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Thrust reversers are featured to help slow down the plane after touchdown. Moving the thrust levers aft activates the translating cowl to open, closing the blocker doors. This action stops the fan airflow from going aft and redirects it through the cascade vanes, which direct the airflow forward to slow the aircraft.
My question is Shall We Consider Reversers in performance Calculations ?
For airplanes certified under Part 23, both takeoff (AMC 23.55b) and landing (AMC 23.75f) performance can take credit for thrust reverser (T/R) if it's shown to be reliable and safe. As detailed in AC 23-8C, much extra work would be required to take credit for T/R for multi-engine airplanes:
For accelerate-stop distance: demonstration of full T/R with one-engine-inoperative and in a 10kt crosswind and on wet runway for adequate lateral controllability is required.
For landing distance: the landing distance used must be that with one-engine-inoperative; differential braking needed to maintain lateral control for this situation may negate most, if not all, of the advantage of a single T/R.
For airplanes certified under Part 25, only takeoff on wet runway may take credit for thrust reverser (25.109f). Similar situation for landing distance as with Part 23, 25.125c)3) allows credit for T/R, but only for that with one-engine-inoperative; therefore, the same problem as discussed before applies.
I've flown aircraft that do take credit for reverse thrust in their stopping performance computations, and I've flown aircraft that do not. Both FAA certified. I have no idea how to answer "shall we..." in the question, but it's certainly possible to go either way on the calculations.
Maybe the best answer is, you should know the factors and assumptions in your aircraft's performance manual.
