Originally Posted by protectthehornet
wouldn't the plane allow a flare in a normal landing? is this change in law dependent upon gear down as well as radio altitude?????
Of course a normal flare would be allowed under normal law - but as IcePack correctly notes - in normal law there is a maximum angle of attack that is allowed (called Alpha Floor) regardless of where the airplane is at the moment. This "protection function" is disabled in "direct law" and the pilot has complete authority over the pitch attitude of the airplane. Because in this situation, both engines were not shut down (i.e., even though the one engine running was producing very little, if any, viable thrust) the computers were still "in control," and alpha floor could not be exceeded. In other words, the pilot could have had (and I understand that Sully DID have) the side-stick controller at the full aft position, but the alpha floor protection would allow only the computed maximum angle of attack. In this case, had the airplane actually been in "direct law," the pilot would have had the ability to pull the nose up even farther than was allowed here, and that may have allowed a more shallow descent and thereby a less impacting touchdown. Of course that is speculative by any measure. Under most circumstances there would be little, if any, need to exceed a designated angle of attack very close to the ground. For example, landing an A-320 in a full stall cannot be accomplished under "normal law" - but it can be accomplished under "direct law." Why? Because there are precious few reasons why someone would want to land a commercial jetliner in a full stall, and limiting the angle of attack also protects from a tail strike (infrequent, but more frequent than other circumstances) - but it also prevents what would otherwise have been an "over rotation" in this case to, I believe in Sully's mind anyway, lessen the touchdown force for the inevitable water landing. However, loss of envelope protection means all loss of envelope protection; i.e., the bank angle protection would be lost as well.
The "law" that is operational at any given time is related to the status of the computers and the malfunctions or combination of malfunctions existing at any given time. Even though the aircraft OEM cites the probability of a complete computer failure being less than 10, raised to the -9 power, the FAA still requires the pilots to be trained on certain tasks when operating in Direct Law (i.e., without envelope protection), and the simulators used for training must be tested in a representative number of those conditions. There are several sets of circumstances that will put the airplane into operation without envelope protection - "direct law" - notice, I said "sets" of circumstances - that means a combination of circumstances - and I think there are about 10 - 12 such combinations. Unfortunately, some of those combinations can be reached quite quickly in some cases.
Sorry for the "ramble." But I think it important that those who are interested should know ALL the available facts. I think Sully and his FO did a wonderful job of maintaining control of the airplane - but, in
my estimation, the most significant thing he did was, make a reasonable decision, and he stuck to it - even when others were offering other alternatives. My hat was off to him and his first officer - and will stay that way - particularly knowing the predilictions of the airplane under the conditions that crew was facing - and that just makes that hat position more appropriate.