The manual requires a enroute climb-penalty of almost 8 tonnes to be applied. Anyone care to guess why?
As you've mentioned FWSOV I will assume a Rolls Royce installation - but same for other engine types.
The Nacelle Anti-Ice valve is a pressure-regulating type valve. In normal operation it only lets through enough air to maintain 20-28psi downstream of the valve. This only "robs" the minimum amount of air from the compressor.
However, when the valve is 'locked open' it discharges the maximum amount of compressor air available in the anti-ice duct. i.e its like having a 3.5" hole in your IP compressor. Makes it very inefficient, which in turn has a dramatic effect on HP compressor efficiency (and therefore thrust/EGT limit).
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