A simple bush rule
Airplanes get old, pilots get tired, runways are not always in best shape out there, things not always work the way we planned. Here comes the first and probably most handy little rule: even when you calculated and planned everything meticulously you should allow for at least a 20% safety margin. Just in case. If required parameters are not allowing for this safety margin you better start thinking how you could improve performance (throw out some luggage, passengers, fuel, change airplane, wait for weather to cool, whatever...).
Density Altitude
This might sound quite tricky but let me show you a quickie here as well. Every degree of Celsius variation from standard temperature, density altitude (DA) changes by 120 feet. If temperature increases density altitude goes up; if it decreases density altitude goes down. So DA is the pressure altitude plus 120 times the difference between local air temperature and standard. At sea level, the altimeter is 1013 and 25 degrees Celsius, DA would be 1200 feet. Surprised? Add pressure altitude (0, we're sea level) to 120 times 10 (difference of actual and standard temperature) and there you go.
10/20 rule for speed
How much tailwind can you afford yourself? The least the best, but sometimes you can only takeoff or land in tailwind. What you need to know is: if you increase groundspeed by 10%, ground roll will increase at least 20% (depending on airplane it can be even more than that). The faster the longer.
10/20 rule for weight
Like in case of speed a 10% change in weight will cause at least a 20% change in takeoff and landing distance, and the same applies, the actual ammount varies from airplane to airplane. But keep in mind the heavier the longer.
Density effects on performance
For each degree Celsius of difference from standard, the takeoff roll changes by about 1%. Simple, eh? But very useful when you are out on a high field on a hot day.
Low Time Pilots Guide to African Bush Flying - 12.99€
Airplanes get old, pilots get tired, runways are not always in best shape out there, things not always work the way we planned. Here comes the first and probably most handy little rule: even when you calculated and planned everything meticulously you should allow for at least a 20% safety margin. Just in case. If required parameters are not allowing for this safety margin you better start thinking how you could improve performance (throw out some luggage, passengers, fuel, change airplane, wait for weather to cool, whatever...).
This might sound quite tricky but let me show you a quickie here as well. Every degree of Celsius variation from standard temperature, density altitude (DA) changes by 120 feet. If temperature increases density altitude goes up; if it decreases density altitude goes down. So DA is the pressure altitude plus 120 times the difference between local air temperature and standard. At sea level, the altimeter is 1013 and 25 degrees Celsius, DA would be 1200 feet. Surprised? Add pressure altitude (0, we're sea level) to 120 times 10 (difference of actual and standard temperature) and there you go.
10/20 rule for speed
How much tailwind can you afford yourself? The least the best, but sometimes you can only takeoff or land in tailwind. What you need to know is: if you increase groundspeed by 10%, ground roll will increase at least 20% (depending on airplane it can be even more than that). The faster the longer.
Like in case of speed a 10% change in weight will cause at least a 20% change in takeoff and landing distance, and the same applies, the actual ammount varies from airplane to airplane. But keep in mind the heavier the longer.
Density effects on performance
For each degree Celsius of difference from standard, the takeoff roll changes by about 1%. Simple, eh? But very useful when you are out on a high field on a hot day.
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