Are you saying on a normal landing the thrust levers should be closed at 30ft? I reckon I have only seen that done two or three times ever. Would it not increase the chance of either a tail strike or hard landing if you misjudged it?
Yes, that's what I am saying.
Here is what the auto throttle is doing during an autoland for reference:
"the A/T begins retarding thrust at approximately 27 feet RA so as to reach
idle at touchdown. A/T FMA annunciates RETARD."
No, I actually believe that reducing the thrust at 10 feet carries a bigger potential for a tail strike. Typically the tail strike occurs when the pilot gets surprised for whatever reason and pulls back on the controls to and gets a rising radio altitude, speed decaying quickly and pitch rising. Having a nose down moment due to thrust reduction may add to the surprise, especially for the less experienced pilot (add to the situation a dark night, a runway with no centerline lights etc.). Floating is another factor that increases the risk of a tail strike. I believe the scenario is often seen with the 10 feet reduction: RA goes 10, 8, 6, 4, 4, 4, 4... and pitch keeps rising, usually ending with the aircraft falling those last feet for an uncomfortable arrival. I believe this floating is because of the thrust carried into the final part of the flare as written above and/or that a too large input was give to compensate the the falling nose. When a too big input is given, it's difficult to reverse it.
Here is another pitfall mentioned in the FCTM, though I will agree that you have to be really unlucky for this to occur:
If higher than idle thrust is maintained through initial touchdown, the automatic speedbrake deployment may be disabled even when the speedbrakes are armed. This can result in a bounced landing. During the resultant bounce, if the thrust levers are then retarded to idle, automatic speedbrake deployment can occur resulting in a loss of lift and nose up pitching moment which can result in a tail strike or hard landing on a subsequent touchdown.
On the other hand with reduction at 30 feet, the risk is reduced because the aircraft is not going to float and you are not getting any pitch surprises a few feet off the runway. The only risk of a tail strike, that comes to mind, is when reducing thrust early is combined with what typically is a beginner error in the flare itself. Namely to start raising the nose at 50 feet. With such a misjudgment, as you say, I guess a tail strike is possible with any technique. This often happen because the inexperienced pilot is fixated on the following the FD instead of looking out the window. Upon hearing the 50 call and looking out the window becoming surprised at the rising runway and pulling back the controls. This can be avoided by looking out the window early, preferably the last 2-300 feet, keeping the aim point fixed in the cockpit window and only looking in to check the speed. If the approach was stabile when reaching 300 feet there really shouldn't be much reason to look at the FD or ILS anymore.
After the thrust reduction stop the nose from dropping and not more, which is usually a movement on the control which can be accomplished with the wrist. At 20 feet pull very slightly on the controls to bring the nose up (FCTM says 2-3 degs, I never looked so I can't verify this). After that there is actually not much movement required. At 10 or 5 feet maybe a little after adjustment. But I will claim that from the initial adjustment at 20 feet, without further input a quite acceptable landing can be made, because all parameters (idle thrust, speed, descent rate) are correct. The last adjustment is to get it smooth.
The opposers to reducing the thrust at 30 feet, have a fear that aircraft will drop out of the sky like a stone if thrust is at idle. It will not. My guess is that this idea comes from the feeling of the pitch down when thrust is reduced.
A typical bad flare begins with the a slight break at 50 feet, followed by another one at 30, thrust reduction at 10 feet, a touch down zone drifting hasty by below and a drop from 4 feet to avoid entering the FOQA statistics for long landings.
A good flare is thrust to idle at 30-20 feet, keeping the nose from dropping, almost simultaneously lift the nose the notch that the FCTM describes as 2-3 degs at 20 feet and another notch at 10 to 5 feet. Result a nice smooth touchdown in the beginning of the touchdown zone.
Assuming a -700 landing at flaps 30 the normal pitch is 2.5 degs on the 3 deg ILS. If we take the high value 3 deg pitch up at 20 feet from the FCTM and add another highly set 2 degs for the extra notch at 5 feet, we get 2.5 + 3 + 2 = 7.5 degs pitch. Tail strike occurs at more than 12 degs with compressed struts.