Date: 202108
Local time of day: 1801-2400
Location: IAH.Airport, TX
Flight conditions: VMC
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B IAH
ATC / advisory: Tower IAH
Second aircraft: B737 Undifferentiated or Other Model
Aircraft component: Altitude
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Human-Machine Interface; Distraction; Situational Awareness; Troubleshooting
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft; Environment - Non Weather Related; Human Factors; Procedure; Airport
Primary problem: Procedure

Narrative: Received vector for initially an ILS to Runway 8R.  Intercepted LOC just outside JELLI at 5;000 feet.  Upon interception; glideslope captured aircraft autopilot began a 'substantial' descent (1;500 feet/min).  At first I didn't think anything about it. Shortly after; we were cleared a visual approach.  We were fully configured before MATON; and passing MATON I felt low on the approach; it was too shallow and looked wrong; and then I noticed our altitude was 1;200 feet MSL (1;100 feet AGL) when it should have been about 2;000 feet MSL.  Flight Director still showed us on glide path.  I immediately called out we were 800 feet low for some reason.  Suspecting an instrument issue; I disconnected the autopilot; turned off the flight director; and began hand flying.  We had been cleared a visual shortly after LOC capture and were visual; so I completed the final segment flying purely visually.  I even took the precaution of having the First Officer deselect the ILS in the nav radio tuners.  At approximately 1 mile from the runway threshold; I noticed 2 aircraft holding short on the north side of the approach end of 8R waiting clearance to cross.  A B737 was holding short on Taxiway NE; and a E-170 was holding short at NR.  It appeared those aircraft were causing interference of the glideslope signal resulting in the indications we were receiving on our instruments.  Our manual recommends backing up a visual approach with the ILS; when available.  However; good airmanship and situational awareness must always be maintained. An additional item: ATC gave us no call about our low altitude on the approach.  I couldn't find it above anywhere on this form; but I could also consider this an incident which was successfully recognized and resolved in time with no incident. I have seen aircraft waiting to cross untold number of times; but never have I seen glideslope interference such as I saw during this approach.




Synopsis: Pilot reported they were low on glide path during approach and thought they might be having an equipment problem.  Pilot observed two aircraft close to the runway which may have interfered with the ILS glide slope signal.