Wind Change Effects

The unavailability of runways due to strong winds at a port is also a major concern for the IOC, as the level of aircraft movements (take-offs and landings) can be reduced significantly. In addition to more predictable winds and their effects, frontal systems moving through can have a significant effect on runway usage and therefore traffic patterns.

At a major airport with two runways at right angles to each other, the level of movements was designated as 100 movements per hour, given that light and variable wind conditions enabled the use of multi-directions for operations. However, after the passage of a frontal system, the prevailing wind strengthened considerably such that only a single-runway operation became possible. Now with only one runway in use, the movement rate was reduced by some 30-40%. As low cloud on the approach required a full instrument approach procedure, the traffic flow was further reduced. This resulted in aircraft having to enter holding patterns at various points en route to the airport, and with the increase in traffic congestion, extensive holding delays began to occur. Whereas the aircraft had departed their origins with sufficient fuel to cater for the weather circumstances, the additional АТС requirements for holding patterns was far in excess of their fuel reserve capability and a number of aircraft diverted as a result. Accordingly, to mitigate the effects of both lengthy delays and the diversions, the IOC began removing a number of flights from the schedules, thereby providing buffers to help absorb the numerous disruptions.

Thunderstorms

In many parts of the world (e.g., the tropics) thunderstorms can occur all year. In other regions, they are more seasonal (e.g., the USA in summer), but of course can occur at any times of the year given appropriate conditions. The nature of thunderstorms can be extremely hazardous both to aircraft in flight as well as to staff on the tarmac. Operationally, they can create turmoil for airlines. Main in-flight disturbances can include severe turbulence, damaging hail, icing, lightning strikes (which may cause minor damage or equipment failures, and often require the aircraft to undergo maintenance checking after landing) and heavy rain. But at and around airports, thunderstorms can also produce tornados, windshear and dangerous microbursts, any of which can affect aircraft taking off and landing. Therefore, crews endeavour to avoid the worst effects of these phenomena for the sake of aircraft safety and passenger comfort. So, if arriving, they will tend to be held by АТС some distance away from the field until the storms have passed or conditions are deemed safe for continuation of movements. A consequence is that this creates a significant increase in air traffic inbound for the airport as explained in the example above. In addition, departing aircraft may be held on the ground by АТС or the crews may elect to delay their departure themselves until the departure route is relatively clear.

Conditions at airports can also be hazardous. When lightning is observed within five nautical miles of an airport, a standard approach is to remove all personnel from the tarmac. In the cases in which lightning is actually striking the ground, all activities cease, to the extent that aircraft will be stopped by АТС wherever they are (even if during pushback or taxiing), and no movements (take-offs or landings) will be permitted. As thunderstorms pass over an airport, conditions on the ground can fluctuate considerably. Winds can change direction and speed abruptly, which can affect runway usage and therefore approach directions and procedures. In some cases, the severity of the weather results in surface winds of a particular strength and direction, but immediately above (say, 1000 ft), the wind could be the opposite direction with far stronger force. This creates havoc for approaches and landings, and often results in missed approaches (go-rounds), which exacerbates the queue of aircraft waiting to land (and take off) and contributes to inevitable diversions.

IOC response

The duration of a thunderstorm is usually from 20 minutes to IV2 hours (for airmass thunderstorms) or several hours (for multi-cell, steady state thunderstorms). More often than not, most flights will operate with the expectation that perhaps after some delay incorporating holding patterns near the destination, landings are probable. Thus, the first IOC response is to gather some information:

• a) What times are thunderstorms predicted at the airport?
• b) How long are they expected to last? (This can influence a decision to delay departure.)
• c) How many aircraft are due to arrive? (If a high number, air traffic holding and congestion are likely to impact further on the schedule.) d) What alternate(s) is/arc nominated?
• e) How long can the aircraft hold before diverting to the alternate? (With thunderstorms passing through an airport, the expectation is that perhaps after some minor holding, there is a strong probability of landing.)

The options include the following:

• a) The most likely action is to dispatch the aircraft on schedule with sufficient fuel to hold (based on the forecast) for an expected period, using the nominated alternate(s) in case of diversion. This may necessitate offloading payload to accommodate the fuel required.
• b) Sometimes, if storm activity is severe and of extended duration, an option may be to hold an aircraft on the ground at the originating port. This is a viable option in the operation of a very short-haul sector, when it is dictated by available slot times, or it is known that traffic build-up at the affected airport is likely to exacerbate holding times considerably.