FLC vertical mode.

[Alistair_Heaton]

Stick monkey here trying to work out what's going on when you select FLC on a A220/c series.

Can't find any documentation about what it's actually aiming for with a speed reduction decent.

It seems to command a green dot minimum drag angle of attack. It's doesn't slow down or go down. And chucking the gear out is asking for for a VS of 4k feet per min while it sorts itself out. Which is different to other OEM setups apparently.

Basically it's optimised for drift down

Anyone know about what it's doing?

My back ground is turboprops and A220 I have 2k hours on now as first jet type but it seems to be a hybrid of TP and Jet from what others have said.

 

[WKTaylor]

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[Alistair_Heaton]

It's not known in pilot circles I have tried.

It must be a control logic feature. But as such nobody seems to.know who would decide the rules it's designed to.

 

[Sparweb]

AFM's and FCOM's can be sparse on these details, though I wouldn't expect that from Bombardier, who are normally quite thorough (taking note that the C-Series was a Bombardier design before being stolen bought by Airbus).

Do you have access to the maintenance manual and the separate system description manual?

Between the two you should be able to figure it out.

Is this a Collins system?

Sometimes the real depths of the system functions need to be dug up from the system OEM documentation. That requires more privileged access (sorry, not me).

 

[LittleInch]

Basically it's optimised for drift down

Have you got the answer already?

I saw this and thought it was very unlikely that anyone here would know other than the control engineer / team who designed it.

 

[Alistair_Heaton]

There seems to be a need to know policy. It was the same with the q400.

To be fair I haven't seen the logic of how any other flight director mode works in detail. That's why I thought to ask here.

 

[Alistair_Heaton]

the background to this is when your instructed to reduce speed ATC has a requirement or expect you do it at 1 knt per second.

It seems to occur with other types I am told by friends.

A220's wing is that slippery it just doesn't really do anything unless you take it into VS mode and select 300 ft/min then go back to FLC with 5-10knts to go to the target even with the spoilers out.

Turboprops you tend to do above idle power descents otherwise it creates problems with pressurisation and you don;t fit in with the jet traffic so you decend in VS

old school Jets you want flight idle which is a feature of flight level change FLC mode.

Geared fan engines seem to behave differently to both. And there is nothing like "handling the big jets" to give you sensible guidance.

 

[Alistair_Heaton]

This popped up this morning

HONEYWELL AND BOMBARDIER SIGN LANDMARK AGREEMENT TO DELIVER THE NEXT GENERATION OF AVIATION TECHNOLOGY; HONEYWELL UPDATES 2024 OUTLOOK

/PRNewswire/ -- Honeywell (NASDAQ: HON) announced the signing of a strategic agreement with Bombardier, a global leader in aviation and manufacturer of...

www.prnewswire.com

 

[MintJulep]

Here's what Google Gemini thinks.

FLC mode, or Flight Level Change mode, is a flight pitch guidance feature on the Airbus A220 that maintains a constant airspeed while climbing or descending to a pre-selected altitude:
How it works
When FLC mode is engaged, the autopilot will hold the aircraft at the selected airspeed. If necessary, the commanded speed will be sacrificed to avoid flying away from the selected altitude.
What happens to the Pitch Target Marker (PTM)
When FLC mode is selected, the PTM is removed. The PTM is the attitude reference for takeoff and is displayed in pink.

If that's right, seems the wrong thing for a speed reduction decent.

 

[LittleInch]

I was going to remind you that you're talking to a bunch of engineers here, not pilots so maybe a little explanation of what "FLC" is or does would help, plus "green dot"???

When do you need to use it?

Why do you need to slow down? Isn't there a speed control function? (There is on my car....)

Is ATC out of date with their expectations?

 

[Alistair_Heaton]

I agree, and it doesn't feel right if you know what I mean.

I have zero clue how proper airbuses do it but others say its different.

Boeing 737 you want to go down you select alt and then FLC and the power levers come back to idle and it appears no change to the attitude speed reduces to target and then it pitches down to maintain.

With this it descends around 500-600ft/min and the speed very gently wastes off. When you eventually get back to your target speed it will then pitch down to maintain. Spoilers don't do much either apart from make noise and vibration and ping the yellow up on the speed tape. High speed they are effective both for decent and speed reduction in a different mode.

The A220 logic is used on other aircraft types. I have asked bizjet mates and its the same with them that use it.

 

[Alistair_Heaton]

green dot is the calculated airspeed to give you the most efficient least drag angle of attack.

Flight level change is a vertical mode which gives zero thrust on the engines and maintains a target airspeed.

Basically you use it to descend. Your goal is to do it using as little fuel as possible between the start and landing.

The Job of Airtraffic control is incredible they have to work in 4 dimensions to stop planes hitting each other and fit aircraft wanting to go up through aircraft descending to land with 5-10 nautical miles between them flying at 290 knots. And then the local winds change with altitude and thus change the ground speed. When they say slow down they need it otherwise alarms go off. Personally I use vertical speed but apparently its not the way to fly a "jet"

But it has tweaked my engineering mind to understand what's actually going on. And why it differs to say a 737-400 pilots thoughts on how to fly a jet.

To make the whole thing work is team work, it is very impressive sitting waiting to depart at say Gatwick in UK with 10 aircraft on approach running in and they bang one in or one out every 45 seconds.

 

[LittleInch]

Most of this is way over my head, but might make sense to you.... Don't even know if its an A220 or not, but seems to have a lot of explanations in it.

Apologies if I'm teaching you to suck eggs here.

Control your Speed… During Descent, Approach and Landing | Safety First

This article is the conclusion of our theme of speed management during a flight, which began in Safety first Issue #18. We are entering into the descent phase. Our objective is to cover descent from cruise altitude down toward the destination airport and prepare the aircraft for its approach and...

safetyfirst.airbus.com

 

[Alistair_Heaton]

It's good but specifically for the general airbus aircraft of 1980's flight logic.

A220 is the next generation on from it.

Again documentation is general

I suspect even the test pilots won't know the logic. Which is why I asked in an engineering forum frequented by north Americans and Canadians.

 

[LittleInch]

I'm still not sure what it is you're after though? FLC maintains a fixed speed whilst going up or down to your required flight level yes?

But are you asking how to make it change air speed whilst it is doing that?

Seems to me the function (FLC) is doing what it was programmed / asked to do and you want to try and make it do something different?

Seems to me they have tuned this to match the aircraft where it sets the rate of descent to maintain the set air speed whilst having no or little thrust?

Or is the point that if the speed is too high it just descends, levels off and then slows down?

 

[Alistair_Heaton]

sort of...

Its more of what its optimised for.

There might well be an old school aircraft V flybywire thing going on as well as a this is how it works when I flew a Boeing 737 or 757.

I just would like to understand what its doing to be honest. I have a safe work round to make the machine do what i want and the procedures are written so its personal choice which mode you use. I use VS to slow down then use FLC to maintain. Using FLC in decent to speed up is seriously not a good idea. Also configuring high drag services is also a bit sporty.

 

[Alistair_Heaton]

right just to see what you think...

Another old mate from Engineering days who used to work for BAe on the 146 and ATP.

He thinks its linked to which generation the flight director is and the data buses between the different flight management modules and data flow.

So EJet, Airbus, A220 and FBW Boeing flight directors will be optimised for drift down. But has his doubts about the 777, 787 apparently it is.

Previous aircraft types there wasn't a data transfer of performance or actual AoA to the FD so it had set values which were chosen by the test pilots as being useful.

A220 has 4 smart probes and 2 traditional AoA sensors so likely qualified to use the data in a more involved manner than previously aka proper Airbus with its 3 traditional.

HIs dad worked on the Autoland for Tri stars but is now in a nursing home with dementia he would have known...

But that side of certification is way way over the head of a stick monkey....

 

[LittleInch]

I think this is just control logic which takes different inputs and provides an output (mainly the control surfaces) to accomplish an action.

So the FLC, as far as I can see, works on the basis of idle engines, and controls the AoA to achieve a certain air speed (presumably fixed by the pilot?), but it seems it isn't its primary aim which is to, as you say, drift down to the set altitude somewhere close to the set speed, but not so low that you stall. I assume it then powers up the throttles once you reach your fixed altitude?

The VS (vertical speed?) set is a different control philosophy which by the look of it will sacrifice air speed to try and maintain a lower rate of descent (higher AoA) than the FLC mode.

So two different control philosophies which are probably written somewhere, but maybe not given to the pilots, which strikes me as a bit odd, but then maybe they've figured out that to cater for the wide range and technical ability of pilots, they just need to keep it simple?? Like everything, it has its benefits (pilots don't try and fool the control logic) but down sides as they don't fully understand what the control logic is doing (MCAS and AF disaster etc).

Probably doesn't help you, but have you tried anyone in Bombardier? They might actually like to get some decent technically based feedback on what how their control logic works in the outside world.

 

[Alistair_Heaton]

They are basically two vertical modes with different target variables.

FLC flight idle target is an airspeed set by the pilot or FMS if its a 3D management system linked to autothrust. When the aircraft levels off the auto thrust increases to maintain your target speed.

VS is vertical speed so you would set what you want with a speed defined by you or the FMS. Autothrust would attempt to maintain airspeed.

FLC protects you against over speed and under speed.

VS doesn't, which I suspect why its historically deemed not the mode of choice for a jet.

There has been a change in what they think a pilot should know.

FLC works great until you need to change speed to slow down for ATC then its very different to previous generations of flight director. Its also a bit twitchy with configuration changes mainly because the trim system changes. If someone thinks its a good idea to do multiple things at once in a short time period it can generate a MACAS quantity of trim change and decent rate.

And its also inappropriate for a line pilot to step outside the "command" chain with questions like this. It will be said if you need to know we would have already told you..

As for someone at Bombardier I have a plan on that one, the project manager for the "cockpit" system which is fitted to the A220 and biz jets jumpseats when he is flying anywhere. I will question him if I am lucky enough to have him onboard next year. He flew with me once this year.

 

[LittleInch]

So what is the SOP / trained procedure for how to reduce speed when you're in a descent using FLC?

Is there an issue with the current SOP not being able to match ATCs expectation of speed reduction in these types of new jets?

That I would assume is the starting point for the "we've already told you to do it this way" kind of conversation. I'm just curious.