"Do you have any rule of thumb on takeoff and landing distances?" - Mark P.
“One rule of thumb to use is that the Pilot Operating Handbook performance figures for landing and takeoff were provided and confirmed by the marketing department and test pilot for the aircraft company building your airplane and as a result, need to be increased for safety margins.
Important considerations include:
o Aircraft condition-engine, tires, prop, etc.
o Runway contamination
o Runway gradient
o Wind and consistency of the wind
o The effects of humidity on performance
An additional consideration is pilot skill and the ability to hit the recommended airspeed numbers.
With excellent pilot skills (well-practiced) and a known airplane, I personally wouldn’t recommend attempting a takeoff over an obstacle with less than a 25% extra safety margin at a minimum. The considerations above could easily increase this margin to 50 or 60%.
It is also important to understand how temperature and pressure altitude (density altitude) affect performance. Check your Pilot Operating Handbook (for this information).”
Note: A runway is considered to be contaminated when more than 25% of the runway surface area (whether in isolated areas or not) within the required length and width being used, is covered by (a) surface water more than 1/8th inch (3mm) deep, or;(b) slush or loose snow equivalent to more than 1/8th inch (3mm) of water or;(c) compacted snow or wet ice.