[translated by Juergen Matthes]
Naturally you don’t turn a runway around, you “switch runways”, that means according to wind directions and speeds you change the direction of takeoffs and landings. Wind still is a major factor in aviation.
As passenger in a jet you can experience this yourself. Let’s say you are on a flight from Frankfurt to Miami. You have a small portable GPS device with you, and you play with it close to a window during the flight towards Miami, right over the Atlantic Ocean. You realize your speed over ground is displayed as 400 miles per hour. You wonder, your airplane is supposed to travel more than 500 mph. On the return flight, again over the Atlantic Ocean, going east, you play with your toy again and it shows 610 mph. What’s going on?
Very easy: the airplane travels always with the same speed in relation to the surrounding airmass, about 500 mph. On your way to North America you normally will have a headwind with about 100 mph, this results that you travel that much slower. On the way back you travel with 500 mph in an airmass moving itself with 100 or 110 mph in your direction of flight, therefore you are now faster (over ground).
This effect has to be considered during takeoffs and landings. You take off and land preferably against the wind. Number one – it is safer, and number two – you save some fuel. A takeoff with a tailwind means the airplane has to accelerate to a higher speed to achieve the airspeed needed for flight. Let’s say you need an airspeed of 120 knots for take off, a tailwind with 10 knots prevails on the runway, so that requires a speed of 130 knots (on your wheels) to achieve 120 knots airspeed. With a 10 knot headwind you only need a speed of 110 knots (on your wheels).
So it makes sense, to adapt the directions for takeoffs and landings to the wind conditions. Therefore wind and its constant changes is an important factor on any control tower. Now the wind at an airport normally hasn’t the desired effect to reverse its direction completely to the opposite within a short period of time. No, wind acts as it pleases! So it is the responsibility of the tower-crew, to constantly monitor the wind, its tendencies of direction and speed, and to switch runways if needed.
Düsseldorf Tower, a nice day, the wind blows from the west with 15 knots. Approaches and landings are proceeding towards the west, as they normally do under these conditions. The weather bureau has forecasted a change of wind towards the north-west within the next 20 minutes.
This is no problem for takeoffs, they will a bit more jerky. For landings it is a different story, they have to point their nose into the wind (crab angle) until shortly before touchdown when they will straighten out the airplane. For us in the tower cab this is great, we have something spectacular to watch! It is always interesting to watch such landings with a crosswind component.
The wind continues to shift towards the north. Now a good crosswind, the pilots have to show their skill! For crosswinds we have specifications in the tower we have to follow, dictating which strength will require the airport to be closed. But – it’s a long way until that will happen.
But if the wind continues to shift to the east, we will have to switch runways. Naturally it continues to drift to the east. Good air traffic control service means, to be able to handle all imponderabilities and yet provide an orderly and safe flow of air traffic!
The imponderabilities today – it is the period of rush hour, so very high traffic density. The tower controller, the feeder (he is responsible for the sequence of approaching aircraft) and the two approach controllers (handling the northern and southern sectors) are now discussing the best timing to switch runways.
Most times this will result in the statement “after landing of the KLM you can go ahead and switch”. The apron controller guides all aircraft ready for departure from now on to the western end of the runway and strings them at the holding point of the “new” runway.
Feeder announces the KLM arrival at 10 miles final, and there she calls in on tower frequency.
Tower: “KLM 83, Wind 050 with 15 (knots), cleared to land runway 24.”
To understand the following, you must comprehend the relations of speeds. Everyone knows this situation, you are driving on a freeway with 85 mph and suddenly you see a speed-limit sign showing 50 mph. What will you do? Some will take their foot off the gas-pedal and let the car slow by itself until it reaches 50 mph (or slightly faster). They take into consideration that they might be speeding well after the speed-limit sign, but this procedure will aid the traffic flow because nearly everybody is acting like this. But there are also those who will step on the brakes passing the sign and therefor obey the speed-limit precisely.
Both behaviours are present in aviation too. We also have speed-limits, which have to (should) be obeyed. Approaching an airport there is a limit of 250 knots IAS below 10 000 feet, then minimum clean (the slowest speed an aircraft can fly without deploying flaps) and finally any speed-limit an air traffic controller issues to ensure separation to other traffic.
In ATC even small errors get on your back, sometimes a bit later, but they surely do! The decision to switch runways after the landing KLM and the designated time gap before the first landing on the new (now opposite) runway had been selected a bit too short! A Lufthansa Airbus A310, who called in at 10 miles final to the runway facing east, was way too early.
KLM, on the other side, was a slowpoke! She was overly correct and had slowed like stepping on the brake when getting the first glimpse of the speed-limit sign, in short she still needed a while for her landing.
The situation was now (pardon my language) – shitty! KLM was on final westbound, Lufthansa A310 on final eastbound (remember: same physical runway), approximate meeting point was the western end of the runway. This constellation is called a “double decker!”
Solution to this problem is quite easy: You instruct the A310 to go around (overshoot), a sharp left turn afterwards, but you cannot be sure that it will work out. KLM could be forced to overshoot as well (by some unforeseen factor), and then you would have a near miss or even worse!
In our case all went well, the Lufthansa A310 overshot and thundered across the hospital, KLM landed safely! Right away the telephone connecting the tower to the outside world rang, and sister Agate (one of the head-nurses of the hospital) was on the line complaining about the tremendous unbearable noise.
This telephone-call, which happened every once in a while, was major punishment for all of us up here! Sister Agate was always right! She chewed us, kings of the airport, up and spat us out in pieces! What a disaster to our confidence.
I had the (doubtful) pleasure to get to know sister Agate personally, she actually was a fine woman taking care of her patients. After I explained to her, that we also took care of our clients, she relaxed a bit and future telephone calls from her were less mortifying …