VNAV and energy management

[Will]

Real world heavy pilots: how much do you trust VNAV to manage your energy in the descent?

Let's say you were cruising at an ECON speed of 0.86, and you start going down on the descent path. Your first restriction just an at-or-above crossing at 11,000 feet, followed by a second restriction at 210/8000. Based on how close the two waypoints are, in order to make the 210 kts, you'd need to start slowing before the first waypoint, otherwise you cross the second one high and/or fast. Can VNAV handle this?

Now, assume the first restriction is simply 210/8000, without the first waypoint. Do you trust VNAV to manage the energy properly or do you find yourself wanting to speed-intervene just to be sure?

[IefCooreman]

If the arrival has many altitude/speed restrictions I always use full on VNAV (with all possible forecast data completed in the FMC). I'd rather have the fmc complain and pull the speedbrake, then bust an altitude or speed restriction. Although in reality I would listen out on the frequency and conform with ATC any speed restrictions as slowing down that high to that speed is not something we like to do. If we need to go down, we need to keep the speed away from minimum clean speeds or even VNAV will have problems.

I do admit I am a VNAV addict. In 90% of the descents with no speed restrictions (apart from the 250/10000), pilots tend to follow the vertical path, but disregard the calculated deceleration and forecasted winds (if they even put them in the FMC) and then complain the path is "off' when flying their own decelerations. If the legs page shows the FMC will decelerate at inconvenient points, it will also for me end up in non-VNAV modes though.

[emerydc8]

Hi Will. To answer your question, VNAV is only looking at the fix directly in front of it. Usually the arrivals are built to allow VNAV to follow it, but there are certain arrivals where you almost always have to use speed brakes to make it, like going into SFO on the SERFR arrival from the south.

Another case could be where you manually insert a hard altitude on an approach after the last fix of an arrival, so it will go from SPD|VNAV PTH as you descend prior to crossing the last arrival fix and IDLE|VNAV SPD immediately after crossing because it's already behind the profile. The VDI will then show you are high.

The strange thing is that, although VNAV is only looking at the fix in front of it, if you make any changes in altitude or speed to fixes down line, the VDI will still disappear as it recalculates a new descent profile. Too bad they couldn't have designed it to make down-line re-calculations in the background so you don't lose the VDI if you make a down-line change.

Jon

[Will]

Thanks gentlemen, that's helpful.