Date: 201207
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Initial Approach
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: ILS/VOR
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Training / Qualification
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: Cleared visual approach in VMC; autopilot engaged; intercepted heading in descent out of about 3;000 FT; glide slope centered; and set zero altitude when cleared for visual. VOR/LOC captured; pressed APP; and glide slope captured. Then; about 1;100 FT five miles out; I realized the runway looked a little flat; disconnected autopilot; and leveled off. Then; got an altitude alert from Tower; but had already leveled off; and continued to intercept normal glide path for normal landing. Glide slope needle was stuck on center indicating we were on path and autopilot was coupled. We recycled my HSI switch and then got an accurate glide slope reading; so must not have been a signal problem. First Officer's glide slope was working properly. Did not write it up because recycling the HSI switch fixed the problem and could not duplicate the sticking. Never thought an autopilot would couple to a stuck glide slope needle. I should have set 1;500 FT FAF altitude in the window and crosschecked with DME. Better crew coordination of cross checking glide slope needles and call outs would have also helped.


Second narrative: During visual approach autopilot on the Captain's side directed the aircraft to follow the localizer/glide slope by selecting Approach. Everything seemed normal until approximately 1;500 FT. I noticed we were unusually low at the same time as the Captain. While I was attempting to reconcile why our indications didn't match; the Captain disconnected the autopilot and began to intercept a normal glide path. I verified my glide slope needle was accurate and I selected his NAV VOR switch from VOR to NAV and back. The indication was normal after this. During this time Tower issued an altitude alert. The Captain regained a normal flight path and landed safely. I should have been monitoring my glide slope in addition to the visual profile.


Synopsis: B737-300 flight crew reports descending below a normal approach path during a visual approach when the glide slope bar freezes in the centered position. The First Officer's indications function normally; but are not being monitored and the low approach is not detected until five miles from the field at 1;100 FT.