It’s my understanding that the autopilot in aircraft which have a flight director will use input from the flight director in order to “know” how to control the plane. If the flight director is switched off, the autopilot will not function correctly, or only in limited fashion.
Therefore, every time you want to use the autopilot, the flight director will need to be switched on.
So… Why does the flight director have an independent switch, allowing the autopilot to be on with the FD switched off? To me, this makes no sense. Shouldn’t the activation switch for the AP also, by default, activate the FD if it’s off? It’s kind of analogous to having a switch to turn on your steering wheel in a car separate from the rest of the car. You never drive without using the steering wheel, therefore the steering wheel is “on” by default.
Am I missing something here? Is there a situation where you would deliberately want to use the autopilot without input from the flight director?
I used to instruct in G1000 equipped 172’s and having the FD off cleans up the PFD and can help avoid confusion in some situations where the FD is doing what the autopilot is set to do but you’re hand flying. Students could easily get confused and follow the FD when they’re not suppose to.
It’s also a good skill to have being able to follow a corse or approach without it as not every airplane will have a FD equipped
In IFR flying I would have it on if I’m hand flying as following it is just easier. But VFR it’s not needed
There are some systems that behave a little differently in this regard. Do you have a specific plane in mind?
One reason might be to declutter the screen, another is that some planes have redundant FDs that can be operated independently, in tandem, or in various reversionary modes by the pilot flying and pilot not flying.
FDs and APs need to be understood within the context of the aircraft that they are installed in. In some aircraft turning off the FD is merely changing what is displayed to the pilot, and in others it is altering the input to the AP. Would help if folks asked about specific aircraft, and in turn, specific models in the sim?
OK, specifically the Airbus A321. The autopilot doesn’t behave properly unless the FD is on. According to videos I have seen, the same is true of the 737 max 8.
I can understand decluttering the screen if the pilot is hand flying, however…. in the situation I am describing, the pilot is NOT hand flying, because I am describing a scenario where the autopilot is in use. So that doesn’t really answer it. My question is not “why would you turn off the FD when hand flying”, it’s “why would you have the autopilot on without wanting the FD on at the same time?”
I am not familiar with AP in Airbuses and Boeings, sorry, take what I say with a grain of salt. I am familiar (IRL) with GA Garmin autopilot in the G1000 system, the GFC 500 system, and the older King KFC 150 autopilot. In all these systems the AP will not work if flight director is turned off. That is, pressing the FD button will turn off the flight director AND disengage the autopilot. There is no other way to “tell” the autopilot how to steer the plane than through the flight director.
Like @CharlieFox00 said, with the big jets there might be different / degraded modes for the autopilot that do not use the FD. But no matter what the mode is, I would expect that there will be always SOME indication to the pilot as to what is driving the autopilot and what the mode is. If you just turn off the FD and the autopilot remains engaged, but there is no indication what it is doing, I would suspect a bug in the simulation.
Yeah, I always assumed with the modern airliners, FD off + AP on is an invalid combination. I’m not sure how else the AP would know what to do without FD enabled. Are there backup systems to provide direct input to the AP?
In some aircraft, the FD can be driven by two different flight control computers, one for each side of the cockpit, similarly connected to independent hydraulic systems. Sometimes one can act as a master, the other side slave, or they can be synced, or they can go into reversionary modes, acting independently (with several fail-safes). If one side is giving erroneous info, or unnecessary info, or the flight control computer fails, it might not be a bad idea to turn that side off and let the other side be the primary.
Even with a single flight control computer you want a separate switch for the FD and the autopilot. The reason is you might want to use the FD without the autopilot so you need a way to turn on “FD, no autopilot” mode.
As to why the autopilot does not automatically engage the FD, the answer is it is not how it is certified and it would not be really practical. What steering mode (horizontal and vertical) would it engage with? There is no information if the FD is not on, and a reasonable default (e.g. roll and pitch hold) is almost never what you really want, in any phase of flight. So the only practical and safe way to engage the autopilot is to turn the FD on, verify/program the steering mode, then turn the autopilot on. This assumes FD/AI-driven autopilot of course.
Sounds like a buggy A321. In an Airbus you should turn the FDs off if you are not following them while flying with AP off, because doing so forces the autothrottle into SPEED mode so you don’t get an nasty surprises (overspeed or stall). Turning both APs and FDs off also resets the autoflight modes which can be useful in some circumstances. The FDs should always be on when the AP is (and they are on automatically at power up), but the AP functions fine with the FDs off.