Twins vs. Singles

The Great Debate

August 1, 1998

A practical look at an age-old argument

Buying an airplane is much like buying a car. You know what you want and you know what you need. You may want a Ferrari but need a minivan. Many times, however, buyers bite off more than they can chew when they show up at the dealer to buy an economical hatchback and drive off in a gas-guzzling sport-utility vehicle decked out with running boards and leather seats.

Pilots tend to be a little more practical-minded compared to the typical car buyer. It's not often that a buyer flies off in a Beech Duke after kicking the tires of the used Piper Archer next to it — or would he? Several questions need to be asked when considering the purchase of an airplane — after all, it is probably the second-most-expensive purchase you'll ever make. One of those key questions has spawned one of the greatest debates in aviation, bar none. Whether it comes up while shooting the breeze in the airport restaurant or when you're face-to-face with an aircraft broker, that fateful question, "twin or single?" has to be answered.

Economics

The cost of ownership may make short work of the twin/single decision. Don't be fooled by low purchase costs — buying a twin is the easy part. You can get an old Cessna 3 10 or Beech Baron for just a little more money than that required for their single-engine stablemates of the same model year. It's the maintenance and fuel burn of the twin that make ownership hairy. It's safe to assume that the twin will cost three times as much as the comparable single in terms of fuel, maintenance, insurance, and engine reserves.

Availability of aftermarket modifications is another point worth considering when making the twin/single decision. Recently, we greatly enhanced the usefulness of AOPA's Beech A36 Bonanza by installing tip tanks under a supplemental type certificate held by Beryl D'Shannon. Not only do the tip tanks greatly increase the Bonanza's range, they also allow for a 183-pound maximum gross weight increase regardless of whether the tips are filled. The result is a far more versatile airplane that can fly as far as the twin-engine 58 Baron at 75-percent power and even carry a few more pounds of payload. Like the twin, this particular A36 is quite well equipped with a standby vacuum system, radar, Stormscope, and air conditioning. Add deicing boots or the TKS deicing system and you'd have a serious all-weather airplane — as long as that one engine keeps running.

That one engine

Here, as well, is where the debate may quickly end. There are those who simply can't trust one engine and for whom nothing less than two engines will do. A few years back much of the editorial staff was ogling over a new PC-12 that Pilatus had brought to AOPA Pilot for evaluation. A veteran charter operator on the field stopped by for a look at the gargantuan turbine single and asked the $64,000 question, "How much?"

"$2.7 million," I said.

"All that money and you still only have one engine?" he replied.

"Yeah, but it's a PT-6," I said, leaning on knowledge that the Pratt & Whitney has an impressive reliability rating. He looked at me as a grizzled old-timer looks at a teenage punk and related three incidents in which turbine engines had failed on him in flight.

"I'd still put my money into a used [King Air] 200," he grumbled as he walked away.

Others draw the line on buying a twin when they consider the terrain over which they'll be flying. Those regularly flying over miles of water or mountainous terrain are definitely going to give the twin an extra-hard look. Operations in mountainous areas also bring attention to service ceiling. The twin's service ceiling is generally far higher than a comparable single's, allowing better routing options for flight over high terrain where minimum en route altitudes (MEAs) are beyond the altitude capabilities of a single. On the other side of the coin is the fact that the pilot of a normally aspirated twin could be in just as much trouble as the pilot of a single if an engine quits. Most normally aspirated twins' single-engine service ceilings are below that of mountainous terrain in the West. Confidence in engine reliability may lead the twin pilot to file a route with MEAs far above the single-engine service ceiling. Time to start thinking about turbocharging now. Heck, while you're at it, tack on pressurization, too.

Need for speed

From somewhere back in your training you may recall a rule that says to double an airplane's airspeed, you must quadruple the power. With that in mind, it's no surprise to find out that true twins (those outfitted with double the power of their single-engine derivatives) don't outperform the comparable singles by any huge margin. In some cases the single-engine variants outperform the twin, as is the situation with the Piper Comanche 400 and Twin Comanche.

A perfect pair to pit in the twin/ single dual is the Baron and Bonanza. With double the power, the 58 Baron musters a realistic 195-knot cruise speed at 75 percent power. The single-engine Beech A36 Bonanza can manage about 170 knots at the same power setting and is burning only half the fuel, or 16 gallons per hour. So the Baron's extra 25 knots comes at a penalty of double the fuel burn and engine maintenance. Are those 25 knots worth it? That depends.

Twins outperform singles significantly in climb rate, which leads to reduced block-to-block times. Let's use a 400nm no-wind trip to and from sea-level airports as an example of the difference between the Bonanza and Baron. After a gross-weight takeoff, the Baron, at its 136-knot cruise climb, requires 6.5 minutes, five gallons of fuel, and 15 nm to reach 7,000 feet. Meanwhile the Bonanza, at its 11 0-knot cruise climb, takes 9.5 minutes, burns 3.25 gallons of fuel, and covers 17.5 nm getting up to 7,000 feet. Average rate of climb to 7,000 feet for the Baron is 1,100 fpm, while the Bonanza ascends at 750 fpm.

After level-off, the Baron, with its excess power, rapidly reaches its cruise speed, while the Bonanza can take several minutes to stabilize in cruise. After a 500-fpm descent, the Baron arrives in the traffic pattern at the destination after a total of two hours and four minutes of flight. The Bonanza will follow 19 minutes later — not a huge difference, but certainly noticeable. The Bonanza's slower climb rate is pronounced when comparing the block-to-block times with the twin.

When you taxi to the pumps, though, the Bonanza fights back. The Baron will have used 43 percent more fuel — 66.5 gallons versus 38.5 gallons. At $2.25 per gallon, that's a difference of $63. As you can see, if you don't fill the seats in the twin, you may as well throw money into the fuel cells.

The safety question

The safety aspect of the great twin/single debate has been beaten to death in various articles over the years but needs to be addressed. A 1979 National Transportation Safety Board study showed that pilots of twins are lousy at maintaining control of the airplane after an engine has failed, especially just after takeoff. Most of the accidents were loss-of-control (Vmc) spins, making the twin statistically more dangerous to fly than a single when it came to the number of fatal accidents. In overall accidents the single was worse than the twin because of that category's higher rate of forced landings. But most engine-failure accidents in singles resulted in survivable forced landings.

In a single, the engine-out after-takeoff procedure is far simpler and consists of flying the airplane and running through the checklist to be sure that the failure wasn't pilot-induced. After that it's simply a matter of finding a good place to set the airplane down and resisting the temptation to pull back on the yoke. The flurry of decisions required of the twin pilot is quite daunting and demands a high level of proficiency for the outcome to be successful. Is there enough runway to land straight ahead? Is the density altitude too high to establish a suitable climb? Are the gear and flaps up? The twin pilot has to carefully contemplate each takeoff in order to make the correct decision in a timely manner.

The fatal accident problem in twins stems from a number of sources. Some pilots continue to fly the airplane like a twin even though it loses 80 percent or more of its climb capability (see " Single-Engine Savvy," March 1997 Pilot), Others get confused in the cockpit and hurriedly make a wrong move, such as feathering the propeller on the good engine or simply forgetting to fly the airplane.

In a nutshell, the twin is safer than the single if the pilot is proficient in single-engine operations and the airplane is within weight and balance. Getting proficient on one engine in a twin is best done in a simulator that mirrors the type twin you'll be flying. Simulator and recurrent training will do worlds of good in such a situation, but be prepared to factor the four-figure-per-year cost into the price of that twin you are lusting after.

Double trouble?

Two engines and redundant systems are great for in-flight reassurance, but the glass-is-half-empty crowd would argue that there is twice the opportunity for something to go wrong. Two vacuum pumps double the chance that one will quit and strand you somewhere. While you're at it, you can double the opportunity for a spark plug to get fouled or an injector to get clogged, etc. At overhaul it's time to really pay for that second engine. Not only is there an extra engine; there are all of the associated accessories — vacuum pumps, prop governors, hoses, engine mounts, and baffling, to name a few. Oh, and don't forget to overhaul those propellers.

It is mostly the twin owners who make the argument that a twin's engines will last longer since they don't have to work as hard. For example, since they climb so well, twins spend less time in high-power flight configurations. Still others make the argument that the twin can operate at lower power settings and still maintain speeds better than that of the comparable single at 75-percent power or better. We haven't seen any concrete evidence that babying an engine will make it last longer, but the argument does make some sense.

Justification

If you consistently need to carry a big load out of short strips, then the twin is the better way to go. Will you consistently fill four or more seats and the baggage compartment? Do you need to take those people on trips longer than 600 nautical miles? If "yes" is the answer, then run the numbers on purchasing that twin.

Also consider the comfort of having extra baggage compartments. Nose and nacelle baggage compartments can handle golf clubs and skis, freeing up space in the cabin for humans. On such trips in a single, the useful load may actually be there, but seats could be unavailable because of the cargo's sheer bulk. Clever packing may work, but passengers are hardly at ease when a ski pole or golf club runs amok in the cabin during turbulence.

Fuel capacities are another point worth considering. Twins generally store more fuel and can stay aloft longer while still making respectable true airspeeds. They have the excess power to carry all of that fuel to altitude and, if power is restrained, each engine can sip that fuel at very low rates, providing nautical-mile-per-gallon figures close to that of a comparable single at normal cruise. For example, a Twin Comanche with all of the fuel tanks available by supplemental type certificates could stay aloft for an entire workday when the power is reduced — and still cover the ground at 145 to 150 knots.

In addition, the twin makes those westbound trips a little easier to handle, given that its excess power has more effect against those prevailing westerlies.

Is the debate over?

The merits of both airplane types can be argued indefinitely. The single's efficiency is so practical to some yet the twin's brawny performance and machismo may be irresistible to others. Careful analysis of the specifics of each type airplane will help you to make a more informed decision. After all, you don't want to botch the second largest purchase you'll ever make.


E-mail the author at pete.bedell@aopa.org.