B757 flight crew experienced an FMC navigation error departing PBI; requests Radar vectors for the remainder of the flight. The CDU displayed two messages off and on throughout the flight; IRS NAV only and Purge Updates-POS 2.
Synopsis
B757 flight crew experienced an FMC navigation error departing PBI; requests Radar vectors for the remainder of the flight. The CDU displayed two messages off and on throughout the flight; IRS NAV only and Purge Updates-POS 2.
Narrative
After takeoff as the pilot flying; I called for LNAV at 400 FT AGL and followed my flight director directions to make a right turn. The initial departure went normal. The Tower instructed us to contact PBI Departure. Upon initial contact; the Controller asked us where we were going. Apparently we were not on the TBIRD 1. Our box looked good and we were on the magenta line; but clearly; according to the Controller; we were off course. We advised the Controller we were following our navigation and we were on target. The Captain (pilot not flying) then advised the Controller that our navigation was in error. We then received vectors for the remainder of the flight. In addition; as a crew; we attempted to evaluate what may be wrong; but were unable to come up with any answers. That said the CDU did display two messages off and on throughout the flight. The first was IRS NAV only and the other was Purge Updates-POS 2. As a last note; on final approach our GPWS went off telling us to pull up. We were VFR; on glide slope and evaluated our situation determining that A) we were safe and B) this was related to the IRS showing our position to be miles from where we actually were. The Captain selected the Terrain button on the panel in front of him silencing the warning. We landed uneventfully.
Second reporter narrative
We indicated we were cleared via the TBIRD 1 departure and asked if that is what it looked like we were doing; he said no. From there; ATC was aware of our diminished navigational capacity and accommodated us with vectors for most of the remainder of the flight. Notes: During preflight crew confirmed the route waypoints were correctly loaded in the FMC. Both VOR flags were in view for most of the flight. CDU displayed 2 messages: either 'Purge Updates-POS 2 or IRS NAV only.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.