I think the situation he's describing is the exact situation I find myself in. I took the written about a few days before the new rules went into effect, and right now I only exclusively fly single engine aircraft (though I'm single digits shy of 500 ME). The question being, can one take the practical in a single engine aircraft, get an ATP cert with SE privileges, and then sometime later on down the line apply for a ME rating as an addition to an existing certificate and not have to take a second written? To me, the answer that would make the most sense is "yes," otherwise the FAA is treating the ATP class ratings as essentially two completely different certificates.
It's actually not the FAA doing it - in the aftermath of Colgan, Congress specifically told the FAA to increase certification and training requirements for the ATP multi-engine rating (the one airline pilots have).
Be that as it may, I think what is relatively clear is that, if you get a single-engine ATP and later want to get a ME rating added to it, you will have to take both the CTP and the ATP-ME written. In that scenario, it's really the CPT and the time and expense going into that which is the big deal.
The two provisions I think are the relevant ones are:
ATP-AME to ATP-ASE: 61.165(e)
(e)
Additional class rating within the same aircraft category.
Except as provided in paragraph (f) of this section, a person applying for an airline transport pilot certificate with an additional class rating who holds an airline transport certificate in the same aircraft category must --
(1) Meet the eligibility requirements of Sec. 61.153, except paragraph (g) of that section;
[(2-4) other requirements]
(You are watching me think out loud. Scary, huh? Obviously, you can't take this to the bank or even buy a cup of coffee) 61.153 is the list of experience requirements including the certified course for the multi. Paragraph (g)in 61.153 is the requirement for the written test. This makes it sound like what I originally thought - if you already have an ATP-ME, you do not need to take the ATP-SE written for the add-on.
OTOH, it doesn't work the other way
ATP-ASE to ATP-AME: 61.165(f)
(f)
Adding a multiengine class rating or airplane type rating to an airline transport pilot certificate with a single engine class rating. A person applying to add a multiengine class rating or airplane type rating to an airline transport pilot certificate with an airplane category single engine class rating must --
(1) Meet the eligibility requirements of Sec. 61.153;
(2) After July 31, 2014, pass a required knowledge test on the aeronautical knowledge areas of § 61.155(c), as applicable to multiengine airplanes;
[(3-5) other requirements]
Notice, no written test exemption here and definitely no CPT exemption.
Most people rushed to take the ATP written to avoid the CPT for the multi, not to avoid the written; might as well use it for that if you can.
