Operating off contaminated runways is one of the most difficult areas that a commercial pilot has to face. Ice, snow, slush and standing water all degrade take-off performance to varying degrees and require reductions in allowable weights. In line operations it is often difficult to obtain exact information on the depth, density and extent of the contamination, and it is equally difficult to measure coefficients of friction. Therefore the Flight Manual adjustments for these conditions are conservative. Snow, slush and standing water affect both acceleration and stopping. Ice and wet surfaces affect only the stopping.
Take-off performance is based on (a) the take-off run – all engines operating; (b) the take-off run with one engine failed at V1; (c) the take-off distance – all engines operating; (d) the take-off distance with one engine failed at V1; and (e) the emergency distance – the distance required to accelerate on all engines to V1 and then to stop. On most take-offs none of these are limiting and even when operating at max weight on a limiting runway usually only one of these is limiting. Therefore it is normal for there to be a range of allowable V1 speeds, although the performance data available to the pilot will only show one V1 in order to simplify the task.
When considering take-off performance, one is balancing the risks involved in stopping versus the risks of continuing the take-off after an engine has failed. On the large 4 engine aircraft that I used to fly (VC10, B707 and B747) it was always considered that a rejected take-off at max weight from V1 on anything approaching a limiting runway was a major emergency in its own right. And that was in addition to whatever had caused the pilot to stop in the first place. First, the pilot is faced with a difficult and unexpected decision (even though he/she should be primed for it); second, really prompt and ‘aggressive’ action (including immediate maximum braking) is needed; third, wet runway braking action is always slightly uncertain; and finally, after stopping, the brakes and tyres will be very hot and may even catch fire. Each of the first three issues can lead to an overrun off the end of the runway, and the fourth may need evacuation of all the passengers.
Bearing in mind that usually there is a range of possible V1s (even though the pilot uses only one) discussion took place back in the 1960s to see whether it would be sensible to use a lower V1 on wet runways. This was led by an excellent performance engineer in BOAC who argued that a small reduction of V1 speed in wet conditions would (a) allow more runway distance for stopping and (b) reduce the kinetic energy to be dissipated (this being related to the square of the speed). This idea was eventually adopted by the UK and later by the FAA and JAA.
Of course the trade-off was to have a lower screen height (35ft reduced to 15ft) but it was agreed that this was a small sacrifice to pay and would only occur on a few occasions as there a number of safety factors that give a ‘bonus’ to the pilot. Firstly, the reduced V1 will usually be within the range of normal V1 speeds; secondly, the aircraft will usually exceed the ‘book figures’ and will therefore exceed the required minimum of 15ft; and thirdly, take-off performance is predicated on a loss of thrust at V1, the chances of this happening in the few critical seconds approaching V1 are very small. Thus, the likelihood of passing the screen at less than 35ft, even when using a reduced V1, is very small indeed.
In addition to all of the above, most pilots also felt that, provided the aircraft was controllable, it was best to get airborne and take the problem into the air. Even in the case of an engine fire, and especially on podded engines, it was considered to be more easily dealt with in the air than when stationary on the runway.
Even though a 15ft screen height seems very little clearance, missing an obstacle by only 5ft is preferable to going off the end of the runway when trying to stop – there are too many runways with unnecessary obstacles, ditches etc. that can cause major damage.