Small Aircraft
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor
Flight Crew Last 90 Days 150; Flight Crew Total 4500; Flight Crew Type 2000
Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Chart Or Publication
MVFR conditions prevailed with relatively strong winds. We were assigned the NRVANA 1 Departure; which is an RNAV (GPS) departure from Runway 14R. The departure includes a climb on an initial track of 135 degrees to 700 feet; then direct NRVANA. Positive course guidance is not available on this departure until reaching 700 feet. On or about the time we reached that altitude; we entered IMC conditions and we were turned over to Seattle radar. As we managed the hand off; we began to notice a divergence from the outbound course. Over the next two minutes; while making heading corrections; the divergence continued. During this time; it became apparent that we had entered strong cross winds. As best as I could tell; the maximum divergence was about 0.6 nautical miles which was displayed on the navigation display as a one quarter scale deflection. Seattle ATC gave an immediate left turn instruction; wherein we immediately corrected an additional 40 degrees of heading. The rest of the flight was normal. My concern is a possible loss of separation. While we remained within the lateral boundaries of the BFI Class D airspace; our cleared altitude reached 2200 feet before we were able to diverge back to the center line. This could have resulted in a traffic conflict with aircraft arriving at SEATAC. I recommend the departure be redesigned to allow for positive course guidance be provided at 600 feet versus 700 feet; which is the standard for an RNP 1 Departure. Given the complexity of the airspace in this region; positive courser guidance at a lower altitude would reduce the risk.
GA flight instructor reported a slight course divergence due to strong cross winds could cause a possible loss of separation.  Reporter recommended the departure procedure be redesigned.