Using LVL CHG vs VS

[pcubine 143]

That said, I do my step climbs using FLCH. But I use that mode not because V/S is dangerous, it's just cause I prefer it.

I use the FMC to perform step climbs. I press the CRZ button on the CDU which brings up the ACT ECON CRZ page. I enter the altitude to step to and the FMC calculates the distance and time until the step climb can be made. When it says NOW or I see S/C on nose of the plane I do the following:

 

(1) Change cabin altitude to the new altitude on Overhead Panel

 

(2) Dial the altitude up to the desired altitude in the MCP. Doing this also enters it on the scratch pad of the ACT ECON CRZ page.

 

(3) LSK that to L1 and press execute.

 

(4) The FMA switches from FMC SPD | LNAV | VNAV PTH to N1 | LNAV | VNAV SPD and then upon reaching the new altitude reverts back to what it was before the climb.

 

I believe any method that works, is safe, and gets the new altitude into the FMC is acceptable. This just happens to be the way I do it.

[Vagabondo 23]

 

 

(1) Change cabin altitude to the new altitude on Overhead Panel

Boeing recommends setting the pressurization controller to the highest planned level for the flight during preflight. They say this guards against problems during the climb if forgotten about and reduces workload.

 

(3) LSK that to L1 and press execute.

You can also just press ALT INTV (obviously if configured). That will automatically set the FMC CRZ ALT to whatever you have set in the MCP if the MCP is higher than the current FMC setting and initiate the climb with N1 and VNAV SPD FMAs.

[pcubine 143]

You can also just press ALT INTV (obviously if configured). That will automatically set the FMC CRZ ALT to whatever you have set in the MCP if the MCP is higher than the current FMC setting and initiate the climb with N1 and VNAV SPD FMAs.

Does this get the new altitude on to the legs pages in the FMC?

 

Thanks

[Vagabondo 23]

 

 

Does this get the new altitude on to the legs pages in the FMC?

Not during a climb, it just sets the cruise altitude in the FMC. During a descent, pressing ALT INTV will normally delete the next altitude constraint in the LEGS page that is higher than the altitude currently set in the FMC. I say normally, because in the real aircraft there are arrival procedures where ALT INTV does not remove the restrictions, which I have never been able to get to the bottom of. I don't know if this encoding of the arrivals is replicated in NGX as I haven't tried it.

[ark 1,288]

WOW, this has been a very instructive discussion, at least for me. I appreciate the insights.

Thx,

Al

[rfresh737 36]

Well, I guess I'll throw in my 2 cents. BTW, I like spin737's answer the best: "Chicks dig it." Too funny.

 

From a training perspective in the full flight simulators the one thing we always started training the new pilots on (I say new but they could be 5,000 pilots coming over from another jetliner or they could be low time jet pilots) was this basic AP question: What's controlling speed? Pitch or throttles?

 

On the MD-90 V/S was used for altitude changes less than or equal to 2,000 feet. More than that, then LVL CHG was to be used. Boeing on the 737NGX goes with 1,000 feet or less use V/S otherwise use the other. I'm sure like many of the other practices, this value will vary from carrier to carrier.

 

Anyway, the main difference between using these two modes of altitude change is pitch controls speed with LVL CHG and throttles control speed with V/S.

 

So, our excerise in the sim would be the following:

 

(1) Level at 5,000 feet. I'd play ATC and tell the crew to climb to and maintain 6,000 feet (I'll use the Boeing standard here). I would watch to see if they used V/S to climb and during the climb I would ask "What's controlling speed, pitch or throttles?".

 

(2) Level at 6,000 feet. I'd play ATC and tell the crew to climb to and maintain 9,000 feet. I would watch to see if they used LVL CHG to climb and during the climb I would ask "What's controlling speed, pitch or throttles?".

 

We instructors would do this a few times in the early sim sessions until it was clear they understood when to use which mode, what was controlling their speed and how to do basic climbs and descents.

 

This was all pretty basic stuff for the autopilot but we had to get it out of the way before moving on with the sim sessions.

 

As for my own use when flying my PMDG 737NGX, on approach I like to be in V/S mode if my altitude step down distances are short between waypoints, as it gives me more control on my descents. If the distances are spaced out OK, I'll just stay in VNAV.

[Vagabondo 23]

 

 

As for my own use when flying my PMDG 737NGX, on approach I like to be in V/S mode if my altitude step down distances are short between waypoints, as it gives me more control on my descents. If the distances are spaced out OK, I'll just stay in VNAV.

 

We never fly step downs on approach, always a constant descent and add 50' to the MDA for a possible go-around. Even for arrivals, we will attempt to keep a constant descent going barring altitude restrictions and ATC, which usually means VNAV or VS depending on if ATC speeds or vectors changes the standard arrival. Once we are on the profile under vectors with speed control it is LVL CHG with speedbrake as required and if likely to go below the profile, back to the VS to reduce the descent again.

 

The major difference between LVL CHG and VS on approach in the real aircraft, especially when caught high on the profile, is how quickly the AP reacts to inputs, LVL CHG is generally too slow to react to inputs, VS will change the pitch very quickly is is useful for recapturing the profile from above (provided there is enough drag). VS is not very good when under speed control and will often hover at the bugged speed +10 kts. It is tempting hold the thrust levers closed, but this is not recommended, I usually handfly or if there is room above the current flap speed, just reduce the bugged speed below the ATC speed.

[pcubine 143]

WOW, this has been a very instructive discussion, at least for me. I appreciate the insights.

Thx,

Al

Al please sign your full name per these Forum Rules http://forum.avsim.net/topic/361816-pmdg-forum-rules-please-read-before-posting/.

 

To have your signature automatically attached to your post do this. Left click on your user name in the upper right of this page. On the next page left click on Edit my Profile. On the next page left click on Signature and on the next page in the box under Edit Signature enter your full name and left click on Save Changes. Now your name will be on every post you make.

 

Thank you.

[pcubine 143]

You can also just press ALT INTV (obviously if configured). That will automatically set the FMC CRZ ALT to whatever you have set in the MCP if the MCP is higher than the current FMC setting and initiate the climb with N1 and VNAV SPD FMAs

I have tried this procedure and it is doing essentially the same thing I outlined in post #16 but with less effort. Once it has been determined that it is appropriate to do a step climb and the new altitude is dialed in the MCP the altitude also goes to the scratchpad on the ACT ECON CRZ page. When ALT INTV is pressed on the MCP the FMC changes pages to the ACT ECON CRZ page with the new altitude on L1. The plane starts to climb and when it reaches the new altitude the FMC switches back to the ACT ECON CRZ page with the new cruising altitude in it and on the ACT RTE LEGS page beside the remaining waypoints. All of this in the FMC is done behind the scenes.

[rfresh737 36]

"...and the new altitude is dialed in the MCP the altitude also goes to the scratchpad on the ACT ECON CRZ page..."

 

And for those who may not be aware, this only works when you are climbing. If you are descending, you have to manually set the lower altitude in the MCP window and if needed, manually into the CDU CRZ page