28 May 2014: intersections of M641 air route and ping rings
On 26 May Australian Transport Safety Bureau (ATSB) published a document "Considerations on defining the search area", including a brief paragraph on Air Routes:
"There is only one published north-south air route in the south-eastern Indian Ocean. Air route M641 connects Cocos Island to Perth and has four waypoints. The air route crosses the area where the four acoustic signals were detected."
The following list includes the four waypoints for the M641 air route:
CC VOR S12 12.0, E096 50.5
IKASA S20 00.0, E103 29.2
UXORA S26 00.0, E109 13.7
MERIB S30 04.0, E112 45.1
The Google Earth snapshots below show the intersections of the M641 air route with the 00:11 UTC ping rings. Two rings are shown: one produced by Duncan Steel (blue arc), and the other (green arc) is taken from the Hishammuddin Hussein charts. The difference between the two rings is 26-27 km (in the area near the current search area).
The snapshots also show circles, with radius of 600 nmi = 690.47 miles, around three locations (RAAF Learmonth, Christmas Island, and Cocos Islands), discussed in previous updates pointing out the equidistant location from the these three centres.
The red lines show segments of the M641 air route, with yellow placemarks showing the waypoints.
A close-up view is below. The IKASA waypoint is very close to the equidistant location. The intersection of the M641 air route with either of two ping rings (Duncan Steel's or Hishammuddin Hussein's) is about 15 km away from the equidistant location, as well as the IKASA waypoint. Several placemarks at the bottom part of the snapshot show approximate locations of acoustic detections by Ocean Shield (#1 and #2 on 6 April 2014), as well as the location of Ocean Shield on 14 May 2014, upon its return to the current search area.
There is no well-defined ping ring for the final (partial / logon) signal from MH370 at 00:19 UTC. The snapshot below approximates this ring by a wider white arc, in order to show the Ocean Shield's locations relative to the 00:19 arc.
In summary, it may be suggested that the current search area was defined by
a) extrapolation of the 00:11 ping rings to 00:19 arc (the wide white arc);
b) considerations of M641 air route as a possible route taken by MH370;
c) proximity of the intersection between the (a) white arc and (b) M641 air route (red line), to a possible hydroacoustic signal detected by Cape Leeuwin station.
Needless to say, in the absence of solid evidence on hydroacoustic signals from Cape Leeuwin, the consideration (b) is the only basis for the current search area. This consideration, however, is just an assumption, and requires further explanations of why MH370 flight would follow the waypoints of M641 air route.