BLUF - Looking at adding an airplane multi engine ATP rating to my ATP Helicopter rating. It's been a long time since I have applied for a new rating and the regulations are just as confusing now. Currently a ATP-H, CMEL, CSEL, CFI/CFII in airplanes and helicopters. SIC type rating in the Citation. Fly part 135 helicopter SPIFR and airplanes part 91. Looking at the requirements for an add on ATP rating I am having a hard time deciphering if I need to have the 121/135/141/142 classroom training (61.156) since I already hold an ATP. Can any current instructors or DPE's assist? If needed I can get the PIC Citation rating but would rather pick up my ATP airplane rating in my C310.