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?

Can you be more specific? What were the FMAs? Was your cruise speed slower than your decent speed? Did you descend before, at or after TOD? Did you speed intervene? Where was the decent path? Did the FMC recalculate? Did you add any constraints that might have forced a geometric path?

In HDG SEL it should still cycle waypoints and calculate the path based on the distance to the next waypoint so it could be that your use of HDG SEL was entirely coincidental.

It’s probably similar to Boeing in general: even if not in LNAV, such as when in HDG SEL, the aircraft still calculates the path if it’s still in VNAV(also other modes), using the aircraft’s current position to the first active waypoint.

As for why the rate of descent is high when starting descent — you need to look at the details of what you did at that moment. For example:

• If you’re in FMC speed mode and set the lower altitude late, the aircraft will begin descending after the normal TOD. But if it’s not too far past TOD, the aircraft is still in VNAV PATH even though in reality it’s above the path, so it pushes the nose down to increase the ROD and rejoin the path. That’s why the ROD is high. It’s not related to HDG SEL.

• But if you set the lower altitude early before TOD, you won’t encounter this issue.If you observe the CDU, you’ll see that the aircraft will begin its process of descent a few miles before the TOD.

Or if you used MCP SPD that was higher than the cruise speed, this could also have happened. More details would be needed to know exactly what occurred.It depends on what you did that day.

If you’re not flying in LNAV, in holding, then use another mode. Or if you want to fly VNAV, then fly VNAV SPD instead of VNAV PATH, such as using MCP SPD or adjusting thrust manually

Thanks both.

I think descent might have been exactly coincident or very slightly after the calculated ToD, so it could well have been the jet chasing to get down on the path. With a VNAV descent in HDG SEL your VSD shows nothing apart from the vertical flight path vector so I couldn’t tell what profile it was aiming for….thinking over 4000fpm was a bit extreme I intervened with an average RoD which would have worked but the high RoD may well have been the jet momentarily chasing down to a shallower profile.

Learning point is probably to ensure you request descent well before the calculated ToD in order to avoid barrelling down at ToD.

Airbus treats this very differently…as soon as you pull heading, the jet just leaves you to your own devices and kicks you out of managed descent.