How to quickly change altitude in VNav approach?

[kneighbour]

I did an approach into Sydney tonight, and I did not notice that one of the waypoints on the approach was at the wrong height in the FMC. The descent was something like this Point A=FL70, point B=FL80, point C=FL60, point D=FL40, etc. Actually, this was the RIVET2 approach into 34L, so no idea why the heights were wrong.

I got to point A ok, then the plane took off and climbed to 8000. I tried to alter the height in the FMC, but I got a faulty input message or something similar. Anyway, these points were very close together, so I decided to go with it and just slow the aircraft right down. I was still in full VNAV mode at this point, so speed was 240 and the aircraft was clean. The MCP Altitude was set to 4000 (the ILS approach height).

I tried to press the SPEED switch, but it would not engage. Obviously in the wrong mode to allow selection. I guess I could have gone to V/S mode (which I presume would work), but I wanted to keep the VNAV doing it's thing for a bit longer. Actually, in the end, I did turn VNAV off and stuck with LNAV. But was getting a bit behind the aircraft at this point.

It is actually a bit more complicated, of course. I disabled VNAV, went to SPEED mode, dialled in 220 and the plane started to pitch up (to slow down from 240). So I actually gained altitude whereas I REALLY wanted to lose some. I got it all together in the end, but purely by luck more than any skill.

Any suggestions as to the preferred procedures in this case?  I have a sneaking suspicion a real pilot would have gone totally manual.

[TDPlane]

I have similar experiences with descent. Most of the times, I quit VNAV altogether and head down using vertical speed or FLCH. Switching between VNAV and V/S mode is hard to accomplish. The devs have already admitted that VNAV is flawed. They're working on it ...

[cmbaviator]

3 hours ago, kneighbour said:

I did an approach into Sydney tonight, and I did not notice that one of the waypoints on the approach was at the wrong height in the FMC. The descent was something like this Point A=FL70, point B=FL80, point C=FL60, point D=FL40, etc. Actually, this was the RIVET2 approach into 34L, so no idea why the heights were wrong.

I got to point A ok, then the plane took off and climbed to 8000. I tried to alter the height in the FMC, but I got a faulty input message or something similar. Anyway, these points were very close together, so I decided to go with it and just slow the aircraft right down. I was still in full VNAV mode at this point, so speed was 240 and the aircraft was clean. The MCP Altitude was set to 4000 (the ILS approach height).

I tried to press the SPEED switch, but it would not engage. Obviously in the wrong mode to allow selection. I guess I could have gone to V/S mode (which I presume would work), but I wanted to keep the VNAV doing it's thing for a bit longer. Actually, in the end, I did turn VNAV off and stuck with LNAV. But was getting a bit behind the aircraft at this point.

It is actually a bit more complicated, of course. I disabled VNAV, went to SPEED mode, dialled in 220 and the plane started to pitch up (to slow down from 240). So I actually gained altitude whereas I REALLY wanted to lose some. I got it all together in the end, but purely by luck more than any skill.

Any suggestions as to the preferred procedures in this case?  I have a sneaking suspicion a real pilot would have gone totally manual.

 

Normally after entering the flight plan you should :

* check the flightplan with the map selection on the EFIS and see if there is no discontinuities......

* Check that the contraints in speed and altitude for each waypoint is respected ( check your airport chart).

 

for you point B, you should have checked the altitude on the airport chart and entered it in the CDU in the leg page, if the chart doesn't provide any altitude contraint for WPT B, just dlete the altitdue and the FMC will recalculate a new value between FL 070-FL060

Edited May 24, 2016 by cmbaviator
typos

[kneighbour]

8 hours ago, cmbaviator said:

 

Normally after entering the flight plan you should :

* check the flightplan with the map selection on the EFIS and see if there is no discontinuities......

* Check that the contraints in speed and altitude for each waypoint is respected ( check your airport chart).

 

for you point B, you should have checked the altitude on the airport chart and entered it in the CDU in the leg page, if the chart doesn't provide any altitude contraint for WPT B, just dlete the altitdue and the FMC will recalculate a new value between FL 070-FL060

Well, obviously I realise all of this. But mistakes happen, even in real life, I presume. I also realise that I could have removed the restriction - but at the time it did not work. Whether this is another bug, or I entered it incorrectly I do not know - it was a tense moment there! My first thought was it was a bug (I have had HEAPS, so I kind of expect them).

I was seeking opinions on the best way to attack the problem as it occurred. In hindsight I think I should have just disabled VNAV, which I did in the end anyway. 

 

[stefans85]

On 24.5.2016 at 0:08 PM, kneighbour said:

Any suggestions as to the preferred procedures in this case?  I have a sneaking suspicion a real pilot would have gone totally manual.

Indeed. Normally you get speed constraints or get adviced to not descent this fast and you increase the throttle (in VNAV descent or FLCH it is in thrust hold with AT in sleep state) a bit so you mostly are in flight change or even VS mode. On most flights I was inside cockpit they began descent with VS -1000fpm to get the engines to idle slowly to extend their life a bit and switched then to Flight Change. VNAV is used pretty rare in europe especially on busy airports I was said.

Some years ago I was on jumpseat on a night flight on a 757 with only some cargo and parcels and no passengers. The captain asked for landing (way above G/S because late descent). Tower "You're pretty high" "We'll made it". Engines Idle, Spoiler deployed -3800fpm and gear out and flaps on shedule. We descended until some seconds after radio altimeter come to life with -3800 on VS.