Low-Level Wind Shear

Operations Duty Managers

Wind shear events are often tactical events that are difficult to anticipate.

• The GFA is the primary source and when indicated Operations Duty Managers may use low-level wind plots to provide better temporal resolution. These charts may be used in supervisor briefings and will usually be printed for quick reference to the affected specialties.
• ATC is advised of wind shear by flight crew (PIREPs).

Wind shear can create many challenges.

• Active runway changes.
• Tolerance for crosswinds can be reduced, particularly if runways are also wet or contaminated.
• Increased spacing between aircraft leading to reduced airport arrival rates.
• Risk of aircraft diversions.

Regional airports may struggle with wind shear as some only have a single runway to offer landing and departing traffic. While diversions are known to occur due to low level wind shear it is uncommon.

Compression on final is a discussion point and requires close monitoring with terminal ATC. NTMU/terminal supervisors will adjust spacing to account for compression.

• Depending on demand, additional spacing requirements can lead to the implementation of TMIs to account for the loss of throughput at a standard arrival rate.
• This is where discussions with CMAC forecasters become key: understanding the start and end times of the strongest low-level wind shear, the forecaster’s confidence, and potential variability in conditions help from a planning perspective and allow for adjustments to be made strategically, both from the flow management side and also managing the number of open sectors and anticipated workload for terminal ATC.
• Compression is often seen in CYUL with the impact of the Saint-Lawrence River valley keeping surface winds from the NE while winds aloft associated with the low-level jet are from the S/SW. This creates strong tailwinds followed by headwinds on final and generates the compression and potential go-around scenarios.

Low-level wind shear can also lead to go-arounds and the need to hold aircraft refusing an approach. This can increase the workload and lower the throughput in any given hour, and can lead to flow issues in periods of high demand. Understanding where the wind shear is, how intense it will be, and how long it will last is a priority from a Operations Duty Manager’s planning perspective when working major hubs.

Low level wind shear usually comes to NTMU’s attention via the terminal supervisors as it causes the requirement of increased spacing.

When wind shear on approach occurs, there is a potential of the loss of lift. Usually two things will occur, the catch-up factor on approach will increase and aircraft will fly a faster air speed on approach but will slow down considerably on final, giving have vastly different ground speeds at different altitudes and the risk of compression.

• This means they will require greater spacing when turning final to ensure minimum separation over the threshold.
• This alone should not affect rate, as rate is calculated by the space required between aircraft over the threshold of the runway. However, these conditions are always very variable, and the space needed from the start of approach varies greatly and is not very predictable, so increased spacing is required to ensure minimums over the threshold usually.
• Additionally, the increased spacing usually leads to aircraft having a longer and longer final (pushing out of the terminal approach box) making it difficult for the arrival controller to maintain his aircraft within his control space.
• Overall, this usually leads to lower rates. The bigger the change in ground speed due wind shear the bigger the effect will be. Additionally, the speed of the wind on the ground, as depending how fast the wind speed is on the ground, it may dictate the runway as usually we land into wind, so usually it will be increased performance commencing final approach (being pushed) and decreased performance nearer the threshold (headwind) causing catchup.
• However, if the winds on the ground are calm or very low, it may be beneficial to have the head wind commencing final and which allows for decreased performance then increased performance and less speed differential which allows for less impact on the operation.

ATC (Major Tower)

• Heightened risk for take-off rejection due to wind shear warnings (especially the case with the new C-series), which increases workload for tower ATC.
• Risk also goes up for go-arounds and overshoots, which has the same impact in increasing controller workload.
• Increased workload passing wind shear PIREP to subsequent aircraft.
• Understanding when conditions are expected to start and end is crucial for ideal situational awareness and staffing.

ATC (Regional Tower)

CYAM (Sault-Ste-Marie, ON) example: if reported by an arriving aircraft this information is relayed to YZ ACC (area control centre) and other aircraft in vicinity per ATC requirements. At CYAM we’ll often see surface winds below 10KT but above 30KT at Circuit Altitude (1000 AGL). This can impact VFR circuit sizing and speeds but doesn’t often correlate with a low-level wind shear report.

Low-level wind shear is one of the more dangerous threats to aircraft as this phenomenon is invisible and can occur under numerous different atmospheric and geographical circumstances. When L LVL WS occurs in the immediate vicinity of an airport, it will impact aircraft during the most critical phases of flight, departure and approach. It is vital that any aviation stakeholder that encounters this condition must swiftly report it to the nearest flight service station or FIC, so that an Urgent PIREP can be issued. With this critical safety information at hand, CMAC forecasters can take action by issuing a SIGMET and closely examining their existing forecasts for any required amendments.

Some indicators for L LVL WS development are easy to detect: thunderstorms, particularly those with high bases over 6000ft AGL, can generate microbursts. With isolated “airmass” thunderstorms, one can easily see these massive downdrafts and the resulting outflow – which is one form of L LVL WS. With more organized frontal thunderstorms or larger mesoscale convective systems, rain and mist frequently obscure visibility to the extent that one might easily fly straight into a microburst. Bear in mind that these are the obvious forms of L LVL WS generation.

Other conditions, unique to designated mountainous areas, can lead to the formation of nocturnal low-level jet streams. This phenomenon is often times associated with low-pressure systems and also common during summer months along the southern aspects of the Rocky Mountains in Alberta, and during winter months further north, along the Mackenzie Valley in the Northwest Territories. Fortunately, this type of L LVL WS occurs under vertically stable conditions (no convection present) and tends to be perceived as a very strong, steady wind once you’ve entered the jet. The pilot of an affected aircraft will note strange performance characteristics (dramatic increased or decreased performance, with little or no turbulence). Pilots may find this startling if not forewarned and may encounter difficulties with excessive unplanned fuel consumption as a result of extra power and extended manoeuvring required to safely extricate their aircraft from the jet flow.

FIC

An FSS working in a FIC must have an excellent understanding of theoretical meteorology and the geography of their AOR, in order to understand how L LVL WS may be formed under different seasonal or even diurnal variations at airports.

Low-level wind shear impacts terminal controllers. As soon as a wind shear report is received all subsequent arrivals must be advised and the tower is advised to pass along information to departures.

• Higher possibility of go-arounds due to low-level wind shear. Pilots may even start refusing the approach which can lead to significant capacity challenges with multiple holds required.
• Low-level wind shear also often occurs with other significant weather (snow, freezing precipitation, thunderstorms) and can exacerbate the dangers associated with its occurrence.

ATC must also be aware of how this will impact workload.

• On approach, aircraft might not want to reduce their airspeed too much or too early, so ATC must make sure to leave additional room between aircraft to allow them to fly the airspeed they want while avoiding separation loss if an aircraft is unable to reduce speed due to wind shear. This additional spacing requirement can impact capacity negatively as it lowers the hourly throughput possible at the airport.
• Wind shear also affects a few of terminal ATC vectoring rules. It’s not the only condition, but having no low level wind shear is one of the conditions that allows ATC to vector aircraft to a shorter final than the standard interception point, which would be 1 or 2 miles before glideslope intercept.
• For example, with good visibility and ceiling conditions, ATC are allowed to tell a pilot “You’ll be joining the final 8nm from the threshold”, and as long as the pilot is okay with it, there is no problem. However, as soon as there is suspected or confirmed wind shear, ATC can no longer do this and must vector the aircraft to join the final at least 1 or 2 miles before the glideslope intercept.

CYUL-specific: wind shear is often seen with surface north-easterlies (for example 060/15), and a strong tailwind from the southwest on approach (for example 240/40). This happens quite frequently. There isn’t much that can be done about it, but every time aircraft have high ground speeds on final, along with a strong headwind on the ground, ATC know that wind shear is a possibility, though a smooth transition may also occur.

• ATC will advise pilots if preceding aircraft report no shear, “expect a strong tailwind on the approach, negative wind shear” on initial contact, to aid situational awareness.

• Low-level wind shear is forecast but controllers mostly react to it when confirmed by pilots. The main impacts at the major hubs are possible overshoots and often a necessity for extra spacing on the localizer since aircraft can lose a lot of ground speed at low altitudes. The pilots must be provided all known information about any possible wind shear, and it is expected that operations will not run as smoothly or efficiently as normal.
• En-route controllers are made aware of low-level wind shear through PIREPS, either transmitted through the automated system or via a call from the terminal sectors. Controllers will pass information along to pilots so they can prepare and will often be asked to provide extra spacing between incoming aircraft to help the terminal deal with the low-level speed decreases. If the wind shear is strong enough to cause overshoots at the airport, en-route controllers may have to hold inbound aircraft to alleviate the stress on the terminal.
• Smaller regional airports will have low-level wind shear information transmitted by PIREPs. Controllers will ensure that any inbound aircraft to a regional airport will have the most up-to-date information so they can plan their approach. Smaller aircraft may choose to divert to an alternate destination, especially if the wind shear is combined with marginal weather conditions.

Pilots are advised of low-level wind shear on the ATIS (low-level wind shear advisories in effect) or often by proceeding landing, departing aircraft, or ATC (Tower Flight Service Station (FSS)). There are different levels of low-level wind shear. It is not uncommon to hear an aircraft report a 10kt loss at 1,000ft or something similar.

Low-level wind shear associated with strong surface winds at locations susceptible to mechanical turbulence is also common and expected at many airports such as CYTZ (Billy Bishop, ON) with a northwest wind landing runway 26 and many of the mountainous airports. Low-level wind shear reports in the vicinity of convective weather require particular attention as microbursts are capable of exceeding aircraft performance.

Mitigations:

• Takeoff - Pilots will use the lowest takeoff flap setting possible, full rated thrust versus reduced thrust and if runway length allows, for a longer take off run to build extra airspeed prior to rotating the aircraft. This procedure ensures the aircraft becomes airborne in the highest energy state possible. For reports of a severe wind shear, takeoff would be delayed. Takeoffs in the vicinity of severe weather may need to be delayed as well.
• Landing - Pilots will use the lowest landing flap possible and add the maximum (10kts in our operation) airspeed additive to V_REF to maintain a high energy state. Landings in the vicinity of severe weather are delayed as well. A report of wind shear from a proceeding aircraft will usually result in a decision to conduct an early go-around.
• Maintaining a high energy state allows for the energy to be traded off for climb performance if a decreasing performance wind shear is encountered.
• Jazz jet aircraft have wind shear warning systems that will advise the flight crew of an increasing or decreasing performance wind shear. Standard operating procedure is to conduct a go-around for both warnings as there is a concern an increasing performance wind shear could turn into a decreasing performance wind shear and catch the aircraft in the worst possible state (power at idle).
• Dash 8 aircraft do not have a wind shear warning system and use aircraft performance to determine when wind shear is encountered.
• On some aircraft, onboard weather radar has the ability to assess if wind shear is possible ahead and provide crews with a warning. If this occurs prior to V1 crews can safely reject the takeoff or if during landing a missed approach may be required.
• If the standard go-around power is not sufficient to arrest the rate of descent and ground contact is imminent crews will use emergency power, maintain the aircraft configuration and attempt to fly out of the warning.

Low-level wind shear affects general aviation (GA) pilots mainly during takeoff, approach and landing.

• Many GA pilots don’t check the GFA, or winds aloft and only base their decision making on the METAR/TAF, especially if it’s just a local flight and not a navigation. It is essential to promote safe and proper preflight planning techniques, including the proper assessment of available forecast products and consultation with the local FIC.
• Low-level wind shear can sometimes be indirectly identified by comparing surface and upper winds as well as by identifying the presence of a low-level jet. When possible, a METAR or PIREP are good indications of wind shear, however, when neither are available, all other weather products and services should be used to properly evaluate winds.
• All GA pilots should be aware of the effects of wind shear, especially during takeoff and landing. Pilots are commonly taught as a rule of thumb, that when they might be in the presence of strong, gusting or changing winds, they should increase their airspeed by a certain amount relative to the change in wind speed, and this of course would apply in the case of low-level wind shear. The risk is that this rule of thumb doesn’t apply to every situation.
• It is helpful in creating a larger safety margin for the case of decreasing performance wind shear, but for increasing performance wind shear, pilots may not realize the impact during their approach, they will likely end up approaching too high and if this was unexpected, they may not take immediate corrective action, resulting in a decision to overshoot.
• Furthermore, a less experienced pilot may try to return to their initial approach path by overcorrecting and/over controlling the aircraft, creating an increase in risk factors.
• A good GA pilot will consider the change in speed and direction of the wind before the approach phase and will create a plan of what actions they will take if/when they encounter it. However, many less experienced pilots will not consider this and will simply react to the change, which reduces their safety margins.
• Floatplane pilots in particular may not have access to METARs and TAFs and may look for alternate wind forecasting tools commonly found on the internet. Often an entire suite of weather forecasting products are used to provide a complete picture of the wind forecast, including use of upper air wind speed and direction forecasts.