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A B737-800 was dispatched with a single VOR. While conducting a coupled CAT I ILS; the aircraft oscillated rapidly at glide slope capture and descended below the glide slope. The autopilot was disconnected and aircraft control returned to normal.
| Attributes | |
| ACN | 893839 |
| Time | |
| Date | 201006 |
| Local Time Of Day | 1801-2400 |
| Place | |
| Locale Reference | ZZZ.Airport |
| State Reference | US |
| Environment | |
| Flight Conditions | VMC |
| Light | Dusk |
| Aircraft 1 | |
| Make Model Name | B737-800 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Initial Approach |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | Approach Coupler |
| Person 1 | |
| Function | Captain Pilot Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) |
| Events | |
| Anomaly | Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Inflight Event / Encounter Loss Of Aircraft Control |
Narrative:
Dispatched with inoperative #2 VOR; as per MEL; VHF navigation transfer switch positioned to both on 1 with HUD selected to CAT I for the approach. Autopilot 'a' engaged. [We were] cleared for ILS runway xx with instructions to maintain 3000 ft until established on localizer. After autopilot captured localizer; app mode was armed to capture glideslope with flaps 5; speed approximately 180 KTS. Glideslope captured and initially appeared to track normally. Aircraft then began to oscillate in pitch and rapidly descended below glideslope. Autopilot was disconnected and aircraft was manually returned to glideslope; during which time ATC issued alert to check altitude. It is unclear whether this malfunction was associated with the positioning of the VHF navigation transfer switch as it relates to the MEL item; but all ILS information is from the #1 receiver only in this configuration. According to the MEL; this configuration is normal and acceptable unless HUD CAT ii or CAT III approaches are conducted. To the best of my knowledge; this issue did not involve crew error or procedural deviation.
Original NASA ASRS Text
Title: A B737-800 was dispatched with a single VOR. While conducting a coupled CAT I ILS; the aircraft oscillated rapidly at glide slope capture and descended below the glide slope. The autopilot was disconnected and aircraft control returned to normal.
Narrative: Dispatched with inoperative #2 VOR; as per MEL; VHF NAV transfer switch positioned to BOTH ON 1 with HUD selected to CAT I for the approach. Autopilot 'A' engaged. [We were] cleared for ILS Runway XX with instructions to maintain 3000 FT until established on localizer. After autopilot captured LOC; APP mode was armed to capture glideslope with flaps 5; speed approximately 180 KTS. Glideslope captured and initially appeared to track normally. Aircraft then began to oscillate in pitch and rapidly descended below glideslope. Autopilot was disconnected and aircraft was manually returned to glideslope; during which time ATC issued alert to check altitude. It is unclear whether this malfunction was associated with the positioning of the VHF NAV transfer switch as it relates to the MEL item; but all ILS information is from the #1 receiver only in this configuration. According to the MEL; this configuration is normal and acceptable unless HUD CAT II or CAT III approaches are conducted. To the best of my knowledge; this issue did not involve crew error or procedural deviation.
Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.