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jgoggi

Always overshooting if levelling off after takeoff/go around.

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Hi, I need to stress a behaviour that I have by now extensively experienced and that needs to be fine tuned.

After applying TOGA for takeoff or go around, if you want the airplane to level off say at 3000 or 4000 ft and use V/S (with A/P on) and SPEED on A/T, the airplane overshoots that altitude even by 1000 ft (with consequent ALTITUDE ALERT), even if I select a low vertical speed and the speed remains low. Then, several seconds after overshooting, he starts descending to the selected altitude. It's like the airplane realizes too late about the commanded level off altitude.

I hope this can get fixed in a next update.

Thank you.

 

 

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Hi James,

 

Are you sure your not asking too much of the airplane? I know toga should give you a massive amount of thrust, and if you are wanting to level off at a low altitude the autopilot modes you mention might not be able to do what you want them to. What weight are you? Again if light then toga will make you climb like a rocket, The autopilot is very good, but has its limitations. What speed are you selecting? The lower the speed the less likely the autopilot will be to to not bust the altitude, as you may be asking it to slow down and stop climbing, this all takes time to do.

 

I,m sure there will be a much more succinct answer than my ramblings for you

 

Cheers

 

Neil warren

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Are you sure your not asking too much of the airplane? I know toga should give you a massive amount of thrust, and if you are wanting to level off at a low altitude the autopilot modes you mention might not be able to do what you want them to. What weight are you? Again if light then toga will make you climb like a rocket, The autopilot is very good, but has its limitations. What speed are you selecting? The lower the speed the less likely the autopilot will be to to not bust the altitude, as you may be asking it to slow down and stop climbing, this all takes time to do.

I,m sure there will be a much more succinct answer than my ramblings for you

 

Hi, James,

 

In addition to Neil's rather succinct answer, for takeoffs you need to use a derated thrust to avoid overshooting and overspeeding.  The more standard way to take off is to use VNAV, not V/S, seems to me.  It will then lower thrust automatically at the preselected altitude, which should be well below your first initial target altitude, and the A/T will switch from THR REF to SPEED as you approach your first target altitude.

 

On goarounds, I haven't experienced excessive acceleration or altitude overshoots, as I usually have quite a bit of flaps still deployed.

 

Mike

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Thank you guys for your reply. Ok, months ago I opened a ticket because of the go around altitude overshooting and Ryan replied confirming that problem. Now I experience it also during takeoff, but I am not asking too much, because after lift off I select SPEED (say 180 knots) and vertical speed (say 1000 ft/min) and the airplane correctly starts climbing stabilized with 1000 fpm at 180 knots, so not a big demand and, above all, it is stabilized and has plenty of time to reach 3000 or 4000 ft and level off. In any other phase it would level off correctly, because it's quite easy with that thrust and v/s, but after takeoff or goaround no, it realizes after overshooting and descends back, so that's an issue.   

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Ok, months ago I opened a ticket because of the go around altitude overshooting and Ryan replied confirming that problem.

 

Hi, James,

 

I'm curious to know if that was before the most recent release.

 

 

 


after lift off I select SPEED (say 180 knots) and vertical speed (say 1000 ft/min) and the airplane correctly starts climbing stabilized with 1000 fpm at 180 knots, so not a big demand and, above all, it is stabilized and has plenty of time to reach 3000 or 4000 ft and level off.

 

I haven't tried this method, so I don't know if I would have the same issue.  What happens if you use VNAV and set the same speed and altitude restrictions in the flight plan - say 180 knots and 3000 at the first waypoint?  I'm kind of thinking that using V/S right after takeoff isn't a very standard procedure and might be causing problems.

 

Mike

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I have experienced overshooting after go-around but never after takeoff.

In some place (on IVAO) I was cleared only to 3000ft initially and set that altitude on the MCP and I never experienced overshooting with VNAV engaged after takeoff even when levelling off at 3000 or 4000ft. I never tried with V/S though.

 

I would think that this issue is not shared by everyone so I' m not sure it would need a service pack actually.

 

 

 


Now I experience it also during takeoff

 

It seems that the issue with after takeoff, is a new one? Did you experience it before?

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I use V/S if for example I am doing a training flight, taking off, levelling at 3000 and landing back in the same airport. This issue is never experienced if one does a normal flight...

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The use of V/S and SPEED during a departure is something that is pretty non-standard. Not sure why you are using this, why you would assume it was normal, and why you would assume "normal" behavior in a non-normal condition: you have a lot of thrust, in two very "dumb" modes.

 

Use FL CH or VNAV to get the expected behavior.

 

 


In some place (on IVAO) I was cleared only to 3000ft initially and set that altitude on the MCP and I never experienced overshooting with VNAV engaged after takeoff even when levelling off at 3000 or 4000ft. I never tried with V/S though.

 

That's why...

You used the proper mode for the operation.

 

I use V/S if for example I am doing a training flight, taking off, levelling at 3000 and landing back in the same airport. This issue is never experienced if one does a normal flight...

 

Likely because you're using a weight that's more normal, in the correct mode, with de-rated thrust.

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I took part in some real flight tests, not with the 777 but with the 737 and the pilots, if they had to check something at low altitudes, they used V/S. Apart from this, I can't understand why the airplane, in V/S (with low rate of climb) and SPEED mode (with low speed), overshoots the altitude, while it has plenty of time to level off... It doesn't matter if the airplane comes from a takeoff or a simple climb, it should not do that, period.

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it should not do that, period.

 

Based on your many, many hours in the real thing, I'm assuming...

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Mr. Goggi,

 

There is nothing wrong with the PMDG B777. 

 

Anytime TOGA is used the aircraft is hand flown.  Departures are hand flown.  All Go-arounds are hand flown. 

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Yeah, I can't wrap my brain around using V/S during a go around maneuver. That's goes against all my (limited) knowledge.

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After applying TOGA for takeoff or go around, if you want the airplane to level off say at 3000 or 4000 ft and use V/S (with A/P on) and SPEED on A/T, the airplane overshoots that altitude even by 1000 ft (with consequent ALTITUDE ALERT), even if I select a low vertical speed and the speed remains low.

May I ask for the field elevation of the actual airport where you apply TO/GA for take off or go around?

Could it be that the APT ELEV is just below 1000ft and your T7 has just passed thrust reduction altitude (hight) and than being forced to pitch and throttle down quite abrupt?

In addition automatic thrust reduction/acceleration and the respective A/T action in conjunction with a light aircraft (T/O on touch-and-go traffic patterns or G/A after normal route with reserve fuel only) would require to select derated thrust.

I will test now your method of "V/S + A/T-Speed" instead of VNAV or FLCH which I normally use. (Immediately after T/O and in a second run after passing thrust reduction altitude.)

Maybe the result is really a surprise.

IMO airspeed selections between 190 and 230kt (Flaps 5 or 1) and V/S selection between 1000 and 2000 ft/min should be possible without overshooting.

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OP I feel your pain. I have experienced this a few times in the past. The behavior during this phase seemed illogical from a common sense point. Now I can understand if it was a matter of the aircraft configuration being changed and messing up the path/arc logic (some form of calculation involving current altitude, target altitude, rate of climb/descent and a few other parameters) that is calculated by the afds for altitude capture (for example making major configuration changes during ALT capture could cause an under shoot or overshoot of the altitude). Sorry but anytime I talk about the technical stuff that happens in the background I get all excited and what I want to say makes sense in my head but comes out jumbled and poorly put together.

 

Anyway the last time I did an autopilot go around, the missed approach altitude was 5000 and I decided for whatever at the time to use v/s. Now the speed was stable, the v/s was stable at the established rate, then somewhere during ALT capture, the engines accelerated to almost full thrust and the v/s shot up through +4000 fpm, overshot by almost 2000ft before coming back down to 5000 ft.

 

Keep in mind however that this was before the most recent update which I believe was to contain a fix for this and I haven't done a G/A since, but I will the next time I fly a full flight and report back, with possibly a video if it happens.

Based on your many, many hours in the real thing, I'm assuming...

With all due respect, and a lot is due trust me I highly respect your knowledge and your input on this forum, but sometimes one should not always be so quick to dismiss things. I know you require cold hard absolute facts when certain claims are made but I've learned sometimes common sense (which is a rare thing these days) and intuition are sometimes very valuable. Take for example on initial release how persons were complaining that the fbw behavior wasn't quite right.

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Guys, I understand that certain issues can depend on many factors and there may be a lot of discussion about them, but this issue is clear and there is little to say, other than facts. Forget about takeoff or go around, think of the following configuration: A/P on, A/T on in SPEED mode (say 180 to 200 kts), V/S with a shallow climb, the airplane overshoots the assigned altitude by 600 to 1000 ft.  It's cristal clear...

 

As to the go around procedure, you advice not to use V/S. Ok, try and follow the Boeing procedure in order to level off at 4000 ft: "1. At flap retraction altitude, set speed to maneuvering speed for the desired flap. 2. After flap retraction, push VNAV (or FLCH), VNAV activates with A/T in THR REF (or FLCH activates with A/T in THR mode)". The airplane will reach 4800-5000 ft or more, then will realize and descend... So, no way...

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