Takeoff decision speed (V1) defines a pilot’s go/no-go decision during takeoff, and it is a critical part of any discussion on takeoff safety. When compared to its more sedentary cousins—rotation speed (Vr) and takeoff safety speed (V2)—V1 is a bit fickle. Depending on conditions and other factors, its value can move up or down.
As a hypothetical example, at a given takeoff weight, configuration, runway condition (slope and wet or dry), and environment, an ATC-directed runway change to a shorter runway can result in a lower V1. Conversely, a longer runway using the same criteria may yield a higher V1 that may reduce the minimum runway required for takeoff. Likewise, a dry or wet runway condition will move the value of V1 to the right or left, respectively. But why?
For discussion purposes, V1—the decision speed—in the no-go, or stop case, is defined as the maximum speed at which the pilot must decide to reject a takeoff to stop on the remaining runway (or runway and stopway), whereas in the go case it is the minimum speed that a takeoff can be continued on the remaining runway (or runway and clearway) after failure of the most critical engine. (Note: The FAA definition of V1 is included in Advisory Circular AC 120-62 and other sources.)
Inclusion of a stopway or clearway in a takeoff performance calculation is easily forgotten when pilots use automated takeoff performance calculators. These tools provide an output to solve a complex takeoff performance problem without providing the details built into the equation. It’s easy to forget about some of these basic principles that are important when discussing aircraft performance and takeoff safety.
Before we get too deep into the conversation, here is a refresher on some key terms:
Takeoff runway available (TORA) is the runway available for takeoff.
Clearways are an area beyond the TORA free of obstructions (over land or water) used to reach the required “screen height” in the event of an engine failure on takeoff. Clearways are required to be 500 feet wide and must be less than or equal to 50% of the TORA. Not all runways have a clearway.
Screen height is the minimum height to achieve before the end of the takeoff distance available (TODA, equal to TORA + the clearway). For Class B aircraft—turbine-powered with nine seats or more—the screen height is 35 feet (15 feet for wet runways) and the aircraft must attain V2 by this height.
Stopway is an area beyond the TORA that can be used to stop an aircraft during a rejected takeoff. Not all runways have a stopway. A stopway cannot be used for landing purposes.
Accelerate stop distance available (ASDA) is the TORA plus the stopway.
Take off distance required (TODR) cannot exceed the TODA. All engines operating TODR include a 15% safety margin when calculating the minimum required runway length (TODR x 115%). The one-engine inoperative TODR does not include a 15% safety margin. TODR—both all engines operating and one engine inoperative—are the basis for our go case discussions.
Accelerate stop distance required (ASDR) cannot exceed the ASDA. ASDR is the basis for our stop case discussions.
Remember there are three factors that define the minimum required runway distance for takeoff. These include the accelerate stop distance required (ASDR) and the required all-engine and engine-inoperative takeoff distances (TODR). The value of V1 is largely dependent on these performance figures.
Balanced V1
When ASDR is equal to the TODR, the resultant V1 is called a balanced V1. A balanced V1 gives the minimum field length required for a given weight, called the balanced field length. On a runway without a stopway or clearway, the ASDA is equal to the TODA.
In theory, when using a balanced V1, the distance required to stop during a rejected takeoff just before V1 is equal to the distance required to continue the takeoff and attain V2 at the required screen height should the engine failure occur at V1.
One concept to remember is the impact on aircraft performance—at a given weight and conditions—if V1 is increased or decreased.
In the stop case, a lower V1 value allows the aircraft to accelerate to V1 quickly and stop in a shorter distance. A higher V1 value requires a longer distance to accelerate to V1 and requires a longer distance to stop due to a higher energy and a greater braking effort to stop the aircraft. If ASDR is equal to the ASDA, then V1 cannot be increased due to runway length (ASDA) limitations.
Of importance, in the event of an engine failure during a stop case, a pilot must reject the takeoff before reaching V1—afterward, the aircraft may not stop within the ASDA.
In the go case, a lower V1 value increases the one-engine takeoff distance required (TODR) due to slower acceleration after an engine failure to attain Vr and V2, whereas a higher V1 value decreases the engine-inoperative TODR due to a shorter period required to accelerate on one engine to attain Vr and V2.
Unbalanced V1
When ASDR is different than TODR, the resultant V1 is called an unbalanced V1. An unbalanced V1 allows for a higher takeoff weight by taking advantage of any runway, stopway, or clearway that is available in excess of the balanced field length.
An unbalanced V1 affects takeoff performance (TODR) due to a lower engine out acceleration (using a two-engine aircraft as an example). To illustrate this, let’s use a hypothetical base set of V-speeds as V1 of 130, Vr of 140, and V2 of 145 knots. In the case of an engine failure at V1 (130 knots), the aircraft must accelerate on one engine for 10 knots to attain Vr (140 knots) and 15 knots to attain V2 (145 knots).
In subsequent examples, V1 will be adjusted and Vr and V2 will remain constant.
Lowering V1 by 5 knots (125 knots) will increase the engine-inoperative TODR since it will now take longer to accelerate the additional 15 knots to Vr (140 knots) and 20 knots to V2 (145 knots) on one engine.
Increasing V1 by 5 knots (135 knots) will decrease the engine inoperative TODR due to the shorter period required to accelerate 5 knots to Vr (140 knots) and 10 knots to V2 (145 knots) on one engine.
Earlier Examples
In the first hypothetical example, ATC assigned a shorter runway than originally planned, and the only change based on the takeoff performance calculations was a reduction in the V1 speed. This reduction in V1 was the result of the ASDR value becoming the limiting factor on the new shorter runway.
In the subsequent example, a longer runway was available, thus a higher V1 was used to reduce the engine-inoperative TODR and allow for a higher takeoff weight based on excess runway length.
In the last example, a wet runway reduced the V1 speed. This is due to a longer ASDR based on reduced braking capabilities on a wet runway. The wet runway may also reduce the maximum allowable takeoff weight for that departure. The reduced V1 speed may increase the TODR due to an increased amount of time required to accelerate to Vr and V2.
Reasonability Check
Runway excursions during takeoff continue to occur at an uneasy rate. Often, these events involve an aircraft rotating late and either departing the paved surface of the runway or climbing through runway approach lighting or instrument landing aid structures and causing significant damage to the aircraft.
Alarmingly, these events usually involve an aircraft that did not have any significant airframe or engine malfunctions. Best practices require flight crews to independently verify and then cross-check the takeoff performance data.
The next time you review auto-generated takeoff performance data, take a moment to analyze what you’re looking for and determine if the results seem reasonable. Some general observations include tightly grouped V-speeds on very long runways; a larger spread between V1 and Vr on shorter runways; and a reduced V1 on wet or contaminated runways.
The opinions expressed in this column are those of the author and are not necessarily endorsed by AIN Media Group.