Managing Severe Turbulence

An A320 aircraft was facing severe thunderstorms on approach into its destination airport. Trying to find their way to the final approach path, the crew passed the boundary of one of the thunderstorms by approximately 4 NM. The aircraft was suddenly caught by a  significant updraft followed by a downdraft, resulting in  a g-load close to zero and the disconnection of the autopilot. Both pilots were surprised by the shift of the g-loads but they did not react on the sidestick. Assessing and accepting the minor altitude deviations, the flight crew then reengaged the AP and landed safely. There were no injuries to any passengers or crew.

The flight crew actions were in accordance with the  FCTM recommended techniques when encountering turbulence. After the initial updraft and AP disconnection the flight crew resisted the potential instinctive reaction  to use manual inputs on their sidesticks to fight against the turbulence. This limited the risk of over-control on the sidestick, allowing the A320’s flight control laws to cope with the effects of the turbulence. 

The cabin was already secured for landing with everybody seated and seatbelts fastened, which was a key factor in the prevention of injuries to passengers and cabin crew.

Several phenomenons create turbulence. Here is a list of the main contributors and how to anticipate, detect and avoid them when possible.

The first  and most obvious is the convective weather where air is heated by the earth’s surface. Hot air rises and causes strong air displacements. Convection associated with high humidity leads to the formation of thunderstorms that can cause turbulence. 

Flight crew must anticipate any potential route deviation and plan extra fuel to avoid any expected storms shown on the weather forecast  analysis during their pre-flight preparation. The weather forecast should be regularly updated, especially during long haul flights, because meteorological conditions can be changeable.

Storms contain a large quantity of liquid water that can be detected with the on-board weather radar . Knowing the capabilities and limitations of the weather radar installed on the aircraft is essential as well as being familiar with the techniques for using the weather radar to optimize the chance to detect convective weather.

Severe turbulence can be met inside a cumulonimbus cloud. However, it can also be met outside of the cloud as we have seen in the event described above where the aircraft was 4 NM outside the boundary of the storm. As a rule of thumb, storm cells should be avoided by 20 NM laterally and preferably upwind to avoid risk of encountering hail. A storm cell must not be overflown by less than 5000ft separation. Avoiding the storm cell by flying around it is preferred  because turbulence can extend well above the visible top of a cumulonimbus. High vertical expansion cells with top over 25,000 ft should not be overflown due to potential of strong turbulence.

Clear Air Turbulence (CAT) is due to the difference of speed of air masses at high altitude. Severe turbulence is generally encountered at altitudes higher than 15,000ft when flying across  the boundary between the two masses.

The on-board weather radar cannot detect CAT as it does not contain water droplets. Using the weather forecast is the main method to predict when CAT may be encountered during a flight. Flight crews may also be  informed of the potential to encounter CAT from pilot reports sent from aircraft that have previously flown through the affected areas to ATC and to the Operators’ operations control centre. There are turbulence information sharing platforms that have been developed by airlines or third parties to share turbulence data and provide real time information to flight crews about the locations of turbulence.

Windy conditions in mountainous areas can cause air to be directed upwards by the face of the mountain that causes a wave effect downwind of the mountain range. Severe turbulence may be encountered when flying through theses waves. The effects of mountain waves can be felt up to 100 NM downwind of the mountain range and up to the cruise altitude of airliners.

Mountain waves are predictable in certain mountainous areas when there are specific meteorological conditions. it is important that operators inform flight crews when the conditions are likely to cause mountain waves on the planned flight path. Pilot reports are also invaluable to help inform other aircraft that may be approaching an area where there are mountain waves. 

Lenticular clouds (fig.1) observed downwind of a mountain range is a good indicator that mountain waves may be encountered in the area.

The pressure difference between the upper and the lower side of an aircraft’s wing creates a wake vortex at its wing tips. Wake vortex may cause severe turbulence depending on the weight of the aircraft generating the vortices and the distance from it. The typical signature of a severe wake vortex encounter is a small roll initiated in one direction followed by a much more significant roll in the opposite direction.

To reduce the risk of a wake turbulence encounter, the flight crew must comply with the aircraft separation minima.

An upwind lateral offset can be used to avoid entering wake turbulence if the flight crew suspects that the aircraft may encounter it.

Ground obstacles such as mountains or buildings can create turbulence that can affect aircraft trajectory at lower altitudes in windy conditions during takeoff and landing phases. 

Some airports are known to be susceptible to turbulence in certain wind conditions due to its surrounding infrastructure, hills or mountain ranges in close proximity . Operators should ensure that their pilots are kept informed when turbulent conditions are expected at the departure and/or arrival airports. 

Efficient coordination and communication between flight crew and cabin crew is essential to safely manage turbulence. It begins with using common terminology in precise and specific communication, both before and during the flight.

Turbulence is classified into three categories. To ease identification, each category is based on the impact to the aircraft’s trajectory and the effects felt in the cabin. Using common terminology ensures that the flight crew and the cabin crew share the same understanding of the level of turbulence expected. This enables the cabin crew to perform the appropriate duties in order to effectively manage the cabin during turbulence. 

(fig.2) The turbulence scale

The preflight briefing is the opportunity for flight and cabin crews to discuss the forecasted weather and the possible effects on flight conditions together.

The flight crew will inform the cabin crew of  any expected turbulence events  and provide the estimated flight times and locations of possible turbulence. 

When approaching an anticipated area of turbulence:

• The flight crew should advise the cabin crew on how much time is available to secure the cabin and galleys, as well as informing them of the level and expected duration of the turbulence encounter.
• A Passenger Address announcement requesting the passengers to return to their seat and fasten their seatbelt should be made. 
• The cabin crew should ensure they inform the flight crew when the cabin is secured.

When entering an unexpected area of turbulence, the flight crew must switch the seatbelt sign ON and make an announcement to the cabin requesting passengers and crew to fasten seatbelts immediately using the Passenger Address system.

It is important that the flight crew informs the cabin crew when the aircraft is clear of the severe turbulence so that cabin crew can check for passenger injuries,  give first aid if necessary, calm and reassure passengers and check for any cabin damage. The purser should then provide a cabin status to the flight crew detailing the number of injuries and any cabin damage.

Flying through turbulence is sometimes unavoidable despite the best efforts to prevent this. The flight crew must use the recommended procedure to limit the impact of the turbulence on the aircraft’s trajectory and limit the risk of injury to passengers and cabin crew.

Any loose objects in the cockpit must be cleared or secured before entering an area where turbulence is expected. Shoulder harnesses should be firmly fastened and locked.

Autopilot is designed to cope with turbulence and will keep the aircraft close to the intended flight path without the risk of overcorrection. The recommendation is to keep autopilot ON during a turbulence encounter. A pilot may be tempted to “fight against turbulence” when manually flying the aircraft and may overreact to sudden changes in the trajectory in some cases.

The flight crew should consider autopilot disconnection if autopilot does not perform as desired. 

The turbulence penetration speed/Mach, also known as Rough Air speed/Mach (VRA/MRA), can be found in the QRH. This speed provides the best protection against reaching structural limits due to gust effect whilst maintaining a sufficient margin above V_LS.

V_RA/M_RA should be used in severe turbulence. Managed speed can be kept when in light or moderate turbulence. 

On A300/A310 aircraft, the flight crew should disconnect the autothrust and set the target thrust to maintain V_RA/M_RA.

Choosing a lower FL enables the flight crew to increase the aircraft’s margins before buffet onset.

If the autopilot disconnects on a fly-by-wire aircraft, the flight crew can still  utilize the advantages of the fly-by-wire technology to cope with turbulence. If the sidestick remains in its neutral position, the aircraft’s flight control system will compensate for turbulence effects by aiming for a 1g flight path and a constant roll attitude. Therefore, if the pilot is only making careful and considered corrections to counter any significant deviation from the intended flight path, this will allow the flight controls to stabilize the aircraft, whereas continuous pilot sidestick inputs could induce further destabilization.

The pilot must not “fight the turbulence” in manual flight to maintain the aircraft’s trajectory or altitude. Only applying smooth sidestick/control column inputs and allowing some reasonable variations from the intended flight path will reduce the risk of overcorrection that can cause unnecessary accelerations, which may increase the risk of injury to passengers and cabin crew.  

Do not use rudder to counter the turbulence if in manual flight. Violent rudder inputs can cause additional aircraft trajectory destabilization and stress on the aircraft structure. 

The flight crew may observe temporary overspeed situations when encountering severe turbulence due to the changes in wind intensity or direction. The flight crew must not overreact to temporary overspeed excursion since the use of VRA/MRA ensures sufficient margins to structural limits. The recommendation is to keep the autopilot ON and autothrust ON and accept the temporary overspeed excursion. 

The use of autothrust and managed speed in final approach enables the aircraft to benefit from the Ground Speed Mini function that will adapt the managed target speed to the wind variation close to the ground.

The flight crew must make a logbook entry to report any severe turbulence encounter so that maintenance crew are alerted to perform the necessary inspections of the aircraft before the next flight.

It is also recommended to report severe turbulence events to Airbus to assess the effects of the high loads on the aircraft and assess what checks may be necessary before commencing the next flight.

Once advised by the flight crew of an anticipated turbulence, the cabin crew should prioritize their duties based on the time available before the turbulence encounter in order to best prepare the cabin, as per CCOM recommended procedure:

• First, they must stow and secure large items such as trolleys and  remove bottles from the cabin and galley surfaces. Any hot liquid must be safely disposed of  
• The cabin crew must then secure the cabin and ensure all  lavatories are unoccupied
• Once the cabin is secured, the cabin crew must secure the galleys
• Cabin crew must then return to their station, fasten their seatbelt and inform the purser that the passengers and themselves are secured
• Then the purser must inform the flight crew that the cabin is secured.

Most injuries in the cabin happened to passengers or crew members not seated with their seatbelt fastened during severe turbulence. Cabin crews are more exposed to risk of injury due to sudden turbulence because they are often standing during service. The cabin crew must ensure their own personal safety first if sudden severe turbulence is encountered. The cabin crew must take the nearest available seat and securely fasten the seat belt. The nearest seat may be a passenger seat.

When the flight crew confirms that the aircraft is clear of the turbulence, the cabin crew can leave their seat, check with  passengers for any reports of  injury, provide first aid if necessary and reassure other passengers. The cabin should then be checked for damage.

Once the situation assessment is done, the purser must report any injury or damage to the flight crew.