Weight and Ground Effect
Most aircraft accidents occur during the take-off and landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. In this section of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight. Hopefully we help the pilot ensuring safe operation during these phases of the flight as the rules require that of the pilot in command.
The effects of increased weight of the aircraft on stall speed or take-off run might be more than some pilots bargain for. More so when the runway is not perfectly flat or level or the grass is long and wet.
The weight of an aircraft directly influences the stall speed. A rule of thumb is that 10% increase in weight equals 5% increase in stall speed. And this has its effect on the performance of the aircraft. This is the formula: VS new = VS old weight x √(new weight / old weight).
Normally, lift-off speed is 15% above stall speed. Thus if weight increases -> higher stall speed, your lift-off speed increases too. As a result more time is needed (higher weight -> slower acceleration) to get to that lift-off speed and you will use more runway at the same time. The general rule is that a 10% increase in weight means 20% more runway needed for take-off.
During landing the same effects apply. A heavier aircraft has a higher approach speed (1.3 VS, and stall speed is higher) and therefore needs more runway to stop. Rule of thumb: 10% more weight means 10% more runway needed when landing.
Into Ground effect
Flying within one wingspan of the ground has the effect of reducing the induced drag from the wing. The result is that the wing is more efficient (wingtip vortices are reduced) and generates less drag. During take-off the aircraft 'wants' to fly sooner and during landing it refuses to land and you will need more runway (floating) before touchdown, and every knot above the recommended 1.3 VS will increase this effect even more.
During take-off roll the pilot can be under the impression that the aircraft is ready to fly. Again caused by ground effect which reduces induced drag. But when the aircraft leaves ground effect it can settle back onto the runway due to a sudden increase of induced drag. Best remedy is to lift-off and remain in ground effect, accelerate to VX or VY then continue climb out. I call this a step climb, it is also used when climbing out over obstacles from short runways.
TODR / ASDA
Take-off distance required (TODR) must not exceed 85% of the take-off run available. Or roughly that 2/3 of your lift-off speed must be reached before 50% of the runway is used. These decision points make it easy to abort your take-off before anything nasty happens. Just check the accelerated stop distance available (ASDA) and the clearway before departing.
On landing, ground effect can result in floating. Even more so when higher (or too high) final approach speeds are used. And as far I can see it, most private pilots come in too fast. The result can be an overrun of the runway resulting in a go-around. Use at minimum 1.3 VS with the applied gust factor as a final approach speed.
Turbulence during landing or take-off and within ground effect can result into a stall. Make sure to add half the gust factor to your approach or take-off speed and be mentally prepared for this. Just make sure that you can clear obstacles and that enough runway is available.