ECDIS in Passage Planning: As wide as possible, as narrow as necessary. The relevance of a correctly set XTD corridor for route monitoring
By Marvin Bielek, Nautical Instructor & Content Team Leader at Safebridge and SafeLearn. Marvin Bielek is an experienced Content Creator, Team Lead, and Project Manager with a demonstrated history of working in the maritime industry. He is skilled in Project Management, Customer Service, Maritime, and Navigation. Marvin is also a strong human resource professional with a Diplom focused in Ship Management from HSB Hochschule Bremen – City University of Applied Sciences.
ECDIS – the new "miracle" solution
The most underestimated features of an ECDIS are usually the voyage monitoring capabilities – it gets pretty busy on the bridge as soon as there is some traffic around, and there might be little time to access information on the ECDIS. Nonetheless, the correct use of such voyage monitoring features starts even before setting sail: during passage planning. While the implementation of electronic navigation led to the adjustments of many procedures and processes on the bridge, it did not reduce the importance of careful passage planning!
One of the most distinguished advantages of a double ECDIS bridge is reducing workload for the navigators. However, this does not mean that a safe voyage can be planned in five minutes. The track must be planned thoroughly to facilitate a voyage – including careful placement of waypoints, an accurate arrangement of turns, and reasonable adjustment of the cross-track distance corridor: XTD.
Passage Planning then and now
“Traditional” route planning on paper charts focused a lot on finding the best place for waypoints. A digital tool, ECDIS, simplifies this process, as points can be moved and adjusted with just a few clicks – instead of erasing them with rubber and redrawing them. On the other hand, once all waypoints were drawn and connected, the route creation was completed. Additional objects, like no-go areas, were drawn onto the paper chart for a full passage plan.
A route is more than the sum of its waypoints
On ECDIS, there is more to the route. The turn radius has to be adjusted so that each turn can be done with a reasonable rate of turn for the respective planned speed. This takes additional time, and at times the route creation on the ECDIS may take longer than on the paper chart. But the process has one crucial advantage: All thought that was put into the passage plan during its creation does not have to be put when executing the voyage. When executing a turn, the officer in charge will already be provided with important information. It reduces the workload in one of the most critical moments.
The same applies to creating an appropriate cross-track distance corridor. A common mistake is to use the same cross-track distance for the whole route. This often means the XTD corridor’s distance depends on the most confined waters, such as the size of the channels leading out of the port of departure or into the port of arrival. Sometimes this means the corridor will be a few hundred meters only. As soon as the route is in more open waters, this small-sized XTD would be limiting. In reality, the vessel can operate in a much wider and yet still safe corridor around the track line. Limiting this corridor means either:
The navigator will try to stay in the small corridor, limiting his possible actions for collision avoidance and adding additional danger to these situations. The navigator will leave the small corridor to have better choices for collision avoidance maneuvers but then has to carefully check if the surrounding area is safe for navigation – adding additional workload during a critical situation and again increasing the risk. During the automatic route, the issue is that some dangers along the route might not have been detected, as they were outside of the small XTD corridor. Basically, you reduce your capability to detect dangers along the route in open waters.
In some other cases, the planned XTD is wide enough for open waters. However, then the wide corridor does not match the channel size during approaches. In those cases, buoys, beacons, shallows, or even part of land areas might be inside the cross-track corridor. This defeats the purpose of creating a corridor that is safe for navigation. If the navigator cannot rely on the XTD corridor to be safe, the area must be checked with extra care before each maneuver – once again, unnecessarily increasing workload and, therefore, risk.
Of course, unsafe objects inside the XTD corridor would show up during the automatic route check. However, if the XTD is set too wide, the number of messages will be exorbitant, leading to alarm fatigue – a different topic altogether.
How can we overcome this no-win situation?
It is actually relatively easy to overcome this no-win situation, but it involves some work:
“Set up the XTD correctly for each leg!”
This does not mean that no two legs can have the same XTD. If they are within the same buoyed channel, within the same port approach, or both during an ocean passage, of course, they can have the same value. But the XTD value should be appropriate for the area and the conditions there.
If the vessel leaves the port through a river or buoyed channel, the corridor should be as wide as that channel, or the safe part of the river – without including shallows or buoys inside the corridor. Once the vessel is outside of such a channel, the XTD should be wider. There is no “rule of thumb” how wide it can be in a coastal area – but the wider, the better. The limitation is the available safe space. The route – and the XTD corridor – could be limited by a traffic separation scheme or the area’s physical features. If the route is free of such limitations, for example, during an open sea voyage, the relevance of the XTD decreases.
A possible rule of thumb is to make it wide enough to ensure collision avoidance maneuvers can be done following the ship’s ISM and standing orders without leaving the corridor. For example, if the rule is that the navigator should keep 2 nautical miles distance to an oncoming vessel, a corridor of only 1 nautical mile will not work out. Of course, it is not a big risk to leave the corridor in the middle of the ocean – but it doesn’t require much work to have a wide but safe corridor there either!
What about an additional safety margin outside of the XTD corridor?
Many recommendations include an additional safety margin outside of the XTD corridor. The basic idea is that the corridor is used for the free movement of the vessel. When the vessel has to give way, it will then leave the corridor, using the additional safety margin. That is not a bad way to do it. As with most things, there is not “THE ONE WAY” to do it correctly.
If this is the common practice on the ship and all navigators know it and can easily identify this safety margin, it can work perfectly fine. If they are not sure about the safety margin or misinterpret it during a maneuver, it might be dangerous.
Another way is to execute the maneuvers within the corridor. Of course, this could get more difficult if the vessel is already sailing close to the corridor’s outer edge before the maneuver starts. If the vessel is in the middle of the corridor, though, sufficient space is provided and clearly indicated. On the other hand, this might result in navigators sticking exactly to their track line – a bad practice that most of us encountered on one voyage or another.
I can’t count how often I heard a VHF message telling us that we should move – even though we had the right of way! – because the other vessel did not want to leave its track line. As with most things in life, moderation is the way to go! Don’t “hug” your course line – but don’t sail on the outer edge of your corridor either, if you can avoid it.