Photo credit: (c) Jose L. Ghezzi
Multi-engine aircraft pilots must undergo training to maintain their proficiency. Insurance policies typically mandate recurrent training every year or two. And ideally, we should train more frequently and to the level prescribed in the Aircraft Certification Standards (ACS).

However, training in multi-engine aircraft presents unique risks, particularly with piston twins, and not all instructors and trainees are prepared to effectively manage these risks.

During routine non-training flights, we meticulously plan for the scenarios associated with an engine failure. For example, we don’t rotate before V_R , we only use runways longer than our accelerate-stop distance, we don’t depart if our VYSE climb gradient won't get us on top of obstacles & terrain with one engine inoperative, and we don’t attempt single-engine go-arounds below 500 feet.

But the possibility of an actual engine failure also looms during training maneuvers such as slow flight and stalls. Perhaps this is unlikely, but a failed engine at the wrong moment could result in a most perilous and catastrophic outcome – the spin. In a failed engine scenario at speeds below V_MC but above V_S, the aircraft will begin to rollover and yaw towards the inoperative engine. That is recoverable with early intervention and sufficient altitude. However, if the aircraft reaches V_S before reaching V_MC, the rotating moment caused by the operative engine combined with the stall will result in a spin.

When a multi-engine aircraft is spinning, the angular momentum caused by the weight of engines and fuel displaced from the vertical axis will almost certainly overcome full opposite rudder deflection. Traditional recovery procedures like PARE (Power, Ailerons, Rudder, Elevator) are not guaranteed to be effective. Applying power in a spin will flatten it. If you spin your twin, consider yourself a test pilot. And those guys use parachutes. For these reasons, the FAA states in AC-61-67C paragraph 200 that "because of the possible catastrophic consequences, single-engine stalls should not be demonstrated or practiced in multi-engine airplanes." The FAA provides us with a more developed discussion of multi-engine stalls and spins in Chapter 13 of the Airplane Flying Handbook.

Imagine you are performing an approach-to-landing power-off stall. During the recovery, you apply forward pressure to restore the angle of attack and to build airspeed, and you rapidly advance both throttles. Perhaps you instinctively apply right rudder anticipating P-factor and torque. But your right engine falters while your left engine develops full power, and suddenly you have a wild yaw and roll to the right. Or imagine you are performing an accelerated stall with a 45˚ bank to the left and your left engine fails. Either of these scenarios or others like them could happen during a training flight, and suddenly you are at the precipice of a spin.

Have you and your instructor planned for such scenarios?

Preventing Spins

There are specific precautions instructors and trainees can take to remain safe while conducting these types of training maneuvers. Every training flight should begin with a safety briefing that covers (a) these kinds of scenarios, (b) the precautionary measures for avoidance of loss of control in flight, and (c) a pre-determined plan to recover from an upset. The briefing should address the instructor’s readiness to immediately pull both mixture levers any time asymmetric thrust or unexpected yaw is detected during a training maneuver being conducted near V_S and V_MC.

By immediately pulling mixtures, asymmetric thrust is immediately eliminated, and an orderly upset recovery can be performed from there.

The briefing should also identify who is responsible for performing which tasks during the upset recovery. Is the instructor PIC or is it the trainee, and what will be the supporting role of the non-flying pilot? For example, the plan might be that once mixture controls are pulled, the instructor calls for control of the aircraft, unloads the G’s, levels the wings, and recovers from the dive. At the same time, the trainee releases control and begins calling out airspeed and altitude.

The trainee has an additional responsibility to ensure that the instructor’s left hand is always resting on top of both mixture levers during:

• Take-off roll
• Stalls (power on, power off, and accelerated)
• Slow flight
• Steep turns
• V_MC demonstration

If the trainee sees otherwise, he or she should respectfully tell the instructor to get that left hand on the mixtures and be prepared to pull them if one engine becomes inoperative. At that point, it might also be a good idea to recite what happens next during an upset recovery.

Preventing Stalls During the V_MC Demonstration

The V_MC demonstration is controversial because it brings us very close to the operating limits of our aircraft, and there have been numerous fatal training accidents related to this maneuver. There is legitimate debate about its utility, but for now it’s in the ACS and we need to be proficient in its execution.

We never want to stall before reaching V_MC. The published V_MC will always be higher than the published stall speed in take-off configuration. So what's the worry? Remember that V_MC is not a fixed speed. It will increase or decrease considerably depending on aircraft configuration and atmospheric conditions. For example, in non-turbocharged aircraft, V_MC decreases with density altitude as the operative engine loses effectiveness, yet V_S remains relatively constant.

At some point, the two speeds cross over setting the conditions for a catastrophic loss of control inflight. On a hot summer day, you may not be able to conduct the V_MC demonstration at a safe altitude without risk of a stall.

In turbocharged aircraft, an operative engine that is not developing full power can also bring you to a stall before V_MC. For example, this could happen due to a leaking wastegate, a problematic controller, or even by not having the operative engine throttle fully forward.

Other factors that could lower V_MC include inadvertent gear extension while configuring for the maneuver, or excessive banking into the operative engine. Any of these factors alone or in combination could substantially lower V_MC dangerously close to or below V_S. Take look at the V_MC Table to review the factors that raise or lower V_MC.

Again, have you and your instructor planned for such scenarios?

The FAA states that instructor should manipulate V_MC higher with a technique called “blocking the rudder”. The instructor applies opposite rudder pressure to prevent the trainee from reaching full rudder travel. This effectively raises V_MC and provides more separation from V_S. The technique has no bearing on the what the trainee sees at V_MC, it just happens with a wider margin from the stall. Another technique is to eliminate or reduce the bank angle during the V_MC demonstration which also raises V_MC and provides more separation from V_S.

In conclusion, multi-engine training maneuvers do have risks. But they are manageable when both the instructor and the trainee have discussed these risks in thorough pre-flight safety briefings, have agreed to use techniques keeping the aircraft at a wider margin of safety from these risks, and having an agreed upon plan in the event any of these risks become a reality.