Nick Dobda

LVL CHG what not to do

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I was level at 8000 slowing to 180 from 240 (about 220) when I decided I needed to get down to 5,000 as quick as possible. So, I rolled the alt back to 5000 and clicked on LVL CHG.

I watch the speed drop 200, 190… I expect it to start to nose over and dive down to 5000. But no pitch-


Speed 180…170…160…150… still no pitch! I can take it no more, I disconnect the throttle and mash them up and finally observe as the plane NOSE DIVES to bring the speed back up.

I know LVL CHG will pitch to descend as much as possible without raising speed once it hits the speed I have dialed in, but I figured it would have recognized somewhere around 200 to start pitching… instead it didn’t start pitching till it was down around 150 (30 below what I had set).


Is this the way it works in real life? Did I do something wrong? From now on in this scenerio I’ll just roll the dial and  finesse the V/C to maintain speed.

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Level Change will arm autothrust after pulling back power to idle, and will adjust pitch for MCP Speed. You can add power to your discretion but you will therefore lower your rate of descent. Reading from what you have stated no it should not have worked that way. Did you verify MCP SPD in you pitch Flight Mode Annunciator ?

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No I did not verify in the FMA - but the process I describe was precisely what I did.

I was already engine idle slowing from 240 to 180 level at 8000 using VNAV (I was on STAR decelerating to hit a speed restriction). Instead of finishing the massive downwind leg in the star I decided to take a shortcut and intercept the localizer I was flying past. I rolled the alt back to 5000 and selected LVL change verifying 180 in the speed display - then changed from LNAV to HDG rolling the heading to intercept the ILS. My hardware was pulled back to idle.

I understand it should have pitched off 8000 to hold the MCP speed at 180 when the speed dropped to 180.

I would have thought the level change mode would have had the foresight to begin pitching down prior to 180 as to maintain 180. Instead it continued to decel to about 150 before finally realizing it needed to pitch down. When it realized what was going on it pitched way down to get back up to 180 resulting in a 2000fpm + rate.

Just seemed that the FMC was behind what was going on. I was curious is this a result of a sim limitation or I did something wrong. Would the real thing have realized what was going on prior to 180 and begin pitching down?

 

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The level change should hold MCP speed and retard throttles.  It is true that in VNAV modes, the A/T will delay responding until 10-15 kts below set speed, so maybe your pitch mode was not MCP SPD.  I use this mode quite often and have never seen my speeds drop below MCP SPD, but always look at the FMA anytime you wonder what the automatics are doing.  I normally use level change below 10000, applying thrust as desired to control rate of descent all the way to glide path intercept altitude.  Sometimes there is a delay of sorts because the rate of descent will increase while the throttle is in retard mode, and additional thrust cannot be added until the retard changes to arm mode.  Then one has to bump the thrust up to maybe 60% N1 to slow the decent and then decrease thrust to prevent leveling.  This is messy so I always disconnect A/T when using level change and avoid the decent rate cycles.  Set 55% N1 and then adjust as required for desired rate of descent with pitch commanded by the MCP SPD setting.

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Thanks for your thoughts - I am still confused about the importance of what was displayed in the FMA - I guess I should start paying more attention to it.

Given the sequence of engaging / disengaging various modes, I would think it should be easy to deduce what should have been in the FMA.

VNAV / LNAV on the magenta and right on the vertical slowing from 240 to 180 to hit the next upcoming waypoint precisely at 8000 and 180. Somewhere prior to that waypoint - decelerating between 240 and 180 and slightly above 8000 I roll the MCP alt to 5000 and click LVL CHG and set the speed to 180. Then roll to an intercept heading and click HDG.

If I understand the concept of LVL CHG the pitch mode should have been MCP SPD, and it should have pitched over to maintain 180. But there was a delay and the pitch didn't start happening until the speed was 30 or so knots below where it should have been.

I guess perhaps because I cannot paint the full picture it will be difficult for others on here to figure out what happened exactly.
Looking back I should have done what I normally do that is:


Roll down the altitude to 5K click V/S and play with that to maintain speed down to 5K. From my experience in the sim it seems to me the plane's reaction to V/S is more instant (the pitch change is immediate when rolling the V/S). Mind you my only knowledge comes from the NGX and the sim. I would have thought since the plane reacts so quickly when you change the V/S it would be able to react quickly when the speed drops to what is displayed on the MCP instead of reacting 30 knots later

Normally I don't use LVL CHG but I figured I'd try it last night - so I did and it didn't go well. I just thought the ALT CHG would have done the rolling of the V/S for me, but the delay (still not sure why there was a delay) threw things out of whack and into chaos.

 

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I encourage you to play with and become comfortable with level change.  The problem with V/S mode is that there is no speed protection, and I've been told by line pilots that it's use is very limited.  It's there as a hold over from earlier systems.

Agree, when you selected level change with aircraft at 180 indicated and MCP SPD set to 180 the pitch response should have been immediate.  The FMA is important because it tells you what modes the automation is in, in this case the pitch mode is important.  On the topic, usually if a procedure has a speed limit of 180 it is an AT OR BELOW speed.... and the VNAV mode actually works on the principle that all speed constraints are AT OR BELOW.  I presume you had some flaps out?

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34 minutes ago, downscc said:

I encourage you to play with and become comfortable with level change.  The problem with V/S mode is that there is no speed protection, and I've been told by line pilots that it's use is very limited.  It's there as a hold over from earlier systems.

I have to say that V/S in descent is probably used on 99% of flights here. Maybe a European thing, but no airline or pilot I know of over here has an issue with it in descent.

In the climb, I agree with you, but I would argue that in descent you are far more likely to get caught out in FLCH than V/S: there's no speed protection in ALT if you forget to re-engage the A/T, or if the autopilot is disengaged prior to level-off either :cool:.

Far safer and more well-protected in SPD | VS than IDLE or HOLD | FLCH in the descent IMO, and if you're going to start disconnecting the A/T in order to achieve exactly what V/S gives you then you are really, really unprotected.

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36 minutes ago, skelsey said:

Far safer and more well-protected in SPD | VS than IDLE or HOLD | FLCH in the descent IMO, and if you're going to start disconnecting the A/T in order to achieve exactly what V/S gives you then you are really, really unprotected.

Food for thought.  I just talked this over with a friend and I think I'm going to try to use a throttle option whereby I could override the clutches in any mode, not just ARM or HOLD.  This eliminates the  need to disconnect the A/T to avoid the delay during retard.  The 777 handles this very well but it has differences in how it handles thrust to provide ride comfort.  V/S in descent on 99% of flights sounds convincing but I remember well the lectures I received from RSR on the use of V/S and thereupon started trying to understand how best to use level change.

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59 minutes ago, downscc said:

Food for thought.  I just talked this over with a friend and I think I'm going to try to use a throttle option whereby I could override the clutches in any mode, not just ARM or HOLD.  This eliminates the  need to disconnect the A/T to avoid the delay during retard.  The 777 handles this very well but it has differences in how it handles thrust to provide ride comfort.  V/S in descent on 99% of flights sounds convincing but I remember well the lectures I received from RSR on the use of V/S and thereupon started trying to understand how best to use level change.

Being able to override the clutches without disengaging altogether certainly makes things better. To me it's a question of looking at the potential pitfalls and outcomes: in the climb if you select a V/S that is greater than the aircraft's performance capability and you get distracted/fail to notice, the aircraft will pitch for the V/S with maximum thrust, speed bleeds off and Bad Things happen. Hence using a speed-on-pitch mode like VNAV or FLCH makes sense because you will always have maximum thrust, the aircraft will pitch (i.e. down) to maintain speed, and if for whatever reason you've selected an altitude which exceeds the aircraft's current climb capability all that will happen is the aircraft will level off in FLCH/VNAV with full thrust on and maintain the selected airspeed at the expense of altitude: untidy, but safe.

However, in a descent the situation is reversed. Here, if you are in a speed-on-pitch mode you are descending towards an ALT capture with idle thrust and/or the A/T clutches disengaged (i.e. HOLD mode). If for any reason the thrust levers do not come back up when the aircraft levels off, for example if the A/T has been/become disengaged, or if the A/P is disengaged (or becomes disengaged) and the aircraft levelled manually/rate of descent reduced without properly going in to ALT CAP etc etc then the thrust will stay at IDLE, the aircraft will pitch for altitude and the airspeed will disappear - equally as bad a situation as the V/S scenario in a climb (and in effect this "FLCH Trap" is what did for Asiana). In V/S, however, the A/T remains actively in SPEED mode at all times and therefore the risk is greatly reduced (because in a descent you are never going to exceed the thrust capability of the aircraft).

It is an interesting discussion for sure - I'm always reluctant to say never use any particular mode, and I certainly not saying you should never use VNAV or FLCH in the descent -- I do regularly, particularly in the initial descent, but tend to switch to V/S to finesse the final stages. The key as always with automation though is understanding the pros and cons and spotting the trapdoors before you fall in.

Speaking personally I find V/S a very useful mode in descent -- used correctly and judiciously it can help you manage your energy in a very effective and efficient way, as it gives you much better control over the ratio of slow down:go down. If you're slightly high as you approach the glide, for example, V/S can make sure you keep coming down at a good rate whilst reducing speed albeit at a lesser rate than if you selected a lower speed in FLCH, with the thrust levers at idle and the A/T in SPEED mode so there's no danger of doing an Asiana. Conversely if you're a little low and fast then selecting a slightly lower V/S than you might get in FLCH will allow you to regain a nice CDA profile with minimal thrust change, a nice controlled speed reduction and, again, A/T in SPEED mode to keep you safe.

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16 hours ago, downscc said:

On the topic, usually if a procedure has a speed limit of 180 it is an AT OR BELOW speed.... and the VNAV mode actually works on the principle that all speed constraints are AT OR BELOW.  I presume you had some flaps out?

Yes - in that particular flight I had flaps 1 as I was dropping the speed from 240 to 180. I went to flaps 5 when I decided to take the shortcut and jump into LVL CHG mode.

The problems happened when the speed dropped past 180 and the plane forgot to pitch to maintain the 180. I guess if I had been monitoring the FMA I would have known a more complete story as to why it missed the nose over to maintain speed. As you said if the pitch was instantaneous at 180 or even above the speed would have been maintained and I would have started dropping in altitude quickly to 5000 and all would have been good. That was the root of my original question - why the delay, is it a limitation of the sim or does the real plane behave this way.

The supplemental conversation you guys had was great. I've fallen into some of these traps when not in the proper mode and not paying attention - the limitation being I am the only one in the cockpit and I got distracted for whatever reason.

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Wow. You guys make this complicated. 

Nick, you screwed up something. It wasn’t the plane, unless you found a sim bug. 

V/S does have speed reversion.

Its LVL CHG, not FLCH.  

ARM is the mode, not HOLD.

VNAV works well, especially on STARs with alt/spd restrictions. 

I’ll edit this after I get some coffee to remove the cranky tone. 

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38 minutes ago, Spin737 said:

Nick, you screwed up something. It wasn’t the plane, unless you found a sim bug. 

That's what I'm trying to figure out - if I did something wrong or its a sim bug. I was hoping someone else had a similar experience to verify what happened to me.

I'm not ruling out that I screwed up. In fact sure highly likely I screwed up. Unfortunately I was not monitoring the FMA so we may never figure out if I screwed up or its an actual "bug" (I assume I screwed up unless otherwise confirmed).

This is the click sequence:

VNAV / LNAV on the magenta and right on the vertical slowing from 240 to 180 (flaps 1) to hit the next upcoming waypoint precisely at 8000 and 180. Somewhere prior to that waypoint - decelerating between 240 and 180 and slightly above 8000 I roll the MCP alt to 5000 click LVL CHG, select flaps 5 and set the speed to 180. Then roll to an intercept heading and click HDG.

All was going good until the speed crossed 180, kept decreasing and did not pitch. Around 150 it finally started to nose over, but at that point I was panicking and mashing the throttles up and disconnecting everything to take back control.

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I agree with Simon on this but it can be a contentious issue.

I flew the single aisle Airbus for 12 years and no one really had a issue with VS at all, it was rarely used in the climb for obvious reasons and often used in the descent around Europe to try and get that coveted Continuous Descent Approach (CDA).

On joining the Jumbo a few years back I gave a number of guys an accidental coronary by pushing that VS button and they'd start twitching uncontrollably and mumbling "but you've got no speed protection". Initially I though I'd contravened some SOP so stopped using it (and made some rather inelegant approaches because of it). As I'm more used to the long haul lifestyle I can see where they're coming from, the chances are you're flying the approach when you either should be asleep or you haven't slept for very long, in both cases the tiniest distraction or inattention can leave you in a perilous condition so why take the risk.

VS has it's uses but I like to brief my better half of my intentions before pushing it lest they start twitching; and then he's worried about the mode I've selected, I'm worried about his heart and no one's watching the aircraft.

Equally, we fly to the US a lot and the airspace is so busy we assume that if they tell you to descend to an altitude, descend to it as quick as you can, they are probably planning on trying to get someone over the top of you soon so don't hang around, CDA be damned, FLCH is normally best for that. It's not like it's going to plummet, at normal approach weights under 250kts you're unlikely to get much more than 1200 ft/min anyway.

Personally (and if the discussion comes up and the Capt is more relaxed about such cowboy behaviour) I have no issue with VS at all, yes, you don't have speed protection so you have to make sure you're not asking the aircraft to do more than it can do (climb or descent) but it's a nice smooth mode to use. Step climbs are much quieter and more gentle if you just VS up at about 800 ft/min, the only drawback is the engine note changing as the speed fluctuates; as against applying full climb power, pitching up to get just shy of 1800 ft/min, have the aircraft nodding about as you climb through various wind layers and the speed fluctuates before going into Alt and pulling the power right back again.

I know this is the NGX forum (and Matt has very succinctly summed it up for the 73) so all the above is a little OT so to bring it back to the original problem...

Nick, we'll never know what the problem was but you've highlighted a very important lesson in flying airliners...

Check your FMA's

It's easy to worry greatly about which buttons to push but really you should be thinking about what modes you want to be in. Then the only way to check whether you've pushed the right buttons is to read the FMAs to see what mode you are actually in. It was described to me as : the MCP is rumour, the FMAs are fact. If you're not sure what the aircraft is doing, check the FMAs. If the FMAs aren't what you want/expected then start pushing buttons to change them, if the FMAs are right and it still doesn't do what you want then you're going to have to intervene.

Don't scream "BUG!" either, the real one doesn't always do it right as well, I was flying just last week and the aircraft went into SPD LNAV ALT but was slow to put the nose down, I clicked it out as it became apparent it wasn't going to level off in time and manually levelled off, we peaked about 150ft high before returning. Nobody noticed, no extra paperwork, the aircraft didn't do it again for the rest of the flight, no idea what it was, real life isn't as sterile and black and white as these sims make out.

HTH

Ian Webber

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I guess I'll just have to re-create what I did and watch the FMA to get the full picture. If I can recall here is where I was:

It was the SUNSET4 approach to Vegas. LNAV / VNAV engaged. Approaching SUNST to hit the restrictions (8000, 210). Slowing airspeed and slight descent. I decided to take a short cut and head over for the ILS 26R, aiming for HAWKO

I rolled MCP altitude window to 5000, clicked LVL CHG, set speed to 180, lowered flaps to 5. Changed the heading to point towards HAWKO and pressed HDG.

Watched the speed drop to 180 and beyond. Started "twitching" (LOL) when the speed was 160 and the plane was not pitching. At about 150 it started to nose but I had a coronary and started taking control back wondering why didn't it pitch at 180??

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18 hours ago, Nick Dobda said:

I am still confused about the importance of what was displayed in the FMA - I guess I should start paying more attention to it.

Always pay attention to the FMA, it will tell you what the airplane is actually doing vs. what you asked it to do.

The saying goes, "The MCP is what you bid, the FMA is what you got."

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On 12/18/2017 at 2:15 PM, Nick Dobda said:

Thanks for your thoughts - I am still confused about the importance of what was displayed in the FMA - I guess I should start paying more attention to it.

We have a saying: "We live and die by the FMI(FMA)" (Flight Mode Indicators) 

MCP is where you order take-out, FMA is where the food gets delivered.

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