On 3 December 2020, when a Dassault Falcon 900 (OE-IMI) being operated by Avcon Jet on a non-revenue positioning flight from Vienna to Kangerlussuaq touched down at destination in night VMC, directional control was immediately lost and a veer to the left began with no apparent braking capability. The aircraft was kept on the runway but having suspected tyre deflation, it was then taxied clear and stopped where inspection disclosed seriously damaged main gear assemblies.
A Serious Incident Investigation was carried out by the Accident Investigation Board (AIB) of Denmark. The AIB removed the SSFDR and the SSCVR from the aircraft. After downloading and playback, SSFDR data were of good quality and useful to the Investigation, but as the SSCVR had not been stopped after landing it did not include that from the Vienna departure which had been overwritten as a result of the recorder not being isolated after the landing at Kangerlussuaq. It was also found that the recording of the Cockpit Area Microphone (CAM) was unusable throughout because of “severe pollution of the signal due to a very poor dynamic of the sensor”.
It was noted that the 45 year-old Captain in command was a Dutch licence holder who had a total of 6,774 hours flying experience including 2,700 hours on type of which 2,400 hours had been accumulated at the operator involved. The 43 year-old Captain acting as First Officer was an Austrian licence holder with a total of 5,352 hours flying experience which included 1,500 hours on type all of which had been accumulated at the operator involved.
Approximately 90 minutes prior to departure, personnel towed the aircraft out of a hangar and positioned it to the general aviation area. The surface temperature was MS01. When they arrived at the aircraft, the pilots observed “light contamination on the aircraft and the beginning of light precipitation” and decided that de/anti icing was required. The aircraft was taxied to a remote de icing stand where Type I and Type IV fluid treatment was performed. The crew reported noting “a few millimetres of snow/slush within the General Aviation area” and “light contamination of snow/slush on the remote de-icing stand” but otherwise the taxiways were reportedly clear of any frozen deposits as was runway 16 from which a standing start takeoff was performed six minutes later.
Once airborne, the airframe anti-icing system and the brake heating systems were selected on but after approximately three minutes climb including passing through a thin cloud layer, both systems were selected off as FL080 was approached. The remainder of the flight en-route was uneventful and on initial radio contact with destination APP, the flight was advised to expect radar vectors for the runway 09 LOC approach with a possibility of a visual final approach in benign weather conditions apart from the MS15 temperature and MS20 dew point. The runway surface was reported as having a 25% cover of 3 mm of “compacted or rolled snow” and 10% cover of 3 mm of ice with a measured braking action of 0.85 by Tapley Meter which was equivalent to ‘good’.
Although visual contact with the runway was obtained it was decided to complete the LOC approach which was stabilised. The brake heating system was not selected on as part of the descent checks being marked (as for use after takeoff) merely “as required”. On touching down on the 2,810 metre-long 60 metre-wide runway, it immediately became clear that directional control was impossible. A veer to the left began with a perception by the pilots that no braking capability was available. However, as the speed reduced, directional control was regained without leaving the paved surface and due to their suspicion that the aircraft had been exposed to a deflated tyre, they decided to vacate the runway and stop to await an inspection.
This found that three of the four main gear tyres had failed with their wheel rims damaged as a result with only the inner right wheel tyre remaining inflated (see the illustrations below). The left main gear hydraulic brake unit lines and the left main gear bay door were also damaged. Disintegration of the three wheels had also caused superficial damage to the runway and exit taxiway surfaces.
The damaged Left Main Gear. [Reproduced from the Official Report]
The damaged Right Main Gear. [Reproduced from the Official Report]
It was noted that had the same event occurred on a narrower runway than the 60 metre wide one at Kangerlussuaq, then “the potential loss of all hydraulic pressure at an early stage, or a more severe loss of aircraft directional control, could potentially have resulted in a runway excursion and consequently an accident”.
Why It Happened
An extensive examination of the aircraft brake units both on site and subsequently led to the tyre bursts being attributed to the ingestion of melted snow during takeoff from Vienna. This then froze during the flight and prevented the brake discs from freely rotating during the subsequent landing at Kangerlussuaq. The flight crew failed to eliminate this freezing by only operating the brake heating system for approximately three minutes rather than the recommended minimum of ten minutes on departure (or until the top of climb) and did not activate it at all during the descent inbound. This resulted in touchdown with three out of four blocked brakes and consequent tyre failures.
It was considered that the origin of the frozen brakes was the failure of the crew to follow the recommended procedure for use of the brake heating system which when selected on uses a bleed of heated air from the airframe anti-ice system to direct such air at the two main landing gear assemblies. It was found that this system is an option which is only installed if the purchaser of a new aircraft requests it. The corresponding checklist entries for its use after takeoff or in descent were found to be limited to “as required” accompanied by a footnote ‘caution’ in the descent checks reading “when the brake heating system is used (if installed): the minimum required N1 with two or all engines operating must be increased by 1%”. After any use during descent, the system was required to be switched off in the subsequent approach checks.
The brakes involved were carbon brakes which it was noted “are more subject to freezing” than older steel brakes with the AIB “unaware of scenarios with blocked steel brakes at touchdown due to ice”. It was further noted that “one reason for carbon brakes being more exposed than steel brakes is the higher porosity of the carbon disc surface” which extends drying time because more water can be absorbed by the discs. The occurrence of previous similar frozen brake events on other large business jet aircraft types including one on a Bombardier BD700 Global Express were noted to have prompted the issue of Safety Information Bulletins by the EASA, the FAA and Transport Canada alerting operators to the frozen brakes risk. At the same time, it was found that on request from the EASA, Dassault had produced (and subsequently revised, most recently in January 2021) a Significant Event Review (SER) on frozen brakes on the Falcon fleet which concluded that “the freezing of one or more brakes has never led to critical consequences” and that “in the case of multiple tyre burst, skidding on the wheel rim provides significant braking”.
It was noted that although the AFM for the Falcon 900 had been approved by the EASA, the corresponding “Operational Documentation” (“Crew Operational Documentation for Dassault non EASy” (abbreviated to “CODDE 2”)) including the Normal Procedure Checklists was required to be “reviewed” by an NAA during an operator approval process. The Investigation found “specific inconsistencies between the CODDE 2 and AFM normal procedures” in respect of the use of the brake heating system. A comparison of the relevant part of the CODDE 2 and corresponding AFM content led to the conclusion that “the interaction on application of the brake heating system between the AFM normal procedures" (“as necessary”), the CODDE 2 “Flight procedures” (“it is recommended”) and the related “Operations on contaminated runways” represented grounds for “subjective flight crew decisions”. This was considered true even for a type-experienced crew such as the two Captains in the investigated event.
It was considered that in the CODDE 2 “Flight procedures” for situations where a brake heating system is installed, the parallel use of the words ‘recommended’ and ‘as necessary’ “might have downgraded and diluted the preventive flight safety effect of these specific procedures".
Two Safety Recommendations were made as a result of the findings of the Investigation as follows:
- that the European Union Aviation Safety Agency in cooperation with Dassault modify in a more directive and explicit manner the AFM Normal Procedures (including the use of the brake heating system) and that the aircraft manufacturer accordingly modifies the ‘Crew Operational Documentation for Dassault non EASy’ (CODDE 2) in respect of the brake heating system. [DK.SIA-2021-0001]
- that the European Union Aviation Safety Agency in cooperation with Dassault re-evaluate the initial airworthiness ‘Post-Failure Situation Sheet’ for blocked wheels and the continued airworthiness ‘Significant Event Review’ for frozen brakes. [DK.SIA.2021-0002]
The Final Report was issued on 29 November 2021.