthy state as it exceeded the airframe certification design load limits during the upset encounter. Therefore the aircraft is considered to be damaged substantially.
During a BFU investigation of the airplane no outer damages on fuselage, wings, and empennage, including control surfaces, were visible. There was no evidence of leakages (oil, fuel).
The inside of the passenger cabin showed damages on the seats and the panelling, as well as traces of blood. The armrests of the four seats in the front, installed in club arrangement, were either deformed or had fractured.
On the left side of the cabin two oxygen masks had fallen from their casings.
Photo of the interior of D-AMSC after the upset (Photo: FlightServiceBureau) from The Aviation Herald
Medical and Pathological Information
According to the operator, four passengers were treated at the hospital in Muscat.
One passenger suffered head injuries and a broken rib; another passenger had fractured a vertebra. The two passengers and the flight attendant, who had sustained minor injuries, suffered bruising and a fractured nose, respectively.
The two other passengers and the pilots remained unharmed.
Safety Case for Wake Vortex Encounter Risk due to the A380-800
An ad hoc Steering Group (SG) and a technical Work Group, comprising representatives from Joint Aviation Authorities (JAA), Eurocontrol, Federal Aviation Administration (FAA), Airbus and Det Norske Veritas (DNV), was set up in 2003 to specify safety requirements to ensure Wake Vortex Encounter (WVE) risk from the Airbus A380 will be acceptable. A safety case (A380 SG, 2006a) and supporting documentation has been produced.
Among others the following recommendations have been made:
Investigator in charge: Jens Friedemann
Excerpted from Bundesstelle für Flugunfalluntersuchung – German Federal Bureau of Aircraft Accident Investigation Interim Report
On Mar 18th, 2017 The Aviation Herald received a draft of an EASA safety information bulletin regarding this accident, stating:
With the increase of the overall volume of air traffic and enhanced navigation precision, wake turbulence encounters in the en-route phase of flight above 10 000 feet (ft) mean sea level (MSL) have progressively become more frequent in the last few years.
The aim of this SIB is to enhance the awareness of pilots and air traffic controllers of the risks associated with wake turbulence encounter in the en-route phase of flight and provide recommendations for the purpose of mitigating the associated risks.
The draft reasons:
The basic effects of wake turbulence encounter on the following aeroplane are induced roll, vertical acceleration (can be negative) and loss or gain of altitude. The greatest danger is an induced roll that can lead to a loss of control and possible injuries to cabin crew and passengers. The vortices are also most hazardous to the following aircraft during the take-off, initial climb, final approach and landing.
However, en-route, the vortices evolve in altitudes at which the rate of decay leads to a typical persistence of 2-3 minutes, with a sink rate of 2-3 metres per second. Wakes will also be transported by the wind.
Considering the high operating airspeeds in cruise, the wake can be encountered up to 25 nautical miles (NM) behind the generating aeroplane, with the most significant encounters reported within a distance of 15 NM. This is larger than in approach or departure phases of flight.
The encounters are mostly reported by pilots as sudden and unexpected events. The awareness of hazardous traffic configuration and risk factors is therefore of particular importance to anticipate, avoid and manage possible wake encounters.
The draft issues following recommendations:
As precautionary measures, operators and pilots should be aware that:
– As foreseen in Reg. 965/2012 AMC1 to CAT.OP.MPA.170, the announcement to passengers should include an invitation to keep their seat belts fastened, even when the seat belt sign is off unless moving around the cabin. This minimises the risk of passenger injury in case of a turbulence encounter en-route (wake or atmospheric).
– As indicated in ICAO PANS-ATM, for aeroplanes in the heavy wake turbulence category or for Airbus A380-800, the word “HEAVY” or “SUPER”, respectively, shall be included immediately after the aeroplane call sign in the initial radiotelephony contact between such aeroplanes and ATS units.
– When possible, contrails should be used to visualise wakes and estimate if their flight path brings them across or in close proximity.
– When flying below the tropopause altitude, the likelihood of wake encounter increases. The tropopause altitude varies (between days, between locations).
– Upwind lateral offset should be used if the risk of a wake encounter is suspected.
– Timely selecting seat belt signs to ‘ON’ and instruct cabin crew to secure themselves constitute precautionary measures in case of likely wake encounters.
In the case of a wake encounter, pilots should:
– Be aware that it has been demonstrated during flight tests that if the pilot reacts to the first roll motion when in the core of the vortex, the roll motion could be amplified by this initial piloting action. The result can be a final bank angle greater than if the pilot would not have moved the controls.
– Be aware that in-flight incidents have demonstrated that pilot inputs may exacerbate the unusual attitude condition with rapid roll control reversals carried out in an “out of phase” manner.
– Be aware that if the autopilot is engaged, intentional disconnection can complicate the scenario, and the autopilot will facilitate the recovery.
– Avoid large rudder deflections that can create important lateral accelerations, which could then generate very large forces on the vertical stabiliser that may exceed the structural resistance. Although some recent aircraft types are protected by fly-by-wire systems, use of the rudder does not reduce the severity of the encounter nor does it improve the ease of recovery.
– Make use of specific guidance available through AOM for their specific type(s)/fleet.
ATS providers and air traffic controllers should:
Enhance their awareness about en-route wake turbulence risk, key factors and possible mitigations, based on the information provided in this document and other relevant material. This could be achieved through flyers, e-learning, and refresher training module.
Possible risk mitigations may consist of:
– Make use of the wake turbulence category (WTC) indication in the surveillance label and/or the flight progress strip (whether electronic or paper), and observe closely separated aeroplanes that are at the opposite extremes of the WTC spectrum;
– As the best practice, provide traffic information, advising “CAUTION WAKE TURBULENCE”, when you identify that a ‘HEAVY’ or ‘SUPER HEAVY’ wake category traffic is climbing or descending within 15 NM of another following traffic;
– Manage en-route traffic crossings such as, when possible while preserving safe tactical management of overall traffic in the sector, avoiding to instruct climb or descend to ‘HEAVY’ or ‘SUPER HEAVY’ traffic within 15 NM distance from another following traffic;
– If at all possible, avoid vectoring an aeroplane (particularly if it is LIGHT or MEDIUM category) through the wake of a HEAVY or SUPER HEAVY aeroplane where wake turbulence may exist.
- Bundesstelle für Flugunfalluntersuchung – German Federal Bureau of Aircraft Accident Investigation Interim Report. State File Number: BFU17-0024-2X. Published: May 2017
- EASA Safety Publications Tool. Safety Information Bulletins
- The Aviation Herald
By Laura Victoria Duque Arrubla, a medical doctor with postgraduate studies in Aviation Medicine, Human Factors and Aviation Safety. In the aviation field since 1988, Human Factors instructor since 1994. Follow me on facebook Living Safely with Human Error and twitter@dralaurita. Human Factors information almost every day