Helicopters Magazine

Singles and Twins

July 29, 2009  By Ken Armstrong

At a recent helicopter function, some operators approached me for opinions on single- versus twin-engine helicopters. Apparently they were essentially being forced to acquire twin-engine helicopters by an oil and gas company that commonly contracted them.

At a recent helicopter function, some operators approached me for opinions on single- versus twin-engine helicopters. Apparently they were essentially being forced to acquire twin-engine helicopters by an oil and gas company that commonly contracted them. The operators wanted to stay with their existing fleets of single turbines and older medium-twins and were concerned about the overhead costs with new twins – especially those with Category A capabilities. Let’s consider the pros and cons among the selections. You be the judge.

The recently certified Bell 429 light twin is powered by two PW207D engines.

Category A (CAT A)
First, we will define the Class version of CAT A performance requirements for twin-engine helicopters (in a simplified manner). In this case, the helicopter must provide adequate performance to guarantee that, in the event of an engine failure, the flight can continue safely. Moreover, the helicopter must manage the failure of one of its two engines at any given moment while maintaining satisfactory safety margins, especially during the takeoff or landing phases. To achieve this CAT A helicopters must fly a relatively specific profile during takeoffs and landings to ensure the helicopter will be able to fly away or make a safe landing on the pad. Observers may have noticed the unusual takeoff flight profile of a CAT A helicopter as it lifts off and begins to climb while backing up. Under these conditions, if an engine fails during this initial portion of the takeoff, the helicopter can descend and move forward to a relatively normal engine out landing manoeuvre. Once it achieves its specified height in this procedure the nose is lowered and the helicopter accelerates in a flight profile that is guaranteed to provide a safe fly away should an engine fail during the takeoff.

The Agusta Grand is powered by two PW207C turboshaft engines. 


On the EC135, operators can choose between two types of engines: Arrius 2B2s or PW206B2s.

The Arrius 2B2, manufactured by Turbomeca, is one of two engine options offered by Eurocopter on its EC135.

On the Agusta Grand, each PW207C is rated at 466kW maximum continuous power.

Sounds great – but what is the cost? Well, CAT A helicopters are expensive because they tend to have larger engines, which are derated and tend to have higher specific fuel consumption.. This is because the engines in a twin powerpack are typically “loafing” under most conditions and best SFCs typically only occur near peak power output. Derating the engine’s rated output so as not to exceed transmission torque limits allows a significant extra margin of power to be called upon when one of the engines takes a “time out,” and this is where CAT A helicopters obtain their “guaranteed” safety margin. Keep in mind, the CAT A capability only exists if the helicopter is flown within the weight limits and flight profile specified by the manufacturer. It should be noted that some CAT A helicopters must be flown at a lesser gross weight than their maximum to meet the guaranteed ability to fly away or safely land after an engine failure. Moreover, prospective purchasers of CAT A helicopters should realize that some flight profiles will not allow CAT A capability. Place a helicopter at the end of a long line hovering over tall trees with a heavy load and an engine failure in most cases will result in a lot of tree chopping. A very alert pilot might be able to jettison the load quickly and perhaps fly away from this emergency, but many, if not most, would not react quickly enough to avoid catastrophe.
The financial bottom line with CAT A is this expensive machine will cost more to fly a pound/mile than a single and most other twins. But, you should ask: “What price safety?” If there are glimmers of financial hope, one is that operators may find they do save money on insurance rates (as a percent of insured value) because of the extra safety associated with CAT A helicopters and the preferential premium rates they can attract. Also, minimizing accident rates and damage due to forced landings in unsuitable terrain can be avoided and this can also result in long-term savings – in many ways.

Oftentimes, the decision to invest in a CAT A helicopter really boils down to a customer’s desire to enhance safety for its crews. However, they need to step up to the bank wicket and provide approximately a 30 per cent premium over single-engine operations to cover the additional operational costs. While customers desire this increased safety level, most operators are not prepared to commit to an expensive CAT A helicopter unless the client provides an adequately lengthy contract to allow for higher helicopter acquisition costs. At other times, operators in foreign countries will be mandated into using a CAT A helicopter because the specific flying operation mandates them under federal regulations. (See ICAO Annex 6 and JAR OPS 3 for these considerations.)

Twin Engine
In this comparison, the non-CAT A twins are essentially between the devil and the deep blue sea. While many twins offer an ability to continue flight after one engine is secured, they don’t do particularly well when engine failures occur at low speed during landings and takeoffs – unless they are lightly loaded. Unfortunately, when engine failures occur at these times they are often followed by an accident. Worse still, at high gross weights some twins often aren’t able to maintain altitude in cruising flight and as the expression goes, the operating engine leads them to the scene of the accident. To be completely honest, even in these situations, given the availability of a fairly open landing area, a perfectly safe landing can be made with a marginally powered twin with a run on landing or even a “sagging” hover landing with a drooping rotor and engine reaching “topping” power.

But, the considerations are even more complex between twins and singles. Similar to singles, twins have only one of the following: tail rotor, tail rotor gearbox, tail rotor drive shaft, and other critical components. The failure or imminent failure of one of these in most cases will result in an autorotation – regardless of the number of engines installed in the helicopter and whether it has CAT A capability! So, picture a pilot faced with a tail rotor drive shaft failure in flight – he/she is facing a no-engine approach to landing at the termination of the flight. Not much benefit to a second engine there….

Some will argue that twins are twice as safe because the loss of an engine leaves the pilot with that second powerplant. In fact they aren’t. Statistics show that twin-engine aircraft suffer more than twice as many engine outs and there is a good reason – or, in truth, it’s a bad reason. Pilots will often continue to fly a machine that is experiencing an engine issue on the basis that they always have the other engine in the event of a failure. In the 1970s, a top-notch company operated a turbine twin which had two igniters per engine for even temperature starting. One day, the number two engine would not start at a remote site. AME troubleshooting found that both igniters had failed. An igniter was quickly removed from the number one engine and installed in number two allowing the helicopter to depart the confined area for the company’s base. On contacting headquarters, it was subsequently learned that neither they nor the engine manufacturer had igniters in stock. With a choice of grounding the machine and losing revenue, that helicopter continued with only one igniter operating in each engine – for the entire summer! In the event of an engine failure of say, number two, and a coincidental failure of the single igniter, the engine would not have been incapable of a restart. (There were no applicable minimum equipment lists [MEL’s] in those days.) And don’t think for a moment that both engines can’t fail. Many readers will recall the two-engine failure of a Bell 212 in the Maldives on a night fight takeoff that resulted in 12 fatalities. For instance, consider the potential of water in a main fuel cell that can turn a heavy helicopter into a glider in a fraction of a second – no matter how many engines. So, you might ask, am I opposed to flying a twin and consider them unsafe? Heavens no! I would rather be flying a twin than a single when an engine quits….

Single Engine
There are many reasons most operators continue to operate singles and avoid twins. There was a time in the late ’70s when twins were beginning to impact the market and many observers thought the days of the single-engine turbine were numbered. However, customers were ill prepared to pay a 30 per cent or greater tariff premium for the implied safety of a twin. Moreover, turbines were so reliable that one was unlikely to have one failure every 100,000 hours – unless you let pilots fly them. Most engine failures were really the result of aviators inadvertently turning off the fuel valve or topping up the tanks with water or running them out of go juice. Truth to tell, engine failures due to component failure are relatively rare. Twins are not immune to pilot mental malfunctions or maintenance personnel errors either. During my first civilian checkout in a new twin whilst

flying a slung load through a mountain pass, the chief pilot decided to show me how the crossfeed worked. Unfortunately, his shutting off the fuel valve to one of the engines was not part of the checklist and he subsequently left me dealing with a drooping rotor while dealing with a real emergency engine start up in threatening conditions. Incidentally, that was the same helicopter that would consequently end up operating for the summer on one igniter per engine….

Another reason most operators shun twins is their inability to justify the higher acquisition and maintenance charges associated with twins. They feel a sure way to go broke involves buying a fleet of twins on the premise clients would be glad to pay for slightly increased safety. Singles are also much less maintenance intensive. Add a second engine and there is considerably more down time involved with inspections and more likelihood of lost revenue, as there are approximately twice as many instances of grounding due to the potential for another unserviceable engine and associated extra gearbox considerations.

There are no easy answers on this topic of singles versus multi-engine helicopters. Our industry is largely driven by our customers’ desires. However, they need to ante up to fund more complex helicopters if they want the implied safety benefits. Moreover, it may not be politic to say so, but, we may have already approached near perfection in achievable safety. While it is our task to mitigate risk in our industry, it is virtually impossible to completely remove threats associated with flying heavier than air machines at dizzying heights above ground. Consider the fact that an average of 1,260 aviation fatalities occur yearly in the word while there are more than 100,000 fatalities due to surgical error in hospitals. Don’t read much about the latter in the press, eh? (My mother, father, step-father and father-in-law were all victims of this great killer.) Once again, this begs the helicopter question, “What cost safety…?”

I don’t have the answers, because they vary for each set of circumstances when it comes to the selection of single versus twin. But, the bottom line is that any operator considering entry into twin-engine operations should accomplish a detailed cost analysis that looks at the guaranteed revenue prospects versus all the additional overhead associated with twin operations.


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