Narrative:

This scenario involves the frdmm one arrival into dca; an arrival that has numerous altitude parameters and several defined speeds; obviously designed for a VNAV capable aircraft. With all programming confirmed and forecast winds loaded; we were vectored off the arrival three times and assigned different altitudes and airspeeds then directed to rejoin the profile and set up for the river visual from the north to runway 19. ATC needs to appreciate that the VNAV system calculates an energy optimized descent profile and when disrupted requires pilot intervention to re-establish. Additionally; the protocols for altitude protection are different when on the VNAV path and when using other descent options. Shifting back and forth between the two is labor intensive and increases the risk of violating an altitude/speed restriction. In our case the weather was VMC but the addition of weather avoidance considerations would have added another challenge to the process. We did a great job complying but found ourselves continuously programming; resetting parameters; and confirming we would be able to comply with the next restriction- two of us fully engaged for 20 minutes. It will only be a matter of time before a ball gets dropped. If cleared for a descent profile this complex; do not mess with it. If an aircraft must be asked to leave the profile; don't ask them to return to it. I will not accept this amount of mixed descent requirements in the future.

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Original NASA ASRS Text

Title: B737-800 Captain detailed the problems that arise when VNAV equipped aircraft are vectored off the arrival and subsequently asked to return to the published arrival.

Narrative: This scenario involves the FRDMM ONE arrival into DCA; an arrival that has numerous altitude parameters and several defined speeds; obviously designed for a VNAV capable aircraft. With all programming confirmed and forecast winds loaded; we were vectored off the arrival three times and assigned different altitudes and airspeeds then directed to rejoin the profile and set up for the River Visual from the north to Runway 19. ATC needs to appreciate that the VNAV system calculates an energy optimized descent profile and when disrupted requires pilot intervention to re-establish. Additionally; the protocols for altitude protection are different when on the VNAV path and when using other descent options. Shifting back and forth between the two is labor intensive and increases the risk of violating an altitude/speed restriction. In our case the weather was VMC but the addition of weather avoidance considerations would have added another challenge to the process. We did a great job complying but found ourselves continuously programming; resetting parameters; and confirming we would be able to comply with the next restriction- two of us fully engaged for 20 minutes. It will only be a matter of time before a ball gets dropped. If cleared for a descent profile this complex; do not mess with it. If an aircraft must be asked to leave the profile; don't ask them to return to it. I will not accept this amount of mixed descent requirements in the future.

Data retrieved from NASA's ASRS site as of July 2013 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.