Altitude set in altitude select but changed by 1000' without flight crew input.

Attributes
ACN	155065
Time
Date	199008
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : lax
State Reference	CA
Altitude	msl bound lower : 7000 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : lax
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent : approach descent other
Route In Use	arrival : profile descent
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : atp pilot : commercial
Experience	flight time last 90 days : 250 flight time total : 9000
ASRS Report	155065
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : atp pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : crossing restriction not met non adherence : published procedure non adherence : clearance other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

We were descending into lax on the civet profile descent with the copilot flying. Approach control cleared us for the runway 24R approach to cross denay intersection 8000. The copilot put this into the mode control panel and I turned and looked up the frequency for the ILS. When I looked up the autoplt was leveling off at 7000' as we passed denay intersection 1000' low. The mode control panel had jumped 1000' by itself while I was distraction for a short time. This had happened numerous times on the medium large transport advance and needs to be fixed immediately. I recommend the inner and outer knobs be reversed in function so if it jumps it will only move 100' instead of 1000'. Also more positive detents on the knob. Callback conversation with reporter revealed the following information. It is possible in turning the altitude set knob to not feel the knob set in the detent, in other words to leave it on top of the knurl and although the correct altitude number is showing the control can fall back or forward into the notch and change the altitude. At least this is what I discern to be the problem with the thousand ft selector. One must be extremely careful to properly notch the selector when selecting an altitude.

Original NASA ASRS Text

Title: ALT SET IN ALT SELECT BUT CHANGED BY 1000' WITHOUT FLC INPUT.

Narrative: WE WERE DSNDING INTO LAX ON THE CIVET PROFILE DSNT WITH THE COPLT FLYING. APCH CTL CLRED US FOR THE RWY 24R APCH TO CROSS DENAY INTXN 8000. THE COPLT PUT THIS INTO THE MODE CTL PANEL AND I TURNED AND LOOKED UP THE FREQ FOR THE ILS. WHEN I LOOKED UP THE AUTOPLT WAS LEVELING OFF AT 7000' AS WE PASSED DENAY INTXN 1000' LOW. THE MODE CTL PANEL HAD JUMPED 1000' BY ITSELF WHILE I WAS DISTR FOR A SHORT TIME. THIS HAD HAPPENED NUMEROUS TIMES ON THE MLG ADVANCE AND NEEDS TO BE FIXED IMMEDIATELY. I RECOMMEND THE INNER AND OUTER KNOBS BE REVERSED IN FUNCTION SO IF IT JUMPS IT WILL ONLY MOVE 100' INSTEAD OF 1000'. ALSO MORE POSITIVE DETENTS ON THE KNOB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. IT IS POSSIBLE IN TURNING THE ALT SET KNOB TO NOT FEEL THE KNOB SET IN THE DETENT, IN OTHER WORDS TO LEAVE IT ON TOP OF THE KNURL AND ALTHOUGH THE CORRECT ALT NUMBER IS SHOWING THE CTL CAN FALL BACK OR FORWARD INTO THE NOTCH AND CHANGE THE ALT. AT LEAST THIS IS WHAT I DISCERN TO BE THE PROB WITH THE THOUSAND FT SELECTOR. ONE MUST BE EXTREMELY CAREFUL TO PROPERLY NOTCH THE SELECTOR WHEN SELECTING AN ALT.

Data retrieved from NASA's ASRS site as of July 2007 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.