NTSB and Rudders

[Wino]

FDXmech

While some other older aircraft use the blocker system (notably the md80 and the 727) they are used in conjuction with reducing the hydraulic pressure from 3000 psi to 800 psi. Thereby limiting your ability to do damage, and making the sensitivity somewhat constant.

The A300-605R A320 A330 and A340 all use a blocker system and maintain 3000 psi at all times. Indeed the A300 tail is not mass balanced and is prevented from fluttering by constant hydraulic pressure of 3000 psi.

If the pilot flying stepped lightly on one rudder to assist in roll he would have gone to the floor at 250 knots and not known how he got there. If he tried to correct he would have almost instantly hit the other stop. I doubt he would have reversed again, but then the other pilot might have tried to put in a rudder input. very quickly you get 4 throws of the rudder right there.

The A300-605R rudder system is DANGEROUS at 250 knots. The tail is incapable of withstanding the stress, and the controlls are trap for a POI. The A300 B4 with its ratio rudder load limiter was a much better design.

The blocker system is much simpler to implement than a complicated ratio changer system. However without the additional safety of depowering the rudder it is a menace. The composite design of the rudder and verticle stab do not allow the rudder to be depowered as the hydraulics are used to balance the rudder. So Airbus cut a corner and 265 people are dead. I expect that out of this accident there will be a short term fix of limiting the throw of the rudder further (probably forcing an engine derate as a result) and a long term fix of requiring rudder load limiters to be of the ratio type or if a blocker is used it is used in conjunction with depowering the rudder.

Cheers
Wino