An IR is valid for 12 months from the date of the Skill Test. It may be
revalidated before it expires or
renewed after it has expired. From the holder's point of view, there is no difference between revalidation and renewal so far as the content of the test is concerned, although the validity of the subsequent rating will be different.
If a rating is revalidated within the three months preceding its expiry, the new rating will be valid from the expiry date of the previous one. If, however, the revalidation is done more than 3 months prior to expiry or if the rating is allowed to expire, the new rating will be valid for 12 months from the date of the proficiency check.
Example: IR issued on 1 Jan 2010 and valid until 31 Dec 2010
If the revalidation proficiency check is completed prior to 1 Oct 2010 then the new validity will run for 12 months from the date of the check (e.g. 10 Sep 2010 to 9 Sep 2011).
If the revalidation proficiency check is completed between 1 Oct 2010 and 31 Dec 2010 then the new validity will run from 1 Jan 2011 to 31 Dec 2011
If the renewal proficiency check is completed after 31 Dec 2010 then the new validity will run for 12 months from the date of the check (e.g. 10 Jan 2011 to 9 Jan 2012).
Alternate validations may be completed in a suitably qualified FSTD. Since the first validation (the initial IR Skill Test) is invariably done in the aircraft, it follows that the first revalidation (but not renewal) may be done in an FSTD. Renewals may not be done in an FSTD.
There is nothing to prevent the MEP Class Rating being revalidated (or renewed) in a suitably qualified FSTD. Unfortunately there are,
AFAIK, no MEP devices of sufficient fidelity to qualify under JAR-FSTD(A) for this purpose.