It is to prevent resonance that will damage the turbine wheel. It is only applicable to certain third stage wheels which will eventually have to be removed from service. Transients through the range are permitted but steady state operation in that range above 85 shp are restricted to 60 seconds. See RR CEB A 1400, it explains the replacement requirements.
Last edited by Gemini Twin; 23rd Aug 2012 at 16:57.