Beyond "dead foot, dead engine"
From our first days as student pilots of small single-engine trainers we yearn to fly twins. The allure of multiengine flying is compelling, what with the promise of extra power, more speed, better climb rates, systems redundancies, and — well, let's face it, ramp and sex appeal. But while twin-engine flying definitely has its advantages, it has a dark side, too. It demands extra care on the pilot's part, and learning a whole new set of planning and procedural skills. Failure to understand and practice them can be fatal.
When two become one
At the crux of multiengine flying issues and dilemmas is a great big what-if: Namely, what if an engine fails or stops running? And what if that engine quits at the most critical time — during the takeoff phase, right after liftoff?
It's at times like these that you quickly discover how multiengine flying's "advantages" can turn into deadly traps. A piston twin with one operative engine doesn't lose half its climb capability — it loses about 80 percent! Where once you climbed out at, say, 1,200 fpm, now you stagger into the air on one engine at a meager 200-fpm climb rate. This implies obvious problems should compliance with a departure procedure's climb gradient restrictions be a factor. In other words, you may not be able to outclimb any nearby high terrain or obstacles.
Airspeed can easily decay under this reduced-power condition, too. Multiengine airplanes weigh more than singles, and the extra mass means that any loss of thrust translates into sagging airspeed (and the severity of any impact will be greater). With a windmilling propeller, there's plenty of extra drag to compound the airspeed problem.
Last — but certainly not least — are the control problems that derive from airspeed losses in engine-out (many times called one engine inoperative) situations. Basically, this boils down to an asymmetric-thrust situation: If one engine is producing thrust and the other isn't, then you've got an airplane that wants to yaw and roll in the direction of the "dead" engine. To stop that yaw, you've got to apply an appropriate amount of rudder force to the pedal that corresponds to the "good" engine. Thus the origin of the expressions "dead foot, dead engine" and "working foot, working engine" as memory aids in identifying a failed engine.
V MC — don't go there
Here's a key concept: Rudder effectiveness depends on airspeed. As long as your airspeed is high enough, rudder pressure will stop any yawing or rolling when an engine quits. This brings us to a discussion of a critical airspeed — V MC.
V MC is a V-speed that's defined as the minimum control airspeed with the critical engine inoperative, or in airplanes with counterrotating propellers, one engine inoperative (OEI). If you maintain an airspeed above V MC, you should theoretically have enough rudder power to counteract any engine-out yawing and rolling.
In twins with propellers that both rotate in the same direction (most often clockwise as viewed from the pilot's seat) the critical engine is the left engine. Why? Because it's the descending blades that produce the most thrust, so the biggest thrust vectors are at the right edges of the rotating propeller disks, as viewed from above.
The left engine's maximum thrust zone is closer to the airplane's vertical axis. The right engine's, however, is far out on the right wing, well away from the vertical axis and able to produce a much greater moment. In this scenario the left engine is the critical engine because its failure will result in the right engine's generating a higher yawing moment. Much higher should the right engine have failed. This, in a nutshell, is why the left engine is called the critical engine.
More recent multiengine designs use counterrotating propellers. In this scheme, the left propeller turns clockwise, the right counterclockwise. You'd still have plenty of asymmetric thrust if an engine were to lose power, but the critical-engine problem is eliminated because both propellers have their maximum thrust sectors closer to the centerline of the fuselage.
Centerline-thrust airplanes — the Cessna 336 and 337 Skymasters come to mind — are free of V MC problems because these airplanes' thrust lines are closely aligned with the longitudinal axis of the airplane. Therefore, there's no asymmetric thrust. But those who earn their multiengine ratings in centerline-thrust airplanes have their pilot certificates endorsed for centerline-thrust airplanes only. And there are still the poor climb rate, propeller drag, and failed-engine identification problems to deal with.
V MC, by the book
By regulation, manufacturers must determine V MC by duplicating a certain configuration during flight tests. This configuration includes:
- A windmilling — or unfeathered — propeller on the dead engine. In airplanes with clockwise-rotating propellers, this engine is the left, or critical, engine.
- Maximum continuous power on the operating engine.
- Landing gear extended.
- Flaps at the takeoff setting.
- Maximum gross weight.
- Most aft center of gravity.
- Establishment of zero sideslip toward the operating engine. (For minimal drag and best single-engine performance the airplane should be slipped enough to keep the rudder ball between one-half and three-quarters of a ball-width out of the center position, toward the operating engine. In addition, you should set up a 5-degree bank into the operating engine — "raise the dead," as some instructors say.)
Test pilots slow the airplane in this configuration until they note the airspeed where an uncommanded heading change occurs. This is the sign that directional control is being lost. Any further loss of airspeed will result in an uncontrollable yaw, a roll in the direction of the dead engine, and probably a rollover. There's only one way to stop the yaw: Close the throttle on the good engine and lower the nose to regain the airspeed you need to restore rudder effectiveness.
Students working on their multi-engine ratings have to do essentially the same drill, in effect making them test pilots. This has its dangers, and there have been fatal accidents where a practice V MC demonstration turned into the real thing — especially at low altitudes. Concerns over the safety of this training maneuver led to the establishment of another airspeed — V SSE. This stands for safe single-engine airspeed, and it gives you a margin of safety. Usually it's five or so knots higher than V MC, so a student who holds V SSE can be reasonably assured that these must-know single-engine drills won't develop into departures from controlled flight.
Real-world V MC
Too often, pilots get the idea that V MC is a static number, one that never varies. Wrong. There are a number of variables that can make an airplane's V MC higher or lower than the number appearing on the airspeed indicator (it's marked by a red radial line). To prevent inadvertent forays into an airplane's real-world V MC, it's important for pilots to understand these variables. Here are a few examples of conditions that produce a V MC that's higher than published in the pilot operating handbook:
- Low gross weights, because the yawing moment is stronger when there is less mass to propel.
- Landing gear retracted. When the gear are extended they act like miniature rudders and help stabilize the airplane. When retracted, asymmetric thrust effects are increased.
- At higher angles of attack, because of a higher drag profile. This means a swifter loss of airspeed at the moment of engine failure or stoppage.
- In a sideslip. Twenty years ago, instructors taught their students to center the inclinometer's rudder ball as part of configuring a twin for best single-engine flight. No more. Rudder deflection and frontal drag are at a maximum when holding a wings-level attitude with the ball centered. In this configuration V MC can rise by as much as 15 knots over the published V MC, and the chance of a single-engine climb can be nullified altogether.
- Out of ground effect, where drag is higher.
- Low density altitude. Engines produce more thrust in denser air, so asymmetric thrust is greater when an engine loses power. Conversely, high density altitudes lower V MC because engines and propellers don't produce as much thrust in thinner air.
Here's something else to bear in mind when practicing V MC demonstrations. At altitude, V MC decreases because of thinner air and a consequent thrust reduction. But the airplane's stall speed remains the same. At some given altitude a twin's V MC and stall speed converges. A practice V MC demonstration at too high an altitude can turn into a disaster because the aircraft may begin its uncontrollable yawing at the same moment it stalls. To avoid this, many instructors limit their V MC work to density altitudes below 4,000 feet or so.
So, how often do standard conditions exist? How quickly can you identify, verify, and feather a sick engine, or establish zero sideslip? How often will your airplane be at its most aft center of gravity? The answers are most likely "never, slowly, and not very often." This gives you an idea of how critical it is to stay away from V MC, recognize its onset, and plan your takeoffs so as to prevent V MC problems and maximize your airplane's climb performance.
Takeoff decisions
Bearing all this in mind, how do you perform a takeoff so as to minimize exposure to V MC and maximize your ability to climb once you're committed to liftoff? More terms come to the fore.
Pilots should, first of all, perform all the necessary preflight calculations of the airspeeds and runway requirements. Many times you'll hear the term V 1, or takeoff decision speed, when referring to takeoff benchmarks. This is the maximum speed in the takeoff at which the pilot must take the first action (i.e., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. This distance is the distance required to accelerate to V 1, then stop the airplane on the remaining runway. Accelerate-stop charts in the performance sections of many POHs let you calculate if you'll have enough runway to stop, based on atmospheric conditions and the airplane's weight.
V 1 has another definition: It's also the minimum distance in the takeoff, following an engine failure, at which the pilot can continue the takeoff and achieve enough altitude to clear the FAA-standard 50-foot obstacle, or comply with any other required altitudes in a takeoff profile. The distance needed to commence the takeoff run, reach V 1, have an engine failure, climb out on one engine, and comply with climb requirements is called the accelerate-go distance.
Many multiengine piston airplanes don't publish accelerate-stop or -go charts. You're more likely to have this planning information for multiengine turboprops and jets. Even so, it's worth emphasizing how much runway can be consumed before liftoff, and understand that you may go off the end of the runway if you experience an engine problem in the takeoff run, know that you won't have enough of a climb rate to clear any obstacles, and wisely close the throttles and brake for all you're worth. It's much better to hit fences and trees under control at 60 knots than to lose control in the air at 50 feet agl and crash.
Takeoff procedures
In multiengine piston airplanes the safest takeoff procedures usually (check your airplane's POH for the official word) include:
- Calculating that you have enough runway to safely take off and clear obstacles, whether you lose an engine at a critical time or not.
- Accelerating to V MC plus five knots.
- Lifting off.
- Retracting the landing gear once a climb has been established.
- Accelerating to V YSE (best single engine rate of climb speed) or V XSE (best single engine angle of climb) and maintaining these speeds should an engine quit after liftoff.
- Accelerating to an en route climb speed once terrain or obstacles are no longer factors.
What if an engine quits?.
On the takeoff run, before V MC-plus-five or V 1? The answer's easy: Close the throttles and brake, maintaining directional control on the runway.
Right at V MC or V 1? In piston-powered airplanes it's probably best to follow the advice just given, because your single-engine climb rate and airspeed deterioration may be such that it's better to go off the end of the runway and take your licks under control than it is to attempt a sickly climb. In multiengine turboprops and jets, it's usually best to continue the takeoff, because single-engine climb rates are significantly better.
Just after liftoff, with obstacles or terrain dead ahead? Because of their anemic single-engine climb rates, it may be best for pilots of piston-powered airplanes to close the throttles, maintain a safe airspeed, and perform a forced landing straight ahead. In other words, follow the same procedures you'd use if you were flying a single-engine airplane. In higher-performance twins, maintaining V YSE or V XSE will probably ensure a safe climbout.
Bottom line
Multiengine airplanes can offer very valuable safety benefits. We always hear about takeoff dangers and accidents, but you never hear about the many times that twins lose an engine and flights continue safely — because no accident happened!
That said, it's vital to practice single-engine climbs and engine-out procedures for those times when an engine conks out at the most dangerous phase of flight — climbing out just after takeoff.
It's also worth remembering that many engine-failure accident scenarios follow a different script. Many times the first event in the accident sequence happens with an engine failure in the en route phase of flight. The pilot safely manages the engine problem, only to crash when he botches a single-engine landing, when excessive floating (there's less drag with a feathered propeller, and the airplane is slipperier) can lead to runway overshoots. Or a decision to go around is made too late, airspeed is allowed to dissipate, and/or there's insufficient power to climb away safely.
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Safety Strategies
Check your pilot's operating handbook for your airplane's particulars, but here's the standard drill for coping with an engine-out situation. In cases where an engine is producing partial power, it may be best if you climb to a safe altitude before going through these steps — and an engine shutdown may not be advisable at all.
Recognition of a problem. This can take several crucial seconds as you come to realize that you've lost an engine. Typically, you'll "swim in glue" while you come to terms with the realization that you do, indeed, have an emergency. Test pilots are spring-loaded to recognize an engine-out condition, but average pilots take awhile longer. This is why it's so important to practice engine-out procedures.
Maintain no less than V YSE or V XSE. If the engine problem occurred on takeoff, then you must adhere to V YSEor V XSE if you want the best climb performance.
All engine levers forward. This means throttle, propeller, and mixture controls. This is a quick way to ensure you develop maximum power for any upcoming single-engine climbs. In turbocharged or turbine-powered airplanes you must make sure you don't exceed any manifold pressure, torque, temperature, or fan speed limitations.
Identify. Identify the sick engine by the dead-foot, dead-engine method, or simply step on the rudder offering the most resistance. Back up your identification — if there's time — by consulting the cylinder head temperature or other engine temperature readings, propeller rpm, or manifold pressure gauges. The sick engine will be running colder, may be turning slower, and may be losing manifold pressure.
Verify. This is extremely important. You must verify that you've truly identified the sick engine. You don't want to feather the wrong engine, correct? To verify the bad engine, pull back on the suspect engine's throttle. If there's no change in control pressures, then you've just identified the culprit. Turboprops with autofeather systems (no-go items in most airplanes equipped with them) take the guesswork out of this and the next step by automatically feathering the propeller of the dead engine.
Feather. Grab the bad engine's propeller lever and pull it back to the Feather detent. This will let the propeller blades turn about 90 degrees and streamline them with the relative wind. They'll stop turning and reduce drag significantly. Reaction time can be critical in some airplanes, so check your POH for feathering details. It's not uncommon to have propellers that must be feathered before rpm drops below a certain value, say, 800 or 1,000 rpm. Let the rpm drop below that, and propellers that use oil pressure and counterweights for feathering may not have enough oil pressure to feather the propeller blades all the way.
Retract landing gear.
Retract flaps. Especially if necessary for improved climb performance.
Establish zero-sideslip configuration.
Climb, if necessary.
Complete engine-shutdown checklist.
Prepare. A single-engine landing at an airport with a suitably long runway may be necessary. — TAH
Common accident scenarios: Multiengine mayhem
Though much has been made of the dangers of engine failure after takeoff,
multiengine accident statistics paint a more wide-ranging list of accident causes. These include the traditional killers — continued VFR flight into instrument weather, fuel starvation, and low-altitude stalls, to name a few — but it's still instructive to review the most common types of engine-out accident setups:
- Failing to maintain V YSE or V XSE after experiencing an engine failure or power loss immediately after takeoff. The temptation is to raise the nose in an attempt to improve climb performance. Instead, airspeed bleeds off and you can enter the V MC flight regime.
- Failing to feather the ailing engine's propeller or follow engine-out procedures. A windmilling propeller is a huge source of drag, and ruins both cruise and climb performance. Also, V MC increases with a windmilling propeller. Similarly, a piston twin with an engine out and gear extended probably won't have any climb performance at all.
- Misidentifying the problem engine. This is often the result of rushing to feather and secure a bad engine. In the press, stress, and haste of the moment it's all too easy to grab the wrong levers. Moral: Take the time to properly and methodically identify and verify a problem engine — but don't waste too much time if you're close to the ground and have poor single-engine performance.
- Freezing at the controls. In the face of the confusing and overwhelming sensations that accompany an engine-out situation, some pilots lock up and do nothing. Practicing engine-out procedures is the best antidote for this.
- Botching the single-engine landing. With a feathered propeller, the airplane will be slipperier and thus more apt to float and consume longer runway than normal. Moral: Pick a runway long and wide enough for what could be a lengthy landing roll.
- Excessive bank in the pattern. Stall speed increases as bank angle increases, so fly your single-engine patterns wide, at a safe airspeed (no less than "blue line," or V YSE), and with conservative bank angles.
- Getting too low on final. If anything, stay a little high on the glidepath as you fly the final approach course. You don't want to sink so low that you'll have to add power on the good engine and risk the chance of your airspeed's dipping below V YSE.
- Delaying a go-around decision. If you must make a single-engine go-around, then make the decision early on final. Piston-powered multiengine airplanes have single-engine climb rates as low as 200 fpm — and that's on a good day. This kind of performance may not be enough for you to transition to a climb configuration (retracting gear and flaps) without sinking toward terra firma or obstacles. Depending on density altitude and other adverse factors, you may not be able to climb at all, so make your single-engine approaches as precise as possible. — TAH
