Several years ago, when attempting to gather more detailed information regarding the Airbus autoland logic, than is currently available in the FCOM and AMM, an Airbus flight control engineer shared the following:
The autoflight system will utilize a crab to compensate for a crosswind until approximately 45 feet. At this point, the align control law engages. The goal of the align control law is to reduce the crab angle, while maintaining the track of the runway centerline. For the A340-500/600, the autopilot will decrab up to 70 percent of the initial crab angle, with a limiting decrab of 7 degrees. According to the engineer, up to a 7 degree bank angle will be used to counter any drift during the decrab.
It was decided to limit the maximum crab angle during touchdown to 3 degrees. Combine that 3 degree angle with the limiting decrab of 7 degrees, yields a maximum crab angle prior to the engagement of the align control law of 10 degrees. This is what was used to determine the maximum crosswind limit of 23 knots, for an autoland in the A340-500/600.
As vilas has pointed out, Airbus has produced several presentations and documents devoted to crosswind landing techniques. While these sources give a reasonably good explanation of how to deal with crosswinds in the Airbus, I do believe there is room for additional clarity.
There are several potential negative transfer issues for pilots who are transitioning from traditional aircraft to the Airbus. The crosswind landing is one of these potential problem areas. As pilots flare and apply rudder pressure to remove the crab, they often apply and hold an opposite roll input. This leads to a greater bank angle than desired. Attempts to then reduce the bank angle typically are made with sidestick inputs that are too large. The result is PIO in roll, especially in gusty conditions. This type of PIO during the flare has been captured in numerous videos of Airbus crosswind landings.
When a bank angle is required to control drift, only a very small roll input is required until the proper bank has been attained. Once this bank angle (no more than 5 degrees) has been establish, the sidestick roll input is neutralized. At this point you are accomplishing a slideslip, but you are not utilizing the “cross control” technique of a more traditional aircraft.
I have found this thread quite interesting, and hope that it continues.
Last edited by Aztec Kid; 25th Nov 2014 at 09:42.
Reason: spelling error