Date: 201412
Local time of day: 1201-1800
Location: LGA.Airport, NY
Weather elements / visibility: Snow
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight phase: Initial Approach
Airspace: Class B LGA
ATC / advisory: TRACON N90
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Procedure; Weather
Primary problem: Weather

Narrative: Enroute to LGA we had monitored ATIS. Arrival ATIS was approximately 1000-1500 overcast 1 mile -SN. Wind was 290 at 14G23. LGA was using LOC 31 but with low vis had to switch to ILS 4. Approach advised 'Braking Action Fair by an Embraer.' I used ACARS to obtain updated landing data.The data came up with a Fair Braking landing roll of about 6400' (don't have the exact info right now). This is on a 7001' runway; no overrun; max braking; min flare; etc. The data also reported the wind as 290 at 14kt and computed zero tailwind for runway 4. This defied logic as 290 deg and 044 deg would clearly produce a tailwind. I quickly ran the winds through a calculator; a 99 cent APP on my iPad; and came up with 22 knot crosswind and 8 knot tailwind. OK; the lawyers won't like that so I found the wind chart in our manuals and came up with 22 knot crosswind and 5 knot tailwind. Either way we were attempting to land in fair braking and an 8 knot tailwind. I had been emailing through ACARS dispatch and he told us there wasn't a tailwind on runway 4 so his data isn't accurate either. This increased our landing data to just over 6800'.We went into holding to get a better handle on this; to determine what had fair braking; and to try to get them to turn the airport around which they would/could not do.Discussion and some confusion ensued due to a recent company message regarding reevaluating braking action based on the type of aircraft reporting vs. your own aircraft. We discussed our own experiences following and flying these reporting aircraft and whether we should give it a try. From a safety stand point there was way too much 'what if'ing' taking place.An A320 landed and reported good braking and approach reported winds of 310 at 12 gust to 22 or there about. Now there wasn't a tailwind so we reluctantly joined final approach. Configured; about at the marker when [another aircraft] landed in front of us and reported 'braking action good; well; actually; good first part of runway fair last part.' We went around and diverted uneventfully.There is clearly a glitch in our automatic ACARS landing data. First; it does not use the gust factor when computing the wind component. I don't know any pilot that would not factor that worst case scenario in especially when landing in less than ideal weather. It also does not accurately compute tailwinds. In our scenario Runway 4 (heading 044 deg) and wind of 290 degrees at 14 kts or 23 kts you still have a tailwind. At max landing weight for LGA; Fair braking; 15% margin in the data you are looking for trouble by not being advised that there is a tailwind. It would have been the difference between stopping at the end or in Flushing Bay. I also believe we need a Part 121 educational program to clarify pilot reports or to at least expand upon the terminology and establish an industry wide consensus as to what constitutes Good; Fair; Poor braking as well as chop; turbulence and the variations between them. Some of this is addressed in various documents yet we still as a profession are lazy or 'non-standard' in the application of these terms.




Synopsis: B737-800 Captain reported diverting because runway and wind conditions at LGA were not favorable. Reporter also stated the automated ACARS landing data was inaccurate.