Date: 201212
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: 8
Light: Night
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Approach Coupler
Component problem: Design; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Human factors: Training / Qualification; Time Pressure; Distraction; Workload; Confusion
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: This event occurred during approach to Runway 28L. The ceiling was approximately 1;500 FT MSL; requiring ATC vectors to the approach. There was other traffic in the area and we were vectored to final behind them. ATC had slowed us to 150 KIAS; which; in our type of aircraft; requires landing gear down; and flaps 15. This is a high amount of drag to have; especially in level flight. I was the pilot flying. I was given an intercept heading to join the localizer; and cleared for the approach. The autopilot was engaged; and I armed the VOR/LOC mode. It was clear that the localizer was going to capture at about the same time as the glide slope was intercepted. As soon as the FMA changed VOR/LOC to green (captured); I selected the approach function; and the glide slope indication went to green (captured) almost immediately. The glide slope indicator then trended downward; showing that we were getting high on the desired path. The autopilot; instead of pitching down to follow the glide slope; began to pitch up. The airspeed then began to decrease below the minimum for the configuration. I had to add thrust significantly; disconnect the autopilot and manually push the nose over to increase the airspeed and try to regain the proper glide slope. I regained it successfully; and flew the remainder of the approach in a stabilized fashion; but the aircraft did not perform as it should have. Performing in the opposite of what it's supposed to do; in a low airspeed; high drag situation is very dangerous. This is not the first time that I have witnessed this kind of behavior in this type of aircraft. This is the third or fourth time that I have seen this; at different airports; but all on ILS frequencies. I have discussed this error with the Captains at the time; and I always get a response such as; 'Well; it's a -300.' This is completely unacceptable; and counter to safety. I can't understand how this equipment can be certified for use in IFR when it behaves in this fashion. Perhaps it's just in a series of autopilots; or series of aircraft; but it seems that a tracking by Maintenance or investigation of some sort is warranted. As we move more and more to a higher level of safety and automation; attitudes like; 'Well; it's a -300;' don't seem to have any place in our system. I have also seen these airplanes nose over suddenly to capture the glide slope if you happen to be above it. The pitch over moment is quite severe; and causes alarm to passengers and crew members alike. That may be a separate issue; but it seems like there is a regular deficiency with the autopilot system in this type of aircraft. I believe that this needs to be investigated by the Maintenance Department. Request that when crews witness this behavior; note the time; approach; configuration; frequency; etc. to see if there is a pattern. Have the autopilot system in the affected/reported aircraft checked for proper function. Consult with Boeing to see if there are other reports of this type of behavior.




Synopsis: As a B737-300 captured the glide slope; the glide slope indication trended high so the autopilot pitched up causing the airspeed to decreased below maneuvering speed. The First Officer manually disconnected the autopilot to pitch over while adding power.