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WilloW_737

VNAV SPD & VNAV PATH - annouciators on PFD

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Hello:

 

VNAV provides VNAV Speed and VNAV Path. 

 

If you want to use VNAV SPD, you select the SPD INTV switch on the MCP and then dial in the required speed into the speed window.  Is the speed window being open the only way to determine the aircraft is in VNAV SPD?  I ask as VNAV PATH is illuminated on the PFD whether I have the speed window open or closed.

 

To return to VNAV PATH (in other words close VNAV SPD), is it only a matter of selecting SPD INTV button again (speed window disappears), or do you have to also push VNAV to activate VNAV PATH?

 

A bit complicated.  I hope an expert can weave his/her way through the above.  Thank you,   WilloW

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Is the speed window being open the only way to determine the aircraft is in VNAV SPD?

To determine in which mode are you now, you MUST look onto the FMA. If it says VNAV PTH, then it's obviously VNAV path autopilot logic.

 

With common VNAV option modeled on NGX, VNAV SPD can be automatically engaged when the aircraft is above the profile or to prevent overspeed.

 

I ask as VNAV PATH is illuminated on the PFD whether I have the speed window open or closed.

You can also select VNAV SPD usin speed intv button. But not always. Check FCOM vol.2 11.31.38 (page 888) for other conditions that apply here.

 

or do you have to also push VNAV to activate VNAV PATH?

You don't need to do it if VNAV spd was working with speed intervention.

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Hello:

 

VNAV provides VNAV Speed and VNAV Path. 

 

If you want to use VNAV SPD, you select the SPD INTV switch on the MCP and then dial in the required speed into the speed window.  Is the speed window being open the only way to determine the aircraft is in VNAV SPD?  I ask as VNAV PATH is illuminated on the PFD whether I have the speed window open or closed.

 

To return to VNAV PATH (in other words close VNAV SPD), is it only a matter of selecting SPD INTV button again (speed window disappears), or do you have to also push VNAV to activate VNAV PATH?

 

A bit complicated.  I hope an expert can weave his/her way through the above.  Thank you,   WilloW

 

As rsvit says, you've got to look at the FMAs to confirm what mode you're in.

 

I think you may also be confusing VNAV Speed and Speed Intervention. They are separate things. Speed intervention can be used in conjunction with VNAV Path or VNAV Speed.

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If you use SPD INTV on the MCP, you don't necessarily go to VNAV SPD. Your FMA will announce MCP SPEED though.

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Your FMA will announce MCP SPEED though.

If we're talking about speed section of a FMA then on descend it can be MCP SPD or RETARD and ARM modes announciated.

 

I ask as VNAV PATH is illuminated on the PFD whether I have the speed window open or closed.

 

•If speed intervention is engaged:

•during a path descent with flaps up on an idle leg, VNAV switches to VNAV SPD

•with flaps down, VNAV remains in VNAV PTH

•when a point to point (geometric path) leg is active, VNAV remains in VNAV PTH

•while a vertical angle leg (GP x.xx on RTE LEGS page) is active, VNAV remains in VNAV PTH

•if a vertical angle leg (GP x.xx on RTE LEGS page) becomes active, VNAV switches to VNAV PTH without pilot action.

 

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Thank you for your replies.

 

I didn't realise there were caviats to using VNAV SPD as RsVit has kindly copied for reference.  I had "thought" that by depressed the speed intervention button the system would change from Path to Speed with the appropiate annouciation on the FMA.

 

My circumstance was in descending on approach with flaps deployed.  I couldn't figure out why the FMA remained in VNAV PATH despite the speed intervention button being engaged, and being able to manually alter the speed in the speed window.  I knew about the geomatric path caviet but not the flaps caviet :(

 

I'll read the FCOM in greater detail. 

 

Cheers,    WilloW

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Basically you don't need to worry about whether to use VNAV PATH or VNAV SPD, in fact you have no direct control over it.  The autopilot decides which VNAV submode to use depending on where it is in the vertical flight plan.  So you select VNAV and let the autopilot do its job, monitoring it with the FMA of course.

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Basically you don't need to worry about whether to use VNAV PATH or VNAV SPD, in fact you have no direct control over it. The autopilot decides which VNAV submode to use depending on where it is in the vertical flight plan. So you select VNAV and let the autopilot do its job, monitoring it with the FMA of course.[/quotes]Down low that's mostly true, but coming out of cruise you might choose one over the other. And you can choose.

 

If I'm high, I'll use SPD INTV (VNAV SPD) to get back on path. If ATC assigns a speed, I'll stay in PATH and set the speed in the FMC.

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If I'm high, I'll use SPD INTV (VNAV SPD) to get back on path. If ATC assigns a speed, I'll stay in PATH and set the speed in the FMC.

Personally, in the real aircraft, I seldom use SPD INTV in the descent. My personal philosophy is that if VNAV PATH isn't doing the job I want or if the clearance changes from the FMC profile, then dump VNAV altogether and use LVL CHG or V/S. If staying in LNAV I'll re-select VNAV once back on path and speed.

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Personally, in the real aircraft, I seldom use SPD INTV in the descent. My personal philosophy is that if VNAV PATH isn't doing the job I want or if the clearance changes from the FMC profile, then dump VNAV altogether and use LVL CHG or V/S. If staying in LNAV I'll re-select VNAV once back on path and speed.

How you do it is how a lot of guys I fly with handle it. I'll do it that way, to.

 

However, I read a lot of ASAP reports with guys on STARS busting crossing restrictions because we can set the MCP for the lowest altitude, not the current leg's altitude if we're in VNAV. When they switch to LVL CHG or V/S they forget to reset the MCP and presto, you're writing down a phone number.

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Yeah, I have not come across it myself yet, but the case where the the arrival specifies something like 8000A and ATC give a clearance for 6000, one still has to be at least level at 8000 at the waypoint unless ATC have specifically used the phrase "descend unrestricted" in the clearance. A potential gotcha for sure.

 

As per Boeing's recommended practice, we always set the MCP altitude for the next restriction. Interestingly, I don't recall ever using the "alternate MCP altitude setting technique" (see the FCTM).

 

In the real world, VNAV can be a little bit left of field at times with uncommanded level offs, disconnections, FMC vertical path disagreements and having to cross a restricted a waypoint before ALT INTV will allow the descent to continue (amongst other things).

 

Regardless of which technique is used, there are potential gotchas/complications, so there is no alternative to maintaining good SA and CRM at all times :D

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As per Boeing's recommended practice, we always set the MCP altitude for the next restriction. Interestingly, I don't recall ever using the "alternate MCP altitude setting technique" (see the FCTM).
I think we use the "Alternate" technique because of our RNP approaches. I don't know when they implemented that one since it was before our time.

 

As far as VNAV disconnect, I'd see it in Asia on some STARs because maybe they weren't constructed correctly, but I haven't seen it in the US.

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I think we use the "Alternate" technique because of our RNP approaches. I don't know when they implemented that one since it was before our time.

Ah, that's something different. The alternate technique applies to climb and descent. On approach with VNAV, the MDA+50' (or more than 50' if more than 3 deg) is set on the MCP around 2nm before the FAF and then to missed app alt 300' below the missed app alt. That is standard Boeing as per FCOM1, Normal Procedures, Amplified Procedure, Instrument Approach Using VNAV and Supplementary Procedures, Automatic Flight, RNAV (RNP) AR.

 

BTW, sorry if I over explain, I know you know your stuff, other readers may find this interesting too.

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