An MD88 autopilot transitioned to an uncommanded vertical speed mode; accelerating and descending; w...

Attributes
ACN	838844
Time
Date	200906
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Make Model Name	MD-88
Operating Under FAR Part	Part 121
Flight Phase	Final Approach Initial Approach
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Autopilot
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 213 Flight Crew Total 14700 Flight Crew Type 5885
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Inflight Event / Encounter Loss Of Aircraft Control

Narrative:

First officer was pilot flying; captain was pilot monitoring. Aircraft had a write-up on the VNAV portion of the FMS/autopilot. Aircraft was on autopilot and on vectors for ILS runway 1R on a heading of 280 at 5000FT MSL and 190 KTS. Approach turned us to 340 to intercept the localizer to runway 1R. The first officer turned the heading bug to 340 and armed the localizer. At that time the speed bug went to 240 KTS. Both first officer and captain saw this and captain tried to put the bug back on the assigned speed of 190 KTS but the bug was frozen in place. Turning the speed knob; pushing the mach/speed buttons had no effect. At this time the captain noticed we were also in vertical speed and the aircraft was trying to catch the speed on the speed bug and we had descended approximately 350 ft. The first officer disconnected the autopilot and autothrottles; pushed the throttles up and climbed back to the assigned altitude of 5000 ft. When the first officer disconnected the autothrottles the captain regained control of the speed bug on the mode control panel. The appropriate speed was set for the ILS approach and the first officer intercepted the localizer and made an uneventful ILS landing.

Original NASA ASRS Text

Title: An MD88 autopilot transitioned to an uncommanded vertical speed mode; accelerating and descending; when the LOC was armed for an ILS on a vector heading. The autopilot and autothrottles were disconnected to regain aircraft control.

Narrative: First Officer was pilot flying; Captain was pilot monitoring. Aircraft had a write-up on the VNAV portion of the FMS/Autopilot. Aircraft was on autopilot and on vectors for ILS Runway 1R on a heading of 280 at 5000FT MSL and 190 KTS. Approach turned us to 340 to intercept the LOC to Runway 1R. The First Officer turned the heading bug to 340 and armed the LOC. At that time the speed bug went to 240 KTS. Both First Officer and Captain saw this and Captain tried to put the bug back on the assigned speed of 190 KTS but the bug was frozen in place. Turning the speed knob; pushing the mach/speed buttons had no effect. At this time the Captain noticed we were also in vertical speed and the aircraft was trying to catch the speed on the speed bug and we had descended approximately 350 FT. The First Officer disconnected the autopilot and autothrottles; pushed the throttles up and climbed back to the assigned altitude of 5000 FT. When the First Officer disconnected the autothrottles the Captain regained control of the speed bug on the mode control panel. The appropriate speed was set for the ILS approach and the First Officer intercepted the LOC and made an uneventful ILS landing.

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.

An MD88 autopilot transitioned to an uncommanded vertical speed mode; accelerating and descending; when the localizer was armed for an ILS on a vector heading. The autopilot and autothrottles were disconnected to regain aircraft control.