Correct use of the Flight Director

[Pizza Express]

I think there are lots of people here who fly old generation aircraft like 73s. The A320 family FD is spot on. Rotate to 15 then follow FD.

our company: both on or both off
fd on follow it, not going to follow it fd off.

From what I remember from the 73 days the fds were crap and sluggish. In my opinion the bus fds are way better than the boeing and me as well and I like to keep it nailed.

[STBYRUD]

Someone mentioned earlier that the FDs are pitch and roll target indicators. In the 737 the opposite seems to be the case in my experience, a rate indication seems to be more accurate. They tell you what input you are supposed to make, not what orientation in space the airplane is supposed to be in.

[John Citizen]

Thanks Microburst. This is the type of advice I was looking for.

Sometimes, a given target "expires" within seconds and is substituted by another, and you know it in advance so, you wait for the bar to come to you, instead of blindly following it

How do you know it in advance ?

When do you follow it and when do you wait for it to come to you ?

I have always blindly followed it but now I am starting to realise that this perhaps not what I should be doing, as it causes me to overcontrol by chasing it

[aterpster]

stbyrud:

Someone mentioned earlier that the FDs are pitch and roll target indicators. In the 737 the opposite seems to be the case in my experience, a rate indication seems to be more accurate. They tell you what input you are supposed to make, not what orientation in space the airplane is supposed to be in.

I believe single-cue and dual-cue FDs are different in those respects.

[John Citizen]

If the FD indicates 15 degress nose up and a significant displacement to the right, does this mean :
- you must set 15 degress nose up and bank sharply to the right
or
- you move towards 15 degress nose up (set perhaps 12 degrees nose up) and bank normally to the right and then wait for the FD to move towards you ?

[captjns]

F/Ds of nower days are pretty good for engine failure at rotation. Yes they are pretty good for most regimes of flight too when the MCP is properly set up.

However the cues the F/Ds present are only as good as the information provided them via the symbol generator, FCCs, and navs be they GPS or VHF.

Bottom line the flying pilot must always scan to confirm accurate and reliable information is being presented... situational awareness.

[Capn Bloggs]

If the FD indicates 15 degress nose up and a significant displacement to the right, does this mean :
- you must set 15 degress nose up and bank sharply to the right
or
- you move towards 15 degress nose up (set perhaps 12 degrees nose up) and bank normally to the right and then wait for the FD to move towards you ?

John, you move the controls towards the FD command, the greater the displacement from where you currently are, the more positive you need to be. As you approach the bars, use the controls as required to follow the bars. This may require backing off or slackening your inputs.

I have never flown FDs by saying "it is at 15° Nose Up so that's where I must put the nose". Roll commands are a good example. It doesn't indicate a bank angle as such: there are no bank angle scales in the middle of the AI/PFD. As you roll, the FD bar will appear to come in to meet you, but it's all intuitive and a "just follow it" thing. All the FDs I've flown do not overreact so as long as you are not rough with your handling, there is minimal overshooting. Generally speaking, when hand-flying a LLZ, I find I need to anticipate the FD commands: it's not sensitive enough to "blindly" follow and stay right on the LLZ. It's great with the AP in but not ideal when hand-flying.

My instructor at USAir beat into me: "Follow the Flight Director or turn it off!". Sound advice. Not routinely following the FD is a bad habit to get into IMO. More often than not, it is telling you the right thing, and if one regularly ignores it, say by turning before the PM gets the new heading in, you'll not follow it one day when you should.

[Microburst2002]

Hi John

Pitch
In take off follow SOPs (in A320 15š at an approximately 3š/sec rate, then FD).

It could be stated that flying consists on maintaining a sequence of states of equilibrium. Maintain-change-maintain-change-maintain... all the time. The transition between each two different states is a manoeuver. The transition can occur in many different ways to the same final outcome.

For instance you are descending at 280 kt and 2500 fpm V/S with a given pitch (V/S mode) and thrust (SPEED mode). You ask for "V/S minus 1000" to the PNF. Then the bar goes up. If you lag a couple of seconds, you don't have to rush behind the bar! You are just going to start "late", but there is no hurry. It is not an evasive manoeuvre. You decided yourselve to reduce rate to -1000. It was not the command of God. You will achieve the -1000 when it suits you. No reason to haste. Donīt let the computer push you. Smoothly pitch up. When the rate reaches -1000 fpm, you will have reached the new state of equilibrium. The bar will be in place because you have 1000 fpm. If it moves silightly, the computer has detected a departing trend. Follow it. The computer senses and calculates faster than you. Maintaining a state of equilibrium is not a manoeuver, it is just avoiding to depart from that state with minute inputs.

Instead, you could have pitched up aggresively. The same pitch amount carried out in less time (higher pitch rate) means a higher AoA and therefore a more abrupt and irregular flight path change. Performance will react accordingly. Maybe the computes will determine that it is time to pitch down a bit, now, so you find yourselve pitching down so as to not go below the -1000 fpm performance target.

I would say that, as a rule, a good manoeuvre is such that you transition progresively from the original to the final state as if you were passing through infinite mid states. Like a "quasistatic" change. It is much better for those pilots who still have to trim, specially.

FD "gains" and algorithms are designed in a given way. I don't know which way. But in a specific way. If you follow the bar in the way you are expected to follow it by the designers, then it will work very well!
But who knows that way?

In Roll:
Same applies to rolling into a turn. The bar moves instantly. If the change of heading will be large, the FD will command maximum bank. You roll smoothly to that bank. You will know what the commanded bank is because the bar is in place.

In the case capturing a heading or a track or a course: the FD bar has some mathematical algorithms that depend on the bank angle, the heading rate of change, the altitude and others to determine when to start rolling out of the turn. The more you lag after the FD bar moves, the more agressive you will have to be to avoid overshooting. I deem that after the FD bar moves you still have a second or so to start a smooth roll rate out of the turn without overshooting.


My advise:
Follow the bars or turn the FD off, but don't haste in doing so. Fly as smoothly as you would if you were flying FDs OFF. Once you have the bars in place, don't let them go. They are there to help you maintain the demanded performance without you having to use too much brain resources. The FD is a wonderful tool, specially in approach.
With practice you can fly it perfectly (and not be flown by it). Practice means not only that you have to fly AP OFF regularly... But also with FDs OFF, if they let you. And even without A/THR, if your airline or captain are not scared of that . Only then you will really master flying with the FD.

Hope it helps

[latetonite]

You stick to that modern FD. It even guides the A/P how to autoland. Usually a better job than the pilot. For most pilots it is a bit like playing tennis: discussing which expensive racket while going for their first tennis lesson.

[aterpster]

latetonight:

You stick to that modern FD. It even guides the A/P how to autoland.

In most modern installations both the autopilot and the FD are controlled by the flight control computer (FCC or whatever name an OEM chooses to call it). So, when the auto-flight is not engaged, the FD shows you what inputs will be provided to the auto-flight if engaged. When, auto-flight is engaged then the FD is simply a monitor of auto-flight track.

So, for any mode, including autoland, it's the FCC that is really in charge of the computation for the FD and the autoland maneuver.

[d105]

Originally Posted by John Citizen

How do you know it in advance ?

When do you follow it and when do you wait for it to come to you ?

I have always blindly followed it but now I am starting to realise that this perhaps not what I should be doing, as it causes me to overcontrol by chasing it

On the 738 a few quick examples come to mind.

1. After take-off the FMC provides TO/GA thrust until thrust reduction altitude. Thereafter it goes into N1 mode which usually means a couple of percentage points thrust drop. If you follow the FD during take-off it will take you up all the way to the pitch which holds V2+15. After the drop in thrust, you need to pitch down up to 2° to maintain V2+15 with reduced thrust. Instead of blindly following the FD you pitch for V2+15 at N1 thrust instead. No pitch reduction required and a smoother ride follows.

2. In Europe we usually fly 250KIAS below FL100, as per ICAO airspace classification limits. Above FL100 the FMC commands ECON climb speeds (ranging from 280 tot 320KIAS). the FD will command a pitch of roughly 3° tot 4° to accelerate. This is much too low. When the aircraft reaches ECON speed it needs to pull the nose back up to 5° to 6°. At 310KIAS you can feel that pull in the tail quite a bit. Instead of follow the FD you pitch for 6° at FL100 and let the aircraft slow accelerate to ECON speed. No pitch change required and again a smoother ride for your passengers.

3. The turn to final from an outbound leg of a procedure turn is calculated at constant bank angle from the limiting point of the outbound leg to the final. This can lead to the FMC commanding a turn of 10° or less, which is just ridiculous. Instead you fly through the FD to 30° bank and intercept the final by the shortest route possible. Thereby helping ATC, reducing fuel burn and speeding up your operation. The FD will follow once you are on the inbound course.

The list goes on basically. If you fly the aircraft precisely on the FD your pax in the tail will feel it. The FD is much too jerky, reactionary in nature and in general causes pilot induced oscillation.

The FD is a safeguard, not a "point at me and stop thinking" tool.

[Ocean Person]

The best thing about a Flight Director is the switch that turns it off. I've never encountered a positive FD that really earns its keep as they all seem to arrive late. They clutter up the main picture, degrade positive attitude holding and contribute nothing towards good scanning. Best described as something of a pest. The runway symbol is far more important.

[A37575]

and contribute nothing towards good scanning.

But then you get the counter argument that instrument scanning is not important any more because you simply follow the magic FD and magenta line and that relieves you of the traumatic stress of scanning