Take-off performance Series – Part 1 – Rejected Take-offs

In this issue of “Smoke on Go’s” Professional Pilot Series, we start working towards knowing more about the complex subject of “Take-off Performance”. We are aware that most Smoke on…Go! readers are not airline pilots. However, the knowledge that is imparted here, will, step by step, serve to be of value to those who will someday fly aircraft that are more complex and perhaps also bigger, heavier, and faster than the aircraft they are currently flying.

Let’s have a look at take-offs from runways and air-strips that are able to serve only conventional general aviation aircraft types. For each and every such aircraft and runway, there will be a speed and a geographic point beyond which, in the event of a decision to reject a take-off, the aircraft will NOT be able to be brought to a stop within the remaining length of the runway. The Tiger Moth presents an extreme illustration of the above statement. This is an aircraft that has no brakes at all. It relies on a tiny tail-wheel to create sufficient drag to bring it to a stop. The aircraft may also be steered through a number of “S” turns so as to dissipate energy, and finally, if rougher, but nevertheless usable surface areas to the sides of the runway exist, then the friction provided by such, could help the aircraft achieve a higher rate of deceleration.

The accomplishment of a successful rejected take-off in a Tiger Moth therefore becomes a very “hit and miss” manoeuvre to perform. Executed at a low speed and without having consumed much of the runway’s length, a pilot would, with luck on his or her side, execute a successful rejected take-off. However, taking into account the type and length of the runways or airfields that these aircraft typically operate from and having already attained almost enough airspeed to get airborne with, rejection of the take-off is likely to end up with the aircraft and its occupants overrunning the runway end and then crashing. Severe damage to the aircraft and injuries to those on board could be incurred. After all, situated at the end of any runway could be fences and other more sturdy barriers, roads, ditches, stones, rocks, rubble, ant- hills, trees and shrubs, water courses, the ocean, poles, electrical wires and buildings.

In spite of aircraft having become better equipped and more sophisticated, the odds and chances of accomplishing a successful rejected take-off have improved only slightly. During the 1940’s, general aviation aircraft began to be fitted with wheel brakes. This enhanced braking capability enormously, in fact, way beyond what had ever existed previously. However, In this day and age, THERE IS STILL NO PRACTICAL  WAY  that a pilot flying in a typical operating environment, would be able to determine at what speed or at what point  a take-off could be rejected so that  the aircraft would be brought successfully to a stop within the remaining length of the runway.

These general aviation aircraft, the ones that we all love to fly, are operated not only off runways situated at airports and airfields, but also from strips, both short and long, that exist on the outskirts of small towns, at game lodges, mines and on farms. Data relating to the length, slope, and coefficient  of friction of these runways, is likely to be unavailable or non-existent. The speed up to which the take-off could be rejected, and the stopping distance available would be indeterminable. A rejected take-off from any of these runways WOULD STILL REMAIN A VERY HIT AND MISS OPTION.

For the spectrum of general aviation aircraft that we are discussing here, it is a foregone conclusion that in the event of an engine failure occurring on the take-off roll, the capability of accelerating to the take-off speed and then getting airborne will be lost and that the take-off should therefore be rejected. However, there are various other abnormalities that could occur. One of these could, for example, be the popping open of a door or a hatch. If a pilot deems that in his or her judgement, rightly or wrongly, that it is necessary to reject the take-off and that this procedure could still be commenced timeously,  then the aircraft will either be stopped within the remaining length of the runway, or it is going to go off the end of it. There is simply no certain knowledge as to whether the procedure is going to be successful or not. Within the very short time-span, perhaps only a second or two, during which an  abnormality  manifests itself and during which a decision must be made as to whether to continue with the take-off or to reject it, all that a pilot actually has to rely on for taking the correct action, is instinct and previous flying experience. 

There are a multitude of factors that affect the take-off and stopping distances of aircraft. These are phenomena such as the weight of the aircraft, the surface wind, undulations in the slope of the runway and contamination from standing water, mud, ice, snow or hailstones . ALL of these factors will be revisited in further issues as we progress with the subject of “Take off Performance”.

It was mentioned earlier that almost all general aviation aircraft up to more or less, a certain size and weight, all have “good braking capability”. This is very true, but wheel-brakes, either drum or disc, are almost ALL that these aircraft have that can be used to bring them to a stop. Unlike airliners, they do not have brake anti-skid systems. Nor do they have  thrust reversers and there are no spoilers or speed-brakes that can be deployed so as to provide additional drag.

Airliners have all of these features and they operate essentially from paved runways where length, exact elevation, slope and coefficient of friction have all been established and published. During the certification process of airliners, from those that carry 20 passengers and more, right up to the Boeing 747’s and Airbus A380’s, “accelerate-stop” distances are determined for different climatic conditions, flap settings, runway slopes, weight, wind, runway surface conditions and with brake anti-skid systems that are either operative or inoperative. This information is then made available to pilots in graphical, tabular or electronic form. For each and every take-off that they do, pilots are able and obliged to derive a speed up to which they can effect a successful rejected take-off. 

In the next issue of “Smoke on Go”, we will define the V1 (“VEE ONE”) speed and then discuss “accelerate-go” distances.  Being armed with a knowledge of “accelerate –stop and “accelerate-go” distances, will lead to an understanding of what is required for a continued  take-off and climb and also the concept of “balanced field lengths”.

See you next month!