VNAV computing during vectoring LNAV

[30K 72]

I don't get it. Have a guess but I don't get it.

[Marenostrum 4]

15 hours ago, Spin737 said:

VNAV does not change climb rate. It only commands an airspeed at CLB, CLB-1 or CLB-2. It will level you at the lower of any LEGS restrictions or the MCP.

It's possible I'm missing some option, but in my experience, it's basically full thrust until you get to the next restriction.

I tested it (same aircraft, same fuel, same payload, same climb setting CLB-01, same derate TO-1, same flaps, same wind conditions)

I did a first climb path following the FMC route to go from rwy 05 @EGCC to the first wpt KOXUM. VNAV and LNAV engaged.

I did a second climb path following the vectors by ATC, that is proceeding straight ahead without turning immediately and, after about 3 miles, doing a first turn 90° left and after about 2 min another turn left inbound to KOXUM. So a longer course.. HDG SEL and VNAV engaged

Both path ending at 9800 ft

The table above (for each position there is a picture if you like to see it) shows that I reached 9800 ft after about the same time but at a very different position.In few words (as stated by Dan and Matt) VNAV does not seem to control directy ROC. It simply follows the chosen CLB program and climbs up to the MCP altitude.

Edited July 4, 2018 by Marenostrum

[Spin737 102]

On 7/4/2018 at 3:56 AM, Marenostrum said:

I tested it (same aircraft, same fuel, same payload, same climb setting CLB-01, same derate TO-1, same flaps, same wind conditions)

I did a first climb path following the FMC route to go from rwy 05 @EGCC to the first wpt KOXUM. VNAV and LNAV engaged.

I did a second climb path following the vectors by ATC, that is proceeding straight ahead without turning immediately and, after about 3 miles, doing a first turn 90° left and after about 2 min another turn left inbound to KOXUM. So a longer course.. HDG SEL and VNAV engaged

Both path ending at 9800 ft

The table above (for each position there is a picture if you like to see it) shows that I reached 9800 ft after about the same time but at a very different position.In few words (as stated by Dan and Matt) VNAV does not seem to control directy ROC. It simply follows the chosen CLB program and climbs up to the MCP altitude.

If there was a question as to whether it the plane is following a “path” climb or a speed, you would simply need to look at the FMA. 

If you see N1 and VNAV SPD, you’re climbing based on the N1 (power) limit and the speed restriction from the LEGS or CLIMB page. This combination would not follow a computed ROC,