Thanks a lot ELAC for your very insightful posts. You raise a valid point.
Originally Posted by ELAC
All bets are off though if hydroplaning occurs or something else drops the co-efficient below the accepted values. In such a case the contribution of the ground spoilers would quite likely be higher, but the total stopping distance would be much higher as well making the over-run probably inevitable regardless of ground spoiler position.
The lack of ground spoilers may of course contribute significantly to hydroplaning as the aircraft still creates considerable lift in CONF FULL without spoilers, thus there will be relatively little weight on wheels.
Especially on contaminated runways, the spoilers contribute three-fold to good deceleration performance:
a - By creating additional drag at higher speeds
b - By dumping lift and thus creating higher normal force ("weight") on the wheels
c - b has the additional effect of avoiding hydroplaning and thus creating a much more favourable friction coefficient.
(Friction = Friction coefficient times normal force)