Boeing 787 VNAV
Hi All. I’m fairly new to Boeing from years on Airbus and have a question about VNAV.
We were flying a STAR that had a level constraint somewhere along it. We reached ToD calculated by the aircraft, based on a 265kt ECON speed and requested a descent. The controller gave us a tactical heading and a clearance to descend. Initiating a descent in VNAV and using HDG SEL, aircraft increased its RoD to something ridiculous like 4000fpm+, so I just took it out of VNAV using an appropriate VS, using 3x table to the level constraint point.
Unlike Airbus, which would kick you out of a managed descent if you took it off the lateral path, I understand Boeing allows VNAV in HDG SEL, but I’m confused as to what it calculates and how it could decide a nice leisurely RoD initially when on the lateral path but suddenly increase the RoD massively when going HDG SEL on a not too different heading to the original one.
How does it calculate the VNAV profile when on HDG SEL? What would it do if you subsequently are given an orbit and start tracking away from the constraint WP?