The last two blogs provided great explanations of the height-velocity diagram as it pertains to single engine helicopters. So, let’s now take it a little further into the multiengine helicopter realm.
Just as with singles, you will typically find a H-V diagram for multiengine helicopters in the flight manual. However, unlike the singles, the H-V diagram for the multi is to insure a safe landing OEI (one engine inoperative), and not from an autorotation. Furthermore, whether or not the H-V diagram even applies is dependent on how well the aircraft can perform OEI. This performance is defined in a series of categories. If the multi is full-time Category B (as are all singles), or a part-time Cat B, then a H-V diagram limitation will apply; whereas, if Category A it will not. Basically, Cat A is where OEI performance is so good that the H-V is not applicable. Comparing three very different multiengine helicopters to highlights these differences.
The BO105CBS is full-time Cat B, with marginal OEI performance. Even in ideal conditions (light weight and low density altitude), it can barely hold altitude on one engine. Varying airspeed from Vy just a couple knots results in a descent. Approach and departure profiles AEO (all engines operating) need to be such that a quick transition can be made in accordance with the H-V diagram, in the event of an engine failure.
The Bell 412 is an example of a multi that can be operated Cat A or Cat B, depending on the weight, altitude, and temperature. At lower weights, altitudes, and temperatures it will have good enough OEI performance to qualify as a Cat A aircraft. However, in most day-to-day operations it is typically a Cat B aircraft, which means the H-V diagram would apply.
The AgustaWestland 139 is a true Cat A aircraft, although as with many other Cat A aircraft it is possible to find conditions that will push it into Cat B. The AW139 was largely designed to operate Cat A, in an offshore petroleum support environment with a high useful load (passengers, cargo, and fuel). It is capable of landing and taking off from helipads, while carrying up to 15 passengers, with Cat A performance.
So, what is Cat A? Cat A is where the aircraft has adequate performance capability for continued safe flight in the event of an engine failure, no matter when that failure occurs. While single engine and Cat B multiengine helicopters have no such assurances, the Cat A aircraft is able to ensure that a safe and normal landing can be made OEI at an airport or heliport.
In the event of an engine failure, different types or categories of helicopters dictate different courses of action in order to do the same thing: preserve rotor RPM. No matter the helicopter and its’ number of engines, Nr is the wing and it must be maintained. The single must obviously enter an autorotation. The Cat B multi must fly at or above Vy (best rate of climb OEI) in order to maintain or increase altitude, and then fly to an area where a safe landing can be made. During takeoff and landing while close to the ground and below Vy, an engine failure in a Cat B will likely result in a forced landing. Though not as dire as an autorotation, it is more of an event than the Cat A helicopter. The difference with the Cat A is that engine failure doesn’t dictate a forced landing. In the event of an engine failure during takeoff, a Cat A has the ability to either return to and safely stop at the takeoff area or to continue takeoff, climb and establish forward flight. In the event of an engine failure during landing, the Cat A can either land at the intended landing area or abort the approach and reestablish forward flight. Unlike Cat B, there is no exposure to the possibility of a forced landing, hence no H-V diagram.
(These views and opinions are my own and do not necessarily reflect the views of Era.)