Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 208
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem
Procedure; Chart Or Publication
After getting our clearance to depart SEA via the Mountain Departure 16L; the airport switched runways to north departure on 34R. ATC [then] gave us the Seattle 7.  After looking at the FMS it appeared that it coded a 345 heading after departure but no intercept of the 341 radial off SEA as shown on the plate.  Thus I used raw data and flew the departure with heading select and tracked the radial manually. I believe there is an error with this departure because a 345 initial heading diverges away from the 341 radial instead of towards it. This may be why the FMS does not show an intercept because it is not possible to intercept on a 345 heading.  This should be 341 or something similar to the Mountain Departure which is basically the same.
Air carrier First Officer reported the Runway 34R SEATTLE 7 SID initial departure heading out of SEA diverges from intended outbound course.