Narrative:

I just had 2 clearances (on the same flight) on the robuc 3 arrival into bos that; in my opinion; highlight some problems with 'descend via' clearances that are modified by ATC. The first clearance was given by boston center prior to robuc; namely 'descend via the robuc 3 arrival; speed 290 until robuc'. The word 'until' introduces a slight ambiguity as to what speed to fly over robuc itself...290 or published speed (260)? A better clearance would be..'descend via except speed 290; then published speeds at robuc...' or 'descend via except speed 290; published speeds after robuc'. These have different meanings and are not ambiguous. The word 'until' is ambiguous to me! (Does the expression 'until X' include X or not?). I say it would be better to use 'at' or 'after'; depending on intent.more seriously; on handover to bos approach; I think near robuc; at a speed between 260 and 290 in VNAV path mode; we were told to slow to 220. There was no restatement of altitudes; so my understanding was to continue to descend via; except speed 220. Due to the immutable laws of physics/aerodynamics; it is not immediately obvious that the aircraft was capable of complying with the the altitude windows at sofee and ernei; since the early speed reduction could make the aircraft higher than the upper altitude of the window. There is no easy way to know in advance if compliance is possible. In actuality; we used speed intv to slow to 220; then used speedbrake while monitoring the vsd (vertical situation display) to ensure compliance (or possibly not) with the altitude windows.ATC should be made aware that after an aircraft has begun a 'descend via' clearance; it may well be physically impossible to slow below the programmed descent airspeed and still comply with published altitudes; especially altitude windows with upper limits. Moreover; it may be impossible for pilots to know the aircraft capability in this situation; until speed has been reduced and actual descent rate examined.

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Original NASA ASRS Text

Title: B737 Captain reported confusion after receiving a speed assignment from ATC and described difficulty to meet crossing restrictions after having the speed reduced.

Narrative: I just had 2 clearances (on the same flight) on the ROBUC 3 arrival into BOS that; in my opinion; highlight some problems with 'descend via' clearances that are modified by ATC. The first clearance was given by Boston Center prior to ROBUC; namely 'Descend via the ROBUC 3 arrival; speed 290 until ROBUC'. The word 'until' introduces a slight ambiguity as to what speed to fly over ROBUC itself...290 or published speed (260)? A better clearance would be..'descend via except speed 290; then published speeds at ROBUC...' or 'descend via except speed 290; published speeds after ROBUC'. These have different meanings and are not ambiguous. The word 'until' is ambiguous to me! (Does the expression 'until X' include X or not?). I say it would be better to use 'at' or 'after'; depending on intent.More seriously; on handover to BOS approach; I think near ROBUC; at a speed between 260 and 290 in VNAV PATH mode; we were told to slow to 220. There was no restatement of altitudes; so my understanding was to continue to descend via; except speed 220. Due to the immutable laws of physics/aerodynamics; it is not immediately obvious that the aircraft was capable of complying with the the altitude WINDOWS at SOFEE and ERNEI; since the early speed reduction could make the aircraft higher than the upper altitude of the window. There is no easy way to know in advance if compliance is possible. In actuality; we used SPD INTV to slow to 220; then used speedbrake while monitoring the VSD (vertical situation display) to ensure compliance (or possibly not) with the altitude windows.ATC should be made aware that after an aircraft has begun a 'descend via' clearance; it may well be physically impossible to slow below the programmed descent airspeed and still comply with published altitudes; especially altitude windows with upper limits. Moreover; it may be impossible for pilots to know the aircraft capability in this situation; until speed has been reduced and actual descent rate examined.

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.