Hi,
To convert a JAA PPL to an FAA PPL you have two choices:
1) Obtain an FAA PPL on the basis of your JAA PPL validity (FAR 61.75)
2) Obtain an FAA Class 3 or higher medical and obtain passes in the FAA PPL Written Test and the FAA PPL Checkride (inc Oral).
From then on it gets a little tricky...
Firstly for the FAA ratings.
FAA CPL checkrides are valid for that particular category only.
So to have FAA CPL privilages on both single and multi engine aeroplanes you will have to submit yourself to two FAA CPL checkrides, one on a single and the other on a multi. This is the case even if you had a multi engine rating on your FAA PPL. If you do the FAA CPL checkride on a multi only then you will be limited to multi engine CPL privilages but only PPL privilages on a single.
You require an FAA CPL Single Engine certificate to add a FAA CFI rating. This is the basic instructor rating under the FAA system. To teach FAA IR's and FAA Multi you will require the FAA CFII and FAA MEI ratings accordingly.
My situation is that I had an FAA PPL based upon my JAA PPL. I then added an FAA IR to this and later upgraded this to a non-restricted FAA CPL/IR. This was all on Single Engine aeroplanes. I then added a multi rating to this and as I already had an FAA CPL/IR on singles, I had to take the FAA CPL checkride on the multi and also carried out instrument work to allow IR privilages.
Now the JAR way...
If you have a JAA PPL/Multi and carry out a JAA CPL flight test on any type (ie single, multi etc), this will automatically give you CPL privilages on the other. Ie if you do a JAA CPL on a single and already had a multi rating on your JAA PPL then you will automatically be given the CPL privilages on the multi too.
The JAA instructor ratings are completely different to the FAA instructor ratings. Under JAA you can obtain an FI(R) and after experience this will be upgraded to a full FI. You can add night, applied instrument and aerobatic privilages to the FI rating. Other instructor ratings are IRI etc but this applies to experienced airline crew etc who want to teach IR's but don't hold an FI rating.
If you complete the cpl multi ir and then do the faa commercial in a single does this mean you can fly a twin under faa?
I gather you mean if you complete the JAA CPL/IR (with Multi/Single) and then do an FAA CPL in a single? If you do then no it doesn't. You will have to do an FAA CPL checkride on a multi as well.
Hope this helps.
Charlie Zulu.