Vref, or Reference Speed, is a fundamental measurement used in aviation to ensure the safety of the final phase of flight. It represents a standardized target speed that the aircraft should attain as it crosses the runway threshold. This speed is designed to provide a necessary buffer for the aircraft during its most demanding maneuvers. The calculation of this speed is based on the aircraft’s performance characteristics and is a prerequisite for a stable landing.
Defining Vref and Its Purpose
Vref is defined as the minimum safe speed required for a specific aircraft configuration during the final stages of a landing approach. The speed is determined assuming the aircraft is in its full landing configuration, including the deployment of landing gear and maximum permissible flaps. It establishes a baseline speed intended to prevent the aircraft from inadvertently stalling during the landing flare or in the event of unexpected wind changes. The primary purpose of Vref is to provide a safety margin above the airplane’s stalling speed in the landing configuration ($V_{S0}$ or $V_{S}$). This margin accounts for factors such as wind gusts, turbulence, and pilot reaction time, ensuring the aircraft maintains sufficient aerodynamic lift and control authority.
The Calculation and Regulatory Basis of Vref
The technical derivation of Vref is directly linked to the aircraft’s stall speed, providing a quantifiable basis for flight safety. Regulatory bodies, such as the Federal Aviation Administration (FAA) and the European Union Aviation Safety Agency (EASA), mandate the calculation formula. For transport category aircraft, Vref is legally required to be no less than 1.23 times the stalling speed in the landing configuration ($V_{SR0}$), or 1.3 times the stalling speed ($V_{S0}$) for many other aircraft types. This factor creates a 30% margin above the speed at which the aircraft would lose lift and enter an aerodynamic stall.
This calculation is stipulated within regulations such as Federal Aviation Regulation (FAR) Part 25, section 25.125, which details landing performance requirements. The regulation requires that a stabilized approach, with an airspeed no less than Vref, must be maintained down to the 50-foot height above the runway. This margin guarantees sufficient energy and control responsiveness for a safe landing. Standardizing this calculation ensures that all certified transport aircraft possess a predictable and safe speed margin over a stall during the most demanding phase of flight.
How Pilots Use Vref During Approach and Landing
Pilots utilize Vref as the foundational speed to establish a stabilized approach, where the aircraft’s flight path, speed, and configuration are constant. The Vref value, calculated pre-flight, becomes the minimum speed the aircraft is allowed to fly during the final segment of the approach. A stabilized approach requires the aircraft to maintain a calibrated airspeed not less than Vref as it descends toward the runway threshold, typically crossing the 50-foot mark at this speed.
This reference speed helps pilots establish the correct pitch and power settings necessary for the required descent rate. The crew continuously monitors the airspeed to ensure it remains at or above Vref, which is the intended touchdown speed after the flare maneuver. Using Vref as the threshold crossing speed ensures the aircraft has sufficient energy to maneuver and flare while minimizing the required landing distance.
Factors That Modify the Vref Speed
Vref is a baseline speed derived from regulatory standards, but pilots must perform operational adjustments to ensure safety in real-time conditions. The Vref value is dependent on the aircraft’s weight, meaning Vref decreases as fuel is consumed and the aircraft becomes lighter. Pilots must recalculate Vref based on the actual landing weight to maintain the appropriate safety margin.
The calculated Vref is also adjusted to create the final target approach speed, often termed $V_{app}$ or $V_{fly}$, to account for environmental factors. Adjustments are necessary for several conditions:
- Wind correction, which often involves adding half of the steady headwind component plus the full gust factor to Vref, up to a maximum addition (typically 15 to 20 knots).
- Airfoil contamination by icing or snow, requiring Vref to be increased significantly to compensate for reduced wing efficiency and increased stall speed.
- Reduced flap settings due to malfunction, necessitating a higher Vref because the aircraft’s stall speed will be higher in that configuration.
The Relationship Between Vref and Other Airspeeds
Vref functions as a core reference point, establishing a link between the aircraft’s stall characteristics and its operational approach speed. Its relationship with $V_{S0}$, the stall speed in the landing configuration, is foundational, as Vref is mathematically derived by adding a safety margin to $V_{S0}$. This means Vref always sits above the minimum controllable speed in the landing configuration, providing the pilot with a buffer against an aerodynamic stall.
The speed maintained during the final approach is often $V_{app}$ (Approach Speed), which is Vref plus operational corrections determined by the flight crew. For example, if Vref is 135 knots and the wind correction is 10 knots, the pilot flies the approach at 145 knots ($V_{app}$). $V_{app}$ is maintained until the flare maneuver, while Vref remains the target speed for crossing the runway threshold.
Risks of Incorrect Vref Management
Failing to manage Vref correctly introduces hazards during the landing phase, as flying too far above or below the calculated speed compromises safety. Flying the approach too slowly, below Vref, dramatically reduces the safety margin above the stall speed, increasing the risk of an aerodynamic stall and loss of control. A slow approach also reduces the aircraft’s control authority, making it difficult for the pilot to maneuver and execute the flare successfully.
Conversely, flying too fast, significantly above Vref, is hazardous, primarily by increasing the required landing distance. Excess speed causes the aircraft to generate more lift, which can lead to “floating” in the ground effect, delaying the touchdown point and consuming excess runway. Flying 10 knots too fast can increase the required landing distance by a considerable margin, potentially leading to a runway overrun, especially on shorter or wet runways. A high-speed touchdown also increases wear on tires and brakes and can lead to a hard landing due to reduced time for the flare maneuver.