Date: 201308
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: Windshear; 1.5
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class E ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Critical; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Took Evasive Action
Contributing factors: Weather; Aircraft; Human Factors
Primary problem: Human Factors

Narrative: Lost VNAV inside the FAF while on a RNP approach in marginal weather conditions. Although the weather was below minimums all day and forecast to remain so; after we delayed for an hour we were told the weather was reported at 400 with 1.5 visibility wind 080/18 making RNAV 36 usable. The Captain agreed to go. We were mishandled by the Enroute Controller who kept us high to accommodate other traffic; which subsequently required us to slow to 270 to drop the gear at 12;000 FT in order to regain the profile for the RNAV approach. We were stable on the arrival with VNAV path and LNAV engaged autopilot on for a RNP 1.5. The Captain thoroughly briefed the autopilot approach then walked through the calls and actions we would perform on the missed approach.  We were told to contact the Tower at (FAF). The Tower cleared us to land and when prompted reported the winds now at 080/29 steady. This was a great increase and startled me. The Captain queried if it was within limits. I checked via the iPad; which necessitated numerous page swipes - a major distraction while inside the FAF. When I looked up; I noted immediately that we had lost VNAV. The Captain stated 'We lost VNAV.' I looked at the altimeter and called 'Approaching DA' and immediately 'DA' The Captain asked if I could see anything and I said 'no - only trees' and he called for the Missed Approach. We continued to sink as he applied power and we executed the missed. I thought we were critically close to the trees and within seconds of contacting them. At the gate; the agent told me the passengers were all commenting on how close the trees were.I don't understand why we lost VNAV. I missed the exact moment we lost it due to being distracted with the iPad; but upon recognition; it seemed there was an unexpected brief delay in the processing of the information before the missed was executed. I believe the Dispatcher pushed the flight to depart when it should not have and I believe the combined meteorological factors precluded this being a safe approach. I don't think 38 feet is enough reported ceiling above DA to render a consistently safe outcome and I think a 29 KT direct crosswind adds enough complication that even though it is technically within limits it's an unsafe margin given the winds swirling against the back of the bowl in the bay; and the angle the autopilot must maintain to comply with the 1.5 RNP. To arrive at the MAP; locate the runway through reduced visibility and cloudy conditions with the extreme crosswind angle; with only 38 feet to establish control and descend safely from the already steep 3.75 glideslope; does not make safety sense in my mind. Had I not been distracted by the iPad search for limits (something I would have already had in front of me with paper); I may have noted the VNAV loss and prompted the need for the missed a 1/2 a beat earlier; but I don't know that I actually would have noticed it any sooner as the Captain articulated it himself a half second behind my recognition. Further; the ceilings were not as reported at our location; and the loss of VNAV was a critical component of a safe approach. If we had lost the LNAV instead; I believe we would have been an accident due to the strong crosswind. As a First Officer I can already tell that as a Captain; I would not descend to minimums on this approach under these conditions even with a functioning VNAV. This approach requires a highly alert crew; reasonable winds; accurate weather reporting and reliable and properly functioning equipment; none of which we had in my opinion.




Synopsis: Air carrier First Officer reported CFTT incident following loss of VNAV on approach in marginal weather conditions.