Losing an Engine on Takeoff: Abort or Floor It?

There is a little more than a mile of pavement in front of the pilots;  the flight has been cleared for takeoff.  The Captain advances the power while the brakes are held.  Engine instruments checked, automatic control of the throttles engaged, and the brakes are released.

The computer pushes the engines to maximum takeoff power, and the aircraft begins its rapid acceleration down the runway.  With over half of the runway behind it, and accelerating through 140 MPH, an engine fails.  “Is there enough runway to stop?”  “Can the aircraft takeoff on the one remaining engine?”  “Will it clear the trees at the edge of the airport?”  The crew is faced with this daunting choice, and they react quickly.  It may seem like one has to consider all these factors, or even rely on gut instinct.  But in fact, the decision has already been made.

Engine failure on takeoff is a situation that all pilots both dread and train for. In small twin-engined aircraft, when an engine fails on the takeoff roll, the remaining engine is brought to idle, and the aircraft is stopped on the runway.  If the engine quits just after takeoff, the pilot may have enough runway available to still land safely.  There is a point, based on the judgment of the pilot, that sufficient runway is unavailable.  At this point, the landing gear is retracted, and the pilot would continue to climb out on the remaining engine.  Many of these principles apply to large turbojet aircraft, though the total picture is far more complex.

Per FAA regulations, an airliner must be able to either abort the takeoff and stop on the runway, or continue with the takeoff and climb out on the remaining engine.  There is a point, or rather a speed that defines this point in which the aircraft can safely either be stopped or continue with the takeoff.  This is commonly referred to as decision speed.

Decision speed, which pilots refer to as V1, is calculated for every takeoff.  Unlike the pilot of a small piston twin who must make a judgment call when the engine quits, the Captain of an airliner relies on the science, testing, and calculations from which the decision speed is derived.

Decision speed falls within a range of speeds; the highest of this range is based on the ability of the aircraft’s brakes, which is especially relevant on older intercontinental airliners.  Passing through 160 MPH, these aircraft simply have insufficient braking capability to stop.  If the Captain were to abort above this speed, the brakes would heat rapidly, melt, and around 60 MPH, simply cease to exist.  Off runway terrain and obstructions would eventually bring the heavy aircraft to a halt.

The lowest of this range is based on the minimum speed of which the aircraft is controllable with the failure of one engine.  When an engine fails, the remaining engine will make the aircraft turn in the direction of the failed engine.  This yaw is counteracted by the pilot’s application of the rudder, which directs the airflow over the tail, counteracting the yaw from the engine.  Whereas the force of the thrust from the engine remains relatively constant during the takeoff, the amount of force the rudder can generate is entirely dependant on the aircraft’s speed.  The higher the speed, the greater amount of airflow over the tail, and thus the greater the force generated by the rudder.  There is a speed at which the force of the rudder equals the force of the yawing from the engine.  Above this speed, the pilot is able to control the direction of the aircraft with the rudder.  Below this speed, the remaining engine will overpower the rudder and the aircraft will loose directional control.  This speed, denoted as Vmcg (minimum control ground), plus a small fudge factor, define the low end of the V1 range.  If the aircraft is going to continue the takeoff, it must be controllable, thus the requirement of V1 being higher than Vmcg.

The departure runway is the primary input for the calculation of V1.  Two basic criteria must be met; the aircraft must be able to abort the takeoff and stop on the remaining runway just prior to reaching V1, or continue the takeoff with the engine failure after having reached V1.  If the takeoff is continued, the aircraft must be able to rotate, clear the end of the runway by 35 feet, and climb steeply enough to clear any trees, buildings, and other obstacles.

Other factors that go into the calculations are: engine power, atmospheric conditions, runway conditions, and aircraft weight.  If the criteria of V1 cannot be met, it is not permissible for the aircraft to depart.  To remedy this, the Captain may choose another suitable runway.  If one is not available, the only remaining choice is reduce the aircraft weight, typically by bumping passengers.  Due to the short runway and steep climb required, Washington Reagan National and John Wayne Orange County airports are places where passengers are often left behind due to V1 considerations.

The decision on when to abort the takeoff or continue on one engine is made well prior to entering the runway.  Whether the aircraft slams on the brakes and stops at the end of the runway, or continues the takeoff and returns for landing, the passengers will be returning to the gate.  Safely.

(Source: NYCAviation)