#8b Vmc Demonstration
To examine the flight characteristics of the aircraft while approaching Vmc.
This exercise is not a flight test item, but is conducted during the course of flight training for the rating.
There is an interesting feature of Vmc—as you increase altitude, the indicated speed at which Vmc occurs will actually decrease. This is primarily the result of a loss of propeller thrust as a result of less dense air, as well as the reduction of engine power as the pressure in the intake manifold decreases with increased altitude. Even more interesting is the fact that the indicated airspeed at which an aircraft will stall remains relatively constant, despite increased altitude. It will follow, then, that there is an altitude at which both Vmc and Vso converge—a danger zone if you will. Theoretically, at sufficiently high altitude, a multi-engine aircraft with full asymmetric thrust—one engine developing maximum power, and the other idle—will stall before the Vmc conditions manifest, while at a somewhat lower altitude, the same aircraft will demonstrate Vmc behaviour before stall symptoms appear. In the danger zone, both occur simultaneously, and of course that will spell trouble.1
For this exercise, the aircraft is slowed to approximately 90 MPH and one engine is brought back to idle, and maximum power is smoothly developed on the other. A prominent reference point is noted for the purposes of maintaining directional control, and then the aircraft is slowly pitched upward in a smooth and steady fashion. The simple task assigned to the student is to keep the aircraft straight, and from the start, rudder pressure is required to counter the imbalance in engine thrust. As the speed diminishes further, more and more rudder pressure is required—until, eventually, of course, maximum rudder pressure is exerted. The use of rudder, however, is not the only instrument the pilot may use to counter the undesired yaw—remember the use of ailerons to maintain the 5° bank into the good engine. In order to maintain this bank as the aircraft is slowed, more and more aileron deflection will be required to counter the roll (the aircraft will seek to roll in the direction of the idle engine) that is created by the undesired yaw. Accordingly, as the aircraft pitch is increased, and as more and more rudder is required, the pilot will have to input increasing amounts of aileron deflection. The point at which Vmc is reached is the point at which the aircraft continues to roll and yaw in the direction of the idle engine despite maximum counter deflection of rudder and aileron.
As you will see, there are two—and only two—solutions to overcoming the onset of the Vmc spin. The first is to pitch the aircraft forward (decreasing pitch), but this action is relatively ineffective, even though it should not be overlooked. The second solution is to decrease power on the good engine—in contrast, this is both effective and efficient. If the pilot responds to Vmc simply by pitching forward, the aircraft will not immediately correct itself, primarily because it must first accelerate to increase the aerodynamic pressures on the aileron and rudder surfaces—and there is quite a delay here. The second action—reducing power of the good engine—immediately reduces the thrust that is at the core of the yaw problem—while aerodynamic pressures on the control surfaces are not suddenly increased, far less counter-yaw inputs are required in proportion to the degree at which power is reduced. Since regaining effective control of the aircraft is the goal during Vmc avoidance, however, the power solution—by itself—is not enough—while it will arrest Vmc yaw, enhanced aircraft control will not occur until the angle of attack is decreased by pitching the aircraft forward. Both counter-Vmc measures will be examined in this exercise.
The main lesson to be extracted from this lesson—in addition to the counter-Vmc measures themselves—is the ability to detect the onset of Vmc behaviour in the aircraft. It is as simple as this: if you are flying with one engine inoperative, and full deflection of control services is required, you are likely on the verge of Vmc and its potentially disastrous consequences. In actuality, the warning signs are quite startling—the aircraft begins, by itself, to yaw and roll in the direction that is opposite to your control inputs. The key, of course, is the pilot response—immediately reduce the power on the good engine and decrease the pitch angle of the aircraft. If you are aware of the warning signs, the counter measures can be employed and trouble avoided.
Remember too that an encounter with Vmc will likely not be sudden—as you attempt to maintain altitude after an engine failure, for example, opposing rudder and aileron might be gradually but continuously increased over a period of time—say a few seconds, at least, but hopefully longer—as the aircraft looses airspeed. If the brain is fully engaged, there is time to recognise the condition and respond effectively.2
Finally, you should be aware that there are few times (perhaps only once) during a pilot’s career that Vmc will actually be approached and experienced during flight. In actuality, only a few minutes will be spent snuggling up to Vmc, so be sure to pay attention and learn the ways of the demon.
Be sure the HASEL is conducted thoroughly and efficiently prior to conducting this exercise.
The minimum entry altitude for this exercise is 4000’ AGL; this will provide sufficient altitude for Vmc spin recovery should one inadvertently be entered.
Also as a precautionary measure against inadvertent spin, be sure the cabin is secure and the seatbelts of the crew are properly secured.
- Retard both throttles to the idle position.
- Apply full rudder in the direction opposite the spin rotation.
- Let up all backpressure on the control wheel. If nose does not drop immediately push control wheel full forward.
- Keep ailerons neutral
- Maintain the controls in these positions until spin stops, then neutralize rudder.
- Recover from the resulting dive with smooth backpressure on the control wheel. No abrupt control movement should be used during recovery from the dive, as the positive limit manoeuvring load factor may be exceeded.
1 This exercise will be conducted at approximately 4000’ and you will see that in fact Vmc and Vso are very close. It is not uncommon to have the initial symptoms of stall—onset appear just as maximum counter-Vmc measures are taken. Very likely, with only a slightly higher altitude, Vmc and Vso will occur simultaneously. The initial onset of stall characteristic is something to look for when this exercise is flown.
2 Just remember that full deflection of a control surface after an engine failure means the dangers of Vmc are lurking.