FMC control V ATC

[Arthur Melton]

When an aircraft's FMC is programmed for route, descend, and approach, what are the rules if ATC changes things. Who has priority. I was watching a TV programme recently where it was stated that when TCAS activates it has priority. Thanks.

[Ernesto Alvarez 818262]

you are talking about two different things, FMC navigation and TCAS are two whole different things

 

ATC instructions take priority over whatever you have programmed in the FMC

 

TCAS on the other hand has priority over ATC, you dont wait for the controller to give you traffic avoidance instructions, follow the TCAS. if it says, Descend, you descend (vice versa). then afterwards you let the controller know your action when safe

[Trent Hopkinson]

FMS is flight management (long term. It takes several minutes to program an FMS, and fractions of a second to disconnect it from the flight controls).

 

TCAS is the last ditch effort to avoid a mid-air collision (hence the name Traffic Collision Avoidance System).

The correct response to a TCAS is to turn the entire autopilot off and follow the TCAS by climbing or descending. Immediately. 2 seconds is too long.

 

2 minutes to open the VNAV profile page, type in as many as 5 digits, press a line select button followed by the EXE, then wind the MCP altitude window up or down a few rotations of the selector wheel, then press an override button? well you probably won't get that far before you find yourself unconscious.

 

 

 

So - ATC's job is to prevent collisions, and can ask you to turn, climb, descend, maintain an altitude, slow down etc at will.

TCAS is there to prevent collisions if the ATC did something wrong. It will only provide instructions in the vertical plane (up or down) and is not capable of issuing a turn instruction (left/right) or speed (fast/slow).

 

 

So in order of importance:

 

TCAS overrides ATC

ATC overrides the pilots.

Pilots override the FMS (which was, afterall, programmed BY the pilots to help them.)

*Caveat: Pilots override ATC if they believe something will be unsafe. The ATC-Pilot relationship is, ultimately, an attempt to improve safety afterall! But ATC has a different point of view (Traffic management vs vehicle/energy management)

 

The FMS is an instrument, utilized to help the pilots to plan things in the future. ie: Programming the whole enroute portion of a YSSY-KLAX at the gate, on the ground, at Sydney... the pilot is instructing the computer what HE(she? (*ze??)) wants the plane should do in 12+ hours time.

 

The ATC is your supervisor/boss. ATC has a bigger picture than the pilot. They need to ensure certain rules are kept. mostly to do with how far apart airplanes need to be. If they need you to turn left, or descend early, or slow down so you don't hit other aircraft (or end up within 1nm of another aircraft, which is the same thing as far as the NTSB is concerned) or be the spanner in the works that turned a "every aircraft is being delayed by about 2 minutes" into a "that guy got in early, but now 42 aircraft are delayed by 15 minutes, increasing by 2 minutes every new plane that enters the approach corridor, we're going to have to ground-stop 5 airports for half an hour asap!".

 

And the TCAS is an emergency last ditch effort to not crash. (Along with EGPWS, GPWS, Stall horns, overspeed protection etc)

[Arthur Melton]

So are you inferring that the FMS should not be used when ATC is controlling?? Or alternatively switch to manual control and ask ATC for vectors?.

[Trent Hopkinson]

So are you inferring that the FMS should not be used when ATC is controlling?? Or alternatively switch to manual control and ask ATC for vectors?.

 

No. In fact, ATC will often provide instructions that require the FMS or GPS if you have filed a flightplan that relies on it. ie "continue as filed" or "track direct to (place) then own navigation" or indeed "Cleared to (airport) via the (Arrival procedure) STAR, (place) Transition, runway (runway number)."

The issuance of a clearance delivery via the flightplan you filed, even before you start an engine is an ATC instruction. Which you may find needs an FMS/GPS to comply with ([Mod - Happy Thoughts]uming you filed an rnav flightplan with /F or /G or /L suffix)

 

- Example: Velocity 888, cleared to Melbourne via Wollongong, Runway 34 Right, MARUB3 departure, Wollongong Transition. Initial climb 5000ft, squawk code 3512, departure frequency 123.0

Is an instruction that (since 2016) requires an FMS (now that Sydney VOR on 112.10 doesn't exist anymore - real world... FSX still has one).

 

So in those (normal) cases, the ATC is almost instructing you to use the FMS (if you filed a /A via a non-rnav VOR-to-VOR route, then obviously the understanding is you may not have an FMS)

 

However if you are instructed "Turn left heading 040" then you are expected to turn left, and point your comp[Mod - Happy Thoughts] at the heading 040, and not fiddle about with the FMS, or worse still, just ignore the instruction and let the autothingy just do what you told it to do when you were programming it an hour ago at the gate.

Most airliners have a fairly simple way to make this happen, using the FCU/MCP and a heading knob. Smaller GA aircraft may have an autopilot control interface, which utilizes a HDG push-button and a heading bug on the gyroscopic heading indicator.

 

Every aircraft that isn't a drone is usually equipped with a stick, yoke or other control system that directly manipulates surfaces on the wing or rudder which can also be used to "turn left" or "Turn right" as required, and a comp[Mod - Happy Thoughts] indicator somewhere.

 

VFR flights on the other hand are usually not asked to take up specific headings, as this requires an instrument, and some very basic ultralights may not even have as much as a comp[Mod - Happy Thoughts]. In which case visual reporting points, roads/rivers, other flying objects, runways, or (worst case) cardinal directions like "Northwards" or "Eastbound" may be referred to instead, expecting the pilot to visually see out the window/goggles objects like coastlines etc.

[Andreas Fuchs]

Note: these may be things that a standard GPS in Flight Simulator may not be able to do at all or only in a time-consuming way. This is the reason why lots of members use addons that have FMC-units that have been programmed realistically.

[Trent Hopkinson]

Note: these may be things that a standard GPS in Flight Simulator may not be able to do at all or only in a time-consuming way. This is the reason why lots of members use addons that have FMC-units that have been programmed realistically.

 

or a GTN750 from Flight1 software, installed into something like the RealAir BE60 Turbine Duke v2, a 6 seat twin engine turboprop that can fly 300 knots TAS at 25,000ft, and certainly isn't your trusty Cessna banging along VFR at 3000ft.

[Arthur Melton]

All in all sounds complicated. Thanks

[Simon Kelsey]

All in all sounds complicated.

 

You're the pilot. It's your job to navigate the aeroplane from A to B. If you were making a long distance car journey, you'd probably look at a map and plan a row beforehand, and flying is no different.

 

ATC are there to stop aeroplanes bashing in to each other. To achieve this they may need to either give you some headings and altitudes to fly in the short term, or they may need to re-route you on a larger scale. For instance, the route you'd planned and filed might take you up a one-way street (airway) and you might need to be rerouted on to a valid route. When you get your initial airways clearance on the ground, this is part of what should be sorted out.

 

Once you set off, it's up to you to follow your route. But if you're in your car and a policeman tells you the road is closed and you have to follow a diversion, do you;

 

a) Drive down the closed road anyway because that was the route you planned and it's programmed in your satnav, or:

 

b) Follow the diversion signs until the road is clear and then re-join your route a little further down and resume navigating yourself as normal?

 

ATC are similar. You tell them what you plan to do and where you plan to fly and they will try and facilitate it, but occasionally they might need to direct you around some other traffic. This normally involves vectoring -- heading and altitude [Mod - Happy Thoughts]ignments, where the controller is primarily responsible for your tracking -- but it may also include procedures like SIDs, STARs and routing to various fixes. Any time that you're not on a heading, it's up to you to navigate, whether that's following your cleared route, following a SID/STAR or routing direct to a fix.

 

How you navigate is entirely up to you -- tune a VOR, use a stopwatch, map and comp[Mod - Happy Thoughts] if you like. As long as you fly the route you've been cleared along, it doesn't matter. An FMC makes it easier to navigate because it does the sums for you and most will hook in to the autopilot. But it's still just a tool for you to use.