B737-800 captain reports anomalous FMC responses.

Attributes
ACN	708749
Time
Date	200609
Local Time Of Day	1201 To 1800
Place
Locale Reference	intersection : totec
State Reference	AZ
Altitude	msl single value : 37000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zab.artcc
Operator	common carrier : air carrier
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
ASRS Report	708749
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

In cruise we were given direct ink. When line selected to the legs page it showed as the letter 'D' with no other latitude/longitude; etc; information. Upon second try; line selected ink but sso showed in scratch pad. Third try worked normally. Kept the airplane a second leg with no further anomalies. Usually I might not report this but yesterday we had a dropped fix and we were watching particularly close today with deliberate FMC entries. Having flown FMC's for over 10 yrs now; I have found the B737 to be particularly reliable. Now with 2 days and 2 strange occurrences; I will be watching the FMC really close. Just curious if other reports have increased. Could the software be less stable? Could it be onboard electronic interference?

Original NASA ASRS Text

Title: B737-800 CAPT RPTS ANOMALOUS FMC RESPONSES.

Narrative: IN CRUISE WE WERE GIVEN DIRECT INK. WHEN LINE SELECTED TO THE LEGS PAGE IT SHOWED AS THE LETTER 'D' WITH NO OTHER LATITUDE/LONGITUDE; ETC; INFO. UPON SECOND TRY; LINE SELECTED INK BUT SSO SHOWED IN SCRATCH PAD. THIRD TRY WORKED NORMALLY. KEPT THE AIRPLANE A SECOND LEG WITH NO FURTHER ANOMALIES. USUALLY I MIGHT NOT RPT THIS BUT YESTERDAY WE HAD A DROPPED FIX AND WE WERE WATCHING PARTICULARLY CLOSE TODAY WITH DELIBERATE FMC ENTRIES. HAVING FLOWN FMC'S FOR OVER 10 YRS NOW; I HAVE FOUND THE B737 TO BE PARTICULARLY RELIABLE. NOW WITH 2 DAYS AND 2 STRANGE OCCURRENCES; I WILL BE WATCHING THE FMC REALLY CLOSE. JUST CURIOUS IF OTHER RPTS HAVE INCREASED. COULD THE SOFTWARE BE LESS STABLE? COULD IT BE ONBOARD ELECTRONIC INTERFERENCE?

Data retrieved from NASA's ASRS site as of January 2009 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.