The relevant Legal Interpretation is Hartzell which reads in part:
The Theriault interpretation reinforces the existing requirement that instrument training used to satisfy the aeronautical experience requirements under §61.129 needs to be clearly documented by the applicant for the commercial pilot certificate. The interpretation dispels the notion that holding an instrument rating is, on it own, sufficient evidence that the applicant has fulfilled the aeronautical experience requirements for a commercial pilot certificate under §61.129. However, we anticipate that for commercial pilot applicants who already hold an instrument rating, the hours of instrument training used to obtain that rating will meet at least some, if not most, or quite often, meet all the requirements for instrument aeronautical experience as required under §61.129. The interpretation did not establish an additive requirement for the number of hours of instrument training required to meet the aeronautical experience requirements of §61.129.
It was reiterated in Oord
To allow for training time to count towards both§ 61.65(e) and§ 61.129(c)(3)(i) in cases where it meets the requirements ofboth, as stated in the letter to Ms. Kristine Hartzell dated December 17, 2010, that time must be logged consistent with§ 61.51 and documented in a manner that demonstrates the time counts towards the commercial pilot certificate and ratings.
In its letter to Ms. Hartzell, the FAA explains it is “merely clarifying the requirement that the applicant for a commercial pilot certicate provide evidence that they have met the requirements of§ 61.129.”
To summarize, if training conducted pursuant to§ 61.65(e) meets the requirements of
§ 61.129(c)(3)(i), that time can count towards the five hours of instrument aeronautical experience under§ 61.129(c)(3)(i). However, pursuant to§ 61.51, that time also must be logged as prescribed allowing for verification by the FAA.
One caveat is highlighted in Rohlfing
Flight instructors who provide flight training on the “control and maneuvering of an airplane solely by reference to the instruments” in§ 61.109 are not required to have an instrument rating on their flight instructor certificate. See Legal Interpretation to Taylor Grayson (Jan. 4, 201 0). Therefore, the 3 hours of flight training on “the control and maneuvering of an airplane solely by reference to instruments” in§ 61.109(a)(3) may be applied toward the 40 hours of actual or simulated instrument time under § 61.65(d)(2), but may not be applied toward the 15 hours of instrument training unless the flight instructor who provided the flight training under § 61.109(a)(3) held an instrument rating on his or her flight instructor cetiificate and otherwise meets the requirements of§ 61.65.