A B737 Captain engaged the autopilot Approach mode while on a southeast heading to capture the BUF Runway 23 ILS localizer. The aircraft pitched up as the glide slope fluctuated and climbed 600' before the autopilot was disconnected.
Synopsis
A B737 Captain engaged the autopilot Approach mode while on a southeast heading to capture the BUF Runway 23 ILS localizer. The aircraft pitched up as the glide slope fluctuated and climbed 600' before the autopilot was disconnected.
Narrative
Autopilot A was engaged; and to my best recollection; auto throttles were disengaged. Being vectored for the ILS 23 at BUF; we were on a 060 outbound heading paralleling the localizer course on the north side. We were given a clearance to descend to 2;500 FT. Upon reaching we were given a right turn to 210 to intercept; maintain 2;500 until established; and cleared for the ILS 23. Captain selected approach mode on the MCP and turned the heading selector to 210 degrees. We were 6 miles from the runway and slowing. Captain called for and I selected flaps 5. At this point I saw we had not intercepted the localizer and that the glideslope indication was erratic. At about the same time; 3 things happened simultaneously; we intercepted the localizer; the aircraft pitch up to about 10 degrees attitude and we climbed. I said; 'Watch your pitch attitude.' The Captain did not disengage the autopilot and the aircraft pitched slightly down and then up abruptly to about 18 degrees approaching the yellow pitch limit indicator. I said; 'What are you doing?' He disengaged the autopilot but we had already climbed to 3;100 FT. All this happened rather quickly and we were just inside the outer marker and high. But we were able to become stabilized and on glidepath by 1;000 FT AGL by selecting flaps 40 and we landed normally. This was a confluence of miscues and lack of communication between the Captain and me. The ATIS indicated high scattered clouds; 3 miles visibility; rain; and mist so he briefed the full ILS 23 approach. What we both failed to brief or even notice was the special notice to Pilot's page immediately after the ILS 23 in the commercial charts. This page says not to engage approach mode until established on the localizer because the glideslope is unusable beyond 5 degrees north of the localizer course. The ATIS mentioned the part about the glideslope being unusable north of course as well. When we were being vectored; the visibility had improved to greater than 10 miles below 5;000 FT so we could see the airport the entire time. As we made the turn inbound I failed to notice that the Captain had engaged approach mode instead of localizer mode until intercepting the localizer. We were vectored very close to the outer marker and it seems we were inside of 5 degrees of the localizer at the time the aircraft started pitching up abruptly. But I should have pointed out that he had approach mode selected with the glideslope indicating erratically. The other miscommunication I had with the Captain was not suggesting he immediately disengage the autopilot at the first sign of unexpected pitch oscillations. He did so after the second pitch up almost to the PLI; but I was ready to do so myself if he hadn't. In the final analysis we should have been more vigilant and taken heed of the special procedures pages in the approach charts and the ATIS. Especially after a long day that was approaching 11 hours.
Second reporter narrative
The initial pitch up caught me by surprise and I probably could have prevented the altitude gain by being more aggressive with flight control inputs. Since we were climbing; I didn't feel the need to panic and overcorrect. We were with Tower at the time and nothing was said about the approach. There is a warning page concerning this approach in our charts and it probably needs to be highlighted a little better. If approach is going to vector aircraft from this direction and if the ILS signal is a problem; mentioning this phenomena may help. This may help breaking habit patterns.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.