An A320 flight crew experienced an RA when flying 1000 ft below traffic because of mountain wave and...

Attributes
ACN	715382
Time
Date	200610
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : cim.vortac
State Reference	TX
Altitude	msl single value : 36000
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	artcc : zab.artcc
Operator	common carrier : air carrier
Make Model Name	A320
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zab.artcc
Operator	Other
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 75 flight time total : 23500 flight time type : 1400
ASRS Report	715382
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 210 flight time total : 6800 flight time type : 350
ASRS Report	715502
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe inflight encounter : weather
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : returned to assigned altitude
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Weather Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

ZAB notified us of traffic at 12 O'clock position; 10 mi at FL370; we were level at FL360. Traffic sighted both visually and on TCAS. Aircraft was experiencing very slight mountain wave. At approximately 4 mi and closing we received a TCAS RA 'descend; descend' without any prior 'traffic; traffic.' I disconnected both the autoplt and autothrottles; nosed down within the allotted 5 seconds to 1200 FPM down; traffic crossed overhead while we were at FL358.5; commenced a climb back up to FL360. We topped at FL361.3 during the return to level flight. First officer advised ATC that we had complied with a TCAS RA. Center queried both aircraft as to their indicated altitudes. (The other aircraft had no TCAS indications at any time during this period). We showed FL360 on captain's pfd; FL359.8 on first officer's pfd; and FL360 on standby altimeter. We were asked to call ATC on arrival at phx. I spoke with mr X of ZAB and described what we saw; what we did; and the altitudes involved. Mr X was unaware of the airbus characteristic of flying a 'soft' altitude (+/-50 ft on the set altitude) and I suggested that the event might have been caused by a combination of minor mountain wave; soft altitude flown; and a sensitive TCAS system. He indicated that 'headquarters' might want him to file an 'incident' report; but definitely not a 'pilot deviation' report. Passenger and crew were unaware of the TCAS event.

Original NASA ASRS Text

Title: AN A320 FLT CREW EXPERIENCED AN RA WHEN FLYING 1000 FT BELOW TFC BECAUSE OF MOUNTAIN WAVE AND THE CHARACTERISTIC OF THE AIRBUS TO FLY SOFT ALTS.

Narrative: ZAB NOTIFIED US OF TFC AT 12 O'CLOCK POS; 10 MI AT FL370; WE WERE LEVEL AT FL360. TFC SIGHTED BOTH VISUALLY AND ON TCAS. ACFT WAS EXPERIENCING VERY SLIGHT MOUNTAIN WAVE. AT APPROX 4 MI AND CLOSING WE RECEIVED A TCAS RA 'DSND; DSND' WITHOUT ANY PRIOR 'TFC; TFC.' I DISCONNECTED BOTH THE AUTOPLT AND AUTOTHROTTLES; NOSED DOWN WITHIN THE ALLOTTED 5 SECONDS TO 1200 FPM DOWN; TFC CROSSED OVERHEAD WHILE WE WERE AT FL358.5; COMMENCED A CLB BACK UP TO FL360. WE TOPPED AT FL361.3 DURING THE RETURN TO LEVEL FLT. FO ADVISED ATC THAT WE HAD COMPLIED WITH A TCAS RA. CTR QUERIED BOTH ACFT AS TO THEIR INDICATED ALTS. (THE OTHER ACFT HAD NO TCAS INDICATIONS AT ANY TIME DURING THIS PERIOD). WE SHOWED FL360 ON CAPT'S PFD; FL359.8 ON FO'S PFD; AND FL360 ON STANDBY ALTIMETER. WE WERE ASKED TO CALL ATC ON ARR AT PHX. I SPOKE WITH MR X OF ZAB AND DESCRIBED WHAT WE SAW; WHAT WE DID; AND THE ALTS INVOLVED. MR X WAS UNAWARE OF THE AIRBUS CHARACTERISTIC OF FLYING A 'SOFT' ALT (+/-50 FT ON THE SET ALT) AND I SUGGESTED THAT THE EVENT MIGHT HAVE BEEN CAUSED BY A COMBINATION OF MINOR MOUNTAIN WAVE; SOFT ALT FLOWN; AND A SENSITIVE TCAS SYS. HE INDICATED THAT 'HEADQUARTERS' MIGHT WANT HIM TO FILE AN 'INCIDENT' RPT; BUT DEFINITELY NOT A 'PLTDEV' RPT. PAX AND CREW WERE UNAWARE OF THE TCAS EVENT.

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.

An A320 flight crew experienced an RA when flying 1000 ft below traffic because of mountain wave and the characteristic of the airbus to fly soft altitudes.