Autoland was originally devised to achieve
approach and landing in poor visibility conditions, so it was focussed on
the necessary approach and touchdown/de-rotation and not on the low
friction surface case and certainly not on the variable asymmetries that,
in reality, usually apply with this circumstance. In terms of the firmness
of touchdown, autoland should do a reasonable job and this touchdown will
also be in the touchdown zone. The typical range of autoland vertical
speeds at main gear touchdown is normally considered to be in the range 2 -
5 feet per second/120-300 fpm - firm enough for the contaminated case.
Although autoland de-rotation is optimised for comfort, it can be expected
to take only 4 - 5 seconds for a larger aircraft, which should not be
excessive. Thereafter, the case for keeping autoland engaged after it has
been used for the touchdown may become more problematic. Whilst system
certification testing does consider the cases of asymmetry of braking and
reverse thrust, it is impossible to consider the range of surface friction
asymmetry which is likely to be encountered during the landing roll on a
contaminated runway. And it is certainly the case that autoland is not
designed for the situation where directional control issues may become more
evident as speed reduces, rather than less so as with a normal landing
runway surface. Many aircraft type procedures will therefore advise
considering an autoland with manual reversion after nose landing gear
contact.
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