Flight Plans and MCPs

[The Artful Dodger]

Hello, all:
This is more of an informational question than anything else. I have been looking over the LUA code for all of the MCPs in the library and one thing struck me: There is no connection between any of those code examples and a flight plan. So, faced with that, I decided that one of two things must be going on: Either all of these systems forced Sim pilots to "fly" their own flight plans or, because all of them employed a Flight Director (FD), the FD "simply" fed the flight plan to the autopilot without intervention by the autopilot or the sim pilot.
Are any of my thoughts on this situation fully or partially correct? I would very much like to hear your comments on this matter.
As always, thanks in advance for your time and comments.

Sparky

[SimPassion]

For me the Autopilot logic as you mentioned, which is following the flight plan in your own words, is not inside the MCP hardware, which I would let it as it's named : (M)ode (C)ontrol (P)anel, that's it
We can sum up to this : the Flight Plan Management is handled by the (F)light (M)anagement (C)omputer/(F)light (D)irector and the overall including the Autopilot, forms the (F)light (M)anagement (S)ystem, all these acronyms are named differently depending on aircraft brand, however the logic is mostly the same
This is what we observe in the Boeing B737

So, the MCP allows to set and display values, as well to arm specific Modes, which eventually constitute an interface to interfere with the FMS (not the FMC itself, for which we are using an MCDU : (M)ulti-Function (C)ontrol (D)isplay (U)nit). The autopilot logic is just behind, if such panel is build together with a complete dedicated computer

Here's a simple and easy to follow explanation of the overall flight management system :
https://en.wikipedia.org/wiki/Flight_management_system
And the autopilot part :
https://en.wikipedia.org/wiki/Autopilot

from https://www.sciencedirect.com/topics/co ... ent-system

Please correct me if I'm wrong, I would just let all parts simple in any case
 

[SimPassion]

Another diagram of FMS :

http://www.flaps2approach.com/storage/d ... 0Smith.pdf



Flight Management System

Description

A Flight Management System (FMS) is an on-board multi-purpose navigation, performance, and aircraft operations computer designed to provide virtual data and operational harmony between closed and open elements associated with a flight from pre-engine start and take-off, to landing and engine shut-down.

Most modern commercial and business aircraft are equipped with Electronic Flight Instrument System, which replaces conventional systems and flight deck displays.

An FMS comprises four main components:

The Flight Management Computer (FMC);
The Automatic Flight Control or Automatic Flight Guidance System (AFCS or AFGS) ;
The Aircraft Navigation System;
An Electronic Flight Instrument System (EFIS) or equivalent electromechanical instrumentation.

https://www.skybrary.aero/articles/flig ... ent-system


Autopilot

Description

An autopilot is a device used to guide an aircraft without direct assistance from the pilot. Early autopilots were only able to maintain a constant heading and altitude, but modern autopilots are capable of controlling every part of the flight envelope from just after take-off to landing. Modern autopilots are normally integrated with the flight management system (FMS) and, when fitted, the autothrottle system.

https://www.skybrary.aero/articles/autopilot
 

[The Artful Dodger]

Hello, SimPassion:
WOW! Thank you for such a thorough answer. It really demonstrated to me that my level of understanding regarding the functions available on the flight deck of a modern airliner was sorely lacking! But, I think it confirms my very basic understanding of the process. My only question that remains is this: In the sim, MSFS 2020, does the sim, in the same way as in an actual aircraft, gradually feed the flight plan information to the autopilot so that the aircraft follows the flight plan? And, therefore, that is why there is never any flight plan information included with any of the autopilot software in the library?
Thanks again for such a complete and helpful reply.

Sparky

[JackZ]

In fact I guess that you are confusing the Autopilot CONTROL PANEL (call it MCP or FCU), which is used for “temporary” orders (short term guidance) to the autopilot by the pilot, and the Flight guidance system which is managed as @SimPassion perfectly explained by the FMS that also gives info to the autopilot (long term guidance) according to what is programmed in the FMS flight plan via the “box”( read the MCDU)

I am less familiar with Boeing than Airbuses that I fly for a living, but the FCU allows the pilot to “tell” the FMS (the heart of the system), either:
1- I (the pilot) “have temporary control (short term) of the autopilot” (Also called SELECTED MODE, in Airbus’s jargon), used when you select basic modes such as heading, speed or V/S or FPA manually, and from then the FMS knows that it will deviate from the computed 3D flight plan which is in the box, for operational purposes (ATC request for example or pilot’s manual & intentional deviation of the flight plan (weather avoidance) to name a few)

2- You (the aircraft, ie the FMS) have control of the autopilot and do your magic” (aka MANAGED MODE), and the FMS will act upon the autopilot giving orders to optimise the flight profile in 3D.

So to come back to your initial question, any AM airliner “autopilot” instrument is merely that: a human-machine interface to the flight guidance system, to allow pilots to interact with the FMS and set the mode (or combination of modes, selected or managed) that the FMS has to follow in the short term.
From there you can understand that no Flight plan logic is involved, as it is part of the FMS system.

[The Artful Dodger]

Hello, JackZ:
There is still a piece of the puzzle missing for me. In a real aircraft, I suppose it's a bit easier to think in terms of individual subsystems because you can open a panel and see each one. And, maybe I muddied the water a bit by referring to the individual blocks of code. With a Simulator it's a bit different. In the case of MSFS 2020, I guess there is AI built in that will fly the plane for you. But, that takes everything that makes being a pilot out of it.
So, my question, only considering MSFS 2020, is this: I know, now how to use the functionality of AM to build a Head Up Display; all that's needed are a few subscriptions. I am working at understanding how the panel that Boeing calls a Mode Control Panel works. But I don't have a clue as to how the flight plan information "gets into" the MCP. As no one has attempted to program the box that I believe Boeing calls an FMS, I have concluded that the Simulator must quietly provide flight plan data to the MCP and thus we don't need to program one. Is that correct? If I properly program the MCP, - not an insignificant problem, will that cause my aircraft to follow the flight plan?
I'm sorry I keep getting away from the situation in a real aircraft but in a real aircraft you don't have anything like a simulator, with somewhat murky instructions, sitting behind the panel. I just hope I have made some sense.
Thanks for the help.

Sparky

[SimPassion]

Hello, JackZ:
There is still a piece of the puzzle missing for me. In a real aircraft, I suppose it's a bit easier to think in terms of individual subsystems because you can open a panel and see each one. And, maybe I muddied the water a bit by referring to the individual blocks of code. With a Simulator it's a bit different. In the case of MSFS 2020, I guess there is AI built in that will fly the plane for you. But, that takes everything that makes being a pilot out of it. So, my question, only considering MSFS 2020, is this: I know, now how to use the functionality of AM to build a Head Up Display; all that's needed are a few subscriptions.
I am working at understanding how the panel that Boeing calls a Mode Control Panel.

But I don't have a clue as to how the flight plan information "gets into" the MCP. | Because the Flight Plan will never enter into the MCP |

As no one has attempted to program the box that sits in front of the throttles that I believe Boeing calls an FMS | Nope, this is the MCDU which is the interface between the Pilot and the FMC, with keys and a screen, again not FMS which is the whole system |
, I have concluded that the Simulator must quietly provide flight plan data to the MCP and thus we don't need to program one. | Microsoft and Asobo have build an intermediate way to ease the Flight Plan creation using the World Map. This Flight Plan which indeed is not located in the FMC, as we haven't entered the way-points ourselves manually using the MCDU keys, is somewhere in the memory of the flight simulator and the sim make the aircraft to refer to it. This is only artificial, just an in-sim solution and related situation, nothing more |

Is that correct? If I properly program the MCP, will that cause my aircraft to follow the flight plan? | There's not that much to program on the MCP in this specific case where we don't enter way-points and data in the flight computer, just activate LNAV and VNAV, then switch the MCP Autopilot to ON / ACTIVATED and yes it will follow the Flight Plan defined on the World Map. The MCP is responsible to display and enter flight data, not flight plan, like expected speed, expected heading, expected altitude, expected course, expected vertical speed, keeping in mind aircraft perfs and limits. Then the flight management system refer to these infos, mix them with the requested in memory Flight Plan and trigger the proper adjustment to aircraft systems |

[JackZ]

Still a lot of confusion down here IMHO.
Talking about the sim ONLY (even though the real A/C philosophy is pretty much the same apart from sensors that give real time data), the core system is a program that « flies » the aircraft in 3D by feeding inputs to the flight controls surfaces according to what is needed at a given time:

For example: climb —> increase the AOA (pitch) to increase the lift (plane climbs), monitor the speed and adjust power to maintain it (AOA increases, more lift means more drag, so it needs more power).
An algorithm based on the real drag/lift/power equations of flight does the trick, with a feedback loop and a PID based black box to smooth the transition between each flight phase (climb to level for example).

The algorithm that « flies » the aircraft (call it the autopilot) receives a série of orders and targets to be reached or maintained: speed, heading and altitude, with basic orders such as climb, descend, turn. The autopilot then usually feeds another algorithm that represents « graphically » (through bars or a birdlike symbol) for the pilot on his PFD what is needed to achieve that goal through the basic flight controls (called Flight director) and the throttle (auto throttle).

From there two possibilities: the pilot chooses to manually fly the
aircraft and « follow the bars/bird), OR it gives control to the autopilot, which in turn will follow the bars by acting directly on the control surfaces. Up to now, there is no need to have an MCP, only two buttons are needed Autopilot ON/OFF and FD ON/OFF

Now for the last part:
How does the autopilot receives orders and target values?
Two possibilities:
1- direct entries by the pilot
2- orders and targets values fed by the Flight management system (FMS)

For direct entries an MCP/FCU is needed as a human/machine interface. The pilot enters target speed, heading and altitude and V/S and tells the plane to follow these orders or not (each value can be selected individually with a possible combination)

For orders fed by the FMS, on the Airbus the target values on the MCP are replaced by —- to inform the pilot that the a/c is following FMS orders/targets.

Last and the least: the FMS part of the system is a program that parses through the flight plan entered by the pilot before the flight which contains the intended 3D trajectory with waypoints, altitude and if needed speed constraints. The flight plan us graphically represented on the Navigation display for Pilot’s reference, that doesn’t mean that the a/c will follow it.

When the FMS is navigating (NAV mode), the FMS then « translates » this 3D profile into short term data and targets/orders (ie to fly from this point to this point, I need to fly this heading, compensated with the wind (*).
These orders are fed to the flight director, and if autopilot is on, it will follow these orders, if not the FD bars should be removed as to not confuse the pilot, unless he wants a guidance while manual flying( “ follow the bars”).

These short term target/orders are what makes the a/c follow the flight plan IF feasible (the orders have to be relevant with the flight capabilities of the a/c), or else a message like « TOO STEEP PATH «  will be displayed to inform the pilots.

Here no need to have an MCP per se, except for a button such as NAV ON/OFF (on airliners it’s a bit more complicated than that with more complex modes, but you get the point).

Hope it’s clearer: NO the MCP is NOT intended to « fly the flight plan » as you said, it is merely a Human/machine interface for short terms orders and feedback to the pilots/autopilot.
Plus some switches that simply tell the FMS either « I have control (selected)» or « Fly the plane according to the FPL entered in the MCDU/CDU (managed in Airbus jargon).

Jacques



*: IRL, the wind is fed to the FMS by the Inertial reference system that senses the movement and position of the a/c over the ground then deduces the drift and from there the wind, in the sim it is way easier since the sim knows the generated wind everywhere.

As a side note, the sim uses pretty much the same programming logic as the real a/c with the exception of :
- all the redundancy and safeguards built in in the real one,
- and the lack of sensors per se, since all the data (speed, altitude, attitude) is known from the flight model computations while the real a/c has to find ways to retrieve these information from the real world using different type of sensors.