Narrative:

I was the captain of flight from san to den with the copilot flying. The WX on the ATIS was clear and 10 mi, temperature 77, wind calm, 30.09 landing runway 26L and right. The first officer and I discussed the WX and there were a few cbs southwest through southeast of the field. We mentioned the change of windshear on approach. We turned final for runway 26R approximately 2 mi outside the marker (visibility approach). After passing the marker and completing the final checklist. The tower mentioned a windshear alert north boundary. A medium large transport ahead of us for runway 26L reported an airspeed loss and discontinued the approach. The first officer and I briefed on what to expect and do (carry extra speed if situation deteriorated, to go-around, we are still at 6800'). At 6300', (1000 AGL) we got a 15 KT increase in speed, although this may partly have been due to first officer selecting 30 degree flaps and adding power. And that was the extent of any airspeed, or vertical velocity changes. We requested to land on runway 26L and tower approved. After our 500' SOP callouts the tower called out a microburst alert and we scanned our landing area for any signs of such (ie, dust devils, blowing dust, etc) none were seen, and we made an uneventful landing, reporting this fact to the tower. 2 planes behind us went around (they had airspeed losses). Later we learned that the microburst alerts were 'doppler radar' alerts and our company has a policy that a go-around will be executed with a 'doppler radar' microburst alert. I have enclosed copies of our company flight operations manual pages concerning this area. The first officer and I are not sure now what system the tower said was issuing the alert as we were talking about the approach, WX, and options on short final. I'm sure the ATIS did not have the 'demonstration in progress' NOTAM. However, at the beginning of this month a notice was issued about den having a doppler radar test daily from 1200L-1900L. The first officer and I don't know which system was used, if we needed the ATIS message to put us in the doppler alert most go-around mode or if LLWAS was in play and our judgement of other factors (no blowing dust etc), and known LLWAS system defects would allow a landing. Lndgs are often made at other stations with LLWAS alerts of windshear, ie at sfo it is a common occurrence with no circuit breaker WX problems. I believe the situation occurred because of confusion surrounding the system we were operating under (ie, doppler, must go-around or LLWAS can land using other criteria) and the last min nature of warning. Tower alert on final at about 500' and no visibility perception of danger. Cbs south and cloud bases north but den out in clear. No change in airspeed or other air carrier program windshear clues, ie, vertical speed unusual, excessive power, temperature, dewpoint spread of 30-50 (only the temperature of 77 degree was given on ATIS), blowing dust, etc. The latitude of our program may contribute to this confusion. Can land with microburst warning under one system but not another. Maybe one hard must go-around system or warning trigger (ie, 'microburst' but not necessarily 'windshear'). Another factor is the relative inexperience in position of both of us, I have 23 yrs at air carrier but only 5 months as captain and the first officer has only 5 months as first officer with 18 months at air carrier. During the event we were both focused on landing, and going around only after some triggering event that fortunately didn't happen. If it comes up again I'm sure we will both be more cautious and exercise better or more conservative judgement.

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Original NASA ASRS Text

Title: WHEN WIND SHEAR AND MICRO BURSTS WERE REPORTED TO FLT CREW IT WAS NOT KNOWN AS TO THE SOURCE OF THE WX INFORMATION. REPORTER CITES CONFUSION ABOUT THIS AS COMPANY REGS MANDATED A GO AROUND IF ABNORMALTIES WERE DETECTED BY DOPPLER RADAR BUT NOT IF BY LLWAS SYSTEM. FLT CREW LANDED.

Narrative: I WAS THE CAPT OF FLT FROM SAN TO DEN WITH THE COPLT FLYING. THE WX ON THE ATIS WAS CLR AND 10 MI, TEMP 77, WIND CALM, 30.09 LNDG RWY 26L AND R. THE F/O AND I DISCUSSED THE WX AND THERE WERE A FEW CBS SW THROUGH SE OF THE FIELD. WE MENTIONED THE CHANGE OF WINDSHEAR ON APCH. WE TURNED FINAL FOR RWY 26R APPROX 2 MI OUTSIDE THE MARKER (VIS APCH). AFTER PASSING THE MARKER AND COMPLETING THE FINAL CHKLIST. THE TWR MENTIONED A WINDSHEAR ALERT N BOUNDARY. A MLG AHEAD OF US FOR RWY 26L RPTED AN AIRSPD LOSS AND DISCONTINUED THE APCH. THE F/O AND I BRIEFED ON WHAT TO EXPECT AND DO (CARRY EXTRA SPD IF SITUATION DETERIORATED, TO GO-AROUND, WE ARE STILL AT 6800'). AT 6300', (1000 AGL) WE GOT A 15 KT INCREASE IN SPD, ALTHOUGH THIS MAY PARTLY HAVE BEEN DUE TO F/O SELECTING 30 DEG FLAPS AND ADDING PWR. AND THAT WAS THE EXTENT OF ANY AIRSPD, OR VERT VELOCITY CHANGES. WE REQUESTED TO LAND ON RWY 26L AND TWR APPROVED. AFTER OUR 500' SOP CALLOUTS THE TWR CALLED OUT A MICROBURST ALERT AND WE SCANNED OUR LNDG AREA FOR ANY SIGNS OF SUCH (IE, DUST DEVILS, BLOWING DUST, ETC) NONE WERE SEEN, AND WE MADE AN UNEVENTFUL LNDG, RPTING THIS FACT TO THE TWR. 2 PLANES BEHIND US WENT AROUND (THEY HAD AIRSPD LOSSES). LATER WE LEARNED THAT THE MICROBURST ALERTS WERE 'DOPPLER RADAR' ALERTS AND OUR COMPANY HAS A POLICY THAT A GO-AROUND WILL BE EXECUTED WITH A 'DOPPLER RADAR' MICROBURST ALERT. I HAVE ENCLOSED COPIES OF OUR COMPANY FLT OPS MANUAL PAGES CONCERNING THIS AREA. THE F/O AND I ARE NOT SURE NOW WHAT SYS THE TWR SAID WAS ISSUING THE ALERT AS WE WERE TALKING ABOUT THE APCH, WX, AND OPTIONS ON SHORT FINAL. I'M SURE THE ATIS DID NOT HAVE THE 'DEMONSTRATION IN PROGRESS' NOTAM. HOWEVER, AT THE BEGINNING OF THIS MONTH A NOTICE WAS ISSUED ABOUT DEN HAVING A DOPPLER RADAR TEST DAILY FROM 1200L-1900L. THE F/O AND I DON'T KNOW WHICH SYS WAS USED, IF WE NEEDED THE ATIS MESSAGE TO PUT US IN THE DOPPLER ALERT MOST GO-AROUND MODE OR IF LLWAS WAS IN PLAY AND OUR JUDGEMENT OF OTHER FACTORS (NO BLOWING DUST ETC), AND KNOWN LLWAS SYS DEFECTS WOULD ALLOW A LNDG. LNDGS ARE OFTEN MADE AT OTHER STATIONS WITH LLWAS ALERTS OF WINDSHEAR, IE AT SFO IT IS A COMMON OCCURRENCE WITH NO CB WX PROBS. I BELIEVE THE SITUATION OCCURRED BECAUSE OF CONFUSION SURROUNDING THE SYS WE WERE OPERATING UNDER (IE, DOPPLER, MUST GO-AROUND OR LLWAS CAN LAND USING OTHER CRITERIA) AND THE LAST MIN NATURE OF WARNING. TWR ALERT ON FINAL AT ABOUT 500' AND NO VIS PERCEPTION OF DANGER. CBS S AND CLOUD BASES N BUT DEN OUT IN CLR. NO CHANGE IN AIRSPD OR OTHER ACR PROGRAM WINDSHEAR CLUES, IE, VERT SPD UNUSUAL, EXCESSIVE PWR, TEMP, DEWPOINT SPREAD OF 30-50 (ONLY THE TEMP OF 77 DEG WAS GIVEN ON ATIS), BLOWING DUST, ETC. THE LATITUDE OF OUR PROGRAM MAY CONTRIBUTE TO THIS CONFUSION. CAN LAND WITH MICROBURST WARNING UNDER ONE SYS BUT NOT ANOTHER. MAYBE ONE HARD MUST GO-AROUND SYS OR WARNING TRIGGER (IE, 'MICROBURST' BUT NOT NECESSARILY 'WINDSHEAR'). ANOTHER FACTOR IS THE RELATIVE INEXPERIENCE IN POS OF BOTH OF US, I HAVE 23 YRS AT ACR BUT ONLY 5 MONTHS AS CAPT AND THE F/O HAS ONLY 5 MONTHS AS F/O WITH 18 MONTHS AT ACR. DURING THE EVENT WE WERE BOTH FOCUSED ON LNDG, AND GOING AROUND ONLY AFTER SOME TRIGGERING EVENT THAT FORTUNATELY DIDN'T HAPPEN. IF IT COMES UP AGAIN I'M SURE WE WILL BOTH BE MORE CAUTIOUS AND EXERCISE BETTER OR MORE CONSERVATIVE JUDGEMENT.

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.