It is what is in the approved company opas manual that counts.
Generally the ops manual will comply with the appropriate CAP and/or EU ops.
I have not come across any commercial operators where instruction either in the aircraft (of any weight) or in the sim was not counted towards duty time limitations.
In some airlines, it is the weekly, monthly and annual hour restrictions that limit the hours available to instruct.
However, where those are not limiting, pilots still have to respect the requirements in respect of "days off" and they will find that their company requires that their outside activities do not restrict their availability for maximum duty times.
Pilots also have to respect the law in terms of turning up for work fully rested. It is unlikely that if you complete a busy series of days with an airline and are rostered on a series of rest days that you could claim to be fully rested when turning up at a flight school to work on your first day off.
It is also the case that if you work on your days off that you can not claim to be fully rested at your report time for the next series of days you work for an airline.
This simply means that usually the first 1 or 2 days off have to be just that -off and the 1 or 2 days prior to the next shift pattern have to be fully off also.
Your company also has to be aware of your duty times so that they can ensure compliance with the rules. This means that you have to tell them of the hours spent instructing.
Finally, EU Ops requirements have changed the way that rest is calculated. In the past rest started at the end of the duty period and provided that you had the required amount of rest you were available to work any time from the end of that rest.
Now the case is different in that rest ends at the start of the next duty period and starts an amount prior to that depending on the length of the previous duty.
Imagine you are expected to work for an airline starting at 0700 on Saturday. If you spend 12 hours involved in instructing on the friday then the company is responsible for ensuring that you were on rest from 1900 Friday night. How do they do that?
The only way that I can see a company letting you instruct on your days off is if you have enough duty free days between each period of "work" to allow the company maximum duty times at each end without reaching the 7 day limit. On a 5 on 4 off cycle, you basically can't do it because 12 hours duty 2 days before your airline working period starts reduces the airlines available hours by 12 in that 7 day period!!
I am sure that someone can find a way round the limits but I can't at the moment or at least not a proactical one.
No point is making onself availble next Tuesday to fly some IR students and then have to ring up Monday morning and cancel because the duty times you have done mean that flying Tuesday would put you over the limit!!
The only answer is to find an airline that gives you enough days off between working days so that what you do on your middle days off has no effect of their duty times. Having less duty hours for the same pay helps also!
Regards,
DFC