> Here, the aircraft entered that wedge at a large angle and too close to the airport for a standard turn to bring the aircraft into line with the runway while staying within the safe wedge.
The entry was okay. The aircraft (and the others) was on a STAR (as with almost all arrivals into HK due to the complex terrain/airspace/traffic environment), and appears to have flown it correctly. The tight final turn is normal, but is supposed to coincide with LOC capture. (The final STAR waypoints for most arrivals to 25L/R [C didn't exist then], LOTUS/RIVER, are virtually coincident with the LOC capture point, which is probably the detail that caused this issue to be noticed in HK and not elsewhere.)
Since the LOC was not captured the aircraft did not complete the turn but instead transitioned to HDG mode and rolled out of the turn at that heading. It then, despite not having captured LOC, captured GS and descended, at which point attentive pilots+ATC noticed the issue and dealt with it, resulting in either a go around or a correction and completion of the approach.
> It then, despite not having captured LOC, captured GS and descended.
Ouch. Right. Lateral and vertical guidance are somewhat separate.
Part of the problem seems to have come from the Consistent Localizer Capture system.
Boeing has a patent on this.[1] This uses other data sources (GPS and/or inertial guidance, per the patent) that yield lateral position to get the aircraft onto the approach path. It doesn't just use the localizer beam. The beam isn't wide enough for that if approached at a large error angle. But the system apparently tells the pilot that it's in LOC mode and has captured the localizer when it's really still in CLC mode and trying to find the localizer beam.
Or worse. Boeing writes: "Boeing has received reports that suggest, depending on the
geometry and ground speed of the approach, CLC may activate
for such a short time that the three FCMs fail to synchronize
the engaged autopilot mode and fail to transition to the localizer
capture mode. This may result in the aircraft turning to a
localizer intercept angle of approximately 20 degrees and flying
through the localizer on this track, rather than properly
capturing the localizer. “LOC” will remain on the FMA despite
the failed capture and, in some circumstances, the aircraft may
begin descent down the glideslope while 20 degrees off of the
localizer course.”"
So the displays are telling the pilot that the localizer beam has been captured, while in fact, not only is the aircraft not following the localizer, the Consistent Localizer Capture system has given up and the aircraft is just holding a heading. The wrong heading. Is that correct? How can that not be an alarm condition?
This is puzzling, because Fig. 8 seems to indicate more normal control overshoot, where the CLC system is still trying get the aircraft on the proper approach path after an overshoot. That's bad enough; there's no guarantee of terrain clearance off the proper approach path. But it's at least understandable as control system behavior. Dropping all the way back to heading hold while displaying LOC and not alarming is just broken.
Here's some pilot discussion of this topic.[2] At least one pilot reports seeing this failure. Other aircraft have systems to use GPS/INS data to get lined up for approach, but apparently don't have this problem.
(Not a pilot, used to work for an aerospace company.)
The entry was okay. The aircraft (and the others) was on a STAR (as with almost all arrivals into HK due to the complex terrain/airspace/traffic environment), and appears to have flown it correctly. The tight final turn is normal, but is supposed to coincide with LOC capture. (The final STAR waypoints for most arrivals to 25L/R [C didn't exist then], LOTUS/RIVER, are virtually coincident with the LOC capture point, which is probably the detail that caused this issue to be noticed in HK and not elsewhere.)
Since the LOC was not captured the aircraft did not complete the turn but instead transitioned to HDG mode and rolled out of the turn at that heading. It then, despite not having captured LOC, captured GS and descended, at which point attentive pilots+ATC noticed the issue and dealt with it, resulting in either a go around or a correction and completion of the approach.