Go/Stop Decision Near V1
It was determined when the aviation industry produced the Takeoff Safety Training Aid in 1992 that the existing definition of V1 might have caused confusion because they did not make it clear that V1 is the maximum speed at which the flight crew must take the first action to reject a takeoff. The U.S. National Transportation Safety Board (NTSB) also noted in their 1990 study of rejected takeoff accidents, that the late initiation of rejected takeoffs was the leading cause of runway overrun accidents. As a result, the FAA has changed the definition of V1 in FAR Part 1 to read as follows:
.
V1 means the maximum speed in the takeoff at which the pilot must take
the first action (e.g., apply brakes, reduce thrust, deploy speedbrakes) to
stop the airplane within the accelerate-stop distance; and,
.
V1 also means the minimum speed in the takeoff, following a failure of
the critical engine at which the pilot can continue the takeoff and achieve
the required height above the takeoff surface within the takeoff distance.
Part 1 defines the critical engine as “the engine whose failure would most adversely affect the performance or handling qualities of an aircraft.”
Pilots know that V1 is fundamental to making the Go/Stop decision. Under runway limited conditions, if the reject procedure is initiated at V1, the airplane can be stopped prior to reaching the end of the runway. See RTO Execution Operational Margins diagrams for consequences of initiating reject after V1 and/or using improper procedures.
When the takeoff performance in the AFM is produced, it assumes an engine failure or event one-second before V1. In a runway limited situation, this means the airplane reaches a height of 35 feet over the end of the runway if the decision is to continue the takeoff.
Within reasonable limits, even if the engine failure occurs earlier than the assumed one second before V1, a decision to continue the takeoff will mean that the airplane is lower than 35 feet at the end of the runway, but it is still flying. For instance if the engine fails 2 seconds prior to V1 and the decision is made to go, the airplane will reach a height of 15 to 20 feet at the end of the runway.
Although training has historically centered on engine failures as the primary reason to reject, statistics show engine thrust loss was involved in approximately one quarter of the accidents, and wheel or tire problems have caused almost as many accidents and incidents as have engine events. Other reasons that rejects occurred were for configuration, indication or light, crew coordination problems, bird strikes or ATC problems.
What's important to note here is that the majority of past RTO accidents were not engine failure events. Full takeoff power from all engines was available. With normal takeoff power, the airplane should easily reach a height of 150 feet over the end of the runway, and the pilot has the full length of the runway to stop the airplane if an air turnback is required.
Making the Go/Stop decision starts long before V1. Early detection, good crew coordination and quick reaction are the keys to a successful takeoff or stop.
RTO Execution Operational Margins
A successful rejected takeoff at or near V1 is dependent upon the captain making
timely decisions and using the proper procedures. The data in the following diagrams, extracted from the 1992 Takeoff Safety Training Aid, are provided as a reference. The individual diagrams show the approximate effects of various configuration items and procedural variations on the stopping performance of the airplane. These calculations are frequently based on estimated data and are intended for training discussion purposes only. The data are generally typical of the airplane at heavy weights, and except as noted otherwise, are based on the certified transition time.
Each condition is compared to the baseline condition. The estimated speed at the end of the runway and the estimated overrun distance are indicated at the right edge of each figure. The distance estimates assume an overrun area that can produce the same braking forces as the respective runway surface. If less than the baseline FAA accelerate-stop distance is required, the distance is denoted as a negative number.
Copyright . The Boeing Company. See title page for details.
October 31, 2003 FCT 757 (TM) 3.21
Initial Climb - All Engines
After liftoff use the flight director as the primary pitch reference cross checking indicated airspeed and other flight instruments. If the flight director is not used, indicated airspeed and attitude become the primary pitch references.
After liftoff, the flight director commands pitch to maintain an airspeed of V2 + 15 to 25 knots until another pitch mode is engaged.
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