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Helicopter - Twin vs Single

Issue: 06-2012By Air Marshal (Retd) V.K. Bhatia

The world record for highest landing and take-off was created by a single-engine AS350B3. Incidentally, the AS350B’s another version, AS550C3 Fennec—one of the two shortlisted helicopters for India’s 197 light utility and RSH acquisition programme is also similarly powered—by a single 500 kW (671 shp) Turbomeca Arriel 2B turbine engine.

Of late, the oft debated question and resulting preference for twin/multi-engine fixed wing aircraft seems to have permeated in the rotary wing domain too. The preference for more than one engine hinges on the general belief that there is greater safety in numbers. But unlike the fixed wing aircraft, whether having more than one engine on helicopters provides the additional safety or not is itself a matter of great debate. Why? Because, before we consider the pros and cons of twin-engine versus singleengine machines in the rotary wing domain, there is an apparent need to understand the fundamental differences on how the rotary wing aircraft respond to power loss vis-à-vis their fixed-wing siblings.

It is well known that if an engine power loss/failure occurs, the resulting emergency landings are significantly different for airplanes compared to helicopters. To maintain control of an airplane, its airspeed must stay above the stalling speed of the wing until ground contact. Depending upon the design features and performance characteristics of the airplanes in question, the speeds could vary vastly from 40-50 to 100-120 knots. Typically, some jet fighters of the yore had landing speed in excess of even 150 knots, but jet fighters as a rule are also equipped with ejection seats enabling the crew to safely egress from a stricken aircraft, if required. For others, a forced or emergency landing required a clear approach, either to a landing strip or a cleared flat ground as a possible landing site. Any obstructions (trees, buildings, fences or ground irregularities) if impacted by the powerless aircraft would result in significant crash forces causing injuries/fatalities.

Conversely, helicopters because of their unique vertical take-off and landing (VTOL) flying characteristics require a little more room than the size of the aircraft for an unpowered emergency landing. This is because the helicopter can descend under control after engine failure in a condition called ‘Autorotation’, whereby the pilot decreases the pitch of the main rotor blades to allow them to be rotated by the air flowing upwards through the rotor arc (or disc), similar to the action of wing on a windmill. The spinning main rotor—to understand it in a layman’s language—acts somewhat like a parachute while the aircraft is flown at a highly manageable speed of around 40-60 knot, and maintain a near-constant descent rate. The pilot retains full control of the aircraft and is able to select the most appropriate landing site, especially if the aircraft was flying at a reasonable height above the ground, prior to the engine failure. Nearing the landing site, the pilot flares the aircraft in a nose-up attitude and increases the pitch of the rotor blades, which increases lift. This, under ideal conditions (handled correctly by a welltrained pilot), allows the descent and forward speed to be slowed to near zero before ground contact to allow a gentle touch-down with no injuries to the occupants or structural damage to the helicopter.

The above is illustrative of the fact that because of its unique capability to land vertically with zero forward speed; even in the case of engine failure, helicopters have to be treated differently vis-à-vis the fixed wing aircraft when dealing with the question of power plant(s) installations. In the case of fixed wing aircraft, especially those being used for commercial purpose, it would be desirable to have a minimum of two engines enabling safe recovery of the aircraft in case of failure of one engine. In case of the passenger-carrying airliners, this argument is refined further by not only taking into account the thrust availability with one engine quitting to maintain safe flight, but also in trying to achieve near parity between V-one and V-two speeds, to ensure 100 per cent safety factor in the event of an engine failure at the most crucial point of ‘unsticking’ on take-off roll.

The helicopters on the other hand should be ‘engined’ depending on their missions coupled with all up weight (AUW) alone because of their in-built capability to be able to land safely, practically anywhere in the event of an engine failure. The only exception would be when for extreme safety considerations of the occupants onboard, there is a mandatory requirement for the helicopter to return to a predetermined safe launch/recovery helipad. It is on these grounds that helicopters have been categorised into three performance classes. These in the ascending order are as follows:

  • Performance Class 3: Refers to all single-engine helicopter operations; which require an emergency landing after engine failure.
  • Performance Class 2: Refers to twin/multi-engine helicopters that are capable of continuing flight after one engine fails except that a forced landing would be required following an engine failure between take-off and transition to safe forward speed and in reverse to landing.
  • Performance Class 1: Includes twin/multi-engine helicopters that are capable of continuing flight with one engine inoperative regardless of when the engine fails.