In those situations where I have been involved in analysing performance, weve taken a more pessimistic view. We created a flow chart of likely scenarios which could involve the requirement for extra runway length, the type of pavement (whether grooved or ungrooved or gravel etc.), runway width and runway conditions.
Safety of the aircraft and its occupants is our primary concern and we look at the probability of an event that might increase the take off and stopping distances.
For example, an aircraft reaching V1 that suffers a double engine failure on take off due to say a bird strike, may have limited braking capability from lack of reverse thrust and reduced hydraulic and accumulator braking pressure. A double engine failure is a rare event but this scenario was played out by a Boeing 707 in Sydney in the early 1970s and the aircraft overan the runway departure point. In areas where there is a high bird population, the safety margins are siginifcantly reduced.
In the Qantas Bangkok accident (VH-OJH), we summised that the aircraft could have stopped safely had reverse thrust been available. However, brakes are the primary equipment for stopping the aircraft. In that accident, the descent rate of the airplane remained unchecked and touch down occured "inadvertantly" resulting in ground spoiler deployment. If the decison to continue with a go around had been maintained at that point the aircraft would still have been on the ground at the departure end of the runway in ground effect at twice the speed.
Pilots need to exercise extreme care that they plan an exit strategy based on more than just engine failure.
Good performance planning involves looking at different scenarios and making sure you do not plan for a disaster.
What if this happens should be followed by a serious appraisal - it's not a joking matter. This requires intimate knowledge of aircraft performance.
Working a fully serviceable aircraft of a minimal strip length in ideal conditions can result in complacency.
