737 VNAV Path / Speed

[flyingmed]

Anyone know the criteria for Vnav to change from Vnav speed (with the window open) to Vnav path on a STAR? Only notice to crew seems to be the change on the FMA.

[Tom!]

Altitude restriction on the STAR or interception of a vertical angle leg coded in the FMC I think.

[aa73]

It's a point called the Geo-Something-Point on the arrival. Once past that point you can open up the speed window and it will stay in PTH. Prior to that point it will revert to SPD.

The fix for this is as follows:

Change the first point on the arrival with a BETWEEN crossing restriction to a Hard Altitude or AT OR BELOW. That will create the new Geo-Point right there and you can open up the speed window and stay in PTH from that point on.

If your first fix (TRUPS ,for example) looks like this:

TRUPS 280/FL300 FL240

and you open up the speed window just past that fix, you revert to VNAV SPD and it goes idle.

So you would change it to this:

TRUPS 280/FL300B

Or

TRUPS 280/FL270

Now you can open up the speed window past TRUPS and remain in VNAV PTH at whatever speed you want.

Works great, we do this all the time coming in to Dfw, lax, DCA, etc... All those arrivals begin, and continue for a while, with BETWEEN altitudes. It is essential to do this, because we are constantly being cleared to "descend via the XXX1 Arrival except maintain 310kts." Well, it is a royal PITA to go leg by leg and change the speeds manually...so using this fix takes care of your descent speed from the first point on: you just open up the speed window and stay in PTH. Voila!

[Avenger]

A speed descent cannot automatically revert to a path descent, except during STAR, approach transition, or approach leg with a vertical angle. However, if all required parameters for a path descent are available, a path descent may be manually selected at any time by selecting the PATH prompt on the speed descent page.
38/26 SEP 13 B-737NG Operations Manual Part B 12-11.31.27 Volume 5

[flyingmed]

It only happened once to me which was a few weeks ago. We were maintaining the STAR using Vnav then we were asked for a speed by ATC so opened up the speed window (fma correct with vnav spd) to allow for this, then as we were slowing for flaps with the window still open when the fma changed to Vnav Pth and accelerated to catch the profile. Thanks for the reply

[ast83]

Sounds like you transitioned to 'on approach' logic. FCOM 2 explains when this happens ( within 2nm of an approach point including transitions ). VNAV will revert to PATH even with the speed window open. However there are other criteria which will 'force' VNAV into PATH with the speed window open. One of these is selecting flap which sounds like the condition you may have activated. If you don't have an accurate representation of your expected lateral path depicted on your NAV display then the aircraft will either pitch down (and accelerate) in order to capture a profile, or simply level off if below the profile. The solution here is to ensure you keep you lateral path updated. If not, stick to LVL CHG or V/S.

[ast83]

As you were on a STAR I'm guessing your lateral profile was accurate. When given the speed by ATC you opened the speed window and decelerated earlier than planned. The aircraft pitched for this speed and in doing so deviated above the VNAV profile. Although the FMC CDU displays the MCP selected speed, the PATH is still based on the previously entered (now hidden) speed. You now select flap, VNAV PATH annunciates and the aircraft pitches down to regain the profile - accelerating in the process. What you could have done is simply type the new ATC speed directly into the DES page. Even if you do opt to open the speed window, I'd still recommend typing the new speed into the descent page in order to update the vertical profile with the new speed.

[aa73]

Again guys. No need to type in the speed on the legs page. Simply make the first point on the STAR a Hard Altitude or an At/Below Altitude. Once past that point you can open the speed window and stay in PATH the entire STAR. Even from 200 miles out it still works. Much quicker and easier than typing in the speed on every leg in the FMC.

[ast83]

You're not wrong aa73 but I don't think that is what the OP is asking. I understood we are talking about why VNAV SPD automatically switched back to VNAV PTH when SPD was the desired mode?

[flyingmed]

Yes the question was related to the logic behind the switch over and why it happens. Found some info on the approach logic. 👍

[737aviator]

ast83, that is how I understand it as well. However, to expand on your good advice to always keep the descent speed in the FMC updated with ATC speed restrictions, it's worth pointing out that if the OP has reduced from say 250 to 220 early using Speed Intervent, and then updates the FMC that he will probably be in the region of 3-500ft high on the new profile. Of course early ATC speed restrictions could often mean its a busy day, perhaps delaying vectors coming up anyway, so probably no rush to correct this deviation from path as extra track miles may be forthcoming anyway.
However, if not and he does nothing to solve this (i.e. speed brake), upon selection of flaps, or through other activation the FMC 'on-approach logic' will cause the vertical mode to revert to VNAV Path, with a resulting pitch down and undesired increase in speed as the aircraft pitches to re-establish on the vertical profile.

As one gets to know the quirks of the aircraft you can anticipate this and prevent the finger mashing of buttons on the MCP...just use another vertical mode (presumably Level Change if you're already happy with a thrust idle VNAV SPD descent up until now) until you're happy that you're approaching your VNAV path, then re-engage VNAV. Or if you were below profile, do you really want to level off for a few miles while ATC were expecting you to continue your descent to your cleared altitude....use a low V/S.

[Avenger]

There appears to be some confusion here.. " On approach logic" and VNAV.

When the FMC transitions to on approach logic, the RNP changes to a default of 0.5 NM and enables alerting features:
"UNABLE REQD NAV PERF-RNP" alerting levels are higher we check in the FMC to ensure the RNP is suitable for the approach.
MAA can be set when > 300 feet below Missed Approach Altitude, VNAV will continue to command a descent.
If more than 200 feet below vertical glide path, VNAV commands zero vertical speed until intercepting path
If FMC not in on approach logic, the pitch mode will revert to CWS pitch if a higher MCP altitude is set
The FMC will command level flight in VNAV PATH until reaching the descent path.
FMC will transition into on approach logic approximately 2 NM before the Final Approach Fix

Now, back to OP.

If engaging speed intervention:
- 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 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 one looks at the magenta path indication with the speed window open and VNAV SPD in the FMA you may notice something as the selected speed is higher than the FMC calculated speed for that phase of the profile:ok:

[aa73]

Gotcha, sorry for the confusion.