Double Vision

Two engines can double your pleasure - or add dramatically to your workload


Something about flying a twin grabs your attention. Sitting up high, a fistful of throttles, clear one, turning one – it all adds up to a feeling of power and control.

Thats appropriate, of course, because power and control are the big issues in learning to fly a light twin. Its not a matter of having power and control, its that you can lose them very quickly when something goes wrong. Flying a light twin isnt for everyone, and its not a panacea that takes all the risk out of flying. But with proper instruction, flying a twin is, in some ways, as good as it gets.

Are Two Better Than One?
The biggest myth of light twins is that they are inherently safer than singles because if one engine quits, you can keep flying on the other. In fact, there are only two reasons for the second engine – extra power for extra performance, or having a second engine for training purposes. Redundancy is not the point.

Speed and payload in airplanes are generally limited by total available horsepower. It takes upwards of 400 hp to carry six people, baggage, and six hours of fuel at 170 knots. The problem is engine designers have not been able to come up with a way to produce that much power using horizontally opposed air-cooled engines. The thermodynamics of air cooling make it impossible to build a cylinder much over 90 cubic inches, and engine compartment aerodynamics make it too hard keep more than three cylinders in a row all within acceptable operating temperatures. So for practical purposes at this point, about 540 cubic inches is the limit, which translates to about 300 hp.

If we want more performance than 300 hp can deliver, therefore, we have to add another engine.

But that second engine brings some performance penalties with it. Two 250 hp engines together weigh more and burn more fuel than one 500 hp engine would.

Two engine faces mean more drag than one larger engine face. Add two engine mounts, twice the beefed up structure to hold the mounts, and by the time youre done, you have a 500 hp twin that performs like a 400 hp single. Thats why 300 hp singles like the A-36 Bonanza, Cessna 210 and Piper Saratoga outperform 320-360 hp twins like the Duchess, Seminole, and Cougar.

Essentially, a light twin has two engines because it needs two engines, and when it doesnt have two engines, its going to perform like a really sick puppy. The two numbers that define just how sick that puppy is are single engine rate of climb and single engine service ceiling.

Lets say a 3,800-pound light twin with two 160 hp engines has a 1,600 foot-per-minute climb rate at full power. This requires 185 excess horsepower, which means it takes 135 hp for that airplane to fly level at climb speed.

Now suppose an engine fails and cuts the power output in half to 160 hp. The aircraft has only 25 extra horsepower and the climb rate drops to 217 fpm. A 50 percent decrease in total power results in an 86 percent decrease in climb rate.

This is almost exactly what happens to a Grumman Cougar when one engine quits – except the added drag of the engine-out configuration (rudder and bank angle) lowers the actual climb rate to 200 fpm. And this is sea level/standard day performance. It gets worse as altitude and temperature go up. Thats why light twins have single engine service ceilings of only 4,000 to 6,000 feet.

If youre at cruise altitude and one quits, the only advantage you have over a single is extended glide range. In cruise, you may find yourself descending into whatevers below you, be it ice-laden clouds or cumulo-granite. (Of course, it might also be clear sailing to an airport.)

When planning or making decisions in a light twin, consider that an engine failure will still result in a forced landing if youre over weather or terrain above your single-engine service ceiling; the second engine only delays the inevitable.

On the other hand, light twins with both engines running generally have much better power-to-weight ratios than singles of comparable size. For example, a Cherokee Arrow has 200 HP and a max gross weight of 2,750 pounds, for a power loading of 13.8:1. A Piper Seminole (virtually the same airframe) has 360 HP and a 3,800-pound max gross, for a power loading of 10.6:1. This gives the Seminole a better climb rate and higher service ceiling.

In the summer, you can climb faster to altitudes above the bumps, where you can see and avoid the buildups. In winter, you can cruise above icy cloud decks. In fact, even non-turbocharged light twins will operate satisfactorily into the mid-teens. Since the FARs require oxygen for the crew for flights above 12,500 for 30 minutes, above 14,000 feet for any time, and for all aboard above 15,000, an oxygen system makes good sense even in any light twin. At night, or if youre a smoker or have any respiratory or circulatory deficiencies, knock a few thousand off those numbers.

Engine Out Considerations
Engine-out operations are what makes twin-engine piloting different. When one engine quits in a single, it flies pretty much the same as it did when the mill was producing power. If you dont stall it, you can control it easily all the way to an emergency landing. In a twin, you have two problems simultaneously when one engine quits: performance and control. Not only will the aircraft try to turn around and bite its own tail, but the loss of half the power will make it very reluctant to get up and climb.

With the engines out on the wings, thrust is being produced well off the airplanes centerline. When both engines are working, the asymmetric thrust from one engine balances that of the other. But when all the thrust is being produced six feet off the centerline, theres a yawing moment produced around the airplanes center of gravity.

You have three choices to control the yaw into the dead engine – reduce the power on the good engine or use rudder or bank. In most situations, you do not want to reduce power after youve already lost half. Banking into the good engine also hurts performance because lift is split between vertical and horizontal components. Rudder into the good engine becomes the best means to control yaw.

The only ways to get more yaw power from your rudder are to increase rudder deflection or increase airflow over the rudder, in other words, increase airspeed. Since your rudder can only go so far, there is a minimum speed at which full rudder can stop the good engine from pulling the airplane around. This is that magic number – single-engine minimum controllable airspeed, or Vmc.

Vmc varies considerably depending on a number of factors – some of which you can control, and some of which you cant. And the number posted in your POH/AFM and painted on your airspeed indicator is only valid for one specific set of circumstances. During your training, you will learn each of those factors, how they affect Vmc, and how to make them work for you.

The second issue is performance. You may remember that best angle of climb (Vx) and best rate of climb (Vy) change as altitude increases, because the best performance airspeed depends on how much power is being produced. As density altitude goes up, engine power output goes down. Just as Vx and Vy go down as altitude increases because of reduced power, your single-engine best rate/angle of climb speeds (Vyse and Vxse) are lower than the all-engines-operating Vy and Vx.

The crucial number for one-engine-out performance is Vyse – any faster or slower than Vyse and your single-engine climb rate and maximum sustainable altitude both go down. Thats why the FAA says twins has to have a special mark – a blue line – on the airspeed indicator at Vyse. Its the speed you want to attain as quickly as possible on takeoff, and stay above until landing is assured. Remember that slowing down is always easier than speeding up when youre short on power.

Flying in the Real World
Translating the theory into appropriate cockpit operations isnt too complicated if you remember some basic rules.

First, if youre in cruise, Vmc shouldnt be a concern. In all light twins, Vmc is so much lower than Vyse that it wont be a factor. If you lose an engine, youll have no problem maintaining control as you decelerate slowly to Vyse or something a bit above it in order to maintain altitude (or minimum sink rate). Allowing the airplane to get enough below Vyse to make Vmc a factor will cost more performance than you can stand. And at Vyse or above, you are so much above Vmc that control is not an issue.

Performance becomes the critical question: Can you keep the airplane above whats below you? If the answer is yes, the second engine can transport you to the nearest suitable airport for a safe landing. If not, you are going to have to put it down in the best patch you can find within the distance you can travel before your descent angle intersects the ground. Thats why a second engine does not provide true redundancy in all situations.

In the landing pattern, loss of one engine isnt a recipe for disaster unless you make it one. Many light twin engine-out landing accidents involve overshoots. Pilots tend to fly single-engine approaches higher and faster than normal because they fear getting low and slow on one engine. They either land long and fast, running off the end of the runway, or decide to avoid an overrun by attempting a single-engine go-around.

Unfortunately, a light twin has to trade a lot of altitude for cleaning up, accelerating to Vyse, stopping the descent, and beginning the climb. Pilot often start this maneuver too low, pitching the nose up, and getting below Vmc. The usual result is an uncontrolled fatal crash.

For single-engine landings, a normal IFR approach or VFR pattern is usually easy to do on one engine since you normally use less than 50% power on each engine.

But some engine failures can bite you. Vmc becomes a serious problem on takeoff and transition to climbout. If you lose an engine below Vmc, all you can do is pull back both throttles and do just what you would have done in a single with one engine out. Fail to pull back the throttles, and youre in for a wild sideways ride off the runway. Thats why twins are never rotated until Vmc or some safety value above it. Dont trade the extra yaw control of the nose wheel until youve got enough air over the rudder to control the nose if one quits. Once you have Vyse and the gear up, you can fly the plane out of the situation if your single-engine performance allows (i.e., not on a hot day in Colorado Springs).

Its the space between rotation and Vyse where things get dicey if one engine fails. The airplane has enough speed for control, but the performance wont be adequate for climb until the gear is up and the plane accelerates to Vyse. Most twin pilots wont try to fly if an engine fails before the gear is up – too much drag to accelerate without trading off what little altitude there is.

But even if you do have the speed, you are going to have an extremely low climb gradient even at sea level – about 2 feet up for every 100 forward. Lose one after takeoff but before acceleration to climb speed, and it will take another half a mile to clear the traditional 50-foot obstacle.

The operating handbooks of many light twins have charts showing distance to accelerate to liftoff and then stop, and another showing distance to accelerate to liftoff, lose an engine, and climb over a 50-foot obstacle. Great stuff if youve got it, but many older twins do not. Experience in the training process may be the best tool available, but may be misleading because most training is done well under max gross weight, which means performance will be a lot better than if youve got a full boat.

This puts a whole new light on takeoff planning. In a single, its easy – if it runs, fly, and if it doesnt, land it in the best space around. In a twin, that one running engine can sorely tempt some pilots to keep it flying once its airborne. This predisposition killed the pilot and eight passengers of a Navajo Chieftan. The pilot attempted to fly after an engine failed about the time he reached for the gear. He got behind the power curve, far below Vyse and unable to accelerate, and the airplane rolled over and crashed inverted about halfway down the 8,000-foot runway. Despite having 6,000 feet of runway in front of him when the engine quit, the pilot tried to continue the takeoff rather than pull both throttles back and land comfortably.

Adding the Rating
The nice things about a multi-engine add-on are theres no written exam and no minimum hours required. Your training should have three stages – basic transition, engine-out operations, and instrument flight. The first is essentially a complex aircraft transition course. The second is the part that makes multi-engine training unique.

In engine-out operations stage, you will be introduced to the aircrafts flight characteristics with one engine out, including performing Vmc and engine-out performance demonstrations. The Vmc demo, performed at or above 3,000 AGL, is actually pretty tame. The airplane is set up in the simulated single-engine configuration of one engine at full power, the other at zero thrust, and 5 degrees of bank into the good engine. (Zero thrust is a power setting in which both forward thrust and prop drag are negligible, something like 12 inches and 2,000 RPM.)

Starting at Vsse or Vyse plus 10 knots with gear and flaps up, the airplane is slowed gently (one knot per second) and rudder displacement is increased to keep the nose from yawing until the rudder reaches the stop. When the nose starts to yaw, power on the good engine is reduced and the nose is lowered to recover.

In the performance demo, youll again set up simulated single-engine flight and then try different configurations to see how they affect the climb (or sink) rate at Vyse. These include gear and flaps down (separately and together), dead engine at zero thrust versus windmilling, and varying bank angles between zero and five degrees into the good engine. This will show you which factors most affect performance, and why the engine-failure checklist for your airplane addresses them in the order in which it does. Then, your instructor will start pulling an engine on you at each phase of flight, from the takeoff roll, through climbout and cruise. In each case, you will have to keep control of the plane, perform the steps to clean up the plane and secure the dead engine, and get it back on the ground.

The final phase is instrument flight. This phase is required if you have an instrument rating. If you do not have the instrument rating and later add it, youll have to take the instrument ride in a twin or your instrument privileges will be limited to single-engine only. In the instrument stage, you will perform basic instrument flying (four fundamentals, slow flight, etc.) plus engine failures after takeoff and single-engine approaches under the hood.

If you hold Private-ASEL-Instrument, your multi-engine training also provides an opportunity to upgrade your certificate as well as your rating. While its possible to get your multi-engine rating in only 12 hours or so, your insurance company is almost certain to require 25 hours in type. In that time, you can complete all the aeronautical experience and training requirements to take your multi-engine checkride at the commercial rather than private level.

Since the Commercial-AMEL ride does not include the commercial performance and ground reference maneuvers (eights on pylons, chandelles, etc.), theres almost no difference in what youll have to do. Yes, you will have to take the commercial written, but if youve already done the private and instrument, youll find the commercial exam pretty easy. Then youll have a Commercial Pilot certificate with AMEL and instrument-airplane ratings (which should make your insurance company happier), and private privileges for ASEL. Further, by taking your initial commercial ride in a complex airplane, you can later upgrade your ASEL privileges to commercial in a simple airplane like a C-172.

Multi-engine flying isnt for everyone. The cost and complexity are more than many pilots are willing to bear. But for those who want performance unmatched by a single and a measure of extra safety thrown in, turning two mills can improve the speed and the utility of personal flying.

Also With This Article
Click here to view “Takeoff is the Real Moment of Truth.”

-by Ron Levy

Ron Levy is an ATP and CFI with over 5,000 hours. He is an Assistant Chief Flight Instructor at American Eagle Aeronautical Academy and an adjunct faculty member in Aviation at Wilmington College. He is the Safety Director of the American Yankee Association.


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