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Using LVL CHG vs VS

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As I understand it, the MCP LVL CHG function will attempt to have the a/c climb using the max N1 allowed by the FMC, or descend at idle N1, while maintaining the speed set in the A/T window. The V/S function will attempt to have the a/c climb or descend at the vertical speed set in the V/S window and at the speed set in the A/T window.

 

Since these two functions seem to be very similar I'd like to better understand when they are used in normal operation. Is one typically used more than the other? If ATC calls and requests a climb or descent, which is the preferred function? If you have the V/S capability, why is LVL CHG needed?

 

Thx,

Al

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This is one of the most overthought question from simmers, and it really doesn't need to be.

 

 

 


If ATC calls and requests a climb or descent, which is the preferred function?

 

ATC's primary concern is that you do it soon after they call it.  Method doesn't matter as long as you act "normally."  By that, I mean climb/descend like the aircraft around you - not too high, not too low.  Do what you want.  If ATC needs something else, they'll vector you out of the way, or ask you to "expedite your descent."

 

 

 


If you have the V/S capability, why is LVL CHG needed?

 

One allows you to accelerate or decelerate without changing your V/S.  The other allows max rate of ascent or descent, allowing V/S to adjust as appropriate.

 

If a plane can be landed without the HUGS, why is a HUGS needed?

 

It's an additional tool to help you do your job.

 

 

 

Watch this:

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I only had experience with the 777 so I would answer the question based on my 777 knowledge and I hope there won't be too much of a difference.

 

The biggest difference between FLCH and V/S is FLCH is pretty much like VNAV SPD, basically uses the elevator to maintain speed for a fixed thrust setting that's why the A/T would either be in THR REF, THR, IDLE, HOLD ( on the 777 ) but never in SPD. v/S on the other is constant vertical path is maintained by pitch, therefore A/T always in SPD mode.

 

This implies that there's a speed protection using FLCH, because the speed set in the MCP window is the primary target, the airplane will adjust pitch to fly that speed regardless of engine thrust setting. However in V/S, vertical speed will be the primary target, if let say you set +4000fpm to try to do a cruise climb close to the MAX altitude, the AP will command a 4000fpm rate of climb in the expense of losing airspeed because airspeed can not be maintained at such a high rate of climb at high altitude. As result the speed will decrease to Vmms until the AP stall protection kick in to save the airplane. The opposite applies on descend, if you dial -5000fpm, you will Overspeed the airplane until the Overspeed protection kick in to save the day.

 

Therefore FLCH is a good mode to use for descend from cruising level down to the G/S intercept altitude or the step climb in cruise when VNAV is not available, and it helps to lower the workload. V/S is best to use on at lower altitude to fine tune your descend profile, or to perform a nor precision appr when VNAV PATH can not be used, small step descend, drift down in a holding pattern or to do small step climbs at low altitude shortly after departure below around 10000ft. Or v/s can be used as long as you are sure that the engine will have enough thrust to maintain the selected V/S on the MCP.

 

FLCH is good for slowing down the airplane from high speed because it will pitch the nose up to maintain ~500fpm rate of descend to allow the speed bleed of. The opposite applies for the climb as well, pitch down to accelerate then pitch up again to continue the climb once target speed is reached.

 

FLCH is also used in go around after reaching Accel HT.

 

You can use v/S solely but if you do, monitor your airspeed carefully, and be ready to change the target V/S if the airspeed can not to maintained.

 

 

I had the same question when I first begin flying airplanes with auto pilot. So don't worry.

 

Hope it helps. Happy flying : D

 

Since most people has already answered the question in details, I would just like to add one more touch on the discussion.

 

FLCH (on the 777) is actually try to reach your target ALT in 2 min if not achieve then the thrust will go idle or max CLB thrust to finish the job with airspeed being the primary target as mentioned before. Therefore if you are doing small step climb or descend with 1000ft difference, FLCH will always give you 500fpm going up or down, and A/T mode will be in THR. And now if you wanna climb 2000ft from your current altitude FLCH will command v/S 1000fpm, so on and so on. So in the 777 case I usually use FLCH on departure if ATC gives me small step climb like 1000-2000ft every time to keep it simple and same for descend.

 

The idea of using V/S for small step climb and descend is for fuel saving and passenger comfort as it avoids the thrust goes all the up to CLB the airplane pitch up quite dramatically then in 10 second the AP captures the altitude and the thrust has to roll back and airplane pitch down again etc etc.

 

There's no right or wrong answer, I personally try to minimized mode changes to keep things simple. It all comes down to experience after a while and everyone does it differently.

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LVL CH or FLCH (pronounced "flich") is basically a "pitch for speed", the TARGET is keeping the MCP selected speed and accepting whatever vertical speed results.

 

V/S is a "vertical speed" mode. The TARGET is vertical speed.

 

 

I find myself using FLCH during most climbs and initial descend. During the final part of the descend or once in an area with heavy ATC vectoring and constant altitude clearances I tend to prefer V/S.

 

V/S is, at least for me, a "fine tuner". I use it when the descend is of a couple thousand feet. On the descends instructed on an Instrument Approach Chart, or during final approach on a non-precision procedure, I use V/S (or fly manually).

 

You can see me using V/S on this video (start watching where it is, minute 20:something). The method I use is the following:

 

- On "ALT CAP" I select the next lower altitude. Say the plane is capturing 5000' and the next lower altitude is 4000. I set 4000 on the MCP when I see the "ALT CAP" on the FMA.

 

- On "ALT HOLD" I open the V/S window and check for zeroes. That way my next altitude is sort of "armed".

 

- Once the descend to the next lower altitude should begin, I turn the V/S wheel.

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Well...there are situations where one or the other are more suitable and safer than the other.

 

The most important thing to remember is that LVL CHG is a 'pitch for speed' mode. Speed will always be protected in this mode. Especially important at high altitude, when the wrong use of V/S will potentially put the airplane in a low speed condition, if you don't monitor speed and thrust very closely. It's not really recommended to make extensive use of V/S in the climb for this reason.

 

Equally V/S is very useful and preferable during the later stage of an approach, where a smaller specific vertical speed is required. Usually to maintain a CDA (Constant Descent Approach), when LVL CHG may produce higher and unnecessary rate of descent resulting in early level off. A common simmer mistake on most videos I see.

 

For newbies, a thorough understanding of the FMA is needed to appreciate what the aircraft is trying to do.

 

Adam Turley

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I've heard this "V/S is dangerous" thing numerous times and I don't quite agree.

 

It's dangerous if you put the plane in V/S at a rate of +5000 fpm at 30.000ft, yes, that's dangerous. But the pilot is there precisely to use his judgement and ask "reasonable" requests to the autoflight system.

 

I don't see why doing a step climb using V/S should be dangerous if you input a reasonable V/S and monitor the airspeed, something you should ALWAYS do by the way, regardless of what autopilot mode or level of automation you're using.

 

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.

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I've heard this "V/S is dangerous" thing numerous times and I don't quite agree.

 

It's dangerous if you put the plane in V/S at a rate of +5000 fpm at 30.000ft, yes, that's dangerous. But the pilot is there precisely to use his judgement and ask "reasonable" requests to the autoflight system.

 

I don't see why doing a step climb using V/S should be dangerous if you input a reasonable V/S and monitor the airspeed, something you should ALWAYS do by the way, regardless of what autopilot mode or level of automation you're using.

 

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.

 

At around FL360 the speed margins are pretty small either way. It won't need a 5000 fpm rate to be dangerous. 800 - 1000 fpm will often be too much for the airplane to handle before it becomes thrust limited and speed decreases.

Pilots do use good judgement and give reasonable requests, but even with 1000 fpm V/S commanded a sudden wind change or change in temperature coupled with a few moments distraction from the flight crew (it does happen) will put the aircraft into the amber band in the high 30's.

 

At my airline VNAV is always the preferred climb mode even for 1000ft step climbs. You see what VNAV does then go to V/S if the rate of climb becomes excessive. Excessive being greater than 1500 fpm.

 

Adam Turley 

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At around FL360 the speed margins are pretty small either way. It won't need a 5000 fpm rate to be dangerous. 800 - 1000 fpm will often be too much for the airplane to handle before it becomes thrust limited and speed decreases.

Pilots do use good judgement and give reasonable requests, but even with 1000 fpm V/S commanded a sudden wind change or change in temperature coupled with a few moments distraction from the flight crew (it does happen) will put the aircraft into the amber band in the high 30's.

 

At my airline VNAV is always the preferred climb mode even for 1000ft step climbs. You see what VNAV does then go to V/S if the rate of climb becomes excessive. Excessive being greater than 1500 fpm.

 

Adam Turley

100% agree. Especially climbing into a strong jet stream above OPT, some airplanes with thrust limited can not always power through it. Again not that we can't do it in V/S, but I am lazy and I prefer the added protection from VNAV or FLCH.

 

Sorry I exaggerated a bit with 5000fpm with my previous example as an attempt to point out the potential danger in v/S.

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One more RW use of V/S:

 

Eg. You're on the BAYVU3 in to KSAN and you get get "Cross BAYVU 15000." Surprise, surprise, they left you high and now VNAV wants to go to VNAV SPD. Well, that's not going to help much.You had planned on descending at .78/280, but now that plan is out the window.

 

Here's what you can do: Put 15000 in the MCP. Look at the "Green Banana" arc and dial in a V/S that has you crossing BAYVU at 15000. In this instance you don't care about speed, just the crossing restriction. The airspeed will go up, but as long as it's a good speed (not too rough, not too close to VMO) you should be fine.

 

Like all modes, this requires monitoring, but this is a quick way of solving a problem.

 

Also, I use V/S to have a nice gentle G/S intercept. I don't like high power settings and level flight on approach, so I keep track of my Vertical Bearing (DESC page) to the FAF and use a more sedate V/S once I'm around 3.0 degrees. Smooth. Chicks totally dig it.

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Smooth. Chicks totally dig it.

:LMAO: :LMAO:

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Speed management is everything while orchestrating a descent path to the IAF. Therefore I almost always use FLCH to keep the speed under control. Its just a matter of learning the distance required to lose a certain amount of altitude at a given speed. This is also very useful for speed restrictions such as 250 kias under 10k feet. If the need arises to expedite your descent, just pop some spoilers out to come down quicker without increasing ground speed. For climb I'm almost always in VNAV.

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I bet he's just theorizing.   B)  :rolleyes:  :biggrin:  

I wonder how many phone numbers he collects at the end of the flight :Party:

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LVL CH or FLCH (pronounced "flich") is basically a "pitch for speed", the TARGET is keeping the MCP selected speed and accepting whatever vertical speed results.

 

V/S is a "vertical speed" mode. The TARGET is vertical speed.

Pilots I've met have spoken FLCH as "felch".

 

FLCH is a vertical speed mode too. Target VS is based on the initial change in level and thrust is increased/decreased to achieve it at the selected airspeed. If the target VS can't be achieved the thrust is limited at either reference thrust or idle.

 

VS mode is good for small adjustments in altitude or to fine tune altitude capture. FLCH is better for larger altitude changes where the additional protection and refinement is good. If you need to reach an altitude in a given time or distance then VS is best.

 

Some autopilots have a less sophisticated version of FLCH which commands an increase in thrust to CLB or decrease to idle and then pitch is adjusted to maintain speed. This can give a rougher ride with more rapid thrust changes.

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With a lightly-loaded high-performance aircraft, selecting FLCH climb will result in a rapid climb with uncomfortably (and therefore unacceptable) high deck angles.  In my G-IV and G-V flying, which involved flying a fair number of short legs with light fuel loads in a jet that can pack 10-12 hours of fuel, my technique was to use V/S mode limited to no more than 3000 fpm on initial climb to keep the deck angle under control, and then switch to FLCH climbing through ~10,000 feet when lightweight max climb performance won't slide the passenger's drinks off the table into their laps.

 

Air France flew (flies?) a B777 route from Paris, to Buenos Aires Argentina, to Santiago Chile, and back.  The short legs between SAEZ and SCEL are only ~1.5 hrs...I can only imagine what a FLCH climb would look like in a B777 with a 10-15% fuel load.

 

Bottom line--you're flying the jet...the A/P is just helping you do the job.  If you wouldn't select full climb EPR and horse the nose up to 25 deg nose-high while hand flying, then you shouldn't horse it up there with the automation, either.

 

Regards

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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.

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(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.

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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

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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.

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

Thx,

Al

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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.

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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.

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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.

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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.

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"...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  :P 

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