Boeing VNAV Path

[JammedStab]

Normally opening the speed window changes VNAV Path to VNAV SPD(at least on my type) and the path is no longer followed. But when on approach using VNAV, the speed window can be opened to adjust speed while remaining on VNAV Path if certain conditions are met such as flaps having been extended.

I was also told that you have to be within a certain distance of the "approach", perhaps 25 miles. Is this true?

[Arfur Dent]

Almost right - with a designated VNAV approach in the data base, within 25 miles of TD, past the 'on approach' waypoint and with the minima set in the Alt Sel window, you will remain in VNAV PATH with the speed window open as you command the deceleration. There are probably a couple of other requirements but that's the nuts of it. Reset the Alt Sel window to the missed approach value when more than 300' below said value (ie below 2700' if go around alt is 3000') and off you go! Works well if a little stressful when approaching/passing said 'on approach' waypoint waiting for VNAV SPEED to change to VNAV PATH.:ok:

[mustafagander]

You can tell whether this will be available when you load the approach out of the data base - at least one leg to the RWxxx way point will have a slope written in, 3% or similar. Some approaches have two legs, some three. For two legs "on approach" logic starts 2nm prior to that way point, for three at the first way point with the slope written beside.

It is a VERY good idea to have rough speeds entered for these legs, otherwise if anything changes or you go Vnav Path too early the aircraft obediently accelerates to the speed on the descent page, often 240Kts - oops, bugger.

My mob uses V/S until at the point where on approach logic starts. We tend to use the 1000 auto call to remind us to set the MA alt in the MCP.

[Arfur Dent]

Another point worthy of note is that (with our lot) if you have a RWAY way point at t/d, you can land with the FD still on, otherwise they must be switched off - we use the 500' call to enact that.
Great point about reviewing your settings at 1000' regarding M App altitude. Gets very interesting if you forget AND do a Go Around having left 560' in the Alt Sel window!! Double whammy becoming triple doesn't often happen in real life - only happens in the Sim when someone needs to sign your licence!!!:cool:

[deltahotel]

Depends on the a/c when it goes into 'on approach' mode. The details are in the B12 section 11. On a 767 it happens (among others) when flap is selected, but not on the 757. Either way it allows VNav Path with the speed window open. It doesn't actually need a notional glidepath either and allows a NPA to be flown in VNav Path, monitored by dme/alt gates.

[JammedStab]

Another point worthy of note is that (with our lot) if you have a RWAY way point at t/d, you can land with the FD still on, otherwise they must be switched off -

This is because they will provide proper guidance to the runway, correct. Some people on murky approaches without a RWY WP at E/D might switch to V/S during the visual portion of the approach as a backup reference.

[misd-agin]

On our fleet, both 757(?) and 767, the first flap setting allows 'speed intervention' while remaining in VNAV PTH ('approach mode').


There are about 4 triggers to get into 'approach mode'. Flaps is the easiest to remember and often the first trigger that engages 'approach mode'.


And then there's always the FMA - yeah you asked for something...but what did the 'box' (FMA) actually give you?

[JT8D-17]

On our fleet, both 757(?) and 767, the first flap setting allows 'speed intervention' while remaining in VNAV PTH ('approach mode').


Same on the 737NG

[Denti]

Seems easier with that IAN thingy on the 737 and newer Boeings, simply arm approach mode for (nearly) everything, no need to remember those VNAV gates.

[Jwscud]

That would be if your employer were willing to pay for IAN.

[Arfur Dent]

Interesting thread. I have to agree with 'misd-agin' about changing FMA modes close to the ground - not a good idea Mr Stab. :=

[JammedStab]

Seems to have been helpful. Autopilot is off anyways so in the situation discussed it is either FD off or VS to get useful info.

[Denti]

That would be if your employer were willing to pay for IAN.

Oh, thought it was standard equipment for the last 8 or 9 years. The 737 was the IAN testbed for the newer boeing variants (747-8 and 787) where it is certainly standard.

[de facto]

I thought yanks would be the first to use IAN so they can shoot an NDB just as they shoot an ILS:E

[Denti]

Nah, they love their dive and drive way too much for that. Its just us sissy europeans that want to fly stable continous descent approaches ;)

[stilton]

I trust you are taking the p*ss Denti.


The major American Airline I work for abandoned 'dive and drive' years ago.
It certainly isn't a procedure that I miss or any of my Colleagues !

[Skyjob]

So in summary the from all the posts above:


There are slight differences between models
SPD INTV activates VNAV SPD before the criteria of the VNAV Approach Logic are met
VNAV PTH remains enganged when the VNAV Approach logic criteria have been met
The open SPD window enables/allows manual adjustement and control of the speed
Closing the SPD window will allow VNAV to control the speed, based on actual flap configuration and preselected VREF additive to selected landing flap when set
Flight directors will guide you down the calculated VNAV PTH until recycled from view
Flight directors can guide you down to ruway or MD coded waypoint
Flight directors when crossing this last point can/will command the missed approach pitch/turns (slight difference between models) or remove from view until TOGA selection

[Jwscud]

Oh, thought it was standard equipment for the last 8 or 9 years

Like most things with Boeing, it's a latent capability, but you have to pay for the capability and the FCOM supplement. It does seem daft that they provide all these capabilities in the software, but their commercial department puts all these false price differentials in, similar to the different levels of detail available in the FCOM for different amounts of cash.

VNAV PTH in approach does have one or two oddities that one may not encounter in normal ops, and has confused me in the sim before - if more than 200ft below the path, it will command a VS of 0fpm until back on the path.

[SOPS]

All I know is that the 737NG is much easier to fly VNAV approaches with, compared to the 777.

[JammedStab]

So in summary the from all the posts above:
Closing the SPD window will allow VNAV to control the speed, based on actual flap configuration and preselected VREF additive to selected landing flap when set

Not sure if this is true


Flight directors can guide you down to ruway or MD coded waypoint

What s MD?



Flight directors when crossing this last point can/will command the missed approach pitch/turns (slight difference between models) or remove from view until TOGA selection

I thought commanded a level off but not sure

[Skyjob]

Originally Posted by JammedStab
What s MD?
FCOM 2: Procedure Fix Waypoint Names: MD – minimum descent

Originally Posted by JammedStab
I thought commanded a level off but not sure
Give it a try next time by not recycling FD's
See text from FCOM2 below:

VNAV mode is terminated by any one of the following:
• selecting another pitch mode
• glide slope capture
• reaching end of LNAV route
• transition of glide slope intercept waypoint if G/S is armed
• cross track deviation exceeds twice the RNP value during PTH descent for
an active leg with a database vertical angle and LNAV not engaged

The FMC transitions out of “on approach” under the following conditions:
• selecting TO/GA
• the airplane lands
• the waypoint cycles to the first waypoint of the missed approach
• executing a direct-to waypoint in the missed approach.

For an approach without a runway waypoint on the RTE LEGS page, the VNAV path is calculated to the MDA or a calculated altitude at the missed approach point. The calculated altitude may be below the MDA to ensure a flight path angle and normal threshold crossing height.

Originally Posted by JammedStab
Not sure if this is true
As per FCOM 2:
VNAV will remain engaged at all flap settings, allowing approaches to be flown using the vertical angle guidance. Speed for final approach can be set on the APPROACH REF page.
In practice the above means in combination with the auto pilot & throttle logic that VNAV will:
• command with Flaps extended the speed for selected flap setting based on Flap Position Indicator
• command no higher than flap limiting speed to be allowed to be flown (losing the PTH if required to compensate)
• command Vref30/Vref40 + the additive on final approach as entered into FMC
• provide UNDERSPEED protection with Flaps EXTENDED to 0 kts vs flaps up -15 kts

In summary the above is the safest way to fly the VNAV PTH approach, when FMC commands FLAPS UP speed and thus initiates pitch up for flap extension iaw profile, just select Flaps 1 etc on extension schedule, no need for SPD INTV as VNAV PTH uses FMC speeds which are derived from actual flaps selection. Note: the flap position indicator triggers the FMC to command associated speeds, not the Flap lever.

[Foxe]

The B12 section 11 you talk about is from what book ? Tks

QUOTE=deltahotel;8697078]Depends on the a/c when it goes into 'on approach' mode. The details are in the B12 section 11. On a 767 it happens (among others) when flap is selected, but not on the 757. Either way it allows VNav Path with the speed window open. It doesn't actually need a notional glidepath either and allows a NPA to be flown in VNav Path, monitored by dme/alt gates.[/QUOTE]