For medium size airports, ATC will direct aircraft with a single message like this "Taxi to runway 29 via Papa, Sierra".
I was wondering if it was the same thing for bigger airports (like international airports) which have a lot of taxiways.
I know that if the pilots are not very familiar with the airport, they can ask for controller guidance. But does the ATC can give all the taxiways points like for medium size airports?
Thank you
Yes, it's the same. An airport consists of one or several runways, one or several parking areas (aprons) and a number of taxiways connecting them. All runways and taxiways are uniquely named.
When taxiing around the airport - be it from runway to parking, or vice versa, or between parking areas, hangars etc. - ATC instructs the pilot of the exact route to follow by specifying the taxiway names in order. ATC can also instruct the pilot to stop and wait at a certain position, or to follow or give way to specific other aircraft. Some airports, in addition to named taxiways, have named holding points around the airport which can be used to specify where the aircraft should stop and await further instructions.
Pilots who are not familiar with the airport will have no problem following instructions, since they all have a ground chart (essentially a street map) of the airport layout, with all the taxiway and runway names.
Example of aerodrome chart - Copenhagen Airport, reproduced from AIP Denmark
Some modern airliners even have a moving map which shows the aircraft GPS position directly on a digital airport map in the cockpit. In addition, signs are posted at all taxiway and runway intersections, showing the name of the current and any adjacent taxiways and runways.
Taxiway signs
In extreme cases, ATC can make use of turn-by-turn instructions (progressive taxi), but that is very rarely needed. Follow-me/marshall vehicles are also available, but those are usually not to help the pilot find the way, but rather to provide an extra set of eyes when taxiing very big aircraft, to ensure they stay on the pavement during a narrow turn/tight gap etc.
What happens in the flight deck at an airport with a complex taxiway system (like NY JFK, which has many multiple parallel taxiways and numerous wrong-turn traps) is the pilot taking the clearance will (should) write it down like a flight plan clearance before reading it back (it's a good idea to write down any clearance with more than 3 elements in it).
Before you start to taxi, a crew with good Crew Resource Management practices will than review the clearance and plot the route on the airport chart together, and mutually agree that the route they are taking is the route in their taxi clearance. Then, if there is any doubt on the way to go at any point, call it in to confirm. Such airports have ground radar and know where everybody is on the taxiways, so in the end you can just call and ask directly which way to go if it comes to that.
Taxi schemes vary with the airport, and also with the prevailing conditions. Some airports may issue a taxi clearance to a runway with intervening taxiways, while others may issue a taxi clearance to a holding point, and then issue another clearance as the aircraft approaches that point. Some locations use standard taxi routings with comprehensive airport information that each crew member must be familiar with. Others simplify.
Incheon, for example uses green taxiway lights and the crew will simply be given the initial taxi instruction, followed with "follow the green." Green lights illuminate in the taxiway progressively to guide the pilot, and stop bars or hold short indications will be given otherwise, with verbal direction as well.
Some international airports can be complex, and confusing.
