How Pilots Know Exactly When to Start Landing

If you're like me, then you probably thought that a pilot just looks out of the window and visually judges when to start descent and landing, but it's actually a lot more complicated than that.

On a single-engine plane or one that flies at a reasonably low altitude, pilots land visually because they can see the runway and they learn to get a feel for when they should start to descend, on larger, commercial aircraft, this isn't possible, they fly so high that their descent needs to start well before they see the airport and if they waited until they saw it, they wouldn't have enough time to descend safely.

The pilot, therefore, has to work out distances and make calculations to know exactly when and where to commence their descent.

On the aircraft navigation display, the pilot has the approach for their target airport set up and it specifies a point with the letters T/D, this is the Top of Decent, the point where the descent to the runaway should commence. This marker can be in two different places, the one furthest from the airport is for a Direct, or Straight-in Approach and the one closest is called an Approach and Transition, this one is used so that the air traffic controller can stagger the landings of aircraft arriving at roughly the same time. Due to the fact that the Top of Descent marker for a direct approach is the first one the aircraft will encounter, the pilot will calculate this one first.

First, the pilot needs to know how far they are from the runway, they do this by tuning into the Distance Measuring Equipment or DME that is closest to the airport. Next, they need to calculate the altitude they need to lose, if they are flying at an altitude of 33,000ft and landing at Miami International Airport which is at sea level, they would need to descend the full 33,000ft, but if they were flying to El Alto International Airport in Bolivia which is at an altitude of 13,325ft they would only need to descend about 20,000ft. They now need to calculate the minimum distance they need, in order to descend that amount comfortably, here the rule is for every nautical mile travelled, the plane descends 300ft, so if we have to descend 33,000ft, we'll need 110 nautical miles. Now they have to consider speed, the average approach speed is 200 knots, if they are travelling at 400 knots they'll need to lose 200knots during their descent, a good average for most aircraft is a distance of 1 nautical for every 10 knots, so if they have to lose 200 knots, that's 20 nautical miles. Finally, they need to calculate wind, if they have a tailwind it will be harder to lose altitude, if they have a headwind it will be easier, here, the calculation is 1 nautical mile for every 10 knots of wind, if it's a headwind they subtract it from the total if it's a tailwind ... https://www.youtube.com/watch?v=FYPmCB12knQ