The flaps are used on approach to steeper the A/c descent without increasing the airspeed. What are the conditions that decide the positions of flaps on an approach or can it be used at any position on any given approach path to land. The basic purpose of the laps is to increase the A/c’s approach path, clearing it from the obstacles on the approach path, by increasing the drag caused by the extended portion of the flaps, which causes a downward flow of the airflow, depending upon the position of the wings position extending the flaps either gives a nose up or nose down moment.
I have flown both types of A/c with high wing location and low wing location although my experience on low wing A/c is very limited to just a couple of hours, but I have closely observed even in those couple of hours, the nose moment after extending the flaps.
In a high wing A/c when the flaps are extended the Thrust/Drag couple is disturbed and Drag curve goes above, it causes a nose up moment and whenever the flaps are retracted it causes a nose down moment.
In a low wing A/c when the flaps are extended the Thrust/Drag couple is disturbed and Drag curve goes below, it causes a nose down moment and whenever the flaps are retracted it gives a nose up moment.
Although the flaps causes an decrease in drag speed, but they also cause an increase in drag, which can be hazardous if the A/c has to carry out a balked landing, imagine an A/c on approach with an engine failure with high amount of flaps extended, and suddenly you have to go around because of some cattle or person on the runway, the drag caused by the flaps at such a low altitude with one engine failed and worse of all unfavorable weather conditions can almost cause a disaster.
The factors that decide the flaps position on an approach are as follows-
- Position of Centre of Gravity
- Approach profile
- Runway Length
- A/c Weight
- Meteorological Conditions
- Terrain in the Climb path if A/c is required to Go Around
- Centre of Gravity – The most forward located Center of Gravity causes a nose heavy tendency which requires the flaps usage to be limited to less than full, since the usage of flaps steep the approach and causes the A/c nose to be tilted low, making it almost impossible to flare the A/c to land. A Forward located Center of Gravity cause a nose heavy tendency and at the same time increases the stalling speed. This makes it undesirable to approach with the full flaps with the most forward C.G.
- Approach Profile – The flaps to be used for an approach are also decided by the obstacles on the approach path, if there are high obstacles on the approach, the high degrees of flaps can be used to increase the descent and increase the glide path. This procedure requires extreme care since the Pilot must be careful when to extend flaps, since the obstacles viewed from a slant distance are different from the actual position. The A/c must be well clear of the obstacles before extending flaps, which should be done in steps.
- Runway Length – is also one of the contributing factors to decide the flaps position on approach. I have found it is most comfortable to approach with less than full flaps and is also safer to execute a Go Around in case things don’t go as planned. Since it is easier to retract partially extended flaps in steps at a low altitude, than to retract fully extended flaps. The significant drag increase due to fully extended flaps also requires the Pilot to flare the A/c at a perfect height which is usually at a lower height than the less than full flaps flare height. This occurs because with full flaps selected the A/c drag increases significantly and does not float as with lower degrees of flaps extended.
- A/c Weight – is a major consideration to be used for approach flaps setting since it should be calculated with different flaps settings that whether the carried weight will give a minimum required climb gradient to clear obstacles in case a Go Around becomes necessary. It should be noted that a flaps retraction causes the flight path of the A/c dip below. Since the retraction of flaps causes a change in the aerofoil shape, which decreases the camber as well as wing area (specially in case of Fowler flaps) causing an reduction in lift as well as drag and A/c descents, on retracting the flaps.
- Meteorological Conditions – also decides the degrees of flaps to be used on the approach path. A recovery from wind shear is better controlled with the flaps in less than full position. The wind shear encountered in the full flaps selection tends to further degrade the A/c performance, since the A/c is already operating under high drag caused by high degrees of flaps and secondly, the higher flaps requires the high throttle setting to be used on approach, that means the excess thrust that makes the A/c climb is less with the flaps in full position. The effect of Cross wind is also aggravated in full flaps setting, therefore most flight manuals recommends A/c to use minimum or no flaps setting when approaching in cross wind conditions.
- Terrain in the Climb path if A/c is required to Go Around – also decides the flaps setting for different types of A/c weight. Since we know that the flaps increases drag and decrease excess Thrust required for a Climb. It should give a minimum required climb gradient to clear obstacles in case a Go Around becomes necessary.