On 7 November 2016, an Avro RJ-100 (SE-DSV) being operated by Braathens Regional on a scheduled domestic passenger flight from Gothenburg to Stockholm Bromma as SCW1B had just passed 3,000 feet in the climb to FL 150 in night VMC and was accelerating when airframe vibrations suddenly began. An “emergency” was declared to ATC with a request for an immediate return. Crew actions to reduce speed led to cessation of the vibrations and the air turnback to land on the takeoff runway but in the opposite direction was successfully accomplished with no further vibrations during the approach and crew advising ATC during the final stages of it that operations were now “normal”.
An Investigation was carried out by the Swedish Accident Investigation Authority (SHK). No arrangements to protect the FDR data were made after the flight but equivalent QAR data relevant to the event were successfully downloaded. Relevant data on the 30 minute CVR was overwritten after the crew “forgot” to electrically isolate it after the flight.
It was found that the 49 year-old Captain had accumulated 9,994 total flying hours which included 7,220 hours on type and the 43 year-old First Officer had accumulated 5,584 flying hours which included 3,000 hours on type. The First Officer was the designated PF for the flight but the Captain took over that role at the onset of the severe airframe vibrations.
It was established that prior to the flight, the aircraft had been parked outside for approximately 40 hours during which time there had been rain followed by snow and sleet with the temperature varying between “a few degrees above freezing” and -5°C. It was noted that during this period the wind direction was such that precipitation impacted the aircraft from behind and that the elevators of this aircraft type are usually angled upwards when parked. Independently conducted pre-flight inspections of the aircraft by both the Operator’s engineer and the aircraft Captain confirmed that ground de-icing would be required. The Captain contacted the de-icing team and instructed them to perform “a one-step de-icing of wings, stabiliser, rudder and fuselage”. This was then carried out on stand using an “Elephant” de icing vehicle crewed by two people. Upon completion, the Captain received a verbal report of the start time of the de-icing, the quantity of fluid used and the type of fluid used - Type 1. A full record of the de-icing operation was recorded on the airport CCTV system.
After completion of pushback and engine start, the crew carried out the flight control check specified in the ‘After De-icing’ Checklist which included holding the elevators in a specified position for 30 seconds so as to allow any fluid which might have been in the inside of the elevator to drain out. The subsequent takeoff was from runway 03 in light winds and a surface temperature of -5° C. QAR data confirmed crew reports of excessive vibrations beginning during acceleration as the airspeed passed 214 knots at about 3,200 feet. The commander reported having taken control of the aircraft and disconnecting the AP. The First Officer declared an “emergency” and a wish to return to Gothenburg ATC. The crew then carried out the QRH drill for a ‘Pitch Oscillation’ and as the airspeed reduced as required by this procedure, the vibrations, having continued for a recorded 48 seconds, ceased. When airspeed was again increased, similar vibrations occurred for a recorded 26 seconds, until it was again reduced, this time to about 180 KIAS.
A descent and an immediate left turn were made to position visually for a straight-in approach to runway 21 and the rest of the flight was uneventful with the crew announcing a “normal operations” status to the TWR controller on short final. Once the aircraft was back on stand, the Captain made a defect entry in the aircraft Technical Log reporting “strong sudden vibrations after flap retraction at 200 knots, slowed to 180 knots and vibrations ceased, approach normal”. The pilots stated that they had “forgotten to perform the procedures to preserve CVR and FDR recordings after the occurrence”.
The Operator’s engineer inspected the aircraft in order to establish the source of the vibrations and found that there was “extensive ice contamination on the primary and secondary flight control surfaces, mainly on the elevators, rudder, ailerons and flaps” which was recorded on photographs including the ones below. Contamination was also found on the underside of several flight control surfaces. All the ice was removed and, after a flight test, the aircraft was released for service without consulting the manufacturer to see if any inspections to ensure that no damage had occurred to the aircraft were appropriate. Later inspections of the elevator system detected play in the elevator dampers and drop-links that were beyond normal tolerances but it was not possible to determine whether this deterioration had occurred before, during the course of or after the investigated event.
The rudder and right elevator after the return. [Reproduced from the Official Report]
The right aileron after the return. [Reproduced from the Official Report]
The left flap after the return. [Reproduced from the Official Report]
When the vibrations occurred following the procedure for pitch oscillation meant an airspeed reduction and this led to the vibrations ceasing. However, the speed was reduced to a lower speed than that prescribed in this procedure. The crew subsequently stated that the vibrations experienced seemed significantly stronger, more abrupt and at a higher frequency than the vibrations experienced during simulator training for “Pitch Oscillation”.
It was concluded that prior to the pre-departure attempt at de-icing, frozen deposits on the airframe including flight control surfaces were likely to have initially consisted of “several layers of ice and snow”. Ice contamination of the underside of the elevator and ailerons was probably due to the fact that the aircraft was parked tail into wind and that those surfaces which had ice on the underside had been angled towards the wind.
The frequencies measured during the vibrations were found to have been similar to those recorded during previous occurrences of airframe vibration in flight on the aircraft type involved but were “too high to represent a rigid body motion of an aeroplane of this size”. All the available evidence pointed to the most likely cause of the vibrations being ice loading on the elevators which had increased their weight and set up a dynamic interaction with aerodynamic forces and the structural vibration modes of the entire airframe to create a flutter instability with limited amplitude, a so-called “limit-cycle oscillation” (LCO). It was noted, without any implied criticism of the type certification process, that the aircraft type involved was, by design, potentially sensitive to relatively small mass imbalances and thus “at an increased risk of aeroelastic instabilities such as flutter”, especially those arising from, for example, very small amounts of excess weight on control surfaces such as an elevator servo tab.
The Investigation sought to establish more about the de icing operation which, given clear skies and an absence of precipitation during the short flight, clearly indicated that it had not been effective. It had been carried out by a contractor called ‘Aviator Airport Services Sweden’ (Aviator) and should have been in accordance with the procedures contained in their general De/Anti Icing Manual (DIM) and their ‘Local Procedure Document’ for services at Gothenberg. Their employee who had carried out the fluid spraying stated that the operation being investigated was his personal first one of the 2016/17 winter season and he had not yet completed the annual refresher training for 2016/17 which was required to have been completed by 31 December 2016. He also stated that he was usually the vehicle driver rather than the fluid sprayer and that the Captain ordered a de-icing of the stabiliser, fuselage and wings but that he “clearly remembered” that de-icing of the lower surfaces of the stabiliser and wings had not been included. He described the de icing carried out as “a rushed operation” but remarked that this was not unusual given the pressure to support “every time on time” departures. He also noted that several more departing aircraft had been in the queue for de-icing and that the wind direction that morning relative to the orientation of the parked RJ-100 had been such as to blow the steam from the heated fluid back towards his face making it more difficult to see what effect the spraying was having on the ice concealed beneath a top cover of snow. He accepted that his attempt to confirm that all frozen deposits had been removed whilst achieving this was not a very good way to be sure of this and with hindsight he “concluded that he had probably not inspected the result thoroughly enough”. No separate post-spraying inspection of the aircraft was made.
Further interviews of Aviator de icing personnel at Gothenberg were conducted to help the Investigation form a better understanding of the on-the-ground reality. Despite the general perception of a “safety first” and “no-blame” management culture, the interview findings included the following:
- The ‘post de icing check’ was generally equated with the report that the requested task had been completed and was not also indicative of a discrete check after completion of spraying. However, the DIM explicitly described the post de-icing check as “a procedure to check if there is any remaining contamination”.
- The DIM and the Local Procedures Document were “seldom used by the de-icing operators as reference material”.
- The de-icing operators were generally unaware of the degree of contamination which may have been built up during a longer ground stop. One of the de-icing operators stated that he could sometimes tell that there would be difficult conditions for aircraft de-icing based on scraping ice off the windshield of his car in the morning when preparing to drive to work.
- All of the de-icing operators perceived the working conditions as stressful, especially because they are the last link in the chain before the aeroplane is ready to take off. Therefore one can be blamed for a possible delay. They recognised that there is always an expectation that departures will be on time and although there was obvious pressure from pilots, it also came from colleagues. In many cases they felt that such pressure was “unreasonable” because “not enough time is allocated to the de-icing procedure”. They also felt that the function is generally understaffed quoting the example that “a person who is loading an aeroplane can be suddenly assigned to do the de-icing procedure without being informed of this task in advance”.
The aircraft operator advised that the boarding gate is usually closed ten minutes before departure after which the Captain must have the load and trim sheet for approval and acceptance not less than seven minutes before departure. Only after this acceptance is complete can the doors be closed and de-icing, when required, then commence. The contract between the aircraft operator and Aviator for aircraft de icing at Gothenburg was found to require them to “remove frost ice and snow from aircraft using de-icing fluid” and to “perform final inspection after de-icing/anti-icing operations and inform flight crews of results”. The aircraft operator was a member of the ‘Northpool’ fuel and de-icing audit group and Aviator’s de-icing function for them at Gothenburg had been most recently audited towards the end of the 2015/16 winter season. A finding during that audit that Aviator’s quality assurance programme had not been implemented prior to the start of the 2015/16 season had been addressed by action acceptable to Northpool.
On the basis of the evidence gathered, the Investigation made a series of observations relating to the de icing operation which included, but were not limited to, the following:
- the de-icing order did not cover the underside of the elevators and ailerons because it was not appreciated that there were any frozen deposits there. The inspection of the elevators was conducted from ground level in the dark and was inadequate. It was in any case difficult to detect visually.
- the poor delivery of the de-icing requested was considered to be less a result of a lack of “current knowledge and know-how” than a consequence of the time constraints, including self imposed ones, which created circumstances conducive to rushing to complete.
- there is widespread confusion about whether the “incorporated” method of post de-icing inspection - inspection conducted during the de icing operation - is as effective as a separate inspection afterwards. SHK takes the view that “it is reasonable to assume that the risk of missing residual ice or other contamination is greater when using the incorporated method than when using a separate control”. However, it was noted that the incorporated method is consistent with the former AEA guidelines to which both ICAO and EASA refer and although these are not binding, “they have become normative for both operators and regulators through ICAO’s and EASA’s references to them (and) it is therefore important that (they) are properly evaluated and secured”.
- There is considerable ambiguity as to exactly what a “post-de-icing check” represents. It is certainly not widely seen by service provider personnel as meaning that frozen deposits on the specified surfaces have been removed rather that the required action has been completed. It “should also be noted that the (aircraft) Operator has not specified what method of checking after de-icing should be used despite the applicable regulatory requirements”.
It was formally concluded that the Cause of the event was “partly the fact that the operator lacked detailed enough procedures for performing a complete contamination inspection and that existing routines were not fully applied and partly the fact that the operator had not properly checked, evaluated and controlled the subcontractor’s working methods”.
One Contributory Factor was also identified as “that the de-icing operation had insufficient organisational support to help the staff resist requests for an on time departure and to ensure that the de-icing was properly executed despite actual or perceived time shortage”.
Three Safety Recommendations were issued as follows:
- that the ICAO should investigate and evaluate the risks of recommended methods for de-icing and post-de-icing check, especially the incorporated method as referred to in the ICAO Annex 6, Part I, Doc 9640 and consider and decide whether the reference should be changed. [RL 2017:10 R1]
- that the EASA should investigate and evaluate the risks of recommended methods for de-icing and post-de-icing check, especially the incorporated method referred to in the referenced documents in GM3 CAT.OP.MPA.250 of Commission Regulation (EU) No 965/2012, and consider and decide whether the reference should be changed. [RL 2017:10 R2]
- that the Swedish Transport Agency should evaluate the need to change their supervisory procedures to ensure that AOC holders have appropriate procedures for contamination inspection and de-icing operations. [RL 2017:10 R3]
The Final Report of the Investigation was published on 7 December 2017.