Date: 201701
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: 0.75
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Landing
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Troubleshooting; Human-Machine Interface
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Aircraft; Weather
Primary problem: Weather

Narrative: [The area] was experiencing a severe winter storm. We were flying a 737NG; flaps -40; in heavy rain; moderate turbulence; strong gusty crosswinds; wipers on high; 0.75 mi visibility to ZZZ's runway. At 'minimums' we saw the MALSR's 'going outside.' The HUD worked great and with the runway environment in sight at 200 ft I said; 'landing.' That's when the fun started. Perceiving the runway environment is one thing. Having sufficient depth perception to finesse a normal flare and landing is quite another. The issue is that you need to visually transition to something real outside the window to land the aircraft; which is impossible when you're looking through a 'sheet of water.' I believe the B737 windshield wipers just cannot create enough of an air gap in heavy rain for adequate forward visibility. You feel like you're looking through glycol. Everything is blurry and it frankly is not safe. After having experienced this complete lack of forward visibility at least twice now; I'm convinced that the B737 wipers are inadequate in heavy rain. Tower visibility might be a couple miles; because they can see familiar landmarks on the airfield; but that's without looking through a 'layer of water' on their windows.It wouldn't be difficult to test the real-world ability of the B737 wipers against various rain intensities; an eye chart outside a cockpit window in a wind tunnel and a way to dump various amounts of water on the windshield. Then some real-world flight testing. I've taken a straw poll and pilots with Regional Jet experience reported that the steeper angle of the RJ windows allows the RJ to shed water aerodynamically. RJ's hardly ever need wipers. The Lockheed C-141 didn't have wipers. It used bleed-air to create a boundary-layer over the windshield. The B737 classics have rain-repellant. Why? Initially the fix would be to disallow approaches in +RA. Something needs to be used as the Go/No-Go determinator. The (+) plus in +RA (heavy rain) would tell us to not even try the approach; regardless if the visibility is legal. Once the wipers have been tested in laboratory conditions to determine the actual forward visibility in various down-pour severities; technological improvements can be implemented. Initially the fix would be to disallow approaches in +RA. Then thorough testing of the actual visibility in various intensity rain conditions.

Callback: Reporter is concerned of a wider problem associated with the amount of water and the visibility out the window relating to window wipers. He suggested using rain repellent like product; but then mentioned problems with possible fumes and possible Boeing warranty issues. Reporter recommended testing in a wind tunnel to adjust the window angle. Reporter was asked if it is a problem associated only to the NG series aircraft. Reporter stated the early B737 model had a button inside the cockpit by the window wiper that squirted rain repellent fluid on the window. Reporter also stated that newer aircraft have a film on the window to work like rain repellent; but it wears out in a year or so.



Synopsis: B737NG Captain reported window visibility issues when flying in heavy rain.