Originally Posted by
KazohiroNakamura
According to the regulations, you have to be able to stop your A/C within 60% of the LDA. So a factor of 1.67 on the Landing Distance required. And if the runway is wet is should be +15% so in total is would be 1.92 of the Landing Distance required.
Is this a performance dispatch requirement or do you need to use these figures also once you are in the air?
Keep in mind that it can vary by country. In Canada, there are dispatch requirements but once in the air, you are allowed to go somewhere without the increased runway length of factoring, just the landing distance required as set out by the manufacturer. And remember that while they did not use any reverse on a bare, dry runway to come up with that distance, the test pilots were allowed to descend on a 3.5° glidepath and land very firmly. And, they don't calculate the added speed that you may have for gusts.
For dispatch, you have to do the multiplication times 1.67 plus 15% as you stated earlier if the runway may be wet according to forecast but if you have an alternate that is sufficient length to meet these requirements, you don't have to add the 15%. In other words, the longer alternate runway allows you to dispatch for the wet runway with dry runway distance factor only(1.67 instead of 1.92). Or a shorter length can be used if the manufacturer published wet landing performance that is shorter than the 1.92 times landing distance required but it can't ever be below the dry landing distance(such a situation might happen if reverse thrust, used for wet calculations but not dry, were very effective).
Remember, as wet landing distances are based on using reverse thrust, if you get into a situation where you have to cancel reverse as per procedure for regaining directional control, that is not accounted for.
Contaminated runway ops corrections(based on reverse thrust use) are advisory only.