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.