In RVSM conditions, if either primary altimeter fails you no longer meet RVSM, must declare the situation and descend below 29,000 feet when cleared. Fair enough...
I suppose the UKCAA are then looking at the situation where the remaining primary is more accurate than the standby and there may be a split in the indications. The correction chart would allow the standby altimeter reading to be corrected to enable cross comparison of the two. Lord knows why, but that seems the most reasonable guess. We never had to do this on our B757s or B767s although the same situation applies. Perhaps the B727 standby instrument is known to have a particularly dodgy auxiliary static system?
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Through difficulties to the cinema