Encountering Wake Turbulence At High Altitude, Part 3

A Eurocontrol analysis of wake vortex reports that occurred in airspace above FL285 found that 64% of the cases involved a combination of one or both aircraft climbing or descending.

It is important to understand that the wake vortices descend with respect to the trajectory of the aircraft. For example, where the generating aircraft is climbing at 1,500 fpm, its wake vortices are descending at 500 fpm with respect to the generating aircraft’s flight path. 

Twenty-seven percent of the reported cases involved aircraft in level flight with 1,000 ft. of vertical separation. For a wake vortex to potentially descend over 1000 ft., the generating aircraft must be heavy to produce a strong initial descent rate. This is more likely to happen with an aircraft whose takeoff weight is above 350,000 kg (771,617 lb.).

Secondly, the atmospheric conditions must be favorable for long-duration vortices, otherwise the wake vortices will dissipate before reaching the next flight level.  As noted earlier, these conditions are most likely to exist approximately 3000-5000 ft. below the tropopause.  

Flight testing by Airbus demonstrated that if the pilot tries to counteract the first rolling motion, these control deflections actually will roll the aircraft into the vortex, placing the aircraft on an intercept with the core of the vortex. Worse yet, when the aircraft enters into the core of the vortex, the rolling motion will be amplified by the initial pilot’s inputs. The result will be a final bank angle greater than if the pilot would not have moved the controls. 

(See https://aviationweek.com/business-aviation/safety-ops-regulation/upset-… for further discussion of the extensive engineering-based findings from the Airbus flight research program.)

Wake Turbulence Recommendations
The following is a compilation of recommendations from authoritative sources, to include the European Union Aviation Safety Agency’s (EASA) Safety Information Bulletin 2017-10 (June 22, 2017) titled “En-route Wake Turbulence Encounters.”

Since it is often encountered without warning, pilots will not have enough time to notify cabin crew and passengers in advance. Therefore, cabin crew and passengers who are neither seated or wearing their seat belts have a high risk of being injured. The announcement to passengers should include an advisory to keep their seat belts fastened, even when the seat belt sign is off, unless moving around the cabin. This minimizes the risk of passenger injury in case of a turbulence encounter en-route (to include both wake or atmospheric turbulence.)

Aircraft in the heavy wake turbulence category should utilize “Heavy” immediately after their call sign. In the case of the Airbus A380-800, they will utilize “Super.”

It can be difficult to estimate exactly where the wake turbulence is located. Considering the high operating speeds in cruise flight and the standard 1000 ft. vertical separation in RVSM airspace, wakes can be encountered up to 25 nm behind the generating airplane. The most significant encounters are reported within a distance of 15 nm.

Anticipate the possibility of a wake encounter when other traffic within 5-25 nm appears to be on a similar track ahead, crossing above you, or climbing or descending through your flight path.

Vortices, like anything else airborne, tend to move with the wind, and you must accurately predict how the wind is flowing in order to accurately predict where the vortices are floating. Pilots have actual wind information (direction and speed) available in the cockpit on the navigation display. Winds at altitude can often be 100 kts. or stronger.  In 2.5 min. a wake that is present in a 100-knot wind will drift 4.1 nm.   An upwind lateral offset should be used if the risk of a wake encounter is suspected.  

EASA reminds pilots that the variability in a generator airplane’s rate of climb or descent makes it difficult to estimate exactly where the vortex is. Pilots should expect possible wake encounters when other traffic in the vicinity appears to be on similar tracks ahead, crossing above your level, climbing or descending ahead through your flight path. Consider requesting a change of flight level to cross heavier traffic from above if feasible.  

It is important for pilots to have a high situational awareness with respect to other traffic and the meteorological conditions. Tools available to pilots include the cockpit navigation display, radio communications and significant weather charts for tropopause information.

In addition, pilot eye scans of the atmospheric environment, to include watching the development and dissipation of condensation trails, can help flight crews visualize wakes and estimate if their flight paths will cross.

Encountering Wake Turbulence At High Altitude, Part 1, https://aviationweek.com/business-aviation/safety-ops-regulation/encoun…

Encountering Wake Turbulence At High Altitude, Part 2: https://aviationweek.com/business-aviation/safety-ops-regulation/encoun…