RUNWAY CHANGES
Several taxying aircraft have had to return to the gate due to weight issues following a runway change or failed to meet a departure restriction enabling them to arrive prior to curfew at destination.
Runway changes for ATC and airport operations are not always predictable, but involve many factors. For instance if an aircraft is taxiing for departure with a critical departure slot time and the runway changes, there is every possibility the aircraft will miss that slot. We will endeavour to stretch the system to accommodate but HK ATC can't operate with the primary focus on a very small number of aircraft. ATC must consider the majority. In the case of a forced runway change, there will be no leeway available.
How do we change runways?
A PLANNED CHANGE
We do two types of runway changes. General criteria is that if downwind exceeds 10KT dry, 5kt wet runway, TWR will initiate a 'change at your convenience' to APP. They'll assess the traffic to see if there is an advantageous gap or least disruption point in the sequence. Sometimes it can be rapid within 10-15 minutes, but more commonly 30-45 minutes. TWR is then issued a clearance expiry time for the old runway. With an absolute minimum of exceptions, it is hard and fast. We are very conscious of the fact that if an aircraft is at the hold and needs to be changed, it will be a considerable delay till departure is possible. This is especially the case for 07. It the aircraft is an eastbound departure, converting to RWY25 will involve an 8 minute taxi, probably 10 minutes of delay at the opposite end until the change is complete and add about 30 track miles or more once the aircraft is airborne to be in a similar point to had it departed 07. So we push the traffic as much as possible. This is one area we'd like to highlight to aircrew regarding the time they waste during the last portion of taxy on to the runway and to eventually roll. Crews think they are expeditious, but they waste significant time. It is simply an awareness issue. If we were to have a 10 aircraft queue to depart off the old runway, every crew must play their part to ensure number 10 gets away. A lot of misses come from the poor actions of number 2, 3 or 4 in the sequence. It's not just down to the last aircraft being expeditious.
A FORCED CHANGE
A very quick change due to the passage of weather where the selected runway is just no longer suitable. They are usually preceded by one or two missed approaches.
Many crews seem perplexed that we are operating on a downwind runway. A case in point during recent bad weather. 8 kts of tailwind on RWY 25 for about an hour and we persisted without changing. If a change had have been initiated, it would have caused a double change as the wind eventually returned to a westerly. The average holding at the time was 20 minutes +. To accommodate a double change under those circumstances in a relatively short space of time, would have incurred a dramatic increase in holding. It really is a considered compromise. Sometimes traffic operating in downwind conditions may just be one of a number in the queue to operate on the old runway and a change is already in the pipeline. As a general rule, ATC considers the impact of a runway change on the performance of departure traffic more than the arrivals.
What latitude is there extend the use of a runway to get one or two away on the old runway to avoid a significant time/ cost penalty to those aircraft.
Regarding the expiry times issued to the TWR, we've been instructed not to hassle APP for alterations and it is consistent practice worldwide. If you have an inbound flow and HKIA is at capacity, a runway change severs the flow and the terminal controllers workload spikes significantly trying to delay traffic and create a new sequence for the next runway in the most expeditious and fair manner. So the dictum is that traffic in the air should always have priority to those on the ground. This will impact many sectors and they are all sequencing with each other. It's as if there are many many cogs and you create a plan to merge them, if you alter the timing it has a huge impact not just on time, but safety. Naturally by favouring one or two you will incur a much greater delay to the inbound sequence and the overall traffic handling of the airport.
Questioning Clearance Delivery on future developments
Putting questions to clearance delivery on expected delays or what runway will be in use in 20-30 minutes, has a very low accuracy value. This position can get overwhelmed with workload. It is most commonly volume of traffic and the extensive co-ordination requirements that obviously are not reflected on the frequency. This position has no control over airspace and therefore no jurisdiction. Final approval comes from the relevant sector that the aircraft will depart in. A restriction may be an existing flow on a specific route, minutes in trail of a specific callsign, or a clearance expiry time to allow the blending of that aircraft with through-area traffic. Aircraft request an update where they'll be ready in 5 minutes, then report ready in 30 and vice versa. From the time a call is made questioning departure restrictions they almost certainly will change. Guangzhou may impose a very restrictive departure flow without warning. If you return to call ready to start and you are now number 5 with 15 minutes between departures, your previous request serves no purpose other than for you to consider that ATC has no idea of what we are doing.
It is common during summer for the terminal controllers to be unable to process traffic in an efficient manner. i.e. they get overloaded and for safety reasons, will quickly restrict the flow of traffic to maintain control. For routes V1-V5 are commonly restricted due to deviations shortly after departure. For V10-V12 it may be a separation requirement departing our airspace.
And the red light for the terminal controllers is "stop departures". All combinations of HKATC, Guangzhou, neighbouring FIR requirements or purely traffic unable to depart due CB's on our very restrictive initial departure tracks.
During adverse weather, delays are caused because aircraft can't or won't fly in accordance with their clearance. Remember this the next time you are in weather and you ask to deviate simply for ride comfort through cumulus cloud or rain. Deviating off track will always have consequences. ATC can have a high volume of traffic prepared to maintain track and a small minority not. The impact on airspace capability varies with the number of deviations. So ATC don't delay aircraft, it is always weather or other traffic delaying aircraft. A small anomaly that often gets overlooked.
The person working clearance delivery usually does not have the time to accurately track the weather conditions nor the intentions of the controllers around them that will make that call. Many people working this position are very junior and many have not yet attended an ATC course. They have limited understanding of the complex interactions surrounding a runway change, other than being directed by the supervisor, coordinator or ground controller.
Delays at HKIA can be ad hoc with little warning. So please before you ask the question of clearance in the future, just be warned the answer you get will most likely have so many variables involved that it is not useful. We contend it is not worthwhile asking. Yet many times on this frequency, the workload is dominated by endless calls of this nature. In effect you are only delaying yourself and others by occupying the one person who can get you moving when it counts.