They're very important for jets. Not so important for piston airplanes. Nice to have for high performance piston airplanes though.
Generally speaking, flying a jet without a flight director is very challenging. While holding altitude, a "pixel" or two up or down can result in an immediate and significant rate of climb or descent. Rolling out from a turn a little late or a little early can result in a minor-to-moderate-to-major heading deviation, depending on pilot input. Simply stated, a flight director makes managing the aircraft much, much easier and is basically required for normal operation. An inop flight director would be a go-no item for my operation and most others. It would also "inop" the autopilot in most aircraft.
There are certain profiles which may not "require" the use of one. For example I did landing currency today at FSI in the G450 sim. One of the landings was a visual pattern. I hand flew the aircraft during the climb and asked my SIC to set altitudes, FGP modes (HDG, FLC, speeds) and headings for the climb, crosswind and downwind turns. I also utilized the autothrottles. On downwind the SIC built a 3 mile visual in the FMS with a 1500' altitude constraint at the 3nm fix. I kicked off the autopilot and the autothrottles before turning base and then armed LNAV and VNAV, and asked him to roll the altitude preselect down to field elevation. This allowed me to take advantage of the flight director for both lateral and vertical nav, even though I wasn't using any of the automation.
Normally I'd use the HUD for all of this, which uses different symbology for the flight director, but works the same. But this was an older HUD I in the sim instead of a HUD II which I have in my work airplanes. HUD I = no thrust director. Therefore I used the PFD's single cue FD symbology and the thrust director -- another great tool in jets which feature it, as it helps the pilot make power adjustments for target airspeeds. As I was rolling onto final I figured "what the hell, let's go raw data," kicked off all the lateral and vertical modes and just used pitch and power to get me on the ground. Works okay, but it's a lot of work, a lot of jockeying the power levers up and down, some pitch changes. Wasn't a big deal, but just not ideal for everyday ops. A good sim exercise.
Allow me to reminisce. I know I'm entering TLDR; territory.
One of the more challenging situations I've had to overcome in my career was many moons ago, at least 12 years if I'm not mistaken... ? An old Hawker 400A, not the Beechjet, but rather the legitimately named Hawker 400A and the one I was flying was made in 1969. This airplane had a lot of issues. Don't ask me to explain it, but it was a real thing: if the SIC placed an altitude constraint in the right side box (GNS-XLS if my memory serves) and the PIC then executed a direct-to or any other waypoint change from the left box, it killed the one-and-only Flight Director which was of course reserved for the captain. FD in that airplane was a mechanical contraption, floating over a mechanical ADI. By killed, I mean the FD would repetitively do this: slowly pitch all the way up while increasing bank to the left. Then it would snap back to straight and level, slowly pitch up/turn left until it reached its limits, etc., ad nauseum. This also kicked off the autopilot. It would do this until shutdown.
I was flying with a brand new SIC. Great guy, knew what he was doing, but forgot the "this is a weird, old airplane" brief. (Real brief, by the way.) We were assigned a STAR. The STAR contained one altitude constraint. He placed the one altitude constraint into the box. He didn't mention it, and I missed it. We were given "direct XYZ." I made the D-> command on the left side while on autopilot and boom, the autopilot kicked off and I was suddenly hand-flying while watching this mechanical flight director do its dance. I knew the FD was done.
It is a given that in these situations the weather is always bad. This is to be expected. It's some kind of rule: all (okay, most... ) mx issues will come to bear when you're tired, it's late at night, and the weather's bad. Fair enough on all counts. Sure enough the airport in question was reporting weather at ILS minimums and I was going to have to hand fly this approach with little to no assistance from the automation. Which I did... not saying it was the greatest approach ever but it was gratifying to see the ALS as I was hearing my SIC call "minimums."
Pistons: I've flown piston airplanes with flight directors. They're becoming more common. They work exactly the same, but piston airplanes aren't very difficult to manage from an attitude and power perspective. It's a nice tool, but by no means a game changer for those aircraft.
