B737NG flight crew reported an altitude deviation on arrival into bos.

Attributes
ACN	1489533
Time
Date	201710
Place
Locale Reference	BOS.Airport
State Reference	MA
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737 Next Generation Undifferentiated
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	STAR JFUND2
Flight Plan	IFR
Aircraft 2
Make Model Name	B757-300
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	STAR JFUND2
Flight Plan	IFR
Person 1
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Type 8012
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Type 5102
Events
Anomaly	Deviation - Altitude Overshoot Deviation - Procedural Clearance Inflight Event / Encounter Wake Vortex Encounter

Narrative:

We had a 757-300 in front of us on the jfund 2 arrival into bos. We were in and out of his wake due to the tailwind on the arrival; so as PF I suggested we ask for a 1 mile right offset. Pm agreed; ATC granted. We were 'descend via' on the arrival. I put the offset into the FMC and it began the offset; then said unable after some fix (can't remember for sure...I think it was jfund). Abeam that fix; it went to cws (control wheel steering) right; but stayed in VNAV pth; so I said I would just use cws right to stay right 1.0 manually (VNAV pth was still engaged). Prior to spysd; we received an approach change from ATC. Pm changed it in the FMC (same runway; same transition; ILS 33L now vs light visual 33L) and I agreed and pm executed. The path disappeared (not uncommon while it recalculates momentarily) but this happened just prior to spysd and we descended below 7;000 prior to spysd before the path came back up. About the time I realized we were low (about 200) ATC gave us a 'descend and maintain 3;000' clearance. I don't think there was any conflict with anyone else; but it bruised my ego.

Original NASA ASRS Text

Title: B737NG flight crew reported an altitude deviation on arrival into BOS.

Narrative: We had a 757-300 in front of us on the JFUND 2 arrival into BOS. We were in and out of his wake due to the tailwind on the arrival; so as PF I suggested we ask for a 1 mile right offset. PM agreed; ATC granted. We were 'descend via' on the arrival. I put the offset into the FMC and it began the offset; then said unable after some fix (can't remember for sure...I think it was JFUND). Abeam that fix; it went to CWS (Control Wheel Steering) Right; but stayed in VNAV PTH; so I said I would just use CWS Right to stay Right 1.0 manually (VNAV PTH was still engaged). Prior to SPYSD; we received an approach change from ATC. PM changed it in the FMC (same runway; same transition; ILS 33L now vs Light Visual 33L) and I agreed and PM executed. The path disappeared (not uncommon while it recalculates momentarily) but this happened just prior to SPYSD and we descended below 7;000 prior to SPYSD before the Path came back up. About the time I realized we were low (about 200) ATC gave us a 'descend and maintain 3;000' clearance. I don't think there was any conflict with anyone else; but it bruised my ego.

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.