Image: Glitters Online
As the expression goes, there is nothing so useless as altitude above you, or runway behind you…for good reason! Knowing how much runway you have, and more importantly how much you need, is critical to executing safe take-offs and landings. While commercial flight operations always make these calculations, very often in general aviation, these distances are assumed to be all in order, often without sufficient consideration for external factors, which can alter these distances significantly. So if you are afraid (rightfully so), of finding yourself in an ‘Oh sh*t!’ moment, with those tall trees at the end of the runway magnifying themselves before you, with great rapidity, then read on to prevent this from ever happening…
The first port of call, as such, is to have a look at the POH for your aircraft. Yes, I know it is usually quite a large manuscript, but there is actually a lot of information in there which will keep you, and your aircraft intact. On a more serious note, this is where you can find graphs that will help you work out what take off, or landing distance you need for your aircraft, taking into account various conditions which can change from day to day. Let’s have a look at various conditions that will influence how much runway you need:
This one may seem obvious, but just in case you didn’t get the memo, if you add more weight into your aircraft, it will affect practically all flight manoeuvres, especially your take off and landing. Even though your flight manual says that the Maximum All Up Weight (MAUW) is a certain amount, and your centre of gravity is within limits, according to your weight and balance check, this does not mean that prevailing conditions are suitable for a safe take off, or landing, at the maximum weight.
On the take-off, increased mass reduces the acceleration that an aircraft can achieve, which means the distance required to reach a certain speed will be increased. With more load on the wheels, there is greater contact with the runway surface, and therefore wheel drag increases. Also, remember that the heavier the aircraft is, the more lift is required in order to get the aircraft airborne, and then the initial climb angle is shallower, which means you will need more distance to obtain the safe obstacle clearance height of 50ft.
When landing, the heavier the aircraft is, the faster the landing speed will be, because of the momentum, which in turn, means that you will need more runway to bring the aircraft to a stop.
Air temperature and Density
The hotter the air temperature is, the less dense the air will be. This is bad news if your aircraft is at max weight. Why? When the air is less dense there is less weight of oxygen molecules available for combustion. in other words there is a decrease in thrust which means it will take longer to get up to rotation and lift off speed. Essentially, you’ll need more runway than usual to take off, and be aware, even if you do lift off successfully, your aircraft actually may not be able to climb at the angle, or rate, required to clear the inconveniently placed clump of pine trees that may be situated at the the end of the runway.
If you thought that landing in hotter, less dense air would shorten your landing distance, then you would be wrong. In less dense air true airspeed (TAS) needs to be higher, in order to maintain the same indicated airspeed, therefore it will take longer time to slow down.
Taking off into a headwind shortens the distance required, due to the fact that some airflow, and therefore, lift is already being produced even if the aircraft is stationary. However, this is not always the case, and you may find yourself at an airfield that has a unidirectional runway (perhaps due to an obstacle like a mountain), in which case you may just have to take off with a tailwind. In a tail wind situation, the aircraft is being pushed from behind, and the forward designed aerodynamics take longer to kick in – that is, it takes longer to obtain sufficient air speed to create the necessary airflow over the wings to lift off. So if you must take off with a tail wind, then you definitely need to check that your aircraft will reach the necessary speed to lift off before you run out of runway, based on the direction, and actual speed of the wind. Landing with a tail wind will produce a similar effect of increasing the landing distance as ground speed is higher. Most aircraft graphs already incorporate 50% of the head wind component and 150% of the tail wind component, but be sure to check that yours caters for wind.
The type of runway surface, and whether it is wet or dry have a significant impact on how much runway length you will need. as discussed in a previous article on grass runways, grass will result in a greater take-off and landing distance required due to the lack of friction between the tyres and the surface. If the runway is wet, be it grass, dirt or tar, it will be necessary to increase the take off distance, due to the increased wheel and spray drag. On landing, however, this drag is not created in the same way, and the aircraft requires a greater distance to stop due to the lack of friction with the runway – in other words, water acts like a lubricant and it takes longer for the wheels to make adequate contact with the runway to create wheel drag.
You must add the following surface factors to the distances you obtain from your aircraft graphs:
Take off Surface:
|SURFACE TYPE||CONDITION||MULTIPLY TAKE-OFF DISTANCE BY:|
|Grass surface 20cm||X 1.15||15%|
|Wet at ETA||X 1.15||15%|
|Regulatory 70% LDR||X 1.43||70%|
The slope of the runway affects take off and landing differently – an downhill slope is more favourable for take off as the aircraft will accelerate faster and reach it’s flying speed sooner. However, an uphill will have the opposite effect, causing the aircraft to take longer to accelerate. Conversely, uphill runways are more advantageous for landing as they will slow the aircraft down sooner, and downhills will inadvertently cause you to stop in the airfield fence if you don’t take care.
Take off: You must add 5% to your take off distance (obtained from the graphs) for every 1% of UPHILL slope.
Landing: You must add 5% to your landing distance (obtained from the graphs) for every 1% of DOWNHILL slope.
Here is a final factor that you may not recall – as a legal safety requirement you need to add 70% of your landing distance available in order to obtain your legal landing distance required. Multiply LDA by 1x43 to obtain your LDR.
Hopefully, we have helped you take out the guesswork of knowing how much runway you need to take off and land. Just remember if it doesn’t feel right, don’t do it, or if you have doubts for a landing, go around or head for another field… Who knows maybe the coffee is better there anyway!
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