While the standard Cooper-Harper rating scale is well documented and the way that it should be used is very clear, the case is not the same with all rating scales. The PIO rating scale is one such example.
The most common (standard) PIO rating scale used is the one found in MIL-STD-1797 initially created by Calspan (1981); however the ratings 1-6 are not accompanied by any descriptions, and no extensive guide is found for this scale. Veridian Engineering modified the standard scale (1999) combining it with some descriptions and changing slightly one of its decision tree questions. According to my limited experience, this version of the scale (seen below) can provide more consistent results as the ratings are clearly defined.
The question which arises though, for both the standard scale and for the suggested modified version is the exact definition of “oscillation” and “undesirable motion”. What is the difference?
While (according to my knowledge) it is not officially documented , during the Calspan Variable Stability Training course some interesting points are made on this point and generally on the scale:
1. An undesired motion is an simple overshoot or a small quickly damped cycle.
2. An oscillation is more than half a cycle or one overshoot.
3. The answer to whether an undesirable motion compromised task performance or not is a judgement the test pilot must make.
4. The PIO rating scale is not to replace CHR, but it is an other communication aid between the pilot and the engineers.
An interesting part of a GARTEUR report describing the evolution of the PIO scale and the versions which are currently used is found attached.