There are three significant items that define an aircraft’s ability to make a steep approach and landing. These are given in JAR certification requirements, but in technical terms.
On the approach, the aircraft must be capable of maintaining the steep glidepath in the most limiting configuration e.g. engines spooled up for anti-icing. The aircraft must demonstrate that there is a further 2 degree margin on the approach to provide maneuver capability; maneuver ‘agility’ is also assessed – the quickness in being able to regain the flight path. These parameters are limited by the aircraft’s drag margin – the ability to maintain a constant speed approach in the chosen configuration, thus the use of full flap/airbrake. The maneuver ability depends on control, mass, inertia, and many aerodynamic design issues.
The second item is the ability to flare and touchdown - the ability (amount and quickness) to use the available wing lift to change the flight path. This comes from good wing lift characteristics (good lift-slope gradient, generally slower speed STOL aircraft), and the elevator effectiveness. It may be necessary to balance quick change of lift and the loss of speed during flare, i.e. where wing spoilers are used to add drag for the approach they might reduce the lift available in the flare and add to the speed loss. This in turn may have to be offset by an increased approach speed (cf A318), which then affects landing distance and operating weight.
The third item is the ability to stop in the distance available. This is not necessarily associated with steep approaches, but normally a steep approach is only used into limiting runways, thus the economics of the operation have to be considered.
Other aspects include a demonstration of a landing from Vref -5, a single engine landing and go-around, good flight deck visibility, EGPWS changes, etc.
Flight tests determine the aircraft’s capability which can vary between variants e.g. BAe146-100/200 has a 6 deg clearance, but the 146-300 only has 5.5 degs and a tailwind limit.
renard the steep approach button (IIRC) also changes the FD/Autopilot control laws (146 a GS sensitivity change, but the RJ uses time/RA?)