Single Engine IFR Operations

Over the years, the comforting idea has developed amongst the travelling public that twin engined aircraft are much safer because they have a ‘spare’ engine in case one fails. This quaint idea persists today, despite the severe single engine performance and handling limitations of many light twins, and the fact that the reliability of modern engines is demonstrably higher than the reliability of the human element in aviation. Perhaps the best that can be said for two engines is that they may allow for continuation of the flight to a safe area after a failure of one engine. However, recent experience indicates that in the rare occasions when modern engines fail, all too often both engines fail together due to fuel exhaustion or fuel starvation, ice/snow ingestion, or in the rare case of the two or three Boeing 747 ‘gliders’, the effects of volcanic dust ingestion.

When Lindbergh was preparing for his solo Atlantic flight, he apparently received expert advice to the effect that he should use a twin, or preferably a three engined aircraft. Such an aircraft would give him a safety margin in the event of an engine failure, it was argued. Lindbergh did his sums, and concluded that as no existing multi engined aircraft could continue the flight after losing a single engine at the weight that he would be at for a large section of the flight, he was better off flying a single. Multiple engines would simply increase the statistical chance of ditching in the Atlantic. History records that he completed a successful crossing of the Atlantic in a Ryan monoplane on May 21 1927. It was fitted with a single 200hp Wright Whirlwind engine.

Nevertheless, multi engined aircraft became the norm in the airline industry, with three or four engines being deemed necessary for long over water flights, and twins being the minimum standard for domestic fare paying passengers. Despite the extremely high reliability of modern turbine engines, this situation continued until fairly recently, when commercial pressure forced a reappraisal. Now we see twinjets regularly flying long overwater sectors, with at least so far, none of the dire consequences predicted by the prophets of doom. Much of the engineering emphasis in clearing a particular type for ETOPS operations actually revolves around the reliability of its associated system rather than sheer engine performance. So if engine reliability has reached the stage where two engines, rather than four, are acceptable for long overwater flights, it is fair to ask the question ‘do we still need two engines for the lower capacity third level domestic flights? The facts suggest that we do not.

There is much to suggest that the statistical probability of an engine failure in a modern turbine engine such as the Pratt and Whitney Canada PT6A is significantly lower than the chance of pilot error causing an accident through inattention, failure to follow procedures, or some other failure. The BASI ‘Survey of Accidents to Australian Civil Aircraft 1990’, the latest survey published, illustrates the point clearly that at present the greatest hazard to flight safety is as always the human element.

Of a total of 219 civil fixed wing accidents reported in Australia in 1990, BASI listed the pilot as the ‘first type of occurrence factor’ in 147 cases. Power plants were listed as ‘first type of occurrence factors’ as follows: powerplant reciprocating 19, powerplant turbojet/prop one, powerplant miscellaneous 13.

Of the 219 accidents reported, 131 involved single engined aircraft. Multi engined aircraft were involved in 88 accidents. 98 accidents occurred during private operations, and 27 during flying training. The engine was a ‘first occurrence factor in 33 accidents. The point to be made is that even when we include light aircraft fitted with ageing reciprocating engines, it is hard to see the engine as being a more significant hazard to aviation than other accepted hazards of the industry.

The aviation industry is suffering from the worst recession since World War 2, and the effects are devastating. Operators who want to succeed in the industry need to find ways of significantly cutting costs, but fixed costs are rising sharply, due in part to government cost recovery action and rapidly increasing insurance rates. The only real scope for significant cost savings therefore lies in somehow reducing pilot/crew and aircraft operating costs. Given the high proportion of human factors in our accident statistics, there is little reason to suppose that training can or should be reduced. Salaries and allowances are fixed by law. Insurance rates are unlikely to fall in the foreseeable future. This leaves aircraft operating costs as the most likely source of cost savings.

Aircraft operating costs can be reduced in two ways; either by reducing the maintenance performed (a short term solution, whose supporters are generally identifiable by their high accident rate), or by re equipping with aircraft designed in such a way that they require less maintenance than existing aircraft. The new single turbine aircraft now becoming available, including the Pilatus PC-12, Cessna Caravan and Aerospatiale TBM700 fall into that category. By taking advantage of technological developments over the past decades, they offer both lower purchase costs (you only buy one engine) and lower operating costs (better material selection, and you only feed and overhaul one engine).

It is also arguable that these new aircraft offer higher safety standards than many twin engined competitors, due to the benefits of modern design techniques and technology, and the higher standards demanded of new designs.

There are two other specific areas where it can be argued that the safety of the new turbine singles is improved over twin engined aircraft.

Author’s Note

Readers should be aware that in my consultancy business, my major client is Pilatus Aircraft Limited, manufacturer of the PC-12 referred to (along with the Cessna Caravan and Aerospatiale TBM700) in the article. I have, however, tried to be objective, and have based that objectivity on many hours spent in IMC or well beyond gliding distance of land in single engined helicopters and fixed wing aircraft, where in many cases, my personal survival very probably depended on the reliability of the Allison 250 or PWC PT6A driving the machine. I personally believe strongly in the single turbine engine concept.

Firstly, the singles do not suffer the dramatic handling limitations of twins following engine failure. The March 1993 issue of the Canadian flight safety publication, ‘Reflexions’, indicates that since 1976, Canada has experienced 100 accidents and 43 fatalities involving twin engined aircraft under 5675kg (12,500lb) following an engine power loss. In 17 cases, (ie 17% of the accidents), the pilot lost control of the aircraft, resulting in 37 of the 43 fatalities. There is no reason to believe that the Australian situation would be very different. And pilot handling is not only a factor in light aircraft. There has been a highly publicised recent example of two pilots shutting down the good engine rather than the damaged one in a heavy jet. They were not the first to do so, and will not be the last. Secondly, the single turbine aircraft are required to stall at not more than 61 knots (113km/h) in the approach configura- tion, while their twin engined competitors generally stall at more than 10 knots (19km/h) faster. That may not sound like much of a difference, but when you consider that energy is a function of velocity squared, a 16 to 20% increase in speed at impact could make quite a difference.

These two factors may account for the fact that in Australian charter operations during 1986-1990, per fatal accident, single engined aircraft flew 131% (two and one third times) as many hours as did twin engined aircraft. It also may explain why in that time no serious or fatal accident involving a single engined aircraft on charter operations was attributed to en- gine failure. (This data has been extracted from an article written by Warwick Raymont, published by Australian Aviation, December 1991, page 79. The article makes interesting reading. ) Nevertheless, if the engine of a single engined aircraft does fail, it is going to descend. No question about it. So the risk, small as it is, should be reduced to the minimum practicable, and the residual risk intelligently managed.

Probably the main reason for engine failure in modern turbines is running out of fuel, either through exhaustion, or starvation due to water, sediment, or mishandling of the fuel system. Such failures can happen in any aircraft and can lead to total loss of power no matter how many engines are fitted. This is a pilot/management problem.

Other than fuel exhaustion or starvation, the next most common cause of failure is probably malfunction of the FCU (fuel control unit). The PC-12, Caravan and TBM700 all have manual throttles which allow unlimited operation in the event of an FCU failure.

Sometimes engines fail as the result of heat damage caused by improper handling during start or other operation, often many hours before the failure. This type of failure can be very significantly reduced or eliminated by the installation of electronic trend monitoring. Such a system would record all operating parameters of the engine, removing the possibility of over-temperature starts or operation with the potential for damaging the engine, from going unnoticed by the pilot and maintenance engineers. It would also alert the pilot and engineer to any trend away from the normal parameters of the particular engine, making it possible to adjust, repair or remove the engine well before it came close to failure. Such equipment could be installed for around $15,000, giving a very high degree of comfort.

Finally, in very rare circumstances, engines fail catastrophically due to some manufacturing or maintenance defect. There is no way to guarantee protection from such failures, so the problem must be intelligently managed, to minimise the risk. Unfortunately, this can become an emotional argument. No one likes the thought of a number of paying passengers in a glider in IMC. Consequently one occasionally hears well meaning suggestions that single engined IFR RPT or charter aircraft should only fly within range of forced landing fields! (For consistency, proponents of this solution should prohibit pilots from entering aircraft, because the likelihood of a pilot flying a perfectly serviceable aeroplane into the ground is statistically very much higher than the likelihood of turbine engine failure. ) Technology exists today, in the form of GPS, for the pilot to immediately have access to track and distance to a selection of the closest airfields in the unlikely event of an engine failure, thus reducing his workload, improving his decision making, and improving the chances of a successful engine-out recovery. It is probably worth noting that the still air gliding range of the PC-12 at maximum takeoff weight from (left) Cabin of the commuter configured Cessna Caravan. A major concern by airworthiness authorities in Australia and North America is the rapid ageing of the third-level single-pilot nine-seat commuter fleet. Comprising mainly Cessna 402s, Titans and Piper Chieftains, most of these aircraft will be a quarter century old by the end of this decade with resultant fears as to their airframe/systems viability. Commuters are never maintained to the same high standards as their heavy metal RPT counterparts and so it is justifiably not considered that they should be in regular airline service as long as airliners built to far higher FAR design requirements. Both the 402 and the Chieftain entered service in the late sixties and most in Australasia are already old enough to vote. Cessne, and most people in airworthiness agree, that despite its single engine, the Caravan with applicable certification is a far safer long term option than continually flying what are becoming very tired aeroplanes. (right) TBM thinking follows similar lines though their biggest competitor, the Beech King Air, is of course still in production. TBM claim operating efficiencies of up to a third against the similar sized twins.

30,000 feet is 96 nautical miles (178km), and 68 miles from 25,000 feet! To put the likelihood of turbine engine failure into perspective, it is claimed that in the worldwide Cessna Caravan fleet, there have been three engine failures in over one million flying hours, and that two of those would have been clearly predictable from the engine trend data weeks before the actual failures. There are approximately 8766 hours per year (365. 25 days per year). One million hours equates to 114 years continuously in the air. Three engine failures, two of which were apparently preventable.

The Canadian aviation regulatory body, Transport Canada, examined the question of how significant was the risk of engine failure in the case of a modern single turbine engined aircraft involved in night or IFR operations in IMC. Its Safety Review Team concluded that: the principal hazards and risks identified with the carriage of passengers in a single engined airplane in IMC and at night would not be related to engine or propeller malfunctions, but were hazards which were generic to IFR and Night VFR operations. As a result of this safety review, they found that engine failure on a single engined airplane was considerably less probable than the predictable occurrence of a number of other identified hazards associated with this operation.

Transport Canada has already approved changes which allow single turbine engine IFR and night transport category operations by specific aircraft and operators under prescribed conditions. The US FAA meanwhile is currently engaged in a ‘data review’ in preparation for a formal study of the issue.

Meanwhile, the Civil Aviation Authority of Australia is currently preparing an Aviation Regulatory Proposal to propose that single turbine engine RPT and Charter operations be allowed under IFR in IMC and at night. The ARP should be circulated through the industry during the final quarter of this year, and it is expected that, depending on industry response, legislative action will be completed within twelve months.

The Australian fleet of aged piston engined aircraft is becoming older and more costly to maintain in airworthy condition and should be replaced, but the purchase and operating costs of the few light twin turbine engined aircraft remaining in production make them an uneconomic proposition for many small RPT and charter operators. There are compelling reasons to believe that the safety of the travelling public would actually be significantly enhanced by amending the legislation to allow operators to upgrade to new single turbine engined aircraft at affordable prices.

Surely it is time to reconsider in a rational manner whether the prohibition on single turbine IFR RPT and charter opera- tions still makes sense. + Terry Wesley-Smith runs a specialist aviation consultancy, ADMS Pty Ltd, based in Canberra. Originally from a military aviation background, he has had wide commercial experience in the marketing and sales of helicopters and fixed wing aircraft, and regularly flies single engined light aircraft both for business and pleasure.