Windshear

It’s almost impossible to have missed the fact that changes in our weather is leading to more challenging situations for pilots. One of those is windshear. This article from easyJet provides an overview of some of the key risks and mitigations you should consider in your organisation. If you are interested to learn more about windshear in general, check out these articles from Airbus Safety First and ANAC – Brazilian Civil Aviation Authority.

The annual European storm season

It's not something that happens just at a specific time of year, it can be continual throughout the year. Most importantly it creates some major risks for the aviation community. The European Storm Season is an annual process that is run by the National Mereological Offices of Europe and runs from 1 September to 31 August each year. So far there have been 27 major storms this season with innocent sounding names like Claudio or Philomena.

A number of these storms had very high wind speeds with gusts in excess of 100 kts. Landing in bad weather conditions poses numerous threats, some of the more common ones include windshear or an overspeed while configuring for landing or during a go-around.

This article from easyJet identifies some of the risks associated with windshear escape manouevres based on internal safety reports and summarises key messages for when encountering these conditions. Occurrence reporting is important to help us all learn how to handle challenging environmental conditions.

Windshear

Weather has often been a key contributor to loss of control inflight accidents, in most of the cases involving thunderstorms or icing, while windshear is also one of the most significant threats. It is generally defined as a sharp change of wind direction and/or speed.

There are several causes of windshear:

• Thunderstorms and associated microbursts; this is the most severe phenomenon and may affect the aircraft at a critically low altitude. 
• Obstacles (e.g, mountains or buildings); disrupting the airflow.
• Wake vortices; which can create severe turbulence or severe up and down drafts.
• See breezes or strong temperature inversions; which can cause a sharp IAS variation resulting in an instant loss or gain of energy
• Wind change caused by a frontal system.

While windshear may be encountered at any time of the year, higher risk events are typically observed when associated with low pressure systems and sustained high winds.

Preventing identified risks

Not applying the reactive windshear memory items if the warning triggers.

When trained in the simulator, windshear events are commonly associated with turbulence. However, events on the line can occur under perceived calm conditions, leaving little opportunity for anticipation. Even under such conditions, the warning should be trusted, as the AoA has usually changed considerably, undetected by the crew. Therefore, it is not the time to diagnose the situation. The memory items should be applied promptly. If the PF does not respond (perhaps through cognitive incapacitation or startle), PM is expected to prompt, direct or intervene to ensure the windshear escape manoeuvre is actioned. A key factor is knowledge of how the system operates and this is obviously different for each aircraft types. On the easyjet Airbus’, once a reactive  windshear alert triggers, windshear conditions are latched by the flight computers for 15 seconds. The end of the audio warning “WINDSHEAR” (repeated 3 times) does not mean that windshear has ceased. The message “WINDSHEAR” on PFD is latched for 15 sec. At the end of the 15 sec period, the audio warning will repeat if windshear is still detected. However, we cannot delay reaction to see if the warning repeats, due to the risk of inaction.

Reactive W/S memory items applied incorrectly

This can relate to configuration changes (such as Gear and/or Flaps) being performed during the windshear encounter. The Pilot Monitoring plays a significant role to ensure that the correct memory items and call outs are performed, monitoring the application of TOGA thrust and pitch flown to SRS or target as stated in FCOM. If asked by Pilot Flying for a configuration change while in windshear, the request should be challenged.  Therefore, good communication is essential to keep a common mental model. Once the flight crew agrees that windshear has ceased, configuration can be changed. Do not change aircraft configuration during a windshear encounter! Retracting flaps results in an instant loss of lift and, on some aircraft types, raising the gear will initially add drag due the gear doors operation.

Confusion on crew actions and callouts following the activation of a predictive windshear warning

In a few cases, memory items relating to a reactive warning have observed to be performed following a predictive warning or caution. This results in the configuration being maintained (e.g. Gear Down, CONF FULL), while a clean-up in anticipation of an actual windshear encounter would have been beneficial. 

Transition from a W/S escape manoeuvre to a normal climb or Go-Around

It is worth highlighting that there are effectively two phases: first the windshear escape manoeuvre and then the transition to a climb or Go-Around. This transition can be very challenging as the gain of energy when the aircraft exits the windshear conditions can easily be confused with an effect of windshear or turbulence, bringing risks of overspeed, altitude busts or Go-Arounds flown not iaw SOPs. Keep the FMAs in the scan – when out of windshear and above acceleration altitude the mode will change from SRS. The aircraft pitch demand will result in acceleration which can be misinterpreted as persisting windshear. Because time to act is limited, briefing beforehand with an emphasis on the PM’s role and actions will help to successfully manage the transition. What flight parameters will the PM monitor? Which callout will be used to indicate that windshear conditions have been escaped from? Which callout will the PF use to initiate a Go-Around or normal climb? How will the PM prompt or intervene if PF’s response is not correct? As a reminder, windshear and associated procedures are described in our Airbus manuals (FCOM and FCTM). The FCOM describes the memory items to perform in case of a windshear encounter and the associated callout “Windshear TOGA”. The FCTM contains guidance and techniques.

The key messages for handling Windshear 

Avoidance

• Assess the conditions for a safe take-off or approach-and-landing based on all the available meteorological data, visual observations and onboard equipment. 
• As far as possible, delay the take-off or the approach, or divert to a more suitable airport.
• Be “go-around minded” when flying an approach under reported wind shear conditions.
• Be prepared and committed to respond immediately to a predictive wind shear caution or warning.

Recognition

• Be alert to recognize a potential or existing windshear condition based on all available weather data, on-board equipment indications and on the monitoring of the aircraft flight parameters and flight path.
• Scan instruments for evidence of impending windshear.

Recovery

• If a wind shear warning occurs, apply the recommended FCOM recovery / escape procedure i.e. set maximum thrust and follow the FD wind shear recovery / escape pitch guidance.
• Make maximum use of aircraft equipment, such as the flight-path vector (as available).