B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 14000; Flight Crew Type 10000
Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented
Weather; Human Factors; Aircraft
Strong tailwinds in descent; turbulence; and then entered an area of scattered rain showers. As the Non Flying Pilot; I monitored First Officer during descent. Flying new 737-900 that has a tendency to revert from VNAV Path to VNAV Speed with the slightest airspeed deviation. Caught aircraft at least twice leaving VNAV path and reverting to VNAV speed. First Officer was planning on making landing to Runway 22L. Winds were reported 5 KTS down the runway. Just prior to 10;000 FT; switched to approach control; heard ATC telling aircraft ahead of us now switching to Runway 4 Right for landing. Wind now 330 degrees at 24 KTS. I immediately announced I will reprogram the box (FMC) for 4R and sent for a new ATIS; First Officer began to retune own radio for 4R. I then asked First Officer if new wind was greater than allowed since First Officer was new and High Minimums. First Officer stated 'You have the aircraft and I will look (or something to that extent).' I announced I had control of the aircraft and First Officer fairly quickly produced high mins restriction sheet and we both realized First Officer was prohibited from making the landing. At this point; I reached up and selected autopilot A and looked down to verify AP switched properly via the FMA. At this point I realized the aircraft had yet again tripped from VNAV Path to VNAV Speed and we were just below 10;000 FT approaching waypoint DYLIN which we were supposed to cross at 8;000 FT.
The Captain and I had studied the weather prior to takeoff and printed hourly weather reports as the flight progressed. We had prepared for gusty winds. It was my leg to fly; however; we both agreed that if the winds were gusty that the Captain would perform the landing; due to the fact that I was considered a low time pilot and had not achieved my 75 hours of 737 flight time prior to the beginning of this trip. One hour out we monitored the ATIS and winds were actually light. As soon as we started on the arrival we heard a report of windshear on the field and that the controllers were anticipating changing the landing runways from ILS 22R to ILS 4R. My Captain then asked me what the parameters were for low time pilots and windshear reports. I told him I believed that I was not legal to fly the approach; however; we both agreed that I would look up the exact parameters. I gave control of the aircraft to the Captain and we verified that we were descending on the arrival. I proceeded to look up the low time pilot regulations and it did in fact state that I could not land in reported windshear conditions. I read it out loud and then put my documents away and told the Captain; I'm back. As soon as I looked at the Altitude and the Vertical Path Indicator; I could see that we were high and not going to clear DYLIN Intersection at 8;000 FT. The Captain then verified what I was seeing and we were both confused how the aircraft had deviated from its VNAV path. We ended up crossing DYLIN at roughly 9;500 FT; perhaps a bit lower. It turned out that the automation had left VNAV Path and entered VNAV speed. I attribute this to the 91 knot tailwind that we had on the descent. It was something that we were both aware of; however quite a distance from DYLIN I used the speedbrakes to slow down just a bit more to stay on path. Once I had reached a steady state descent I stowed the speedbrakes and the aircraft had no issue staying on path; until I handed control over to the Captain. In a very short amount of time; the aircraft deviated off path; which is how we found ourselves high at DYLIN.
B737-900 flight crew reports missing a crossing restriction during arrival due to the aircrafts tendency to come out of VNAV path into VNAV speed with the slightest provocation. The descent is unmonitored for a few moments while the FP/NFP switch roles and a new runway is entered in the FMC.