Every takeoff could potentially result in a rejected takeoff (RTO) for a variety of reasons: engine failure, fire or smoke, unsuspected equipment on the runway, bird strike, blown tires, direct instructions from the governing ATC authority, or recognition of a significant abnormality (split airspeed indications, activation of a warning horn, etc.).
Ill-advised rejected takeoff decisions by flight crews and improper pilot technique during the execution of a rejected takeoff contribute to a majority of takeoff-related commercial aviation accidents worldwide. Statistically, although only 2 percent of rejected takeoffs are in this category, high-speed aborts above 120 knots account for the vast majority of RTO overrun accidents. Four out of five rejected takeoffs occur at speeds below 80 knots and generally come to a safe and successful conclusion.
The kinetic energy of any aircraft (and thus the deceleration power required to stop it) increases with aircraft weight and the square of the aircraft speed. Therefore, an increase in weight has a lesser impact on kinetic energy than a proportional increase in groundspeed. A 10 percent increase in takeoff weight produces roughly a 10 percent increase in kinetic energy, while a 10 percent increase in speed results in a 21 percent increase in kinetic energy. Hence, it should be stressed during pilot training that time (delayed decision or reaction) equals higher speed (to the tune of at least 4 knots per second for most jets), and higher speed equals longer stopping distance. A couple of seconds can be the difference between running out of runway and coming to a safe halt. Because weight ceases to be a variable once the doors are closed, the throttles are pushed forward and the airplane is launching down the runway, all focus should be on timely recognition and speed control.
The decision to abort takeoff should not be attempted beyond the calculated V1, unless there is reason to suspect that the airplane’s ability to fly has been impaired or is threatened to cease shortly after takeoff (for example on-board fire, smoke, or identifiable toxic fumes). If a serious failure or malfunction occurs beyond takeoff decision speed (V1), but the airplane’s ability to fly is not in question, takeoff must generally continue.
It is paramount to remember that FAA-approved takeoff data for any aircraft is based on aircraft performance demonstrated in ideal conditions, using a clean, dry runway, and maximum braking (reverse thrust is not used to compute stopping distance). In reality, stopping performance can be further degraded by an array of factors as diversified as:
- Runway friction (grooved/non-grooved)
- Mechanical runway contaminants (rubber, oily residue, debris)
- Natural contaminants (standing water, snow, slush, ice, dust)
- Wind direction and velocity
- Air density
- Flaps configuration
- Bleed air configuration
- Underinflated or failing tires
- Penalizing MEL or CDL items
- Deficient wheel brakes or RTO auto-brakes
- Inoperative anti-skid
- Pilot technique and individual proficiency
Because performance conditions used to determine V1 do not necessarily consider all variables of takeoff performance, operators and aircraft manufacturers generally agree that the term “takeoff decision speed” is ambiguous at best. By definition, it would suggest that the decision to abort or continue can be made upon reaching the calculated V1, and invariably result in a safe takeoff or RTO maneuver if initiated at that point in time. In fact, taking into account the pilots’ response time, the Go/No Go decision must be made before V1 so that deceleration can begin no later than V1. If braking has not begun by V1, the decision to continue the takeoff is made by default. Delaying the RTO maneuver by just one second beyond V1 increases the speed 4 to 6 knots on average. Knowing that crews require 3 to 7 seconds to identify an impending RTO and execute the maneuver, it stands to reason that a decision should be made prior to V1 in order to ensure a successful outcome of the rejected takeoff. This prompted the FAA to expand on the regulatory definition of V1 and to introduce a couple of new terms through the publication of Advisory Circular (AC) 120-62, “Takeoff Safety Training Aid.”
The expanded definition of V1 is as follows:
- V1. The speed selected for each takeoff, based upon approved performance data and specified conditions, which represents:
- The maximum speed by which a rejected takeoff must be initiated to assure that a safe stop can be completed within the remaining runway, or runway and stopway;
- The minimum speed which assures that a takeoff can be safely completed within the remaining runway, or runway and clearway, after failure of the most critical engine the designated speed; and
- The single speed which permits a successful stop or continued takeoff when operating at the minimum allowable field length for a particular weight.
- Minimum V1. The minimum permissible V1 speed for the reference conditions from which the takeoff can be safely completed from a given runway, or runway and clearway, after the critical engine had failed at the designated speed.
- Maximum V1. That maximum possible V1 speed for the reference conditions at which a rejected takeoff can be initiated and the airplane stopped within the remaining runway, or runway and stopway.
- Reduced V1. A V1 less than maximum V1 or the normal V1, but more than the minimum V1, selected to reduce the RTO stopping distance required.
The main purpose for using a reduced V1 is to properly adjust the RTO stopping distance in light of the degraded stopping capability associated with wet or contaminated runways, while adding approximately 2 seconds of recognition time for the crew.
Most aircraft manufacturers recommend that operators identify a “low-speed” regime (i.e., 80 knots and below) and a “high-speed” regime (i.e., 100 knots and above) of the takeoff run. In the “low speed” regime, pilots should abort takeoff for any malfunction or abnormality (actual or suspected). In the “high speed” regime, takeoff should only be rejected because of catastrophic malfunctions or life-threatening situations. Pilots must weigh the threat against the risk of overshooting the runway during a RTO maneuver. Standard Operating Procedures (SOPs) should be tailored to include a speed callout during the transition from low-speed to high-speed regime, the timing of which serves to remind pilots of the impending critical window of decision-making, to provide them with a last opportunity to crosscheck their instruments, to verify their airspeed, and to confirm that adequate takeoff thrust is set, while at the same time performing a pilot incapacitation check through the “challenge and response” ritual. Ideally, two callouts would enhance a crew’s preparedness during takeoff operations. A first callout at the high end of the “low-speed” regime would announce the beginning of the transition from “low speed” to “high-speed,” alerting the crew that they have entered a short phase of extreme vigilance where the “Go/No Go” must imminently be decided. A second callout made at the beginning of the “high-speed” regime would signify the end of the transition, thus the end of the decision-making. Short of some catastrophic failure, the crew is then committed to continue the takeoff.
Proper use of brakes should be emphasized in training, as they have the most stopping power during a rejected takeoff. However, experience has shown that the initial tendency of a flight crew is to use normal after-landing braking during a rejected takeoff. Delaying the intervention of the primary deceleration force during a RTO maneuver, when every second counts, could be costly in terms of required stopping distance. Instead of braking after the throttles are retarded and the spoilers are deployed (normal landing), pilots must apply maximum braking immediately while simultaneously retarding the throttles, with spoilers extension and thrust reversers deployment following in short sequence. Differential braking applied to maintain directional control also diminishes the effectiveness of the brakes. And finally, not only does a blown tire eliminate any kind of braking action on that particular tire, but it could also lead to the failure of adjacent tires, and thus further impairing the airplane’s ability to stop.
In order to better assist flight crews in making a split second Go/No Go decision during a high speed takeoff run, and subsequently avoid an otherwise unnecessary but risky high speed RTO, some commercial aircraft manufacturers have gone as far as inhibiting aural or visual malfunction warnings of non-critical equipment beyond a preset speed. The purpose is to prevent an overreaction by the crew and a tendency to select a risky high-speed RTO maneuver over a safer takeoff with a non-critical malfunction. Indeed, the successful outcome of a rejected takeoff, one that concludes without damage or injury, does not necessarily point to the best decision-making by the flight crew.
In summary, a rejected takeoff should be perceived as an emergency. RTO safety could be vastly improved by:
- Developing SOPs aiming to advance the expanded FAA definitions of takeoff decision speed and their practical application, including the use of progressive callouts to identify transition from low-to high-speed regime.
- Promoting situational awareness and better recognition of emergency versus abnormal situations through enhanced CRM training.
- Encouraging crews to carefully consider variables that may seriously affect or even compromise available aircraft performance data.
- Expanding practical training in the proper use of brakes, throttles, spoilers, and reverse thrust during RTO demonstrations.
- Encouraging aircraft manufacturers to eliminate non-critical malfunction warnings during the takeoff roll at preset speeds.