(a) The critical speeds of any shafting must be determined by
demonstration except that analytical methods may be used if reliable
methods of analysis are available for the particular design.
(b) If any critical speed lies within, or close to, the operating
ranges for idling, power-on, and autorotative conditions, the stresses
occurring at that speed must be within safe limits. This must be shown by
(c) If analytical methods are used and show that no critical speed lies
within the permissible operating ranges, the margins between the
calculated critical speeds and the limits of the allowable operating
ranges must be adequate to allow for possible variations between the
computed and actual values.