Large transport has static discharge in rain then false TCASII RA.

Attributes
ACN	257906
Time
Date	199311
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : sfo
State Reference	CA
Altitude	msl bound lower : 8000 msl bound upper : 8500
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : oak
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : instrument pilot : flight engineer pilot : cfi pilot : atp
Experience	flight time last 90 days : 150 flight time total : 15000 flight time type : 100
ASRS Report	257906
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : atp pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude inflight encounter : weather other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 6000 vertical : 100
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

On descent into sfo our TCASII gave one TA followed by RA at 12-12:30 O'clock, 1-2 mi, 100 ft below. First officer responded immediately and correctly to RA, resulting in climb. Conflict resolved immediately. ATC advised during RA. No traffic within 5 mi nor did we cause traffic conflict by deviation from descent clearance. Note: we had 5 mins prior to RA a static discharge and were in moderate rain. Time frame of TA, RA and resolve less than 15 seconds. ATC had said our altitude readout of RA traffic was correct but distance more than 5 mi. Question can rain or static conditions cause distortion of signal and cause unreliable signals on TCASII.

Original NASA ASRS Text

Title: LGT HAS STATIC DISCHARGE IN RAIN THEN FALSE TCASII RA.

Narrative: ON DSCNT INTO SFO OUR TCASII GAVE ONE TA FOLLOWED BY RA AT 12-12:30 O'CLOCK, 1-2 MI, 100 FT BELOW. FO RESPONDED IMMEDIATELY AND CORRECTLY TO RA, RESULTING IN CLB. CONFLICT RESOLVED IMMEDIATELY. ATC ADVISED DURING RA. NO TFC WITHIN 5 MI NOR DID WE CAUSE TFC CONFLICT BY DEV FROM DSCNT CLRNC. NOTE: WE HAD 5 MINS PRIOR TO RA A STATIC DISCHARGE AND WERE IN MODERATE RAIN. TIME FRAME OF TA, RA AND RESOLVE LESS THAN 15 SECONDS. ATC HAD SAID OUR ALT READOUT OF RA TFC WAS CORRECT BUT DISTANCE MORE THAN 5 MI. QUESTION CAN RAIN OR STATIC CONDITIONS CAUSE DISTORTION OF SIGNAL AND CAUSE UNRELIABLE SIGNALS ON TCASII.

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.