After completing and adding the ARINC 424 NavDb capability to my A320Sim software, the next step was to implement each of the ARINC 424 leg types, also known as Path And Terminator's, as defined in the Attachment 5 of the ARINC 424 specification.
One particular and complex leg type is the Holding Pattern. There are two ways a holding pattern can be flown. Flying right hand turns is the standard holding pattern and flying left hand turns is the non-standard holding pattern.
Entering into a holding pattern is done by flying a holding pattern entry. The following figure describes the 3 different holding pattern entries that exist: direct entry, offset entry (also referred to as teardrop entry) and parallel entry. The heading you are flying to reach the holding fix determines the type of entry that needs to be flown. This is done by dividing in 3 sectors as shown in the figure. Examples of each of the 3 holding pattern entries are also drawn on the figure:
- Sector 1/Parallel Entry: in light dotted line
- Sector 2/Offset (aka Teardrop) Entry: in dark dotted line
- Sector 3/Direct Entry: in dark dot-dash line
In the screen capture video below you can see this in action. For test purposes I am not really flying the (yellow) plane on the Navigation Display (ND), but instead I keep it fixed on the departure airport runway, and vary its heading. This gives the result that the ND is turning around the yellow aircraft and the aircraft itself is always pointing to the top of the screen. This is a handy trick to be able to visualize and test all of the possible headings toward the fix and the resulting entries. As such, the vertical line from the bottom going up to the fix is the heading (as said, always kept vertical).
First I let the heading vary from 0 to 360 for a standard right hand turn holding. Thereafter I change the holding pattern from right hand turns to left hand turns, making it a non-standard holding pattern. For this non-standard holding pattern I then let the heading vary from 360 back to 0.
Watch the entry type switch when we go from one sector to the next sector.