Htydjtk

I'm frequently travelling to Zürich for business and I usually make my schedule so that I have at least an hour to do some planespotting and photographing at the ZRH airport.

Whenever I came to the airport, the runway usage was the same: RWY14 for all the landings, RWY28 for almost all the takeoffs, while RWY16 was used by heavies for takeoffs (usually the flights to Asia or to the USA). I noticed that once a heavy aircraft lines up at RWY16, it waits for some 5-10 minutes before it starts rolling (although it is already way behind its scheduled departure time). The only possible explanation that I could think of was that its engines would cause severe turbulence for the airplanes which are landing at RWY14 and that it has to wait for a significant gap between the landings.

Am I right? Or if not, does anyone know the real reason for these delays?

I don't know, but I have what I think is a reasonable guess.

First, if separation (wake turbulence mitigation) was the reason, then why would ATC block a perfectly servicable runway by having an aircraft line up and just sit there for upwards of 10 minutes? It would make far more sense to have the aircraft taxi and hold short of the runway, than to line up and wait, until there is a likely large enough gap in the traffic. Even if the first aircraft is holding short at the threshold of 16, unable to take off due to say separation concerns, it would still be possible to allow another aircraft to enter the runway from the next taxiway down, backtrack then line up, or to land on 16, if ATC wants to do so. (I would argue that this is probably inappropriate for other reasons, but at least the takeoff scenario looks no more complex than would be a takeoff on 14.)

Second, notice that 10/28 and 16/34 intersect at about a third of the runway from one end. (A third down the runway on 10 and 34, two thirds down the runway on 16 and 28.) Even not considering winds, this would severely limit ATC's ability to safely use those two runways simultaneously, especially for heavier airplanes, as they would need to consider also the situation at the intersection in the case of a successful or a rejected takeoff or, in the case of a landing, an airplane either landing long or executing a missed approach. All of these add complexity and risk.

The ATC mantra is safe, orderly, expedient, in that order. Safety comes first, so anything that compromises safety in the name of expediency is going to be a no-go from the start.

If the airplane was "well behind" its scheduled departure time, yet just sits on the runway for a good ten minutes, my first guess would be that whatever caused the initial delay might be causing further delay before the pilots feel comfortable beginning their takeoff roll, or before ATC clears them to begin the takeoff roll.

As an aside, engines don't cause wake turbulence per se; wings providing significant lift do.

If jet blast is the reason, then there's an easy fix -- jet blast deflectors. Using satellite imagery, I can't spot one for runway 16 at ZRH.

Using Flightradar24, I used the playback function for an Emirates A380 departure to Dubai. I chose Dubai because it isn't affected by delays due to curfews (removing one variable).

It departed right away after lining up (around 1:50 pm UTC), it was preceded by a smaller jet from runway 28, which intersects 16 (both shown below).

One possible explanation for the delays that you witness is waiting for runway 28 to be clear. Flights bound to European airports are on tighter schedules due to airspace congestion, so it's possible they get priority.

An A380's jet blast at takeoff affects 540 m behind the plane (shown below).

The yellow contour is 35 mph (16 m/s) (Source: Airbus).

There is enough space behind runway 16:

The reason I checked is I couldn't easily find heavy departures than coincide with arrivals. But now you have an extra tool for your next spotting.

There are many reasons that departures from runway 16 might be delayed.

If an arrival to runway 14 has to go around, it would not have adequate separation from an aircraft taking off from runway 16. There would need to be a adequate spacing in the arrivals to allow for a departure from runway 16.

Departures from runway 16 will interfere with departures from runway 28. In addition, smaller aircraft departing from runway 28 would have a required delay after a large aircraft departing from runway 16, due to wake turbulence. They may prefer to get some departures off of runway 28 at closer spacing rather than make them all wait for wake turbulence.

As ymb1's answer shows, if both aircraft are departing to the north/east, their paths will also cross in the air. So if the controller decides to get the departures off of runway 28, they would need to wait for the last aircraft to get far enough east that there will be room for the departure from runway 16.

It's also possible that the aircraft have to wait for release so they can be integrated into the traffic flow. If they are significantly late, they may have lost their original spot in the flow, and it could take time before another opening can be made for them. This extends all the way to having a gate available at the arrival airport.

Any of these reasons could cause delays from departures from runway 16. Air traffic control would have them line up and wait so they are ready to go as soon as conditions allow. It sounds like heavy departures are the only use for the runway at the time, and if there's an emergency there are two other runways that ATC can use. ATC knows the traffic flow, and that they can afford to have an aircraft sitting on runway 16 waiting for takeoff.