VNAV descent with higher altitude set in MCP

[guest77]

Hi, i have a question

Let's say that i am descending from FL300 to FL200 with VNAV engaged in pitch mode. What happens if i set a higher altitude, say 35000 in MCP while descending through 30000 to 20000? Does the aircraft continue descend until the next altitude restriction and ALT HOLD at that altitude? (assume that i have an altitude restriction at 15000) or does it descend into terrain and crash?

Thank you.

[katanapilot]

Interesting question, although not a situation you are likely to find yourself in. The following is based on trying to work out the system logic, it's not gospel.....:)

If you were in FLCH descent, and put the MCP altitude higher, the aircraft would CLIMB.

In VNAV PTH, putting a higher MCP altitude in would NOT initiate a climb, as you correctly say the pitch mode is VNAV PTH. I think you would get a 'CHECK ALT TGT' message on your CDU, and the aircraft will now only level off at the next 'hard' altitude: it might be something like MIRSI 210/FL70, for example, and then, further descent will not, IMHO, take place until you re-select a lower MCP altitude.

If there was no hard altitude, then, yes, you are right in that the aircraft would most likely fly down to 50' above the runway, but without any sort of flare, etc, resulting in something that really doesn't bear thinking about!

Consider also the risks associated with putting 0000 in the MCP window for the same reason!

[c100driver]

On the CL B737 you have to have the MCP at 0000 to allow an RNP SAAAR approach. I do RNP SAAAR approaches every other day so it is not really a biggie.

The NG can I believe fly an approach away from the MCP altitude in VNAV, anyone do SAAAR approaches in the NG that can confirm? QANTAS in ZQN?

[5150]

For VNAV approaches we set the MCP to the MDA rounded up to the nearest 100' to initiate the descent once cleared for it.

Once the descent is commenced we then dial the MCP window to the first missed approach hard altitude.

[fourgolds]

Greetings , am converting from Bus to 777 and it just did this.
We had a constraint of FL230 by a certain waypoint. We were then cleared to FL 210 to expedite. So I used FLCH mode . At FL210 ( just before the constrained Waypoint I reselected VNAV to comply with the farther Constraints on the arrival and guess what , she initiated a climb towards FL230 ( with FL210 set in MCP). So a quick selection of FLCH saved the level bust. I guess thats why we do conversion training. I do find the Busses FMC a little more clever , however the Boeing did do what it was asked to do.Merry Christmas