With respect to performance, the main point of a Category A procedure is to provide engine-failure accountability.
With an engine-failure anywhere in the profile, the pilot should be able to return to the take-off point or continue the flight safely (avoiding all obstacles) to a place where a landing can be carried out - all without damage to the helicopter.
For a helipad departure:
- at any point up to the TDP and following an engine-failure, the pilot must be able to return to the take-off point and land the helicopter safely. That means that the pilot has to have sufficient visual cues to be able to conduct the landing, and the helicopter sufficient power to allow a controlled descent and landing.
- from the TDP and following an engine-failure, the pilot must be able to clear all obstacles by a safe margin, climb and continue to the flight to a point where a safe OEI landing can be carried out.
Category A is a certification standard which provides assurance of continued flight in the event of a failure by employing design assessment, to reduce the probability of failure. Engine isolation ensures that one engine failure is unlikely to lead to a second, and fire in an engine compartment can be detected, contained and/or extinguished. These provisions give a level of confidence that the helicopter can be operated for continuous periods over a hostile environment.
Category A requires performance data so that One Engine Inoperative (OEI) obstacle clearance from take-off, through climb, cruise and landing can be calculated. This data should include: mass related take-off and landing procedures; heliport/helideck size limitations; distances and climb gradients (or rates of climb); and one-engine inoperative climb performance graphs. From these procedures and graphs an operator/pilot can establish an OEI flight trajectory.
The failure rates for engines and tail-rotors (should) differ by four orders of magnitude. When reliability of any system does not reach the desired level it is mitigated by the provision of redundancy (in this case two engines).
JimL