A B737-700 pilot reports the prevailing visibility at the runway departure end to be one-quarter mi ...

Attributes
ACN	634619
Time
Date	200410
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : smf.airport
State Reference	CA
Altitude	agl single value : 0
Environment
Flight Conditions	IMC
Weather Elements	Fog
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	ground : takeoff roll ground : position and hold
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 78 flight time total : 9000 flight time type : 78
ASRS Report	634619
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 220 flight time type : 22
ASRS Report	634707
Events
Anomaly	non adherence other
Independent Detector	other flight crewb
Resolutory Action	controller : issued advisory controller : issued new clearance other
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Weather Navigational Facility Flight Crew Human Performance Airport ATC Human Performance
Primary Problem	Weather

Narrative:

This report is a request for clarification on what determines whether RVR or pv (prevailing visibility) is controling. While we were approaching #1 at smf runway 16R for takeoff, the tower reported RVR at 400 ft, 1400 ft, 1200 ft. Not good enough for us to depart. However, upon arriving at the #1 position, it appeared that the visibility was close to 1/2 mi or better. The tower asked how the visibility looked to us, and to be conservative we said that it looked better than 1/4 mi. Tower said 'prevailing visibility is greater than 1/4 mi, cleared for takeoff.' I did a quick check of the departure page and determined that greater than 1/4 mi was good for takeoff. So we departed and indeed the visibility was at least 1/4 mi through the whole takeoff roll. My understanding is that once the tower reports pv, that it is now controling. Further study left me less sure about when pv is controling versus RVR. I have always tried to operate well within the regulations, but this seems a little gray. I wanted to bring this to light so I or anyone else may better understand what is correct.

Original NASA ASRS Text

Title: A B737-700 PLT RPTS THE PREVAILING VISIBILITY AT THE RWY DEP END TO BE ONE-QUARTER MI WITH A 400 FT RVR RPTED. THE TWR CLRED THE CREW FOR TKOF AND THEY DEPARTED.

Narrative: THIS RPT IS A REQUEST FOR CLARIFICATION ON WHAT DETERMINES WHETHER RVR OR PV (PREVAILING VISIBILITY) IS CTLING. WHILE WE WERE APCHING #1 AT SMF RWY 16R FOR TKOF, THE TWR RPTED RVR AT 400 FT, 1400 FT, 1200 FT. NOT GOOD ENOUGH FOR US TO DEPART. HOWEVER, UPON ARRIVING AT THE #1 POS, IT APPEARED THAT THE VISIBILITY WAS CLOSE TO 1/2 MI OR BETTER. THE TWR ASKED HOW THE VISIBILITY LOOKED TO US, AND TO BE CONSERVATIVE WE SAID THAT IT LOOKED BETTER THAN 1/4 MI. TWR SAID 'PREVAILING VISIBILITY IS GREATER THAN 1/4 MI, CLRED FOR TKOF.' I DID A QUICK CHK OF THE DEP PAGE AND DETERMINED THAT GREATER THAN 1/4 MI WAS GOOD FOR TKOF. SO WE DEPARTED AND INDEED THE VISIBILITY WAS AT LEAST 1/4 MI THROUGH THE WHOLE TKOF ROLL. MY UNDERSTANDING IS THAT ONCE THE TWR RPTS PV, THAT IT IS NOW CTLING. FURTHER STUDY LEFT ME LESS SURE ABOUT WHEN PV IS CTLING VERSUS RVR. I HAVE ALWAYS TRIED TO OPERATE WELL WITHIN THE REGS, BUT THIS SEEMS A LITTLE GRAY. I WANTED TO BRING THIS TO LIGHT SO I OR ANYONE ELSE MAY BETTER UNDERSTAND WHAT IS CORRECT.

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.

A B737-700 pilot reports the prevailing visibility at the runway departure end to be one-quarter mi with a 400 ft RVR reported. The tower cleared the crew for takeoff and they departed.