simulating an obstructed static port

[emerydc8]

You are right -- theoretically if you had a crossing restraint at 17,000 and you had a really low QNH it could put you below the 17,000 as you are setting the QNH, but in this case, I am pretty sure that what happened to the Delta flight behind us was exactly what happened to us because we were given the same speed amendment. I don't remember that the QNH was particularly low that day, if at all.

I don't know the 767 modes, but on the 744, as we know, you get VNAV ALT or VNAV PTH when an altitude is captured.

I think most US carriers will set the bottom altitude for a descend-via STAR, so there is no MCP to back you up in the event VNAV puts you low. And when the FMC is re-calculating a new path, VNAV is out to lunch during that period, so it won't stop you from crossing a fix below the published altitude either.

In our case (and I think Delta's too), we were on the MUSCL 3 arrival between JERMN and MUSCL. We were at 280 knots and told to maintain 250 knots. When we overrode the STAR speeds on the LEGS page with the ATC speed, the VDI disappeared from the ND as the FMC calculated the new path. During that time, we were effectively descending at what we had in VNAV 3/3 (310 in this case), so the FMA went from SPD|VNAV PTH to IDLE|VNAV SPD. Since we had 310 in the VNAV page, the aircraft pitched for 310 and started to go below the path. With 7000' set in the MCP, there was nothing to stop us from crossing MUSCL below 17,000'. At that point, we disconnected the A/P and A/T and leveled off at 17,000'.

[Jeroen Hoppenbrouwers]

At least one pilot per aircraft.

Working on that... one pilot aboard, one backup pilot somewhere in a room in a building, attending to 5-10 other flights that are routine and don't need attention except for cruise monitoring (all green? ok) and assistance when requested. It obviously requires full remote control capability. That is an issue. But there are airlines preparing for the principle as of today. Like, changing procedures and acquiring tech that would allow relaxed one-pilot ops in legal emergencies without causing undue stress (call it "legally emergency" just as calling mayday gives you prio, but does not mean you are about to crash, unlike what many media think).

Hoppie

[paradoxbox]

It's the assumption that a robot won't overshoot a 250 speed limit, for example.

I can attest that VNAV on every 767 we have regularly overshoots the speed restrictions on STARs (e.g., leaves you 2 miles from the fix at 310 knots when you're supposed to be at 280). Then it leaves you a nice little DRAG REQUIRED message after it's too late. That's probably what happened to Delta.

...snip...

So, there's still a long way to go with the automation even under the best of circumstances.

Funnily enough, this is something that if a pilot calculated manually on an E6B, or even with a bit of quick mental math, would not happen. I've seen what you describe happen more times than I can count. To the point where I always verify with an E6B or mental math before T/D to make sure that speed is actually going to be possible.

[emerydc8]

If the idea behind VNAV is to save fuel, it has failed miserably. There is almost no leg where speed brakes are not required because VNAV will almost always leave you high or fast. Think about that when you get on a passenger flight and see the speedbrakes used as regularly as a primary flight control. As soon as you pull out the boards, you've just lost the fuel you've ostensibly saved using VNAV.

The DC-8 never had VNAV, and it didn't have speedbrakes either, but we were able to operate that fine just by multiplying the altitude we needed to lose by 3 when passing every thousand feet. We didn't need an E6B. For every 50 knots of tailwind you would add ten miles and also add 10 miles to slow from 300 to 250. Worked better than any VNAV I've ever used.
Jon