I couldn't find what 'Mach 1.0' and 'Mach 0.8' are trying to indicate in the middle of the chart.
Can anybody explain about this please?
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As I read it, the upper arch pointed to by
Mach 1.0 represents the correction necessary at the different pressure altitudes when flying at Mach 1.0, while the lower arch pointed to by
Mach 0.8 represent the correction necessary at those pressure altitudes when flying at Mach 0.8.
i.e. the correction necessary at 30,000 feet at Mach 1.0 is roughly -30, while the correction at that same altitude but Mach 0.8 is only roughly -15.
The M 1 curve is uncorrected calibrated airspeed at the speed of sound at various pressure altitudes and is the limit of the chart, and the M .8 curve is the uncorrected calibrated airspeed at M .8 at the various pressure altitudes, roughly (M# is actually a function of temperature; pressure alt is used as a substitute based on standard atmosphere temperature and lapse rate).
They are just there for reference. If use of the chart required the precise plotting of Mach # as a parameter, there would be an array of Mach # plots, not just the one at .8.