Challenger/A380 Encounter Occurred Amid Rising Wake-vortex Concerns

 - October 26, 2017, 7:00 AM

A European Aviation Safety Agency (EASA) safety information bulletin (SIB) on avoiding en-route wake turbulence suggests that concern about the issue was rising even before an encounter between an Airbus A380 and a Bombardier Challenger 604 in January this year left the business jet a total loss.

The June SIB "enhances the awareness of pilots and air traffic controllers of the risks associated with wake turbulence encounters in the en route phase of flight and provides recommendations and advisories with the purpose of mitigating the associated risks," the EASA explained in the document. The SIB does not reference the A380/Challenger accident. But the agency took the unusual step of following up the bulletin, issued just weeks after release of the German Federal Bureau of Aircraft Accident Investigation (BFU) preliminary report on the accident, with a press release. The release notes that wake-turbulence encounter likelihood "is very low, but cannot be excluded." It also calls out “heavy” and “super-heavy” aircraft, naming the A380 among others, as "more prone to generate stronger vortices."

The bulletin "was not directly a consequence of the A380 event," an EASA spokesman said, but rather "more general information on the wake vortex issues." The SIB is a "mitigating measure" as part of the regulator's sharpened focused on wake encounters, the spokesman added. “There isn’t any other new EASA communication on this topic at this stage.”

Late last year the EASA quietly added wake-vortex encounters to its Safety Risk Portfolio (SRP), a list of mode-specific risks that data show merit specific risk-mitigation focus. Air transport operators were involved in 1,100 wake-vortex encounters between 2012 and 2016, according to EASA data. None was classified as an accident or serious incident.

The January 7 encounter happened as the Emirates Airline A380 and the German-registered Challenger were flying in opposite directions on the same route, L894, over the Arabian Sea. The A380, en route to Sydney from Dubai, was cruising at FL350 on a southerly heading. The Challenger, bound for Al Bateen, United Arab Emirates, from Malé, Maldive Islands, was on the opposite heading flying at FL340. 

Approximately 45 seconds after the Challenger passed beneath and just to the right of the A380, it flew through the widebody's wake, and the smaller jet rolled "heavily" to the left, the BFU report said. The pilots struggled to regain control, losing 9,000 feet of altitude in 45 secondsDuring the altitude loss, the Challenger completed "several rotations," the pilots told BFU. The pilots recovered control of the aircraft at FL240 and diverted to Muscat, Oman. Two passengers were severely injured, while two others and the cabin attendant suffered minor injuries.

The BFU probe continues, but the preliminary report suggests that an area of focus is aircraft separation standards. The 1,000-foot vertical separation met reduced vertical separation minimums (RVSM) standards within the Mumbai flight information region where the occurrence took place, the BFU said. Strategic lateral offset procedures (SLOP), in which pilots are permitted to fly to the side of an air route's centerline to expand separation—and safety margins—were not permitted on L894, the BFU said.  

Separation standards for the A380 were developed on the basis of working group recommendations made in 2006, the year before the aircraft entered service. While the group established new standards for arrival and departure sequencing, it concluded that existing en route horizontal and vertical spacing requirements were sufficient for the A380. "Based on current separation criteria, [wake vortex encounter] risk is considered to be acceptable at this time," a "safety case" produced for the working group said. "However, it is recommended that this issue be investigated further."

Wake-turbulence Risk Studies

The EASA's SIB references several publications that underscore wake-turbulence risk. One, a 2014 study published in Aircraft Journal titled "Improved Understanding of En Route Wake-Vortex Encounters," pointed to several factors that may boost en route risks. Two—the amount and mix of traffic—are related. The Aircraft Journal study cited a "stabilized" annual-movement increase of 3 percent forecast through the end of the decade. "With more movements, the risk of encountering a wake vortex increases quadratically," the study explained. Add an increasing mix of traffic and the risk grows. "Especially when the number of heavy aircraft increases along with the introduction of medium, light and very light aircraft, an increased risk is to be expected with respect to wake vortex encounters during the en route phase of flight," the study said.

Another factor is climate change. Multiple studies referenced in the Aircraft Journal piece indicate that the tropical regions are expanding toward the North and South poles. One ramification is a raising of the tropopause, the boundary between the troposphere and the stratosphere. The altitude of the tropopause varies with myriad factors, among them weather systems, but generally it ranges from 30,000 feet at the poles to 56,000 feet at the Equator. Research shows that wake-vortex encounters are more common just below the tropopause, in the troposphere. "The generally lower stability in the troposphere provides favorable conditions for the evolution of the wake vortex," the Aircraft Journal study said. 

As the tropics expand, the tropopause in European airspace rises, exposing more cruise altitudes to tropospheric conditions. "This could result in more en route wake vortex encounters in the future and may contribute to increased severity of wake vortex encounters," the study said.

The EASA's SIB lists three factors contributing to the elevated risk of en route wake turbulence: crossing traffic climbing or descending in proximity to another aircraft; heavy aircraft generating stronger vortices; and the rising tropopause. The bulletin also underscores the added risk during RVSM operations. "Considering the high operating airspeeds in cruise and the standard 1,000-foot vertical separation in RVSM airspace, wake can be encountered up to 25 nm behind the generating airplane," the SIB said. "The most significant encounters are reported within a distance of 15 nm." 

The January Challenger upset happened in airspace with no radar coverage. According to the BFU's reconstruction of the occurrence using TCAS and other onboard data, the Challenger was 15 nm past the A380 when it hit the widebody's wake.

While EASA is on the record as taking a closer look at the risk of wake turbulence, it has not changed any procedures. The FAA's general guidance, Advisory Circular 90-23G, was last updated in February 2014. In 2009, the U.S. agency's Air Traffic Organization (ATO) issued a notice to controllers spelling out "interim" procedures, including separation standards, for A380 flights. The notice, effective for one year, was updated annually, and in May 2012 the term "interim" was removed from the title. In 2013, the massive Antonov An-225 cargo transport was included for the first time. 

The most recent notice expired in June 2016, and the FAA has yet to issue a new notice. "Nothing has changed," an FAA spokesman said.

The A380 fleet, which numbered about 165 in the middle of last year, was up to 215 at the end of August. Airbus listed 102 more in its backlog.