March 7, 2010
Cat III approach
Just prior to a late night departure for a short, 30 minute flight to our scheduled layover, the weather at our destination was good but not great. It's usually the visibility, not the ceiling, that is the limiting factor with respect to legally beginning an approach. At our destination that night, the visibility was 2 1/2 statue miles and forecasted to stay that way. It was my leg and I, the first officer and a guy who doesn't like to give up a landing for any reason, am not allowed to land the airplane if the visibility is less than a mile. I've never been crazy about this rule, but its intent is to put the aircraft in the hands of the most experienced pilot while flying in the most challenging conditions.
Our flight that night progressed normally, climbing through a thin, stratiform cloud layer that streamed through the landing lights like soft white ribbon revealing a beautiful star filled sky and full moon above. The air was stable and smooth...I could hear the flight attendants beginning their duties as we retracted the last of the flaps and slats. In less than 10 minutes we were cruising at 23,000 ft, completing cruise checklist items, planning our descent and retrieving destination weather. The automated weather system reported visibility of 2 1/2 miles, conditions that were confirmed by the approach controller at first contact. Ten miles later and a mere 20 miles to the airport, the conditions began to change. 20 miles may seem like a lot, but we were still clipping along at 250 kts and would quickly gobble up 20 miles.
With 20 miles to touchdown, we were informed that the visibility on runway 17R, our intended runway, had just fallen to one mile and the visibility on runway 17L was now at 1/2. We had some quick decisions to make. We were set up for an approach to runway 17R. If the weather remained unchanged we could safely and legally continue and I could execute the approach and land, but the lowest visibility to legally fly the approach was 3/4 of a mile. Runway 17L on the other hand, was equipped with a special approach that would allow us to land with a visibility of 600 RVR. (Runway Visual Range). Since the visibility was so different on two runways in such close proximity, we decided to change to runway 17L and fly what is known as a CAT III, autoland approach. This approach is flown all the way through touchdown and to a complete stop on the runway by the autopilot and takes special consideration and planning. Given our distance to the runway, we elected to go-around and give ourselves some extra time to set up the approach.
By the time the controller vectored us around for the approach to runway 17L, the weather on both runways was reported to be 1/4 and falling. Of course, since the vis was below one mile, I had to relinquish control of the jet and my precious landing to the Captain. As we continued the approach and intercepted the glide slope, we each began our required call outs and completed the landing checklist. At 500 ft, I reported "on speed, sink 700" and noticed that the cloud tops were still glowing in the moonlight in my peripheral vision. My call outs continued at 300 ft. as I heard the tower controller announce 17L visibility at 600 RVR..."300" "200"...still above the cloud tops..."100"...finally in the clouds...at 50 feet I reported "minimums" just as the Captain stated "landing" and the autopilot continued the approach as the aircraft announced "50" "40" "30" "20" "10" followed by the best autoland I've ever witnessed. Very nice.
We crawled to the gate that night. The visibility was so poor that we couldn't see the terminal from the center line on the ramp. The ground crew had to walk out to the aircraft to guide us onto the lead in line. It's been a strange winter this year. Low visibility approaches like this one are rare in the U.S. In the last 10 years I have only flown 6 approaches that required an autopilot flown approach and landing. 3 of those were in February 2010.
Posted by APC at 5:15 PM