Some VHF heads are able to change frequencys by themselves.

Attributes
ACN	284543
Time
Date	199409
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : orf
State Reference	VA
Altitude	msl bound lower : 24000 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zdc
Operator	common carrier : air carrier
Make Model Name	B767-200
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 8000 flight time type : 3000
ASRS Report	284543
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Holding at different fixes along route atl-ewr. Left 1 fix and had a frequency change. Checked in on new frequency, then lost contact. 10 mins later got back in radio contact. After checking, I believe the radio changed frequency by itself. This type of head can switch setting without touching the dial. It has an led display and less than position detent on the dial for frequencys. You can be on the frequency and have the proper led display but not in the detent. It can then slide into the detent of the wrong frequency. It should have a better design so this is not possible. Some of our navigation radio's are the same way ie, ILS. Callback conversation with reporter revealed the following information: the reporter flies the B-767 for a major united states air carrier. His aircraft has 2 different type radio heads. One of them has a very definite detent. The other is rather 'soft' going into its detent. The reporter has had this happen again since his report, and he has talked to other pilots that have had the same problem. The head with the 'soft detent is the one that gives problems. The reporter has gone back to the old procedure of writing down the frequency to assure that he has a record.

Original NASA ASRS Text

Title: SOME VHF HEADS ARE ABLE TO CHANGE FREQS BY THEMSELVES.

Narrative: HOLDING AT DIFFERENT FIXES ALONG RTE ATL-EWR. LEFT 1 FIX AND HAD A FREQ CHANGE. CHKED IN ON NEW FREQ, THEN LOST CONTACT. 10 MINS LATER GOT BACK IN RADIO CONTACT. AFTER CHKING, I BELIEVE THE RADIO CHANGED FREQ BY ITSELF. THIS TYPE OF HEAD CAN SWITCH SETTING WITHOUT TOUCHING THE DIAL. IT HAS AN LED DISPLAY AND LESS THAN POS DETENT ON THE DIAL FOR FREQS. YOU CAN BE ON THE FREQ AND HAVE THE PROPER LED DISPLAY BUT NOT IN THE DETENT. IT CAN THEN SLIDE INTO THE DETENT OF THE WRONG FREQ. IT SHOULD HAVE A BETTER DESIGN SO THIS IS NOT POSSIBLE. SOME OF OUR NAV RADIO'S ARE THE SAME WAY IE, ILS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE B-767 FOR A MAJOR UNITED STATES ACR. HIS ACFT HAS 2 DIFFERENT TYPE RADIO HEADS. ONE OF THEM HAS A VERY DEFINITE DETENT. THE OTHER IS RATHER 'SOFT' GOING INTO ITS DETENT. THE RPTR HAS HAD THIS HAPPEN AGAIN SINCE HIS RPT, AND HE HAS TALKED TO OTHER PLTS THAT HAVE HAD THE SAME PROB. THE HEAD WITH THE 'SOFT DETENT IS THE ONE THAT GIVES PROBS. THE RPTR HAS GONE BACK TO THE OLD PROC OF WRITING DOWN THE FREQ TO ASSURE THAT HE HAS A RECORD.

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.