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.
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.
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.
When to reject a takeoff
On takeoff roll 70% of flying speed should be reached at 50% of the length of the runway or the takeoff should be rejected. The reason you need more than half your speed when you’ve only halfway on the runway is that acceleration is not linear.
If wind is 15 degrees to the runway, the crosswind component is 25% of the wind velocity (for ease: at 10 kts wind the cross component is 2,5 kts). If the wind is at 30 degrees, the crosswind is 50% wind speed (meaning 5 kts). If the wind has a 45-degree to the runway, the crosswind component 75% of the wind (7.5 kts compnent at 10 kts wind). In case the wind is 60 degrees or higher you can calculate that the crosswind is the same as the total wind.
Descent planning
Normally a three-degree descent gives aproximately 300 feet per nautical mile (actually 318, but 300 is much easier to use). Dividing the altitude to be lost by 300 is quite easy. You are approaching the field cruising at 6,000 feet and you need to get down (0 feet). The altitude you want to lose is 6,000 feet, which when divided by 300 results in 20. So start your descent 20 nm out.
To determine rate of descent for the 3 degree path, simply multiply your groundspeed by 5. At 120 knots, your rate of descent would be 600 feet per minute. So if the descent should be initiated at 20 nm to lose 6,000 feet (see above). And the groundspeed is 120 knots, which is 2 nm per minute. Then 20 nm will take 10 minutes. And there you go 10 minutes at 600 feet per minute means you’ll lose 6,000 feet.
Happy landings: Cs (maunpilot.blogspot.com)
Pannon Air Service
Diamond Aircraft Hungary
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