Understanding V1

One of the many things a first time jet pilot learns in training is a set of entirely new speeds- V₁ , V₂, V_REF just being a few.  To make the issue even more complex, different manufactures sometimes call the same speed by different names; what Embraer calls V_FS, Cessna calls V_ENR.  Most pilots struggle through these new performance considerations along with a raft of other new topics, and understandably often come out with a shaky grasp of just what exactly these new speeds represent.

V₁, for example, is the very first “V” speed a jet will encounter as it transitions to flight, yet most pilots have an erroneous picture of what this critical speed represents.  Ask the average jet pilot to define V₁, and they will respond along the lines of, “V₁ is takeoff decision speed- the highest speed at which the plane can have an engine fail during takeoff and abort on the runway in the space calculated, or continue the takeoff on one engine.”  This statement, besides being almost universally believed, is completely false.

The critical fallacy lies in the assumption that V₁ represents a decision speed of sorts.  Most pilots believe that if an engine fails at V₁, the pilot is given a space of time (three seconds is often claimed) to recognize the failure, then begin the process of aborting the takeoff- bringing the thrust levers to idle, initiating braking, deploying speed-brakes or spoilers.  What the FAA defines V₁ to be is very different.

FAR 1.2 defines all the V speeds a pilot could ever use.  Telling about the complexity of V₁ is that while most speeds are defined in just a few words- “V_R means rotation speed”, “V₂ means takeoff safety speed”, the definition for V₁ runs for two lengthy sentences.  That definition reads, in part, “… the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance…”  Clearly if the pilot must take the first action at V₁ in order to stop in the calculated distance, the decision to abort must have been made well before V₁.  It also follows that if an engine fails at V₁, there is no way a pilot could instantaneously recognize the failure and initiate the abort.

Indeed, the highest speed at which an engine can fail on takeoff and still allow for an abort within accelerate-stop distance is not V₁ at all, but a speed most pilots have never heard of- V_EF, or engine failure speed.  V_EF is not published by aircraft manufacturers, but rather is used during the testing that determines the V speeds which are published.  The most critical engine is failed at V_EF and the airplane continues to accelerate on one engine while the pilot recognizes that the engine has failed and makes the decision to abort.  Once the test pilot initiates the abort, that speed is noted and becomes V₁.

The practical implication of this distinction is that once V₁ has been reached, it is too late to initiate an abort within the runway distance calculated.  Unfortunately, as V_EF is not published, it is impossible to know exactly when the moment passes which would allow for a successful abort.  For this reason, many operators call “V₁” aloud at 5 knots before V₁, not at V₁ itself.  Once the flying pilot hears “V₁” spoken, the hand on the thrust levers is moved to the control yoke, signifying that from that point on, no matter what occurs the plane will be taken airborne.

Important also to understand is that during the aircraft testing for calculation of accelerate-stop distances, the speed never gets above V₁.  Every second that an airplane is accelerating beyond V₁, the energy the brakes would be required to dissipate during an abort increases as a function of the square of the speed increase.  Putting some numbers to this abstraction, consider the case of a light jet departing at maximum takeoff weight, with a calculated V₁ of 105 knots. 

Just as V₁ is reached, and the pilot erroneously thinks he has his last chance to decide to abort, a loud bang is heard, and the plane pulls to one side.  Unfortunately for the pilot, rather than the perceived engine failure, what has actually occurred is a tire blow-out.   During the few seconds it takes the pilot to react both engines, not one, are producing takeoff thrust, and continuing to accelerate the aircraft past V₁.  If the plane reaches a peak speed of only ten knots beyond V1, the brakes must now dissipate 20% more energy than had the abort been initiated at V₁.  Beyond the fact that there is no certification requirement for the brakes to be able absorb any energy beyond that existing at the highest weight and V₁ combination demonstrated, there is also no performance data to know how much runway would be needed even if the brakes are able to handle the extra energy.  Adding to the chaos, one brake must now absorb all the energy of the aircraft, as the blown tire’s brake has been rendered useless.

Unfortunately many rejected takeoff (RTO) accidents have been caused by such a set of circumstances.  One industry study found that 80% of RTO accidents were avoidable, and the accident planes would have been able to safely continue the takeoff with the problem in effect.  For this reason, most jet operators use a two-phase abort decision tree.   Up to a specified airspeed, usually 70 knots for light jets, the takeoff can be aborted for any reason.  Above this speed and before V₁, if the runway available is less than the runway required plus a defined safety margin (often 50%), the takeoff will only be aborted for an engine failure, engine fire, or the perception that the aircraft is unable to fly.

When there’s a loud bump right at V₁, a pilot has no way of knowing for sure what has occurred, and what the safest course to take is.  But history and certification requirements point to continuing the takeoff as the path most likely to lead to a happy outcome.