MD-11
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Clearance
Chart Or Publication; Human Factors; Procedure
We were filed and flew an overwater route to ANC on B453 airway. I had not operated on this particular airway before. At KYLLE; we received a radio call 'radar service terminated; switch to enroute frequency'. Out of force of habit; I changed the squawk to 2000 and initiated a 1R Standard Lateral Offset Procedure (SLOP) while the First Officer (FO) checked in with San Francisco Radio on HF (we were [using] Controller Pilot Data Link Communications). Approaching KATCH; it occurred to me that I wasn't certain that SLOP procedures were appropriate/authorized for this airway. I looked though the FOM and Jepp General Manual to try to find the answer; however; I was not able to find a definitive answer (those pubs reference some pretty obscure directives; which apparently contain a list of routes where SLOP is authorized). At KATCH; we were transferred to ANC Center. I queried the ANC Center controller as to whether SLOP was authorized on B453. The controller did not know and said that he would check. He came back with something to the effect of 'we don't know'. I removed the offset at that point. It seemed to me that if the controllers didn't know; there was a reasonable chance that SLOP was; in fact; not authorized for that route--meaning that I may well have flown 1 mile off-course for 1 hour and 25 min. We were in and out of radar contact for the time that I had the offset applied and at no point did a controller question our position relative to the airway. After the flight; I tried searching our various manuals (and online) to try to find out if SLOP is authorized on B453 (or; generally; in Canadian Pacific airspace)--but I was unable to find a definitive answer.1. Assumption on my part that SLOP would be authorized on any over-water; non-radar-environment route.2. Lack of clarity in our manuals about where; exactly; SLOP is authorized. This information is buried pretty deep.It might be good to have additional FOM guidance--like 'use SLOP on the North Atlantic Tracks; on trans-Pacific tracks; and wherever directed to use the procedure by ATC. Having FOM guidance that refers us to obscure ICAO documents that are not available to us may satisfy the lawyers--but it does us no operational good.
As a crew we were not sure if SLOP is/was required or an option on said route. We still don't know. We entered SLOP because it is [Company] FOM procedure on Oceanic routes.
MD-11 flight crew reported they flew a lateral deviation on an oceanic route but were not sure if it was authorized.