I think, in simple terms, there are three types of overloading:
- Above the RFM absolute TOM limits - with or without immediate consequences
- Outside the RFM C of G range
- Outside of the required performance range.
1. Will cause - as a minimum - reduced component life for both the airframe and engines, which may cause premature failure prior to their normal life. This may be minor, but costly - such as cracking in structural elements of the airframe - or there could be catastrophic failure at some later point in a component's life. The latter may take an entirely innocent pilot and pax load with it!
2. May not have obvious consequences, but will also include similar effects as above. In a more severe state, though, it may lead to loss of control due to lack of control authority.
3. I would think the effects are obvious, but for some graphic examples, please see the following video clips (with credit to Gary Spender's website):
Video Player » Griffin Helicopters | Mil Helicopter Crashes in the street
Video Player » Griffin Helicopters | A 206LT TwinRanger crashes trying to get
The other element to consider is that any pilot overloading an aircraft - with the exception of incorrectly manifested loads (for whatever reason) - is either a) ignorant of what they are doing or, b) recklessly ignoring limitations. You then have to consider how those traits manifest themselves in other areas of their flying!
Sorry - no anecdotes
