FlyingOnFumes
Nobel WAR Prize Aspirant
I think to get the Inst. Airplane if you already have Inst. Helo you would need 15hrs instrument fixed wing time, take the oral/practical tests but no written test. There are some additional FW reqs but if you have a C-ASEL/MEL I suspect you've probably already gotten those.
A fixed wing Instrument Rating (if you already have a Multi-Engine Land with Instrument or Rotorcraft Instrument) should be only an "add-on" rating (vs. doing another 40 hours again).
I was not aware that no knowledge exam was required for SEL add-on... but then again... if you already have a commercial SEL with Instrument, getting a MEL with Instrument Commercial is also only an add-on rating with no min. time (just to proficiency as determined by the MEI and passing the practical test with an FAA Inspector or Designated Examiner) and no Knowledge exam.
I could have sworn though that Rotorcraft have a separate set of questions than fixed wing that would make it necessary to take a fixed wing knowledge exam (unless the fixed wing is a subset of the Rotorcraft... i.e. the rotorcraft exam bank includes fixed wing stuff too)...