VMC and Critical Engine made EASY - Multi-Engine Ground School

Introduction

This tutorial provides a comprehensive understanding of VMC, or Minimum Controllable Airspeed, and its significance in multi-engine aircraft. It aims to clarify how VMC relates to aircraft control, particularly when one engine fails, and the importance of understanding critical engines in ensuring safe flight.

Chapter 1: What is VMC

• Definition: VMC stands for Velocity Minimum Controllable, which is the minimum airspeed at which a multi-engine aircraft can maintain directional control with one engine inoperative.
• Importance: Knowing VMC helps pilots avoid situations where they may lose control of the aircraft due to insufficient speed.

Chapter 2: Determining VMC

• Aircraft Manufacturer's Role: Determining VMC is the responsibility of the aircraft manufacturer, who conducts tests to establish this critical airspeed.
• Testing Process: Test pilots perform maneuvers including shutting down engines and deploying parachutes to determine the safe VMC speed.
• Example Scenario: Consider two aircraft manufacturers claiming different VMC speeds. A lower VMC is preferable for safety, as it allows better control at lower speeds.

Chapter 3: CFR 14 25.149

• FAA Regulations: The FAA provides guidelines under FAR 25.149 to standardize how VMC is determined, ensuring manufacturers provide accurate and safe information.
• Combat Acronym: The acronym COMBATS is used to remember the criteria for determining VMC:
  • C: Critical engine
  • O: Operating engine with full power
  • M: Maximum gross weight
  • B: Bank of no more than five degrees into the operating engine
  • A: Aft center of gravity (CG) effects
  • T: Takeoff settings (gear up, flaps up)
  • S: Standard day conditions

Chapter 4: Understanding Critical Engines

• Definition: A critical engine is the engine whose failure most adversely affects the aircraft's flight characteristics.
• PAST Acronym: The acronym PAST helps identify factors contributing to critical engine status:
  • P: P-factor (thrust asymmetry)
  • A: Accelerated slipstream
  • S: Spiraling slipstream
  • T: Torque effects

Chapter 5: P-Factor

• Explanation: P-factor refers to the difference in thrust produced by the ascending and descending blades of a propeller.
• Effects: When one engine fails, the remaining engine produces more thrust on one side, causing the aircraft to yaw towards the inoperative engine.

Chapter 6: Accelerated Slipstream

• Understanding: As the aircraft flies, the operating engine accelerates the airflow over its wing, generating more lift, which can influence the aircraft's bank.
• Bank Correction: Pilots must correct for the bank induced by the slipstream using both aileron and rudder inputs.

Chapter 7: Spiraling Slipstream

• Impact on Control: The spiraling slipstream from the operating engine helps provide airflow over the rudder, aiding in maintaining control, especially when one engine is lost.

Chapter 8: Torque

• Torque Effects: When one engine fails, the torque from the operating engine can exacerbate the yawing motion, requiring proper rudder input to maintain control.
• Critical Engine Identification: Typically, in conventional twin-engine aircraft, the left engine is the critical engine due to these combined effects.

Conclusion

Understanding VMC and what constitutes a critical engine is crucial for safe multi-engine flying. Pilots must be aware that flying below VMC during critical phases can result in a loss of control. For further learning, consider obtaining a multi-engine rating or consult the FAR 25.149 regulation for detailed specifications. Safe flying depends on knowledge, preparation, and adherence to established guidelines.