Take off performance series – Part 2- Balanced Field Lengths

In part one on the take-off performance series, reasons were given as to why a rejected take-off in light general aviation and sport aircraft is to a large extent, a very hit and miss manoeuvre. If you missed part 1, read it here.

Part 91 of South Africa’s Civil Aviation Regulations (CAR) in fact prescribes how aircraft having a maximum certified mass of 5700 kilograms or less, must be operated. These are the very aircraft that are used for training, industrial aid and charter operations. They are also the ones that are flown for business or leisure by private owners and they cover a huge range of aircraft, from types such as the Piper Cub, up to light twin engine machines that are capable of carrying about ten to twelve people.

When one reads the CARs, it is evident that the CAA has very little to say about how much runway length Part 91 aircraft should have for the take-off. Basically, the core of what is written in rule 91.08.1 is that the owners or operators of such aircraft must ensure that they take into account the airframe configuration, the aircraft’s mass, the environmental conditions in terms of altitude, temperature and wind, and then also the runway condition and its gradient. That’s it! This is all good advice that is based on good airmanship!

However, once an aircraft’s maximum certified Mass (MCM) is 5700 kg and above, or if it has 19 seats or more, the runway lengths and the associated maximum take-off weights become governed by stringent rules that are applied by aviation authorities world-wide. In South Africa, these would be the CAA Part 121 regulations.

The essence of these rules is that Part 121 aircraft are obliged to adhere to what is called the “balanced field concept.” For a runway of any length, the aircraft needs to be able to accelerate to a speed from which the take take-off can be rejected and still be brought to a stop within the remaining length of that runway. Alternatively, in the event of an engine failure, if the aircraft has already achieved a speed from which a safe stop cannot be effected, it should have the engine power to provide enough acceleration to continue the take-off run. In that event, within the remaining length of runway it will need to get airborne and cross the far end of the runway at a minimum height of 35 feet and at a speed not less than 1.2 of the aircraft’s stalling speed for the particular flap setting that is being used. This speed is aptly called the “take-off safety speed”.

So… A datum, provided as an adjustable marker on the airspeed indicator, displays what is known as the V1 Speed.  As the aircraft approaches the V1 speed on the take-off run, the pilot knows that the length of runway that is going to be available for bringing the aircraft to a stop before the end of the runway is rapidly reducing. The pilot also knows that in the event that the take-off actually needs to be rejected because of an engine failure, the first actions associated with this procedure, namely the retardation of all of the aircraft’s thrust levers, accompanied by simultaneous maximum braking, would have to be taken before or at V1 and not a second later!  The distance required to travel from the starting point of the take-off run to the position where the V1 speed is attained, is known as the “”accelerate-stop” distance.

Once the V1 speed has come and gone, the pilot is obliged to continue with the take-off run, even if an emergency situation is then recognised. If an engine had failed the aircraft would still be  accelerating, but then at a greatly reduced rate, right up until the rotation speed was achieved and the aircraft could be taken into the air. The requirement would be to cross the far end of the runway at a height of at least 35 feet and at a speed no lower than the inflight safety speed. The distance required to continue the take-off run after the failure of an engine at or above the V1 speed and then, at the rotation speed, “Vr”, to get the aircraft safely airborne, is known as the “accelerate-go” distance.

If one adds the “accelerate-stop” distance to the “accelerate – go” distance, this would then be the minimum allowable all engine take-off field length.  Expressed in easier terms, we talk of this length as a “BALANCED FIELD LENGTH”.

The balanced field length can be affected by many factors which include but are not limited to: 

• The aircraft’s mass
• The aircraft’s take-off configuration in respect of the flap position
• The environmental conditions and therefore the aircraft’s performance.
• The condition and slope of the runway and the wind component

One way or the other, these factors serve to either increase or decrease the balanced field length, but for every single take-off, the runway length needs to be, at the very least, the same or greater than the balanced field length

 The international airports at Johannesburg, Cape Town and Durban all have very long runways and the heavy aircraft that fly  intercontinental flights from there, usually require balanced field lengths that are very close to every bit of the runway length that is available.

On the other hand, lighter and smaller airliners that operate short domestic or regional flights off the same runways, have balanced field lengths that are much shorter, even as little as 33%, of what the runway actually provides for. The balanced field length concept therefore allows for take-offs from runway intersections. Such take-offs save on taxi distances, time and fuel usage. They also help ease airport congestion.

In order to determine if the balanced field length for the airport and intended runway to be used is going to be sufficient for the planned take-off, the input of many parameters into the aircraft’s performance data base is necessary.

A positive solution will indicate that the aircraft is within the limits and corresponding V1 and Vr speeds will be provided. A negative result will indicate that the take-off is not sanctioned because certain of the parameters cause the aircraft to be out of limits. These could be issues such as the weight of the aircraft and the associated performance and brake energy limits, or environmental effects such as the current temperature, the density altitude and the current wind component. In this case no V speeds would be provided.

Next month, we deal with the reasons for electing to reject a take-off and the actual method of doing so.  See you then!