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VNAV SPD/PTH Descent?

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VNAV SPD/PTH Descent?

Old 10th Jul 2009, 05:32
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Question VNAV SPD/PTH Descent?

Is there a difference between a VNAV SPD and a VNAV PTH descent? If so, what is it?
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Old 10th Jul 2009, 06:17
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Is there a difference? Yes.

In basic terms it (VNAV) does what it says on the tin:

VNAV SPD: the elevators control the pitch attitude of the aircraft to control speed.

VNAV PTH: the elevators control the pitch attitude of the aircraft to follow a programed vertical path, (e.g. to cross a waypoint at a altitude/Flight Level programmed into the FMC).

Last edited by wiggy; 10th Jul 2009 at 06:44.
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Old 10th Jul 2009, 08:17
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VNAV SPD - Aircraft will descend at the FMC programmed descent speed with the thrust levers closed and will disregard the vertical profile.

VNAV PTH - Aircraft will descent at approx the FMC programmed speed, and will maintain a vertical profile. To achieve this the thrust levers might be advanced to increase thrust to reduce the VS if the aircraft is going to be low on the profile (due headwind usually). Likewise, if the aircraft is unable to achieve it's descent profile (due tailwind usually), then the VS will be increased and you can expect the airspeed to increase beyond that programmed in the FMC. In this case, on B737 Classic, the FMC will advise Drag Required (speed brake) when the airspeed exceeds 10kts above the FMC speed in order for the speed to be managed with the descent.
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Old 12th Jul 2009, 18:13
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VNAV is a finicky beast. In general operations, we use VNAV PATH, as it computes the most efficient descent path based on an idle thrust descent. If the winds in the FMC are wrong, it will allow the airspeed to fluctuate around target speed slightly, before reverting to MCP SPD mode to increase thrust.

Unfortunately, it has no control of overspeeds. Target speed + 10kts leads to a scratchpad message (on the 737NG at least) of DRAG REQUIRED, and its up to you to manage the speed. Ignoring this too long can sometimes lead to the VNAV disconnecting.

Using the Speed Intervention switch will revert to VNAV SPD, where the aircraft will fly a target speed, and in some cases disregard the path. For instance, when in VNAV PATH, if you are above target speed and there is a speed restriction approaching, it gives the path priority.

This can be dangerous too, as VNAV has no VMO or MMO protection built in. If you are 3000 feet above path at say 320 knots, it will pitch for the path, and the airspeed will rapidly increase, and exceed max operating limits.

Horgy
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Old 12th Jul 2009, 22:09
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In addition and at least in my machine, you can not be in VPTH whilst on an ATC heading, the vertical profile can still be displayed but the mode will default to VFLC at which case another direct to can be done to help with the maths and then keep on the profile with the thrust levers (no autothrottle )

My VPTH is calculated on flight path angle so the descent point remains the same assuming a programmed speed the only variable is rate of descent. 3.7 degrees normally has us descending around the same time as the boeingbuses.

Of the 3 or 4 "whats it doing now" moments I have had in 2 years of the aircraft VNAV played a part in each!
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Old 13th Jul 2009, 00:50
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DRAG REQUIRED
and I always thought we'd have to dress like Danny La Rue ! learn something new every day !!
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Old 14th Jul 2009, 13:25
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Or the good old "BUFFET ALERT" - Where's my damn sandwich Boeing?

G-SPOT - We had one such moment into Lamezia this week. VNAV Non Precision Approach being planned, as we were being vectored for downwind VNAV insisted we were 4000" above path, even though we were at 8,000", 250kn with over 40 track miles.

We continue in using LVL CHG or V/S and get an extended downwind, with a turn at 12 miles, so we end up actually low. At 14 miles I start slowing down and ask for Flap 1, we intercept the profile and start a nice descent. I'm about to reduce when BANG, it says we're 2000" high again. What happens? It pitches to 2500 fpm at 10 miles, speed rapidly increases so we're doing Flap 5 240kn speedbrake out at 10 miles, with it in a dive. The speed stabilises but it took all of our effort to get Flap 10, Gear, and the rest out without busting placard speeds. What an effort!

VNAV is a great tool but it's just that. I'd take conventional MCP modes over FMC managed path anyday.

Horgy
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Old 18th Jul 2009, 16:10
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Once you come out of VNAV PTH by changing modes how do you get back into it rather than VNAV SPD? Is it just as simple as closing speed intervene window or do you need to be in LNAV or on the profile or anything else too etc. Thanks.
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Old 18th Jul 2009, 17:26
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Originally Posted by MrHorgy
so we're doing Flap 5 240kn speedbrake out at 10 miles,
- why were you?

D Steve - Boeing or Airbus? What is a 'speed intervene window'? In a B you need
1) LNAV
2) Line select Vnav Path
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Old 18th Jul 2009, 19:03
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BOAC, In a Boeing. (Airbus would be managed descent).

I meant on a descent say passing through FL200 you changed modes from VNAV PTH temporarily, eg to FLCH or VS. How do you then get back into VNAV PTH if you wanted. I dont have a boeing license but have had a few goes in the sim and find the VNAV overcomplicated at times. eg Airbus has managed descent which seems much moe straight forward. By speed intervene window I simply meant if you had used speed intervene by selecting an airspeed (as you would need to if using FLCH or VS), then by arming VNAV and closing that 'window' back to 'managed/vnav' speed does that re-engage VNVAV PTH or just VNAV SPD. If the latter then how do you get back into PTH mid descent? Hope that makes sense.
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Old 18th Jul 2009, 19:38
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If you are on an LNAV track and you select VNAV from your 'speed intervention' mode, you get PATH. Only diff is if you have dropped into VNAV SPEED then you need to reselect PATH.
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Old 18th Jul 2009, 23:44
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Might depend on FMC version, since 10.6 or so we were able to select VNAV again even if on a heading both in classics and NGs if our position was in a corridor of ~20NM around the active LNAV leg. However that is an option best to be used with care, personally i prefer to trust my own descent planning, especially if not on LNAV.
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Old 19th Jul 2009, 11:44
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Steve,

Just hit VNAV again. The default mode for VNAV if the path. What I was hinting at though earlier is that if you hit VNAV and your above the path, the aircraft might simply dive for the path and the speed will rapidly increase, unless you use VNAV SPD.

BOAC - It's what we needed to get the V/S that was demanded by the FMC for the path. I forget exact figures it may not have been quite that dramatic, but we we're 2000" high and it was commanding 2500fpm in the descent to catch it!

Horgy
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Old 19th Jul 2009, 12:20
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BOAC: "Only diff is if you have dropped into VNAV SPEED then you need to reselect PATH."
How do you reselect it?
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Old 19th Jul 2009, 17:30
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Flight Management Computer

In a SPEED descent the 'Speed' (bottom right) prompt becomes PATH
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Old 20th Jul 2009, 13:54
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Thats a 737 FMC. How about the 777??
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Old 20th Jul 2009, 16:11
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For the B737, speed and altitude intervention through the MCP are offered as optional features.

In the B777, when descending in VNAV PTH mode after T/D, speed intervention (i.e., pressing the IAS/MACH selector) results in a mode change to VNAV SPD. Automatic mode reversion from VNAV PTH to VNAV SPD occurs in certain under- or overspeed conditions (before switching to VNAV SPD, the system first allows the aircraft to deviate up to 150 ft from the computed path in an attempt to correct the airspeed). VNAV SPD is also used for early descents ("DES NOW" prompt on the FMS or altitude intervention when within 50 NM from T/D); in that case, VNAV PTH is automatically re-engaged when the computed idle descent path is captured.

In general, descent modes can be divided in two classes: speed-on-pitch and speed-on-thrust. LVL CHG is speed-on-pitch, V/S and FPA are speed-on-thrust. For the VNAV PTH descent mode it is less obvious what's actually happening: in VNAV PTH descent the aircraft basically follows a computed idle thrust descent path. But, while the initial autothrottle action (closing the thrustlevers) appears identical to what happens in LVL CHG, VNAV PTH is not a speed-on-pitch mode! It only appears that way as long as the conditions on which the FMS based its path computations are identical to the actual flight conditions. In fact, VNAV PTH is basically a sort of speed-on-thrust mode, except that it assumes idle thrust for the entire descent and pilot inputs (additional drag or thrust) are required if airspeed corrections are necessary.

This explains why the system automatically switches to VNAV SPD descent (which is a speed-on-pitch mode, although not always by default based on idle thrust, as in LVL CHG) if the actual flight conditions differ from the planned conditions to such an extent that simultaneously keeping the flight path and the airspeed within a certain acceptable bandwidth becomes impossible.

See also this paper: http://human-factors.arc.nasa.gov/IH...Journal1TM.pdf.

Last edited by xetroV; 20th Jul 2009 at 16:37.
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Old 20th Jul 2009, 16:59
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VNAV PTH is automatically re-engaged when the computed idle descent path is captured.
xetroV

Thanks for that detailed reply. So...... On the 777 in VNAV SPD there is no easy was to get back into VNAV PTH?? Unless you happen to recapture the computed idle descent path? Which without changing modes could only be achieved by adjusting speed to control the path (unless I suppose there was a waypoint with an altitude constraint to bring the two back together)
To recapture the computed idle path quicker V/S could be used for a shallow descent if low, or FLCH with a higher speed if high but then you wouldnt be in VNAV so how would it re-engage in VNAV PTH??

VNAV seems way overcomplicated!! I thought the "How to I get back into VNAV PTH" question would be simple but it doesnt seem that way.
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Old 20th Jul 2009, 17:50
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Hi Steve,

My reply was only partly directed at you; I also tried to explain some general principles to the topicstarter. I forgot to say that the MCP speed window can be closed again, terminating the speed intervention, by pressing the IAS/MACH selector once more. This should bring you back to VNAV PTH, as long as automatic mode reversions don't interfere (so it's no use trying to do this when flying 3000 feet above a computed 250 kt descent path while flying 320 kt). Pressing VNAV again, as suggested above, should also work as VNAV PTH is the default VNAV mode (with the same caveats).

But (over)complicated it is! That NASA paper I mentioned stated: "The VNAV function (also known as the PROF function) accounts for the majority of reported human factor issues with cockpit automation". From an engineering point of view, the VNAV mode is probably very clever, but aircraft designers are no aircraft operators.
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Old 1st Jul 2015, 20:07
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Hello All. I'm new to PPRuNe. This was the thread that sparked my interest in joining. I'm an FMS Systems Engineer for Rockwell Collins specializing in the functionalities of VNAV. Hope to learn something about all of you. Thanks.
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