B737 captain reported an altitude deviation occurred on arrival into ord when the crew was distracte...

Attributes
ACN	1524228
Time
Date	201803
Local Time Of Day	1801-2400
Place
Locale Reference	ORD.Airport
State Reference	IL
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B737 Next Generation Undifferentiated
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Commercial Fixed Wing
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 300 Flight Crew Type 4239
Events
Anomaly	Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Inflight Event / Encounter Wake Vortex Encounter

Narrative:

Prior to the approach I had briefed a visual backed up by an ILS to 27L to ord. We were cleared for the approach at 7000 ft. We were approximately 21 miles at 7000 ft. At the last minute I thought best to use VNAV and practice a LNAV/VNAV approach till closer to the airport; then switch to an ILS approach. I thought I had the FMS programed correctly. The airplane started an early descent crossing grabl intersection below the published 7000 ft level. It took me a second to realize we were below the published approach segment because we were encountering some wake turbulence from the preceding airplane. I realized the plane had started an early descent on the approach and I took action to level the plane and return to 7000 ft. Once back on the published approach I converted back to an ILS approach and it was successful. I should have flown the approach as I briefed it to the first officer which was a visual backed up with an ILS. Instead trying to utilize a LNAV/VNAV without prior briefing; my non flying pilot was caught off guard unable to back me up adequately. The unexpected wake turbulence complicated matters and lead to an unintentional altitude deviation.

Original NASA ASRS Text

Title: B737 Captain reported an altitude deviation occurred on arrival into ORD when the crew was distracted by a wake turbulence encounter compounded by an inadequate briefing.

Narrative: Prior to the approach I had briefed a visual backed up by an ILS to 27L to ORD. We were cleared for the approach at 7000 ft. We were approximately 21 miles at 7000 ft. At the last minute I thought best to use VNAV and practice a LNAV/VNAV approach till closer to the airport; then switch to an ILS Approach. I thought I had the FMS programed correctly. The airplane started an early descent crossing GRABL intersection below the published 7000 ft level. It took me a second to realize we were below the published approach segment because we were encountering some wake turbulence from the preceding airplane. I realized the plane had started an early descent on the approach and I took action to level the plane and return to 7000 ft. Once back on the published approach I converted back to an ILS approach and it was successful. I should have flown the approach as I briefed it to the First Officer which was a Visual backed up with an ILS. Instead trying to utilize a LNAV/VNAV without prior briefing; my non flying pilot was caught off guard unable to back me up adequately. The unexpected wake turbulence complicated matters and lead to an unintentional altitude deviation.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

B737 captain reported an altitude deviation occurred on arrival into ord when the crew was distracted by a wake turbulence encounter compounded by an inadequate briefing.