ALDdry doesnot take credit of the reverse, that doesn´t mean that it use is forbiden. So if we use reverses our ALD will be shorter than calculated, and this is good for us.
JAR OPS 1.515 (A) RLD dry= ALD/0.6
JAR OPS 1.520 RLD wet= 1.15 RLD dry
JAR OPS 1.520 RLD cont= the greatest of 1.15 ALD cont/ RLD wet.
ALD cont is calculated with reverses.
ONE EXAMPLE:
ALD dry: 1200m ( no reverses)
ALD on dry runway using reverses: 950 m.
RLD dry: 2000m, minimum LDA to land legally. IF reverses are used 1050 m are left unused (better for us), if you land without reversers only 800m are left.
According JAR 25.125 braking means other than wheel breaks, e.g. reversers can be used when they are safe and reliable (except on dry runway), for determine the ALD.
So,
"ALD" on wet without reversers 1400m, with reversers 1300m
RLD dry (no reverse) 2000m, RLD dry with reverses the same and
RLD wet= 1.15 *2000m= 2300m. IF you use the reversers 1.000m are not used, without them 900m.
ALD cont ( using reversers)= 1850m for the type and depht of contaminant.
RLD cont= the gratest of:
1.15 ALD cont= 1.15*1850= 2.127 m
RLD wet= 2300m.
In this case RLD cont= 2300m
So finally, ALD cont (1850M) is the 80% of the RLD 2300m.