Construction of Nyaribo airstrip in Nyeri in 2011. Safety determines the length of a runway. FILE PHOTO | JOSEPH KANYI |
What you need to know:
Since takeoff speeds are high and the planes require some time and distance to accelerate to these speeds, the runways are long.
In the final scene of Fast and Furious 6, the bad guys attempt to escape by boarding a cargo plane on a runway. The protagonists give chase in their fast cars and manage to prevent the plane from taking off by tethering their vehicles to the wings.
A furious battle ensues, the baddies are killed and the day is saved. The whole sequence takes 13 minutes. Given the takeoff speeds of such a plane would be somewhere north of 160 kilometres per hour, various commentators have been quick to point out that the runway would have to be extremely long.
Even allowing for the fact that the movie cuts to various battles which were occurring simultaneously and other conservative estimates we are talking about a 30 kilometre runway.
We’ve previously covered how Hollywood doesn’t really stick to realism when depicting planes, law enforcement or anything really. However, we are able to suspend disbelief in this scene because we know intuitively that runways are long.
Runways in the real world come in various sizes but those used by normal airliners tend to be long. This is in the range of two to five kilometres with a few on either extremes. The main reason for this has got to do with aeroplane performance.
A plane can only take off when it has sufficient speed to generate the lift necessary to overcome its weight. For a particular type of airliner, the heavier the plane the more speed it requires to get airborne. Since takeoff speeds are high and the planes require some time and distance to accelerate to these speeds, the runways are long.
Engine performance deteriorates with increasing altitude and hence the distance required to reach the takeoff speed increases the higher you go. For that reason the runway in Nairobi is 4.1km, while the one in Mombasa at sea level is 3.3km.
Regulators impose additional safety margins. The runway has to be long enough for a plane to reach a certain decision speed, suffer a malfunction which traditionally has been an engine failure and still have enough room to stop before the end of the runway.
If the pilots continue with the takeoff, they have to clear a certain safety height at the runway end.
Additionally planes which are coming to land have to be able to stop within the paved surface. In general, since planes land at a much lower weight than when they took off and use much larger flap settings to generate more lift for less speed, this is rarely a limiting factor.
However, when the runway is covered in snow or standing water, reduced braking efficiency can mean that the stopping distance affects the amount of weight you can land with.
In rare occasions such as when the Kisumu runway was shortened for repair works, you may have to limit the takeoff weight from your origin so as to be able to stop on the runway at your destination.
Dr Ondieki is a pilot with an international airline.
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