Narrative:

Crossing the final approach fix we descended down to 860 feet MSL. At level off we got a caution obstacle from the GPWS and at the same time a low altitude caution from renton tower. They advised us we were at 660 feet; but our indicated was 860 feet. As we executed the missed approach ATC gave us the altimeter setting of 29.85 which was exactly what we had set. Once on with seattle departure we climbed to 4;200 feet and requested vectors to bfi for an ILS. We also asked ATC to keep an eye on our altitude because we believed that we had an altimetry error and we were IMC. Our altimeters were jumping and not steady during the ILS. Upon landing the aircraft was written up with the maintenance discrepancy. A cessna technician purged the system and noticed water coming out of the lines. The system was tested and returned to service.

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Original NASA ASRS Text

Title: CE680 Captain reports descending to 860 feet during a non precision approach to RNT and receiving a caution obstacle from the GPWS and at the same time a low altitude caution from Renton Tower is issued. A go-around is initiated and the flight diverts to BFI for an ILS. Maintenance finds water in the pitot static system.

Narrative: Crossing the final approach fix we descended down to 860 feet MSL. At level off we got a caution obstacle from the GPWS and at the same time a low altitude caution from Renton Tower. They advised us we were at 660 feet; but our indicated was 860 feet. As we executed the missed approach ATC gave us the altimeter setting of 29.85 which was exactly what we had set. Once on with Seattle Departure we climbed to 4;200 feet and requested vectors to BFI for an ILS. We also asked ATC to keep an eye on our altitude because we believed that we had an altimetry error and we were IMC. Our altimeters were jumping and not steady during the ILS. Upon landing the aircraft was written up with the maintenance discrepancy. A Cessna Technician purged the system and noticed water coming out of the lines. The system was tested and returned to service.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.