October 15th, 2012

Fresh Air Matters... with Capt. Yaw

I was recently asked to explain the differences between TORA, TODA and ASDA. TORA is the ‘Take Off Run Available’, TODA is the Take Off Distance Available’, and ASDA is the ‘Acceleration Stop Distance Available’, terms used in relation to runway length/aircraft take-off performance terminology. 

It may sound simple, but it is actually a complicated matter, and one, that if not well understood, could lead to a serious accident. Runways are critical bits of kit that often get taken for granted, so let us explore what this is all about and why!

First of all ICAO (the International Civil Aviation Organisation) defines a runway as a ‘rectangular area on a land aerodrome prepared for the landing and take-off of aircraft’. That makes sense. But how long is the runway, is the runway what it looks like and is a runway safe to use for a specific aircraft on a specific day?

It is easy to make the mistake of thinking that the runway length is how far it is from one end to the other, but it is not. That is why we have these wonderful terms.

The ‘runway’ is broken down into parts, not all of them are easy to understand, and even pilots make mistakes in their understanding, leading to accidents, some of them fatal.

Let’s first grasp what happens with an aircraft when it takes off. First of all there is the take-off ‘run’, or ‘roll’, being the distance needed to get the wheels off of the ground. Secondly, there is the ‘initial climb to screen height’, or the distance needed to climb past obstacles, including the standard ‘fixed imaginary obstacle at 50feet’. The combined distance is called the TOD or ‘Take Off Distance’. This distance is generally declared as if the aircraft is flown on a dry, stable, well prepared surface (‘a paved, hard surface’ is used as the ‘standard’ surface) by an ‘average’ pilot in ‘good standard conditions’ (that means about 15C, dry, good weather!). Therefore, an above average pilot may use less, and a below average, especially those in training, will use more – often lots more. The take-off distance will also vary based on the aircraft loading, weather and the surface conditions. Wind, temperature, humidity, pressure, runway slope, wet runway, long grass, overweight aircraft, poorly loaded aircraft, engines performance (or lack of it), fuel quality, etc. will all affect the overall performance of the aircraft, increasing the take-off distance considerably, as well as the climb rate – having even more effect on the distance required by the ‘below average’ pilot. Student pilots, low hours pilots and pilots new to the type of aircraft generally exhibit ‘below the benchmark’ piloting skills and often use an additional 50% or more whilst they are building their skills.

Each aircraft has a ‘take-off distance’ required under those ‘standard conditions’, but it has to be related to the runway.

The aerodrome declared Take Off Distance Available or TODA is generally the length of the runway from one end to the other (threshold to threshold). However, at many airfields there is a ‘clearway’, which often looks as if it is runway, but it is not. It is not considered suitable for the aircraft to use for take-off calculations, due to safety considerations (condition of the surface, obstacles, etc.). If the clearway is suitable for stopping on, it becomes called the Stopway and therefore the TODA plus the Stopway would be called the ASDA – Accelerate Stop Distance Available – which means that the Stopway can be used to decelerate in, in the case of an abandoned take-off – but should not be used for any other purpose.

However, it is not that simple. Some airfields have obstacles that change the appropriate use of the runway. If an airfield has obstacles at the end of the apparent runway, it may not allow the whole physical length of tarmac, or other prepared surface, to be taken into account for the declared TODA. An allowance must be given for the aircraft to climb to above the ‘screen’, (theoretical 50feet) ,or actual obstacle height – which may be quite considerable at some airfields. In such cases, only part of the apparent runway can be considered suitable for the Take Off Run, resulting in a shorter TORA. When there are obstacles at the end of the runway, the TODA (Take Off Distance) is critical. In such cases the pilot must ensure that the initial climb can be complete within the declared TODA, or there is a risk of collision with obstacles. Such collisions are not good and can result in injuries and death, both to those in the aircraft and those on the ground. In formula terms we state that the Take Off Distance is a factor of take-off speed, thrust, rolling friction and mass; clearly ‘skill’ is not taken into account in the formula and the ‘average pilot’, a mythical beast, is applied! In Ghana we can expect take-off distances to be 20% - 30% longer than the ‘standard book figure’, even for the average pilot, due to the higher temperatures and humidity – of course, there are many other factors too!

When it comes to Landing, there are similar considerations, and the same issues of real, or the ‘screen height 50 foot’, obstacle has to be considered on all the landing distances. 

All of this becomes more critical in training, since we carry out ‘touch and go’ operations. Here, the student pilot will land, slow down and then take-off again. That take-off again point may be well down the runway and push the climb over obstacles/screen even further out. This ‘go around’ can happen quite late down the runway – and the pilot must be sure that he can clear any obstacles and maintain safe operations as he does so.

At the end of each airfield there are often ‘safety areas’, these may be called ‘undershoot/overshoot’ or ‘RESA’ meaning Runway End Safety Areas. The RESA are not intended for aircraft manoeuvring and should only be used in emergency conditions.

At Kpong Airfield we have a clearing of over 1100meters. However, we consider our runway to be 500m long. In reality we have over 800m of ASDA, with around 150m RESA at each end!

When the experienced pilots operate, we often see take-off rolls of less than 50m, student pilots can take as much as 250m! The same on landing, experienced pilots generally use less than 200m, whilst the learners take up to 500m! 

The same occurs in your business activities. There are tasks that an experienced person can do quickly, wasting little time and resources. However, when you are teaching somebody you consume a lot more of both – and need to be even more safety aware. This is called ‘teaching and learning’, and unless we give that consideration to the learner, and take our time to develop them to become a ‘safe and efficient above average’ person, we will not develop our workforce appropriately – nor safely.

It is easy to think that everybody should do a task ‘the same as me’, but the reality is we need to build in more safety and understanding, especially as we train people, then, and only then, will we see sustainable growth in a suitable manner.

Capt. Yaw is Chief Flying Instructor and Chief Engineer at WAASPS, and lead Pilot with Medicine on the Move, Humanitarian Aviation Logistics (www.waasps.com www.medicineonthemove.org e-mail capt.yaw@waasps.com)