A MD80 flight crew reported that their aircraft's slow FMS processor does not have time to update on an RNAV departure before the 1;540 turn point and as a result turns slight later than ATC expects; causing a track deviation.
Synopsis
A MD80 flight crew reported that their aircraft's slow FMS processor does not have time to update on an RNAV departure before the 1;540 turn point and as a result turns slight later than ATC expects; causing a track deviation.
Narrative
On takeoff leg of this departure; engaged NAV at 400 FT AGL (1;540 FT MSL) and autopilot on at 5;000 FT AGL. Tower called us just above 500 FT and told us it looked like we were tracking left of the course track and we needed to track further right for the initial transition. The aircraft was on a 280 bearing and verified on course on the NAV display and the PFD. FMA NAV TRK; and progress page 2 of the FMS (0.06NM left of track). We were given the frequency change to Departure and upon check in we were requested to call Tower upon arrival. I called. He was very professional and was looking for information on how we flew and tracked the RNAV departure. His concern was that it appeared; from his visual cues in the Tower; that we tracked left before turning right on course. He also explained that this is a common occurrence on our carrier's MD80 aircraft and not on another carrier's. I explained the departure initial heading; when we engaged the NAV and autopilot; and what we monitor to make sure we are tracking the course accurately. His main concern is obviously the safety zone between the north and south runways. I believe I followed RNAV procedures as per company directives. But somewhere in the transition from NAV capture and track and autopilot on for the RNAV departures to the west; that small delay may cause the aircraft to track too slowly to the desired transition course.
Second reporter narrative
On takeoff from ZZZ; after engaging NAV at 400 FT and autopilot at 500 FT with NAV CAP then NAV TRK annunciated; Tower asked us if we were flying the RNAV departure. We said yes. They said we were left of course and needed to turn right. I replied that we were tracking on the SID as depicted. We were switched to Departure Control prior to being released to the next sector. Departure instructed us to call them on arrival. Captain spoke to the Tower after arrival. All cockpit indications were normal with the track deviation page displayed. Tower advises this is typical of all company aircraft and not a sole anomaly with our flight.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.