B737 flight crew reported encountering wake turbulence on final approach to IAH in trail of a heavy jet and as a result the aircraft's autopilot did not join the final approach course; causing the flight crew to fly through it.
Synopsis
B737 flight crew reported encountering wake turbulence on final approach to IAH in trail of a heavy jet and as a result the aircraft's autopilot did not join the final approach course; causing the flight crew to fly through it.
Narrative
On approach into IAH; we were cleared direct RAIDS to intercept the localizer for 26R and cleared visual 26R. We discussed and decided to let LNAV mode turn us onto final then arm the approach mode due to the distance RAIDS was from the field. Winds were indicating calm at 4000 feet. Approach Control notified us we were approximately 7.5 miles behind a heavy" right as we were approaching RAIDS and before he finished his transmission we encountered the aircraft's wake. Our aircraft rolled approximately 20 degrees left then right. I instinctively put my hands on the controls and almost disengaged the automation when it rolled to the right; but it appeared to correct. I monitored for a second as the Captain responded to the controller's transmission that we had encountered it. At that point I noticed the little amber flag indicating the automation had dropped out of LNAV mode into Control Wheel Steering (CWS) and vocalized that it was in CWS. After finishing his transmission; the Captain asked if I wanted LNAV back as the controller noticed and responded that it looked like we were going to fly through final and gave us a 290 heading to rejoin. I engaged heading select; spun the heading in; and armed approach mode to regain the final path as the Captain responded to the instructions on the radio. This happened in a matter of seconds as the wake turbulence encounter happened right as we were turning to final. The deviation south of the final was minimal and we quickly got back on final. After we regained final; the controller slowed us to 180 and then 160 until the final approach fix (we were assigned 210 knots before the encounter) to build more spacing for us off the heavy. The rest of the approach and landing were uneventful."
Second reporter narrative
We were cleared direct RAIDS and cleared the visual approach for Runway 26R. As we were starting our turn in LNAV to join the final approach course over RAIDS intersection; we encountered wake turbulence due to following a heavy jet. We believe ATC told us we were 7.5 miles in trail. There was zero wind being displayed on our navigational display. As we encountered the wake; the aircraft rolled approximately 20 degrees left and then 20 degrees right. This caused the autopilot to revert to Control Wheel Steering (CWS) for the lateral mode. Therefore; the autopilot did not join the final approach course completely causing us to fly through the final approach course. We reported the wake encounter to ATC and told them it knocked off our automation. The controller realizing we were through the final approach course gave a heading to 290 to rejoin the inbound course. We complied and reengaged the automation. The remainder of the approach and landing was uneventful. We do not believe traffic separation was ever in jeopardy nor the safety of the flight. We also don't believe there was much of a way to avoid this situation; hence the report for data collection and awareness for all parties. We feel this was a unique culmination of many environmental factors and not the fault of us or ATC. ATC had limited airspace to vector aircraft due to thunderstorms in the vicinity and 7.5 miles seems like sufficient separation to avoid wake turbulence following a heavy jet.
NASA callback
Reporter stated they were surprised at the effect on autoflight the wake encounter had.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.