A/T Problems

[dimatrixxx 0]

Hi guys,

 

My T7 is not keeping speed on approach... I running the newest pmdg b777 version on p3d v3.2. I read some posts in regards to this issue, so here the answers:

1. A/T overide NEVER

2. Flare overide YES

3. Weather ASN is good (no turb.0, windshear etc. )

 

a/t is working with a big delay so the aircraft is very unstable while final approach. Some times it could lead to a stall (vref + 5 knots is set). What to do???

[downscc 1,327]

Full names in PMDG forums please.

 

While there is a realistic throttle response delay to speed variance due to wind/turbulence, if there is no turbulence the A/T is perfectly capable of holding the set speed. You didn't mention if you were using the MCP to set speed, this is not the NGX that will control speed to landing.

[Mike777 163]

 

 

a/t is working with a big delay so the aircraft is very unstable while final approach. Some times it could lead to a stall (vref + 5 knots is set). What to do???

 

Hi, dimatrixxx,

 

Two questions:

 

1)  Have you had an actual stall?  The autothrottle is supposed to compensate for gusts on its own, when switched on and the normal approach speed is set on the MCP (the FMC doesn't set the final approach speed on its own, as Dan noted).  Take a look at the FCTM beginning at section 1.10, pdf page 41.

 

2)  Are you seeing extreme windspeed/direction changes on final?  You may have some unrealistic weather being introduced somehow.

 

Mike

[Vim Fuego 1]

Hi,

 

OP, this is really down to your ASN turbulence settings I'd suggest.  If you're using default, thats 70% on the Turbulence Effect Scale?  Even in clear conditions i've had problems with that one. On a number of occasions I've had to take manual control in CAT III conditions due to the AT 'falling asleep' on approach and then rocketing up during wakeup mode, and thats with no wind, no apparent tubulence. 

 

Problem is I've no idea how realistic ASN is in it's modelling of turbulence.  It's certainly superior to the native FSX but as I've only experienced real turbulence as a passenger i've no idea how turbulence affects the aircraft at the business end and how it affects (or doesn't) factors like airspeed.  50% turbulence effect scale in ASN is usually fine for me, but I usually fly manual throttle after disconnecting the AP anyway.  After some experimentation a while ago I recall that with 20-25% TES in ASN the AT will maintain speed regardless of conditions.  Obviously the wow factor of flying turbulent approaches is somewhat mitigated as a result, so 'realism' may suffer.

 

Depressingly, the Aerosoft Airbus will hold speed even at 70% TES in absolutely filthy weather conditions, but I expect that may be due to a simpler modelling of AT than PMDG.

 

Cheers

[Alpha Floor 237]

Try to fly the approach without using ASN and in clear weather conditions.

 

Same question as Mike: Did the aircraft really stall? Or did the speed just go down below Vref, which wouldn't be a stall?

 

On the PFD set your Vapp to Vref + 5 (regarless of weather when using AT, no need to add knots for wind adjustment).

[Mike777 163]

Bear in mind that there is a margin of safety between the recommended approach speed and the stall speed.  The British  Airways 777 that crashed at Heathrow due to a dual engine thrust rollback had an approach speed of 136, but actually stalled (with flaps retracted from 30 to 25) at @ 107.

 

Mike

[Alpha Floor 237]

Bear in mind that there is a margin of safety between the recommended approach speed and the stall speed.  The British  Airways 777 that crashed at Heathrow due to a dual engine thrust rollback had an approach speed of 136, but actually stalled (with flaps retracted from 30 to 25) at @ 107.

 

Mike

 

Exactly. It's really difficult to fly the 777 into a stall. With 1g it's impossible in Normal Law. You'll have to pitch up aggressively (more than 1g's) while manually holding the thrust levers down at idle.

 

See how many "protections" there are into a Vapp speed:

1. Vapp is 5 knots higher than Vref
2. Vref can very well be 10 knots higher than Minimum Maneuvering Speed (MMS)
3. MMS grants 40º bank maneuver capability in level flight and can be some 20 knots higher than the top of the barber pole (stick shaker activation speed SSAS)
4. SSAS is still some knots away from the stalling angle of attack at 1g. At this point you're still not stalled and can theoretically maintain level flight.

[dimatrixxx 0]

Sorry, my Name is Dimitri. I will try to describe the main problem with a/t when flying in good weather:

The mcp speed set to 180kt, flaps 5 allready extended, the aircraft at the alt with gs armed. At this point a/t commands about 60-70% throttle because the aircraft produces a lot of drag, so we need much power to maintain 180 kt. Then a gs comes down, when captured I put gear down and set after gear extended flaps 20. The a/t commands late to decrease the throttle (because of gs capture and negative vert speed) and the speed of decreasing thrust is slow (very big engines) so in spite of the fact I was good established on approach when captured gs the aircraft accelarates because as mentioned above everything happens to slow. So the speed increases, than after finaly the a/t set the throttle idle the aircraft is fast (210-230) so it is very very difficult to decrease the speed and get a stable approach... Any suggestions?

 

By the way yesterday I flew from Amsterdam, there was very bad weather with turbulance and windshear. In climb after passing 10000 the speed bug jumped to 290 kt in order to accelerate but the aircraft remained climd on speed of 235 kt which was very very soon from amber line (before stall). The throttles were in clb mode, the vert speed was 1750 ft/min, nose angle at about 8 degrees. All the way up until good weather I holded a hand on a throttle in order to disc a/t and set full trust if the speed decreases too much. After passing bad weather (about FL250) the nose goes down to 2-3 degr and the aircraft accelerated to speed bug... Is this the way how the aircarft flies in reality?

Try to fly the approach without using ASN and in clear weather conditions.

 

Same question as Mike: Did the aircraft really stall? Or did the speed just go down below Vref, which wouldn't be a stall?

 

On the PFD set your Vapp to Vref + 5 (regarless of weather when using AT, no need to add knots for wind adjustment).

It was not a really stall, but I get aural warning which I mean is not ok.

[Mike777 163]

Sorry, my Name is Dimitri. I will try to describe the main problem with a/t when flying in good weather:

The mcp speed set to 180kt, flaps 5 allready extended, the aircraft at the alt with gs armed. At this point a/t commands about 60-70% throttle because the aircraft produces a lot of drag, so we need much power to maintain 180 kt. Then a gs comes down, when captured I put gear down and set after gear extended flaps 20. The a/t commands late to decrease the throttle (because of gs capture and negative vert speed) and the speed of decreasing thrust is slow (very big engines) so in spite of the fact I was good established on approach when captured gs the aircraft accelarates because as mentioned above everything happens to slow. So the speed increases, than after finaly the a/t set the throttle idle the aircraft is fast (210-230) so it is very very difficult to decrease the speed and get a stable approach... Any suggestions?

 

 

Hi, Dimitri,

 

This is most likely not an autothrottle problem.   The 777 is not easy to slow down when it is descending.  You didn't say how heavy it was, but the problem will be worse with a heavier aircraft.  You should try using the speed brake when necessary, as well as lowering the gear and extending the flaps sooner. 

 

 

By the way yesterday I flew from Amsterdam, there was very bad weather with turbulance and windshear. In climb after passing 10000 the speed bug jumped to 290 kt in order to accelerate but the aircraft remained climd on speed of 235 kt which was very very soon from amber line (before stall). The throttles were in clb mode, the vert speed was 1750 ft/min, nose angle at about 8 degrees. All the way up until good weather I holded a hand on a throttle in order to disc a/t and set full trust if the speed decreases too much. After passing bad weather (about FL250) the nose goes down to 2-3 degr and the aircraft accelerated to speed bug... Is this the way how the aircarft flies in reality?

 

 

What vertical mode are you in?  Best for climb usually is VNAV. The vertical mode will display on the FMA at the top of the PFD.   If the autopilot and autothrottle are on, the aircraft will decrease vertical pitch when below the target speed and the engines should go to the set maximum climb thrust (top of PFD should read THR REF).  It sounds like you may be using vertical speed -- in that mode the aircraft won't lower pitch to  accelerate.  Even if the autopilot is off, in VNAV you can follow the FD bars to maintain proper pitch to accelerate.  Also, 235 should not be near stall speed unless you have retracted the flaps too soon.

 

Regarding the airspeed warning on final, 1) are you sure you don't have some weather addon causing excess turbulence and 2) is the autothrottle on, not just armed?  What does the FMA say about the autothrottle mode on final?

 

Finally, PMDG wants you to sign your full name, first and last, in its forums.  Otherwise they may delete your posts.

 

Mike

PS: Another thing that occurred to me:  Are the gross weight and other weights set correctly on the PERF page of the CDU?  If the weights are wrong, you will get  an incorrect final approach speed on the FMC approach page. 

[Alpha Floor 237]

Privet Dimitri

 

Listen to what Mike says, I think it's difficult to judge whether there's a problem or not without having a look at your flight.

 

What you could do is watch MY approach, and say if YOURS is similar. It's an approach from the IAF; have a look at how I manage the airspeeds/flaps settings from the IAF to touchdown. I reach the IAF at Clean Maneuvering Speed and Flaps UP, and start making the aircraft "dirty" from there: https://youtu.be/mFqJyyDHsMg?t=11m57s

 

Pay attention to how the AT reacts to pitch changes to adjust thrust. For example, at 15:30 when leveling off. The speed might fall down below a couple of knots, but the AT in general does a good job of keeping it.

 

 

the speed bug jumped to 290 kt in order to accelerate but the aircraft remained climd on speed of 235 kt which was very very soon from amber line (before stall). The throttles were in clb mode, the vert speed was 1750 ft/min, nose angle at about 8 degrees

 

I don't think that's normal, I mean the 235 knots. Speed should be at 250 or CMS, whichever is higher, when hitting the acceleration altitude. Bear in mind that it takes some time to accelerate from 250 to 320, you might think it's not doing anything but if you wait, you'll see the speed come up. Again, see how a climb acceleration is supposed to look like here: https://youtu.be/i42HSGaSRjg?t=21m17s

 

When passing FL150 the speed bug goes from 250 to climb speed of 320. The aircraft gradually lowers the nose to accelerate. Note the modes on the FMA are: THR REF | VNAV SPD, meaning AT is keeping a reference CLIMB thrust and VNAV is in charge of managing the airspeed via pitch up/down. But he won't level off completely to gain airspeed during the climb, instead it lowers the nose a bit to achieve a gradual acceleration. The green speed trend vector predicts the speed that you will fly at in 10 seconds.

 

 

 

All the way up until good weather I holded a hand on a throttle in order to disc a/t and set full trust if the speed decreases too much.

 

That's good airmanship! Also hold a hand on the column in case you need to pitch down to gain airspeed.

 

 

 

It was not a really stall, but I get aural warning which I mean is not ok.

 

Do you mean the "AIRSPEED LOW" warning?

[dimatrixxx 0]

Hi, Dimitri,

 

This is most likely not an autothrottle problem.   The 777 is not easy to slow down when it is descending.  You didn't say how heavy it was, but the problem will be worse with a heavier aircraft.  You should try using the speed brake when necessary, as well as lowering the gear and extending the flaps sooner. 

 

 

 

 

 

What vertical mode are you in?  Best for climb usually is VNAV. The vertical mode will display on the FMA at the top of the PFD.   If the autopilot and autothrottle are on, the aircraft will decrease vertical pitch when below the target speed and the engines should go to the set maximum climb thrust (top of PFD should read THR REF).  It sounds like you may be using vertical speed -- in that mode the aircraft won't lower pitch to  accelerate.  Even if the autopilot is off, in VNAV you can follow the FD bars to maintain proper pitch to accelerate.  Also, 235 should not be near stall speed unless you have retracted the flaps too soon.

 

Regarding the airspeed warning on final, 1) are you sure you don't have some weather addon causing excess turbulence and 2) is the autothrottle on, not just armed?  What does the FMA say about the autothrottle mode on final?

 

Finally, PMDG wants you to sign your full name, first and last, in its forums.  Otherwise they may delete your posts.

 

Mike

PS: Another thing that occurred to me:  Are the gross weight and other weights set correctly on the PERF page of the CDU?  If the weights are wrong, you will get  an incorrect final approach speed on the FMC approach page.

 

Hey Mike! I was ofcourse in the vnav mode, thats why I'm asking here if this situatiion is normal because normally as you said the nose should go down and the aircraft should accelerate to speed bug. The speed was nearly stall because I retracted the flaps little early but I does it because I though the aircraft will accelerate after passing 10000 feet...

[sosuflyer 30]

You might have trouble accelerating and climbing at higher altitude because of a weather engine problem, or simconnect. I know there is an issue where simconnect makes the air get warmer as you get higher. Try it without clear weather and no simconnect if you do have it.

[Mike777 163]

 

 

You might have trouble accelerating and climbing at higher altitude because of a weather engine problem, or simconnect. I know there is an issue where simconnect makes the air get warmer as you get higher. Try it without clear weather and no simconnect if you do have it.

 

I never heard of simconnect causing such a problem and I don't know how to disconnect it (actually them - there are several versions) without uninstalling.  This would cause many addons not to work.

 

But you do have a good point: sometimes weather engines can cause high temperatures that will make the engines perform poorly.  Dimitri, take a look at the air temp on the EICAS the next time you have a sluggish thrust problem.

 

Mike

[dimatrixxx 0]

Today again this problem. approach into RJAA. Weather is not too bad, no turbulance, wind 12 kt and rain. Vref 30/149. I set it in mcp. At full flaps, gear down and fully ready for landing (3-4 nm final) the speed goes below the speed bug about 10 kt and thats why it was neer amber line. The trust was about 60-70 % n1, so a/t was working (spd) but not enough! Short before final the aircraft finally accelerated ... but too much!!! The speed was about 157 kt, I turned a/t off, throttles idle and touch down with -250 ft/min. If I would not turn a/t off it would be a long landing because I was to quick. It is important too say that the approach was all the way stable, I mean gs was in center all the time, so no big deviation, turns, nose up, nose down etc. ... My Asn turbulance setting is set to 45. I have no idea why this happens.

[Mike777 163]

 

 

Today again this problem. approach into RJAA. Weather is not too bad, no turbulance, wind 12 kt and rain. Vref 30/149. I set it in mcp. At full flaps, gear down and fully ready for landing (3-4 nm final) the speed goes below the speed bug about 10 kt and thats why it was neer amber line. The trust was about 60-70 % n1, so a/t was working (spd) but not enough! Short before final the aircraft finally accelerated ... but too much!!! The speed was about 157 kt, I turned a/t off, throttles idle and touch down with -250 ft/min. If I would not turn a/t off it would be a long landing because I was to quick. It is important too say that the approach was all the way stable, I mean gs was in center all the time, so no big deviation, turns, nose up, nose down etc. ... My Asn turbulance setting is set to 45. I have no idea why this happens.

 

Hi,Dimitri,

 

Three questions:

1) What are ALL of the settings in ASN regarding turbulence.  There are several.

2) What was the exact time of the approach to RJAA?  ASN can be set up to recreate weather at a given date and time.  Also, do you have the METAR from ASN during your approach?

3)  What was the gross weight on approach, and which 777 were you flying -200LR or 300ER?

 

I also want to remind you again that PMDG wants you to use your full first and last name in its forums.  I am not comfortable corresponding with someone who does not follow that rule.  If you don't want to publish your full name in a forum you can file a ticket with PMDG.

 

Mike