B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 205
Confusion
Aircraft Equipment Problem Less Severe
General None Reported / Taken
Procedure; Chart Or Publication; Aircraft
This was a Passenger flight to MMPR that did not result in any ATC deviation. I have a general safety concern about an anomaly that I observed with our FMC and vertical NAV program on three separate flights going into MMPR in three different aircraft in 2 weeks. Our flight plan has us arriving into PVR on J141 from MZT to PVR. There is no STAR for MMPR. The VOR 2 Runway 22 (page 13-5) is flown from the PVR 331 radial to the 15 NM arc with numerous step downs over high terrain to intercept the 036 radial for Runway 22. To set up for the approach we used the VOR 2 Runway 22 KENDU transition. I used this on each of the three flights for the last two weeks. The FMC places a waypoint well outside of KENDU on the 331 radial. It is coded as PVR331 and it automatically includes an altitude restriction at or above 11;000 feet. I have no idea where this coded altitude restriction comes from. It is not on any of the charts (High/ Low; terminal; approach or TMA).Upon beginning the descent from cruise; with the descent winds; transition altitude; and altimeter setting loaded; the FMC always had the same reaction. We were given an initial descent from cruise to FL240; then FL200; and then 11;000 feet; then 7000 feet then cleared for the full approach. Upon the first descent from flight level; the vertical deviation indicators on the V-Path would close; and either touch or would turn yellow. On my last flight the VNAV would not descend and the vertical Required Navigation Performance (RNP) went immediately to 125 feet while the Actual Navigation Performance (ANP) was at some value above this. This continued all the way on the approach while flying the arc; progressively getting better until intercepting final where the ANP values stabilized at what appeared to be normal approach values. One other variable in this is that the altimeter was very low; below 29.40. I flew these approaches in LNAV using Vertical Speed to ensure terrain avoidance while our VNAV indications were showing either out of tolerance; or just in tolerance.I think we need to look at the program data for this approach into PVR and also the communication of the loaded altitude setting in the Descent Forecast page and how that effects the VNAV at lower settings. Although the first indications occurred well above the flight level transition the ANP could barely keep up or did not keep up depending upon the aircraft I was flying. In contrast; when I flew the HAWKZ4 Arrival into KSEA; RNP vertical values remained at 400 on the STAR and did not transition to 125 until the beginning of the approach. I think that the computer is automatically transitioning to Approach mode while still 50 miles from the field with the flaps up and still in the flight levels. The IAF is at 17 miles; just prior to arc intercept on the 331 radial on PVR. These events occurred 20 or more miles from that point; so I think there is a problem with our programing. We may need to look at this at more than MMPR. This may be a problem at more international destinations where we transition from the high enroute structure to the approach immediately. Add night; IMC; thunderstorms; and fatigue and this could be a problem for our Crews. Either that or I somehow messed up two weeks in a row loading this approach; but I don't think so.
B737-700 Captain reported that when the VOR 2 Runway 22 via KENDU is loaded in the FMC; a waypoint (PVR331) is created on the PVR 331 Radial at 50 NM with an 11000 or above crossing restriction. Irregularities with the Required Navigation Performance and Actual Navigation Performance are also noted during the approach; gradually diminishing as the approach progresses. The irregularities occurred on three separate occasions during approach to MMPR.