Narrative:

Approximately 30 DME from bos; ATC cleared us to descend pilot's discretion to 16;000. I mentioned to the international relief officer who was the pilot monitoring that it seemed unusual for ATC not to give us 240 at bos; but that maybe it was because we were at a lower than usual altitude; 280. I checked our FMC legs page and confirmed that there was now no altitude restriction at bos leaving the expected 160 at zzzzz. I double-checked the VNAV page was programmed for .82/310 with the zzzzz restriction planned. The top of descent showed up on the HSI about 1/2 way between bos and zzzzz and I selected des now on the VNAV descent page. I knew we were going to get busy with calling the captain back and our seat swap/bathroom break and wanted to be sure to descend at the appropriate time.just as I finished setting it up; ATC called back amending the clearance to cross bos at 240. A moment later; ATC called again amending the clearance to descend to 180 after the bos restriction. Then another minute or so later; ATC called back again. The pilot monitoring was on the interphone talking to the flight attendants; and he asked me to answer. ATC then amended the clearance again to 190; which I set and pointed out to the pilot monitoring. I kept reprogramming VNAV. Our captain was an lca and I had been discussing the use of VNAV versus flch when things change/get busy on the arrivals and how much work it was to keep re-cruising. He had told me to try des now that it worked really well. I was trying to use that technique and kept reprogramming/reselecting des now with each altitude clearance change. We got a lot of changes pretty rapidly while we were busy with multiple calls from the back and coordinating to use the lavatory and the seat swap.we easily made all the altitude restrictions; but just as the captain called to come back into the cockpit I noticed the airplane getting quiet and saw the airspeed coming back towards 240 knots as we were leveling at flight level 240. I was looking at everything to figure out why. I then hit flch/speed intervene and dialed the speed up to 310. I finally saw on the VNAV descent page I had [incorrectly] programmed (hit the wrong key) on the descent speed the last time I reset it. Instead of .82/310 on the descent page; I had .42/310. I think we got as slow as 235 before the plane started accelerating again. In hindsight; I should have pushed the throttles up manually as it would have recovered to the normal speed quicker.

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Original NASA ASRS Text

Title: B767 flight crew reported inadvertent speed decrease during initial descent.

Narrative: Approximately 30 DME from BOS; ATC cleared us to descend pilot's discretion to 16;000. I mentioned to the IRO who was the Pilot Monitoring that it seemed unusual for ATC not to give us 240 at BOS; but that maybe it was because we were at a lower than usual altitude; 280. I checked our FMC legs page and confirmed that there was now no altitude restriction at BOS leaving the expected 160 at ZZZZZ. I double-checked the VNAV page was programmed for .82/310 with the ZZZZZ restriction planned. The top of descent showed up on the HSI about 1/2 way between BOS and ZZZZZ and I selected DES NOW on the VNAV descent page. I knew we were going to get busy with calling the Captain back and our seat swap/bathroom break and wanted to be sure to descend at the appropriate time.Just as I finished setting it up; ATC called back amending the clearance to cross BOS at 240. A moment later; ATC called again amending the clearance to descend to 180 after the BOS restriction. Then another minute or so later; ATC called back again. The Pilot Monitoring was on the interphone talking to the Flight Attendants; and he asked me to answer. ATC then amended the clearance again to 190; which I set and pointed out to the Pilot Monitoring. I kept reprogramming VNAV. Our Captain was an LCA and I had been discussing the use of VNAV versus FLCH when things change/get busy on the arrivals and how much work it was to keep re-cruising. He had told me to try DES NOW that it worked really well. I was trying to use that technique and kept reprogramming/reselecting DES NOW with each altitude clearance change. We got a lot of changes pretty rapidly while we were busy with multiple calls from the back and coordinating to use the lavatory and the seat swap.We easily made all the altitude restrictions; but just as the Captain called to come back into the cockpit I noticed the airplane getting quiet and saw the airspeed coming back towards 240 knots as we were leveling at flight level 240. I was looking at everything to figure out why. I then hit FLCH/speed intervene and dialed the speed up to 310. I finally saw on the VNAV descent page I had [incorrectly] programmed (hit the wrong key) on the descent speed the last time I reset it. Instead of .82/310 on the descent page; I had .42/310. I think we got as slow as 235 before the plane started accelerating again. In hindsight; I should have pushed the throttles up manually as it would have recovered to the normal speed quicker.

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.