We found out which airfield the aircraft landed at, and now need to pick the right runway.
Selecting the Runway
From basic flight phase analysis we know when the landing roll took place. Using the recorded heading we take the median heading during the landing roll as the direction of the runway. Note that we don’t take the heading at the point of touchdown as this may be offset due to crosswinds.
The airport database carries a list of the runways at each airport, and it’s a fairly easy process to scan through the list of runways to find those that are close to the recorded landing heading. As ever, we get airports with multiple runways with slightly different headings, or parallel runways, which make life more difficult. A careful selection of tolerances allows us to make a best guess, and ensuring that inoperative runways are not listed in the database also helps.
Where we have a recorded ILS frequency, we can check which runway has an ILS Localizer transmitting on that frequency. This is a very good and robust way to determine the runway used for the approach phase.
This is particularly important where landing on parallel runways, as the navigation system accuracy is often not adequate to determine which runway was the correct one.
Sometimes we find that our calculations for the landing runway and the approach runway give different answers. We carry out a check to make sure the two are parallel, and then recognize this as a runway change. The pilot may have used the ILS signals to guide his approach, but then when he can see the airfield, he will change across to the other runway and fly the landing visually.
In fact, we have found some cases where the crew changed ILS frequency to pick up a signal from the second runway as they stepped across and were descending through 500ft. We boldly assume that this was done while visual, and that no sane crew would change ILS approaches in IMC at such a height. Therefore we assign all the ILS safety event detection to the first approach runway, and assume that the approach and landing on the parallel runway is visual.
Where the ILS localizer is offset from the landing runway, we can monitor the approach phase and then assume that the final departure from the localizer is where the crew turn onto the final landing approach heading, so this is not considered a safety event (the same diversion from the localizer for a normal runway would be a real cause for concern). For this reason the airport database identifies which runways have offset ILS systems and these are treated accordingly.