Obstacles, Flaps 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.
Climb-out performance tells us if we can clear that obstacle in our path. The correct use of flaps can or cannot help with that. We will discuss another important factor too: ground effect.
It is advisable to clear any obstacle in your flight path by at least 50 ft. Check your aircraft climb gradient during preflight and gather any pertinent information about the airports you are going to visit on obstacles near the climb out and/or landing path. Check NOTAMs also for any new obstructions not listed on charts or in the AIP.
Calculating the height gain per mile (climb gradient) is easy: Rate-of-climb (ft/min) / Ground Speed (NM/min). For example: 500 ft/min climb and a ground speed of 66 Kts (1.1 mile per minute). Dividing 500 ft / 1.1 NM gives a height gain per mile of 454 ft / NM. This will give you a good indication if you will be able to maintain safe terrain and obstacle clearance. Start with a no wind situation for this calculation, any headwind will increase the climb gradient.
Read more on climb performance as it is important and influenced by a number of factors.
Correct Flap settings
Flaps reduce aircraft stalling speed by increasing lift and it enables you to lift-off at a lower airspeed. Resulting in a shorter ground run. It also reduces the rate of climb (and angle) due to a somewhat higher drag. Always use the recommended flap setting for the given circumstances. Never, ever reduce a flap setting while in a turn or on final. In a turn stall speeds are higher and reducing flaps could mean a low level stall / spin event.
Extending flaps to the first position usually adds quite some lift and a minor amount of drag, the second position will increase lift and drag and the third position adds much more drag and almost no lift at all. The increase in drag helps to stabilize the airspeed and lower the nose of the aircraft. You will need to add power when flaps are fully extended when maintaining altitude.
Slats are leading edge flaps (where as normal flaps are on the trailing edge of the wing) and have the effect of increasing camber, lift and wing area thus reducing stall speeds. These slats either move forward or are extended from below the leading edge automatically or under pilot control. Some slats even extend forward, Fowler slats.
While on approach to a runway with a considerable amount of crosswind, it is wise not to use full flaps on some type of aircraft (old model Cessna's have 40° flaps and the DynAero MCR-01 even has 45°) without proper training from a qualified instructor. On these aircraft the full flap setting does not lower stall very much (maybe a knot or so) but they increase drag enormously. The glide angle will be very steep (helpful when there are obstacles in the way) and you will need to add power to reduce the flight path angle. Do maintain power until touchdown, especially so in crosswinds.
With flaps extended your groundspeed is even lower and the effects of the crosswind will be higher. Possibly resulting in being unable to line up the aircraft with the rudder and banking into the wind at lower speeds to keep the aircraft from drifting. Your only option is then to reduce flaps and approach with a bit higher speed. Make sure enough runway is available.