Date: 201208
Local time of day: 1801-2400
Location: ZLC.ARTCC, UT
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Cruise
Airspace: Class A ZLC
ATC / advisory: Center ZLC
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Confusion
Second reporter function: Captain; Pilot Flying
Anomaly: ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
Result: General None Reported / Taken
Contributing factors: Human Factors; Weather; Procedure
Primary problem: Procedure

Narrative: A B757 was cleared by ZLA to deviate left of course when able proceed direct EKR. The B757 was northbound on the left deviation and decided that he needed to deviate to the right; he made about a 90 to 110 degree right turn to go right around some weather. I informed him that a right turn when he was cleared to deviate left was a pilot deviation; any right turn should have taken the aircraft direct to EKR. He said that because he was cleared to deviate left of course that gave him approval to make any deviations necessary as long as he remained left of his previous flight plan. Two other Air Carriers earlier in the day did similar turns opposite the direction they were cleared to deviate. I am curious if other countries allow aircraft to deviate as they please as long as the stay to the left/right of the aircraft's previous route. On occasion I have cleared an aircraft to deviate up to i.e. 10 miles left or right of their filed route; but I am very clear on my expectations. If an aircraft is cleared to deviate left of course when able direct to a fix; their new route is a direct line from the deviation to the cleared fix; as a result any turn right of that line would require a clearance from ATC. I believe this is a huge safety issue; I don't know if the pilots are so caught up in getting around the weather that they forget what their clearance was or if they think it is no big deal and just work their way around the weather. As a controller when I issue a left deviation I do not separate aircraft to the right of aircraft unless the on course turn could become a factor. Pilots making these types of deviations potentially put themselves and other aircraft in the NAS at a huge collision risk. I believe that the airlines need to educate their pilots so that they are clear on what is expected from them.


Second narrative: Requested a right and left deviation from ATC for thunderstorm avoidance. Recieved clearance to deviate right of course up to 20 degrees and proceed direct to OAL when clear. Deviated ten degrees right of course putting us 5 miles north of course. Abeam first group of storms started a left turn to clear second group of storms. Reported our turn to ATC at which time we were told we were not cleared to come left. At this point I could not come back right due to second storm. Continued left turn and retuned to original course and avoided storm by five miles.


Synopsis: ZLC Controller voiced concern regarding the interpretation of a 'weather deviation clearance' and what turn limitations are in fact authorized from ATC's perspective.