- Defining Vmc
- The good engine is developing full power.
- The aircraft is at its most rearward Centre of Gravity.
- The bad engine is at idle and has not been feathered.
- The engine that has failed is the Critical Engine.
- The aircraft is at maximum takeoff weight.
- The gear has been retracted.
- The flaps are in the takeoff position.
- The aircraft is airborne without the benefit of ground effect.
- The Vmc Spin
In many respects, the pleasures of multi-engine flying revolve around two elements—speed and security. Speed is self-evident; conversely, it is reasonably safe, for example, to cross mountains at night in a twin, provided, for example, weather conditions are right, and options—and the associated planning—exist for a possible failure of one of the two engines. Ironically, however, the two engines that make you faster and safer also present a new danger—generally referred to as Vmc. In actuality, the dangers associated with Vmc are no different from the dangers of stall/spins which the single-engine pilot must live with; the single-engine pilot learns to live happily with stalls/spin risks because of knowledge, skill, and experience—these same things enable the multi-engine pilot to live happily with Vmc. Vmc is a big part of multi-engine training, so let us now dig into this concept.
Vmc is defined as the speed, established by the aircraft manufacturer in accordance with certification criteria, below which, should the aircraft be flying on one engine only, control of the aircraft by pilot cannot be maintained. With the Seneca, when an engine quits during flight, you will immediately lose about 80% of the effective thrust you normally had with two engines.1 Moreover, unless you immediately change and reconfigure the aircraft’s pitch attitude and drag predicament, the airspeed indicator will quickly migrate towards the red radial line that appears on the low end of the ASI—this red radial indicating sea-level Vmc. As the red line is approached—located at the 80-MPH mark on the Seneca’s ASI— the truly unthinkable happens—with full thrust developing from the still functioning engine, the aircraft will autorotate onto its back and spin into the ground with you and your passengers underneath a two-ton burning mess of avgas and splintered aluminium. This is, of course, very graphic, but it is critical for you to learn to apply all of your previously learned fear and precautions about spin avoidance at low altitude to the concept of Vmc (unique to multi-engine aircraft).
The most critical phase of multi-engine flight is those few seconds after takeoff—airspeed is low and the engines are developing their maximum rated horsepower. If the engine quits during or shortly after takeoff, you have essentially two choices. If the speed is below Vmc the aircraft cannot fly; if an attempt is made to proceed with the departure climb, the asymmetric thrust from the one good engine, with its fully developed power, will cause the aircraft to flip on its back. In contrast, if the speed is sufficiently fast, the aircraft could, theoretically at least, continue with the departure and climb on single engine. In and of itself, these choices are straightforward. The problem is, however, that these speeds are not contiguous—that is to say, as you accelerate during the takeoff, you do not pass directly from the “danger” speed range into the “safe” speed range. Instead, there is a grey transition area above the dangerous Vmc speeds, yet below the safe single-engine climb speed.2 It is in relation to this “no-man’s-land” speed range that we must develop a plan of action for every multi-engine takeoff and departure.
First, let us define this dangerous speed range. As we have stated, Vmc for the Piper Seneca is indicated on the ASI by a red radial line, and it appears at 80 MPH. We are also provided with a second radial line on the ASI—the “blue line”—that sits on the Seneca ASI at the 105-MPH position. The blue-line speed is the best single-engine rate of climb speed—Vyse. At Vyse we can safely predict normal single-engine climb performance—which, incidentally, is still marginal.3 On any takeoff, our goal is straightforward—get to Vyse (105 MPH) as quickly as possible and maintain this speed until the possible risks resulting from engine failure are reduced as the aircraft achieves more and more separation from terrain. In a normal two-engine takeoff, power settings should not be reduced4 from “maximum power”5 until all obstacles below the departure route can safely cleared—normally 400’ AAE6—unless special terrain features exist such as obstacles (requiring an extension of the Vyse climb). If we have the engine failure at blue line, and provided we have safe obstacle clearance, we have a very good chance indeed of still making it home for dinner that evening. Until we get to the blue line, however, there are no certainties. Here, then, is the dangerous speed range: between 80 MPH and 105 MPH. If we have an engine failure during takeoff and we are airborne, our job is to make the aircraft ASI needle moves toward the “blue end” of this speed range as fast as safely possible. If we cannot accomplish this task, we are left with only one option for survival—if redline penetration is imminent, the throttle of the good engine must be closed and we must deal with the engine failure as if it occurred on a single engine aeroplane. It is far better to land right-side-up, hopefully in a field, than it is to land pilot/passenger-side-down.
An engine failure between 80 MPH and 105 MPH therefore presents us with the probability of marginal success. At airports with long runways, the risks are reduced considerably—the aircraft will be normally established in a climb at 105 MPH by the time it crosses the departure end of the runway. At Langley Airport, however, the circumstances are quite different—we will not attain 105 MPH until we have crossed the road traffic at either end of the paved runway. We do have a plan of action for Langley departures, and this will be discussed fully later in the section on Standard Operating Procedures. The point here is that should an engine failure occur when the aircraft remains in the marginal speeds—just after rotation—the pilot must respond without the slightest delay—the climb must be immediately abandoned, the aircraft (with a failed engine) levelled, and the gear and flaps retracted so that the erosion of airspeed is stopped, and the failed engine properly stowed by means of “feathering” the propeller.7
All of this requires careful and disciplined planning for any multi-engine launch from Langley Airport, but there are other occasions when Vmc can bite. Consider the consequences of an engine failure during the takeoff roll—before the aircraft has left the ground. Here the aircraft will yaw uncontrollably off the runway, unless of course the power on the remaining good engine is immediately brought to idle.8 Consider also the effects of one engine failing to respond to the pilot’s “maximum power” inputs during an overshoot—where the speed just prior to the touchdown is well below Vmc—the result could be disastrous as the engine which does respond with maximum power promptly rolls the aircraft on its back just feet over the runway.9 Finally, what about stall recovery? Again, the airspeed is well below Vmc during a stall, and if maximum power is immediately applied during the recovery, and one of the engines has failed—or simply “hiccups”—the stall could quickly covert to a Vmc spin, with the same disastrous results.10
So, as you can see, the risk of encounters with Vmc requires that the pilot achieve a different mind-set, quite unlike that which has been associated with single-engine flight operations. The mind-set manifests in such matters as careful planning, specialized takeoff briefings, careful monitoring of power indications prior to and during the launch process, and simple awareness of the dangers associated with the Vmc demon. Let us now examine how Vmc is defined. What is the meaning of that red radial line which appears on the ASI?
When the manufacturer—Piper Aircraft Corporation—originally applied to the US FAA for certification of the Seneca, it did so under performance conditions established by the FAA for aircraft weighing less than 6,000 lbs.—aircraft typically not used in commuter or airline transport operations. “Light Twins,” including the Seneca, are not required to climb with an engine failed, nor are they required to maintain altitude.11 The Seneca POH advises us that this aircraft is able to maintain certain parameters pertaining to singe-engine performance—a single-engine service ceiling of 3650’ ASL, at maximum gross weight, or 5000’ ASL, with a gross weight of 4030 lbs.—so it would appear that we are, at least, not in the category of “climbless” single-engine performance aircraft.12 Nevertheless, all manufacturers of light twins have to demonstrate and publish an accurate Vmc speed in accordance with the FAA’s definition. Essentially, Vmc is defined as the speed below which the aircraft will go out of control when the following conditions exist:
The good engine is developing full power.
Initially you would think that “full power” is a good thing—after all, this is what will get us to our precious 3650’ if the engine quits at maximum gross weight. However, the power developed from one engine when the other has quit is, ironically, the very origin of the dangers of engine failures in a twin. The more power produced on the good engine, the more rudder will be required to stop the adverse yaw that results from asymmetric thrust forces. The thrust vector generated by the good engine is significantly offset from the aircraft’s longitudinal axis, and as a result creates a “turning” moment—or, more accurately, what really appears to the pilot as a yawing moment. As airspeed decreases, rudder authority—used primarily to counter the yaw—diminishes. It follows that a point will eventually be reached at which the yaw can no longer be controlled—the pilot has reached the travel limit of the corrective rudder pedal. Any slower and the aircraft will begin to roll (in the direction of the bad engine) and pitch-up (i.e., yaw produces roll which, in turn produces pitch changes). This is of course the classic definition of “autorotation” associated with spins, and it is truly no different in the case of Vmc. However, you must remember that it is power (or thrust) that causes the Vmc autorotation, and full power from the good engine is therefore the worst-case scenario for getting into Vmc trouble. For this reason, a reduction of power from full-power settings will have the attractive effect of reducing Vmc.
Importantly, the greatest power that can be generated by the “good” engine will occur at sea level, where the air is thickest, and the engine is at its most powerful. For this reason, certification of an aircraft requires that the manufacturer demonstrate Vmc at sea level conditions, and the result, of course, appears on your ASI. Conversely, the higher the altitude at which single-engine maximum power occurs, the lower will be the actual speed at which Vmc occurs. This is one of the few occasions at which high altitude for an engine is in fact a “good thing.” Further with regard to altitude, however, there is a height above sea level at which, should an engine fail, the aircraft will stall before the adverse behaviour of Vmc becomes apparent. Theoretically and practically, however, there is a down side to this—there is also a very tricky airspeed at which Vmc and stall behaviour occur simultaneously, and of course this is not a good thing.
The aircraft is at its most rearward Centre of Gravity.
You will remember from earlier training that a rearward Centre of Gravity places the pilot at the greatest disadvantage with respect to aircraft pitch stability (along the lateral axis)—the more rearward the Centre of Gravity, the less effective “negative lift” of the horizontal stabilizer, and the less effective pilot control. With an engine failure in a twin-engine aircraft, however, the most important feature is the longitudinal distance between the Centre of Gravity and the rudder. As a rule, the rudder is at its least effective state in dealing with the yaw (created by the asymmetric thrust of the good engine) when this distance is small—that is, a most rearward Centre of Gravity The shorter this distance, the less “leverage” the pilot can utilize in keeping the aircraft under control. Conversely, the more forward the Centre of Gravity the greater the rudder authority of the pilot, and the greater the likelihood of success in dealing with Vmc behaviour. Vmc is again determined with the “worse case”—in regard to Centre of Gravity location, the most rearward.
The bad engine is at idle and has not been feathered.
A unique feature of multi-engine, constant-speed propellers is the design feature that enable the pilot to “feather” a propeller—adjust the blade angle so that it is parallel to the airflow.13 The effects on decreasing drag when a propeller has been feathered are dramatic, as you will see during the course of your training. Continuing with “worse case” theme, however, Vmc is determined with the assumption that the pilot has failed to feather the bad engine, resulting in the maximum amount of drag being created. The rearward drag force of the un-feathered, failed engine—combined with the forward thrust force of the good engine—produce the maximum amount of yaw possible for the pilot. The quicker the pilot can get to feathering a bad engine, the greater the likelihood of success in managing a failed-engine situation. Similarly, when a bad engine has been successfully feathered, Vmc is theoretically less than that indicated on the ASI.
The engine that has failed is the Critical Engine.
The concept of a “critical engine” does not apply to the Seneca, as the propellers are counter-rotating—viewed from the pilot seat, that is, the right-engine propeller spins counter-clockwise, and the left engine, more conventional, spins clockwise. This is not the case, however, with other light twins, such as the Piper Twin Comanche or the Beechcraft Baron—with these and other twins, both propellers turn clockwise as viewed from the pilot. The concept of a critical engine is rooted in appearance of asymmetric thrust—or P-factor—as the angle of attack is increased. As the angle of attack is increased, the down-going side of the propeller disk (viewed by the pilot) produces far greater effective thrust than the up-going side. This is what causes the need for right rudder during a climb in a single-engine aircraft. With a multi-engine aircraft that lacks counter-rotating propellers, the asymmetric thrust will cause a starboard-shift in the effective thrust vectors generated by the two engines.14 The effective thrust of the two engines is no longer equally offset from the aircraft longitudinal axis; instead, the effective thrust from the right engine is located at a distance from the longitudinal axis that is greater than the effective thrust of the left engine. Now you have to ask which of the two engines would present the most adverse conditions for the pilot if the other engine fails. If you answer the right engine, you are correct—its thrust has the greatest leverage that must be countered with rudder. But the concept of “critical” engine does not end there, as the left engine is in fact the “critical” engine—if it fails, you are left to deal with the more adverse thrust of the right engine. In determining Vmc with an aircraft lacking counter-rotating propellers, the manufacturer must demonstrate the effects of a critical engine failure. If the non-critical engine is the one that in fact fails, Vmc will be less than what appears on the ASI.
The aircraft is at maximum takeoff weight.
With the aircraft at its heaviest, the requirement for the pilot to generate lift will be therefore at its highest. The higher the weight, the greater the lift required (angle of attack) for a given flight configuration; the greater the lift required, the slower the aircraft, and the closer you are to Vmc. When faced with an engine failure, the “worse case” would be an aircraft at maximum takeoff weight.
The gear has been retracted.
Extended gear has the effect of providing a stabilizing keel similar to a ship. With the gear retracted the aircraft is more susceptible to the uncontrollable yaw that occurs below Vmc.
The flaps are in the takeoff position.
Drag is increased with the use of flaps, and therefore flaps cause greater draw on the precious little remaining thrust from the good engine. With the Seneca, the flaps are not extended in the normal takeoff, but are extended in the performance takeoff variations for short and/or soft fields. Vmc determination is based on the worse-case scenario of a failed engine right after takeoff, so testing for certification is done in the normal takeoff configuration of the aircraft with respect to flap settings.
The aircraft is airborne without the benefit of ground effect.
Vmc would of course occur at a lower speed if the aircraft were under the influence of ground effect. For certification, Vmc is determined where there is no benefit of ground effect.
As you can imagine, these are possible conditions a pilot may be faced with, but remember, the conditions outlined above are the “worse possible” conditions. If we can feather the prop on the bad engine, if we can recover the flap, if the aircraft is lightly loaded with a forward Centre of Gravity, etc., we increase our chances of successfully managing an engine failure. Importantly, the conditions of Vmc certification outlined above tells us that the Vmc depiction on the ASI is really a theoretical product, subject to all sorts of variations that occur during actual flight. The conditions also provide us with guidance—but understanding Vmc we can come up with a workable plan of action for multi-engine takeoffs—the Takeoff Briefing.
The Vmc Spin
Just a quick final note on how Vmc will appear to the pilot. A Vmc encounter will appear as a spin. The autorotation, however, is entered very rapidly—quite different from the somewhat gentle and passive spin characteristics of a single-engine trainer such as the Cherokee. Instead the thrust of the working engine will conspire to tighten and flatten—and essentially stabilise in a deadly fashion—the autorotation movements.15 Like any spin, you will require considerable altitude for the recovery. The key to survival is to recognize the symptoms immediately and to cut power instantaneously. The further the aircraft approaches Vmc instability, the greater will be the force and speed with which the initial yawing movement of the autorotation will occur. Be sure you examine the spin recovery procedures prescribed for the Seneca (p. 3-19 of the POH, and transcribed on p. 69 )—they are conventional.
1 You would think that the loss of thrust would be only 50%, but a sizable portion of the remaining 50% thrust from the one good engine is consumed by the increased drag from the bad engine—particularly the windmilling propeller.
2 You will not be safe, for example, with an engine failure at Vmc + 1 MPH, yet in peril with an engine failure exactly at Vmc; instead, the concept of Vmc is purely theoretical, and in practice appears in gradations based on many factors with affect a Vmc occurrence
3 The performance obtained is based on the assumption that the propeller of the failed engine is feathered, and Piper produces a performance graph (see p. 9-8 of the POH) that is designed to predict singe-engine climb performance; in practical terms, with the best of conditions—sea level density altitude, light weight, etc.—the performance is comparable to the performance of a very heavily loaded Piper Cherokee. So you can imagine what performance will be in less favourable conditions.
4 As Don Nikel of Valley Aero Engines always says, if an engine failure is to occur, it will likely happen when a power setting is changed.
5 Full throttle, propellers set maximum RPM, and mixtures full rich.
6 Above aerodrome elevation.
7 Feathering—whereby the propeller blades are turned parallel to the relative wind—creates a substantial reduction in drag.
8 For this reason, the right hand of the pilot during a multi-engine takeoff must never leave the throttles—the pilot must be always “ready on the draw” to instantly pull back both throttle levers—this exercise is described on p. 53 .
9 For this reason, as will be discussed later, any power increases are made slowly and smoothly by the pilot, and where maximum power is required—as would be the case, for example, in an overshoot, or a touch-and-go, the power is increased in stages—first ½ -power, and then full power.
10 In stall recovery, as well, the power used to recover is advanced carefully in stages.
11 This is in contrast with aircraft certified under US FAR Parts 23 (Commuter) and 25 (Airline), the manufacturer of which must demonstrate the ability of the aircraft to continue with a takeoff should an engine fail during the takeoff roll after accelerating through a specified speed referred to as “decision speed.” (This is examined in greater detail on p. 35 .)
12 The Piper Apache—used commonly for multi-engine training in the 1960s and early 1970s—is reported to have been notorious for its lack of single-engine performance with its 150-horsepower engines. Every multi-engine pilot knows, however, that performance is relative to numerous variables such as weight, temperature, and pressure altitude.
13 Feathering control is not found in single-engine aircraft equipped with constant-speed propellers.
14 The reverse would be the case with a twin-engined aircraft manufactured in Europe, for example, where the engines rotate counter-clockwise (as viewed from the pilot seat).
15 To recover from a stabilized spin the pilot must counter the stabilizing forces—the counter measures that can be exerted by the pilot during a spin recovery procedure are extremely limited owing to a lack of aerodynamic pressure over effective control surfaces. Also remember that while normal spin recovery is demonstrated by the aircraft manufacturer, this is not the case with Vmc-induced autorotation.