Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Type 15000
Workload; Distraction; Confusion; Situational Awareness
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Air Traffic Control Issued Advisory / Alert; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Executed Go Around / Missed Approach; Flight Crew Took Evasive Action
Airport; Environment - Non Weather Related; Procedure; Weather
We initiated a go-around during an ILS to Runway 31R in Dallas [Love]; due to an errant glideslope signal which placed the aircraft below glidepath and triggered an obstacle alert from the GPWS. The approach was being flown uneventfully by the autopilot. At approximately 1900 FT MSL; the glideslope indicator made a rapid downward deflection of approximately 1.5 dots; pitching the aircraft nose down. This was followed by a GPWS 'Caution Obstacle' alert triggered by the building that lies on the approach course; and is charted at 1049 FT MSL. We were in and out of the clouds in MDT rain and visually acquired the building ahead; noting that we were still well above it. I commenced a go-around and was immediately handed off to Departure Control. I told the Controller that I suspected that an aircraft on the ground had penetrated the ILS critical area and triggered the glideslope fluctuation and to please advise the Tower as we were unable to do so during the missed approach. The Tower Controller told him that an aircraft had indeed been taxied into position on the runway as the weather was above 800 FT ceiling and two miles visibility; which are the requirements for protecting the ILS critical area. The ATIS was reporting a ceiling of 1300 FT broken; 2000 FT overcast and visibility of four miles in heavy rain and mist. The second approach was uneventful and we landed four minutes late. During taxi in; I gave the Ground Controller the reason for our previous missed approach.
I called out the obstacle (a high-rise tower) in sight; but because we were still in IMC (there were clouds below and beyond the obstacle); the Captain initiated a go-around. It appears that aircraft in the ILS critical area caused our incident.
ATC cleared on aircraft onto DAL Runway 31R. That aircraft interfered with the ILS glideslope and caused an aircraft on final to pitch down and receive an EGPWS alert. The crew executed a missed approach.