Boeing Aero Contents

Published: July 2000

Rejected Takeoff Studies


BACKGROUND

The RTO manoeuvre has been a fact of a pilot's life since the beginning of aviation. Each takeoff includes the possibility of an RTO and a subsequent series of problems resulting from the actions taken during the reject. Historically, the RTO manoeuvre occurs approximately once each 3,000 takeoffs. Because the industry now acknowledges that many RTOs are not reported, however, the actual number may be estimated at 1 in 2,000 takeoffs. For example, an unreported RTO may occur when a takeoff is stopped very early in the takeoff roll because the flight crew hears a takeoff warning horn, stops to reset trim, then taxis back to the runway and continues takeoff.

According to these statistics, a pilot who flies primarily long-haul routes, such as in our Boeing 747 fleet, may be faced with an RTO decision only once in 20 years. In contrast, a pilot in our DC-9 short-haul fleet who makes 30 takeoffs per month may see an RTO every 7 years. Unfortunately, the pilot in each of these fleets must be prepared to make an RTO decision during every takeoff.

Boeing studies indicate that approximately 75 percent of RTOs are initiated at speeds less than 80 kt and rarely result in an accident. About 2 percent occur at speeds in excess of 120 kt. The overruns and incidents that occur invariably stem from these high-speed events.

A takeoff may be rejected for a variety of reasons, including engine failure, activation of the takeoff warning horn, direction from air traffic control (ATC), blown tires, or system warnings. In contrast, the large number of takeoffs that continue successfully with indications of airplane system problems, such as master caution lights or blown tires, are rarely reported outside the airline's own information system. These takeoffs may result in diversions or delays, but the landings are usually uneventful. In fact, in about 55 percent of RTOs the result might have been an uneventful landing if the take-off had been continued, as stated in the Takeoff Safety Training Aid published in 1992 with the endorsement of the U.S. Federal Aviation Administration (FAA).

Some of the lessons learned from studying RTO accidents and incidents include the following:

HISTORY OF RTO OPERATIONS AT EVERGREEN

Evergreen International Airlines began a study of the RTO manoeuvre in 1991. Resources included information from the FAA and industry studies, notably RTO data produced by Boeing.

Our standard procedure was to use the V speeds generated from Boeing airplane flight manuals (AFM) in the form of speed cards. These cards list the appropriate speeds for a given weight and flap configuration. However, the speeds given provide only the FAA minimum recognition interval. In addition, a definition of V1 was in use that referred to "decision speed." This term implied that the airplane could accelerate to that speed, that the decision to reject or continue could then be made, and that the resulting manoeuvre would have a successful outcome.

All the data we collected pointed toward some weaknesses in this philosophy. In addition, the FAA-approved takeoff data is based on performance demonstrated on a clean, dry runway. Separate adjustments for a wet or contaminated runway are published in operational documents. The takeoff accelerate-stop distance shown in the AFM is based on a specified amount of time allocated to accomplish an RTO from V1 speed. Time delays in addition to those demonstrated in actual flight tests are included in the AFM computations. Simulator studies conducted in the 1970s showed that a flight crew requires anywhere from 3 to 7 seconds to recognise and perform an RTO, especially when the cause is other than a power plant fire or failure. More recent studies with higher fidelity simulations, such as those conducted in conjunction with the development of the Takeoff Safety Training Aid, indicate that the times for the pilot to recognise and perform the RTO procedure are within the time allotted in the AFM.

INITIAL PROPOSALS

Although we did not have a history of high-speed RTOs to use for our data, we determined that a better method must be designed to improve the flight crew's chances for an uneventful RTO. Using the Boeing data, quoted below from FAA Advisory Circular 120-62, we first changed the definition of V1. We used the definition of V1 as:

The speed selected for each takeoff, based upon approved performance data and specified conditions, which represents:

  1. 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;
  2. 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 at a designated speed; and
  3. The single speed which permits a successful stop or continued takeoff when operating at the minimum allowable field length for a particular weight.

Note 1: Safe completion of the takeoff includes both attainment of the designated screen height at the end of the runway or clearway and safe obstacle clearance along the designated takeoff flight path.

Note 2: Reference performance conditions for determining V1 may not necessarily account for all variables possibly affecting a takeoff, such as runway surface friction, failures other than a critical power plant, etc.

The "go/no-go" decision must be made prior to reaching the published V1 (figure 1). As the speed approaches V1 the "go" decision becomes more appealing. Our goal became to identify a reduced "decision speed" to provide increased flight crew recognition time in case of a catastrophic situation. Using the Boeing data, we initially approached the FAA with a proposal to call a reduced V1 the "decision speed" and treat it as a V1 speed. The flight crew would remove their hands from the thrust levers, and the takeoff would continue. The initial proposed speed was 10 kt less than published V1.

We presented this proposal to our principal operations inspector (POI) in 1991. After several months of dialogue and deliberation, it was disapproved because it was too different from certification criteria.

APPROVED PROCEDURES

In late 1992, after we received the Boeing Takeoff Safety Training Aid in draft form, we decided to again seek approval of the "decision speed" concept. This time we chose a speed of 8 kt for a reduction, which added approximately 2 seconds of recognition time. In the worst case the screen height was degraded to approximately 15 to 20 ft. We also expanded our efforts to include a revised airspeed call. We had been using an airspeed call of 80 kt, both for airspeed verification and for power setting completion in the 747. A 100-kt call was added, which indicates entry to a high-speed regime where an RTO would be more difficult and dangerous. We also refined the guidelines for an RTO as follows:

Again with the help of our POI, the revised procedure was presented to the FAA in early 1993 and approved after much discussion. It was implemented throughout our fleet in June 1993.

We believe that this reduced V1 procedure provides a valuable increase in the safety margin over that provided in the AFM in the event of an RTO. At V1, the decision to initiate an RTO must already have been made and the RTO must already have begun. If there is any hesitation, the remaining time may be insufficient to allow a successful high-speed RTO (see information on simulator studies in the previous section, History of RTO Operations at Evergreen). With our reduced V1, we increase the stopping margins on every takeoff. If an engine failure did occur just before V1, screen height is reduced. However, engine failure was not involved in nearly 75 percent of all RTO accidents. In addition, because we fly earlier generation airplanes that lack the automatic inhibit of lower level warnings after 80 kt, the use of 100 kt as a notification of entry to high-speed operations provides the pilots with more incentive to continue a takeoff if a nuisance warning occurs.

During training, our instructors traditionally used simple engine failures to teach the RTO manoeuvre. This technique, however, may condition pilots to think an engine failure is the only cause of all rejects. After the new procedures were implemented, the check airmen were instructed to use other failures, such as tires, warning lights, or system failures, to force pilots to make an RTO decision. In the high-speed regime above 100 kt, rejects should be performed only for engine failure or other catastrophic failure. The takeoff should be continued if non-critical alerts, tire failures, or system problems not related to the safe completion of the takeoff occur. Introduction of these problems requires a decision by the pilots and makes the RTO manoeuvre more realistic.

The reject itself is now taught as an emergency manoeuvre, with emphasis on full braking and correct use of spoilers and reverse as essential to the successful outcome of the manoeuvre.

RESULTS

Since the introduction of our RTO procedures, we have had only one related incident. This incident, however, proves the point of the procedure.

A DC-9 was departing Portland International Airport on runway 10L. Conditions included a crosswind, wet runway, and the airplane at balanced-field maximum weight. Near 100 kt during the takeoff roll, the captain felt something strange occur in the nose area. Because he was not sure if a tire had blown or failed in another manner, he elected to continue takeoff. A noise similar to a deflated tire thump was heard as the airplane accelerated. The takeoff continued uneventfully, however, and the airplane diverted to Seattle-Tacoma International Airport. After landing, it was discovered that the left nose tire had come apart and deflated.

This incident could have had other consequences had the captain attempted an RTO from high speed. Given the conditions of the runway, and the fact that the tire was deflated, the airplane could have been very difficult to stop on the available runway.

The captain reported that, when he first heard the noise from the nose tire area, he remembered our training and cautions regarding a high-speed reject for any reason other than a catastrophic failure.

Summary

Although we sacrifice about 15 to 20 ft of screen height on the DC-9 and less on the 747 if an engine actually fails at V1, the airplane is flying when it reaches the end of the runway. We believe that the procedures and training we have developed, using flight operations data and other information from Boeing and other sources, have helped give our pilots an edge in takeoff safety.

(All references to Boeing studies are from the Boeing Takeoff Safety Training Aid as endorsed by the FAA in 1992, in draft and final form, and other documents produced by Boeing, the Air Transport Association, the FAA, and the U.S. National Transportation Safety Board. Statistics noted in this article appeared in either the draft or final version of the training aid. Doug Smuin, then director of flight training at Evergreen and currently DC-9 captain, assisted in the preparation of this article and initial approval of the RTO studies project.)

ROBERT A. MACKINNON
CAPTAIN, BOEING 747
EVERGREEN INTERNATIONAL AIRLINES

UPDATE ON REJECTED TAKEOFF SAFETY STATISTICS

In 1989 the U.S. Federal Aviation Administration (FAA) urged the aviation industry to take steps to reduce the number of overrun accidents and incidents resulting from high-speed rejected takeoffs (RTO). This led to the formation of an international takeoff safety task force, with members from airlines, regulatory agencies, pilot unions, and manufacturers. The task force produced nine recommendations, including the following three directly related to training:

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