A Line Mechanic reports he had to stop Deicing procedures twice; on an ATR-42 aircraft; because the right engine was running in Hotel mode; even though the Captain demanded he continue.

Narrative

December 2011. I; Mechanic X; in ZZZ; was approached by the First Officer on an ATR-42 aircraft and he told me that the mainline Air Carrier would need me to pull the Ground Power Unit (GPU); I told him OK. Around a half hour later; after the Load team finished loading the aircraft; I noticed that the aircraft was in Hotel mode and had not been deiced yet. So I went to the Captain before the door was closed and I told him that he could not deice the aircraft in Hotel mode on that side of the engine. He said he has done this all the time.So I got out of the aircraft and I called Maintenance Trainer Mr. Y; to ask him about this procedure and he said that the right side should not be done in Hotel mode; so I went back to the aircraft and handed the phone to the Captain and he hung it up. So I stopped the Deice crew and I got back in the aircraft and told him my Trainer Mr. Y said that it should not be done. He told me he's the ***** Pilot in Command (PIC) and he will do what he wants to; so I told him don't curse me; I don't care who you are; but don't curse me. I then got out of his aircraft and let him do what he wanted to do. I thought we were doing Battery starts. The mainline Air Carrier Deice crew came up to me and said they have never deiced in Hotel mode and they were also trained by our Trainer Mr. Y; not to deice the right-side engine [when] in Hotel [mode] or with props turning.All I am trying to do is keep the people safe out on the ramp; and that is what Mr. Y said our job is; to overlook the operation. The following day; I called ZZZ1 and ZZZ2 Line Station Managers. There was a question if there was precipitation on the wings and yes there was visible precipitation on the aircraft. The PIC also had requested to be deiced by the mainline Deice crew. The deice operation was stopped by me in ZZZ twice; because the Deicing/ Anti-icing Manual Vol VI; 14 CFR Part 121 and Company Operations Manual 121-000-000 states under a Caution; 'Prior to Hotel mode activation; the right engine intake; nacelle; propeller and wing will have all snow removed and deiced.' The right-hand engine though was already in Hotel mode and the engine was running before the deice process began. Suggest the pilots should follow the procedure in the Volume-6 Deicing Manual and not curse at the mechanics when we are trying to do the correct procedure.