Flight Crew Air Transport Pilot (ATP)
Workload; Time Pressure; Human-Machine Interface; Distraction
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Airport; Environment - Non Weather Related; Procedure
After being assigned a heading to intercept the localizer on the ILS 19 at KTEB; ATC cleared us for the approach. We were established on an intercept heading at 2;000 FT MSL. I armed LNAV and the autopilot captured the inbound course. Once established on the inbound course and approximately three miles outside the FAF; I armed the approach and the autopilot immediately captured the inbound localizer course. The glide slope indicated armed or white GS mode on the PFD. At this point; I noticed the glide slope indicator on the PFD rapidly move from full scale up to full scale down and then full scale back up again. The autopilot captured the false glide slope and the airplane abruptly pitched up from our captured altitude and began to rapidly climb in an attempt to join the glide slope from below. I immediately disconnected the autopilot and began to pitch the noise down to stop the climb. However; our altitude increased between 300 and 400 FT above the assigned intercept altitude of 2;000 FT. This occurred so rapidly that ATC never noticed the altitude deviation nor did we mention it since we were told to contact the Tower for landing clearance. The remainder of the approach was hand flow with the autopilot disengaged. As I continued the ILS approach; I noticed the glide slope indication make another rapid up and down gyration. It is my belief that these erratic glide slope indications are caused by antenna interference from airplanes as they taxi across Runway 19 on their way to Runway 24 for departure. I recommend a statement be added to the TEB Airport Risk Assessment alerting crews to the potential for false glide slope capture when flying the ILS 19 approach.
The Reporter stated that the Glide Slope was not protected because the ceiling was 1;400 FT overcast; visibility 10 miles. When they became VMC they could see aircraft crossing Runway 1 taxiing to Runway 24 for departure. Their Company now has a procedure in place which briefs pilots about the possibility of this type event during Runway 19 arrivals.
As the ILS critical area was not guarded; the GS is apt to be erratic. The pilot flying could have his hand ready to disengage the autopilot at the first sign of erroneous behavior. As it was; there was only a moments delay and it led to an altitude excursion.
A CL604 reported a TEB Runway 19 false glideslope at 2;000 FT prior to TUGGZ which resulted in an abrupt 400 FT altitude gain; most likely the result of departing aircraft taxiing to TEB Runway 24.