B767 flight crew reported altitude deviations on approach related to wake and atmospheric turbulence.
Synopsis
B767 flight crew reported altitude deviations on approach related to wake and atmospheric turbulence.
Narrative
As Captain Monitoring we were about 25 miles from CVG and told to descend and maintain 5000 ft. on a heading. We were turned onto about a 25 mile final and told to intercept the 27 localizer; maintain 5000 ft. until established on the localizer and cleared for the approach ILS 27. Also we were 5 miles in trail of a 777. Winds were strong at the field; do not remember exact but around 290 at 20 gusting to 32. We added the 15 kts. to approach speed for the target. Winds were slightly stronger at 5000 and we were experiencing turbulence. The FO (First Officer) armed the LOC and we captured it. Next he armed the approach and G/S was armed. Still about 20 miles from the field. RVR reported greater than 4000 ft. Autopilot was on. I was looking for G/S alive on raw data. I felt the aircraft nose over and saw the FMA GS captured. The aircraft started to descend. I told the FO to disconnect the autopilot and climb back to 5000 ft. as we were not close to KAYDE and did not know if he realized we had captured a false GS. Before I could call Approach they called us and said 'maintain 5000 ft.'. We were in turbulence the entire time. As the FO climbed back to 5000 we felt stronger turbulence; possibly from the 777 who was also at 5000 ft. With autopilot disconnected I reached over and cycled both flight directors. I saw the altimeter go through 5000 ft. towards 5400 ft. and told the FO to return to 5000 ft. As Pilot Monitoring I re-armed the approach mode. The LOC captured and G/S was armed again. We did not realize if the G/S captured from equipment interference or the turbulent weather or wake turbulence. The FO decided to hand fly the aircraft for the remainder of the approach. Minimums were Cat 1. We landed without any further problems and taxied to the parking gate. Waiting almost three hours for the gate to be cleared.
Second reporter narrative
I was the Pilot Flying. ATC instructions were to descend and maintain 5000 ft. on a heading which put us approximately 20~25 miles final of CVG Runway 27. We were cleared to intercept the 27 localizer; maintain 5000 ft. until established on the localizer as well as cleared for the approach ILS 27 with a caution for being 5 miles in trail of a 777. Winds were published as speed 22 kts. gusting to 34 with headwind component (can't recall the exact direction) in the final ATIS. Per procedure I added the 15 kts. to approach speed for the target. Winds were slightly stronger at 5000 ft. and we were experiencing turbulence. Aircraft captured the LOC with AP engaged and I armed the Approach mode. RVR was being reported 4000~6000 by the ATC over the comm as RVR was going back and forth within that range due to the storm. I was looking for G/S alive on raw data. At that time I heard the Captain saying we are descending and asked why? I scanned GS raw data and it did not show the GS intercept with the magenta diamond pegged at the top of the scale. Captain asked me again why are you descending which was completely contradicting what I was seeing in the raw data. So as I scanned the ALT in the PFD it indeed showing ALT less than 5000 ft. It got me startled and took few seconds to recover. At Captain's prompt I disconnected the AP and began the climb back to 5000 ft. through the very rough wind condition. I may have overshot the altitude before establishing at 5000 ft. Before we could call Approach they called us and said 'maintain 5000 ft.' We were in turbulence the entire time which was made worse possible from the wake of preceding 777 who was also at 5000 ft. With autopilot disconnected the Captain cycled both flight directors; re-armed the Approach; and FMA was reflecting the correct modes. We did not realize if the G/S captured from equipment interference or the turbulent weather or wake turbulence. I decided to hand fly the aircraft for the remainder of the approach to minimize the possibility for the same thing to happen again. Minimums were Cat 1. We landed without any further incidents and taxied to the parking gate. It was a long 2 hours 23 minutes taxi to the gate due to the winter storm and airport conditions. To my best recollection at the time of the occurrence the AP was engaged with ALT selector at 5000 ft. in the MCP; FMA showing LOC GREEN; APP armed; Vertical component of the FD suddenly dropped to the bottom while Horizontal component indicating slightly right of course; raw data G/S indicator (the magenta diamond) pegged to the top of the scale.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.