All aircraft have to undergo testing to determine how much runway they need in order to land and come to a stop without running out of runway, and how much runway they need in order to be able to safely reject a takeoff.
These tests are conducted:
- With full braking on all wheels so equipped (although with the brake disks and pads worn down right to the metal)
- With full use of airbrakes/spoilers (if so equipped)
- Without using reverse thrust, even if the aircraft is so equipped, despite the fact that, for anything other than a small general-aviation aircraft, in real-world line operations, reverse thrust is essential for a successful landing or RTO unless conditions are damn near perfect.
Given that thrust reversers are indispensable for slowing any larger-than-small aircraft in less-than-optimal conditions (for instance, if the runway is wet or icy, or if you bounce the initial touchdown and lose the next chunk of runway, or if some of the aircraft’s tyres burst during takeoff, causing the brakes on those wheels to work poorly when you reject said takeoff, or if you're landing overweight, or if the speedbrakes can’t be extended because you’re out of fuel and the aircraft’s ram air turbine can’t power more than the basic flight controls, the navball, and the radio, or if you touch down long, or fast, or long and fast, or if you’re making a forced landing and have to take whatever reasonably-flat surface you get), why are aircraft certified without them? If it’s to make sure that the aircraft can safely land without reversers, why not also run additional certification tests where the reversers aren’t locked out, but, instead, the spoilers stay retracted, or some (or all) of the wheel brakes are nonfunctional?