Toggle switch on DME selector rotated in its base so that #1 position was actually #2.

Attributes
ACN	126743
Time
Date	198910
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : zzz
State Reference	US
Aircraft 1
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument pilot : atp
Experience	flight time last 90 days : 20 flight time total : 5000
ASRS Report	126743
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other flight crewa
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Toggle switch on DME selected rotated so that #1 position was in fact #2 position. Wires in back of switch were twisted in such a way that once the lock nut backed off, the switch 'snapped' into the wrong position. All aircraft switches should have notched cutouts to prevent rotation. This type of problem could have caused many unknown accidents on instrument approachs. I suggest it be a checklist item incorporated in your/NTSB/FAA accident investigation checklists and made essential item for aircraft manufacturers.

Original NASA ASRS Text

Title: TOGGLE SWITCH ON DME SELECTOR ROTATED IN ITS BASE SO THAT #1 POSITION WAS ACTUALLY #2.

Narrative: TOGGLE SWITCH ON DME SELECTED ROTATED SO THAT #1 POS WAS IN FACT #2 POS. WIRES IN BACK OF SWITCH WERE TWISTED IN SUCH A WAY THAT ONCE THE LOCK NUT BACKED OFF, THE SWITCH 'SNAPPED' INTO THE WRONG POS. ALL ACFT SWITCHES SHOULD HAVE NOTCHED CUTOUTS TO PREVENT ROTATION. THIS TYPE OF PROB COULD HAVE CAUSED MANY UNKNOWN ACCIDENTS ON INSTRUMENT APCHS. I SUGGEST IT BE A CHKLIST ITEM INCORPORATED IN YOUR/NTSB/FAA ACCIDENT INVESTIGATION CHKLISTS AND MADE ESSENTIAL ITEM FOR ACFT MANUFACTURERS.

Data retrieved from NASA's ASRS site as of August 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.