March 1, 1997
By Peter A. Bedell
With all the benefits multi-engine flying has to offer, there is one major caveat — what happens when an engine quits? To a quick-thinking, well-trained pilot flying an airplane within its limits, probably nothing. To a complacent pilot flying an overloaded twin, possibly disaster. The key to handling an engine failure is a thorough understanding of the aerodynamics of a conventional twin and V MC, the minimum airspeed at which a conventional twin operating on one engine can be controlled.
For multiengine students, understanding V MC can be one of the more complicated concepts to swallow, but designated examiners spend a great deal of time during the oral and practical tests to make sure that the applicant has a thorough knowledge of V MC and the factors that affect it.
Why is this speed so critical? In 1979, the National Transportation Safety Board released its findings that a fatal accident after an engine failure was four times as likely to occur in a twin than in a comparable single. It was certainly a surprise to most pilots — and the general public — who believed that two engines meant double the reliability and, therefore, more safety. But, as the study clearly stated, when twin pilots were suddenly faced with an engine failure, especially shortly after takeoff, the element of surprise and the precipitous loss of performance often wrestled control of the airplane away from the pilot.
Fatal accident rates in single-engine aircraft were far lower, according to the study. The reason: When the engine quits on a single, there is no sudden yaw and roll to fight; the aircraft just keeps flying along straight — albeit in a descent. The pilot of a single-engine airplane dealt an engine failure right after takeoff is required to make far fewer spur-of-the-moment decisions — you know that the airplane is going down; it's just a matter of where. In a twin you are faced with many options at once. Can you land on the remaining runway? Do you continue the takeoff? Do you land and run off the end of the runway? Can you outclimb the mountains with the load on board? It all depends on conditions and pilot training.
What happens when an engine quits on a conventional twin (those with engines mounted on the wings)? You lose half your power and, therefore, half of your climb performance, right? Wrongo. In general, light twins are lame ducks with an engine out. Most of them lose about 80 percent or more of their climb performance following the loss of an engine — and some, like early Piper Apaches and Cessna T-50s, may actually descend at the best single-engine rate of climb. Twins are good climbers because of the excess power that the second engine provides for climb. A Piper Seneca I with two 200-horsepower engines has a total of 400 hp. Let's say it requires 160 hp to maintain level flight. With both engines running that means there is 240 excess horsepower available for climb. After an engine failure, though, the airplane has only 40 horsepower available for climb — a loss of 90 percent of its climb performance. Also, when one of those engines quits, the excess power immediately reverts to excess weight and drag. To counter the weight and drag attached to one wing, the pilot must dial in a healthy deflection of rudder and aileron; this increases drag even further.
As expected, problems occur when pilots continue flying the airplane as if it were still a twin (or a comparable single for that matter) even though the performance is not there. As airspeed inevitably decays, the airplane may eventually become uncontrollable. The magic speed is V MC. Below that point, there is no longer enough rudder and/or aileron authority to counter the tendency of the airplane to roll into the dead engine. If not immediately countered, the roll will continue until the airplane is inverted. This is known as a V MC rollover.
Not many people have lived through a V MC rollover at or near the ground, but one examiner experienced a rollover after an overzealous student yanked back on the yoke at the pinnacle of a high-altitude V MC demonstration in a Beech Baron. He described it as a near-instantaneous snap roll to the inverted position. Reportedly, the roll was violent enough that the examiner smacked his head against the cabin door. Since the right engine was still producing full power, the airplane quickly entered an inverted flat spin at about 7,000 feet. After stopping the rotation by powering up the "dead" engine, the examiner managed to right the airplane at about 1,000 feet agl after performing a split-S maneuver. He and his pupil were lucky. Others haven't been so fortunate. Training accidents accounted for many of the V MC-related twin accidents surveyed by the study.
What causes this to happen? It is easy to see why a conventional twin yaws after the failure of one engine, but why the rolling motion? Think back to your initial stall training. Power-off stalls occurred at a much higher airspeed than power-on or departure stalls. The same goes for the twin; when both engines are blowing all that air over the wing, it can stall at amazingly slow airspeeds and high angles of attack. With power off and the gear and flaps up, a Beech E55 Baron will stall at 79 knots. With the power on in the same configuration, it stalls at 58 knots. If you throttled the right engine back to idle, the right wing would have a stall speed closer to 79 knots because of the lift lost from the engine's thrust and the blanketing effect of airflow caused by the windmilling propeller. This loss of lift causes a roll toward the dead engine. If the airspeed is low enough, there may not be enough rudder or aileron effectiveness to counter the roll (some airplanes run out of aileron before rudder).
The trickiest part about V MC is that it's always changing. With any given condition, V MC could be lower or higher than the published speed. The published speed is determined by using a factory-new airplane on a standard day with a test pilot at the controls. The airplane is loaded to its maximum gross weight, the center of gravity at its rearmost point, the left engine (or the critical engine, if applicable) windmilling, and full power on the remaining engine. The flaps are in the takeoff position and the gear is up. Any change from these conditions and V MC begins to wander from what's published. The following are the factors that affect V MC:
In general, it's a compromise between control and performance regarding the factors that affect V MC and climb rate. If it lowers V MC, it usually degrades performance. First on the mind of the conventional-twin pilot is control. If control is maintained, it's decision time. If the decision is made to continue the flight, performance is the next goal.
Imperative for every twin pilot is to have a plan before every takeoff. What airplane are you flying? What's the load? Where's the center of gravity? What's the strip length? Are there any obstacles? The list of variables goes on and on, and their importance increases as the strip gets shorter and the load greater. A good rule of thumb for many light twins is to use the point of landing gear retraction as your go/no-go decision maker. It works for long and short runways with or without obstacles and is simple enough to avoid any confusion — land if the wheels are down; keep going if they're up or in transit.
On the takeoff roll, it's preferable to rotate at a speed higher than V MC. Some airplanes, however — like the Baron — are hard to hold on the ground at such a high speed. With such airplanes, it's best to rotate when the airplane wants to fly and stay level in ground effect a few feet above the runway as the speed builds. If an engine coughs on the takeoff roll or just after liftoff while the airspeed is below V MC, immediately close both throttles and land straight ahead, regardless of strip length. You may run off the side or the end of the runway, but that's certainly better than facing a V MC rollover, which is most surely fatal.
After accelerating beyond V MC, and if nothing's awry, rotate and establish a reasonable climb attitude that will still allow the airplane to accelerate to blue line (best single-engine rate of climb). Some pilots may think that holding the airplane in ground effect until reaching blue line is preferable, but this has proven to severely penalize the accelerate-stop and accelerate-go distances for a twin. It's better to go for the altitude than airspeed. When departing from long runways, retract the gear when there's no longer enough runway to land straight ahead. On shorter runways, it will probably mean retracting the gear right after rotation, so long as you do so at an airspeed faster than V MC.
Short- and soft-field operations in twins pose more of a threat to V MC-related hazards. The Baron's original pilot's operating handbook suggested using approach flaps for short- and soft-field takeoffs, which results in the airplane's breaking ground at about 55 to 60 knots — more than 20 knots below V MC. Granted, the procedure works and the airplane can be airborne in less than 600 feet at max gross weight; but, as Beech and the NTSB found, if an engine fails in that no-man's land (after rotation but before V MC), the results are usually disastrous. Remember what happened to the previously mentioned examiner who experienced a V MC stall at altitude? Imagine that happening just above the ground.
After the 1979 NTSB study, manufacturers incorporated a V SSE (minimum safe single-engine speed) into the POHs of their twins, and in the case of the Baron, the use of flaps for short- and soft-field takeoffs was abolished. This magic speed was intended to deter pilots, instructors, and examiners from intentionally getting too close to V MC during training flights. But, like V MC, V SSE will change with conditions.
Takeoff and initial climb are, of course, the most critical stages of a flight for an engine failure, but what about other phases? If an engine quits in cruise flight, it poses little danger unless flying over terrain that is higher than the airplane's single-engine service ceiling. In general, you have plenty of altitude, airspeed, and time to deal with the emergency. Don't do anything rash, like feather the wrong propeller.
Likewise, don't do anything suddenly if an engine quits on a stabilized approach — especially an IFR approach. At approach power settings, the airplane would stall long before it reached V MC, and in the soup you don't want to get so distracted that you must perform a missed approach on one engine. Increase both throttles to maintain your approach speed; don't immediately worry about which engine quit. If heavily loaded and speed or altitude cannot be maintained, identify, verify, and feather the offending engine's propeller. It will probably be necessary to retract the flaps (and possibly the gear) to maintain proper approach profile and speed.
The best way to train today for emergencies in twins is to attend FlightSafety International or SimCom Training Centers; each has simulators utilizing actual cockpits for many popular light twins. There, you can try all of the things you never dreamt of doing in the real airplane — and walk away unscathed. It costs some money, but it is certainly less than the value of your life and better than abusing your airplane.
One of the most important things for multiengine pilots to remember is that a twin gives you a second chance, provided you know how to manage that chance. Don't rely on engine-failure statistics as insurance that an engine failure won't occur. The pilot who plans for an engine failure before every takeoff will not be nearly as surprised when the main event occurs.
To obtain a copy of the AOPA ASF booklet Flying Light Twin Engine Airplanes, send a self-addressed 7 x 10-inch envelope with 78 cents' postage to Flying Light Twins Pamphlet, c/o AOPA ASF, 421 Aviation Way, Frederick, Maryland 21701. For simulator training information, call FlightSafety International at 800/608-5620 or SimCom Training Centers at 800/272-0211.
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