Are you an aviation enthusiast looking to deepen your knowledge of aircraft performance and safety?
If so, you’ve probably come across the acronym SMACFUM. But what does it stand for, and how does it relate to aviation?
In this article, we’ll explore SMACFUM and its significance in the world of aviation.
What is SMACFUM?
In aviation, Vmc refers to the minimum controllable airspeed at which an aircraft can maintain directional control with the critical engine inoperative.
This speed is determined by a number of factors, which can be remembered using the acronym SMACFUM. Let’s break down each factor and how it affects the actual Vmc speed.
- S – Standard day at sea level
- M – Maximum power
- A – Aft CG (Center of Gravity)
- C – Critical engine windmilling
- F – Flaps up/gear up
- U – Up to 5° bank
- M – Most unfavorable weight
Let’s take a closer look at each parameter and its significance.
Standard day at sea level
The first factor is a standard day at sea level. As we know from density altitude and single-engine aircraft performance, engine performance decreases as density altitude increases.
This means that as altitude increases, the aircraft will require more rudder input to counteract the yaw from asymmetric thrust, resulting in an increase in airflow or airspeed to maintain directional control.
As a result, Vmc will decrease as density altitude increases, making a standard day at sea level the worst-case scenario where Vmc will be the highest.
Max power
The maximum power factor also affects Vmc speed. The more power there is in the operating engine, the more performance it will have, causing more yaw into the inoperative engine.
This requires more rudder input to counteract that yaw, which in turn requires more airflow or airspeed to have more effectiveness of the rudder.
With more power, both engine performance and Vmc speed increase, making max power the worst-case scenario and Vmc at its highest.
Aft CG
The third factor is an aft center of gravity (CG). When the CG is aft, the arm between the CG and the rudder is shorter, making the rudder less effective.
A higher airspeed is required to counteract the yaw into the inoperative engine, causing the Vmc speed to increase. Conversely, a forward CG would decrease Vmc.
Critical engine windmilling
The fourth factor is critical engine windmilling. When the critical engine is windmilling, it will create more drag than if it was feathered.
This will raise the Vmc speed. In turn, once the propeller is feathered, Vmc decreases.
Flaps up/gear up
The fifth factor is the position of the flaps and landing gear. When flaps are out, they help stabilize the aircraft, which helps reduce Vmc.
On the other hand, less stabilization occurs when flaps are up (in takeoff position), which increases Vmc.
Regarding the landing gear, lowering it creates the “keel effect,” which helps keep the aircraft straight.
The accelerated slipstream behind the engine at full power encounters the gear and creates excess drag, which helps counteract the turning tendency. Therefore, both flaps up and gear up will negatively affect Vmc speed.
Up to 5° bank
The sixth factor is up to a 5-degree bank. When one engine fails, the wings level with the ball centered actually create a mild side-slip because the failed engine causes drag and a loss of lift.
Turning to up to a 5-degree bank toward the operating engine decreases the drag, leading to a better climb performance and improved performance, as well as decreasing Vmc
Most unfavorable weight
The second factor is the most unfavorable weight. The heavier the airplane, the lower the aircraft’s Vmc, while the lighter the airplane, the higher the aircraft’s Vmc.
What is Vmc Certification Requirements 23.149?
Vmc certification requirements 23.149 refer to the minimum controllable airspeed with one engine inoperative, or Vmc, that an aircraft must be able to demonstrate during certification.
This requirement ensures that the aircraft can maintain directional control and remain airborne in the event of an engine failure during takeoff or climb.
How Does SMACFUM Apply to Vmc Certification Requirements?
SMACFUM is a critical checklist that pilots and aviation professionals use to ensure safe aircraft operations before takeoff. Each letter in the acronym represents a critical parameter that must be checked before a plane can safely take off, including Vmc certification requirements.
When it comes to Vmc certification requirements, SMACFUM can help ensure that the aircraft meets the necessary criteria for certification. For example, the “C” in SMACFUM stands for critical engine windmilling, which is directly related to Vmc.
If an engine fails during takeoff or climb, windmilling can cause significant drag and affect the aircraft’s ability to maintain directional control.
By ensuring that pilots are prepared to respond to critical engine windmilling, SMACFUM can help ensure that the aircraft remains stable and under control in the event of an engine failure.
Additionally, the “M” in SMACFUM stands for most unfavorable weight, which is also critical for Vmc certification requirements. The aircraft must be able to maintain directional control and remain airborne with the heaviest weight that it is certified to carry.
By ensuring that the aircraft’s weight is within a safe range, SMACFUM can help ensure that the aircraft meets Vmc certification requirements and is safe to operate.
Frequently Asked Questions
What is Vmc?
Vmc stands for “minimum control speed with the critical engine inoperative”. It’s the minimum airspeed at which directional control can be maintained when an engine fails on a multi-engine aircraft.
How is Vmc determined?
Aircraft manufacturers determine Vmc through a series of calculations based on various factors that affect the aircraft’s performance. These factors are often remembered by the acronym SMACFUM, which stands for Standard Day at Sea Level, Most Unfavorable Weight, Aft CG, Critical Engine Windmilling, Flaps Takeoff Position/Landing Gear Up, Up to 5 Degrees Bank, and Max Power in Operating Engine.
What factors affect Vmc?
The factors that affect Vmc include altitude, weight, center of gravity, engine windmilling, flap and landing gear positions, bank angle, and power settings. These factors can either increase or decrease the aircraft’s Vmc speed.
Why is Vmc important?
Vmc is an important safety consideration for multi-engine aircraft pilots, as it determines the minimum airspeed at which directional control can be maintained in the event of an engine failure. Knowing the Vmc speed for a particular aircraft and operating conditions can help pilots avoid dangerous situations and make informed decisions.
How can I calculate Vmc for my aircraft?
The specific Vmc speed for a particular aircraft can be found in the aircraft’s flight manual. It’s important to note that Vmc may vary depending on the aircraft’s weight, center of gravity, altitude, and other factors. Consult with a qualified instructor or aviation professional for guidance on calculating Vmc for your specific aircraft and operating conditions.
Conclusion
In conclusion, understanding Vmc speed and the factors that affect it is crucial for safe operation of single engine aircraft.
Aircraft manufacturers determine Vmc speed based on several factors including standard day at sea level, most unfavorable weight, aft CG, critical engine windmilling, flaps takeoff position/landing gear up, up to 5 degrees bank, and max power in operating engine.
Each of these factors has a unique impact on the Vmc speed and must be taken into consideration during flight planning and operations.
By understanding and accounting for these factors, pilots can ensure that they maintain directional control during an engine failure and safely fly their aircraft.
