diff --git a/validate_yaml.py b/validate_yaml.py
index 4533034..68df0f9 100755
--- a/validate_yaml.py
+++ b/validate_yaml.py
@@ -29,10 +29,12 @@ def check_trips() -> None:
     print(len(routes), "routes")
 
 
-def check_flights() -> None:
+def check_flights(airlines: set[str]) -> None:
     """Check flights."""
-    flights = agenda.travel.parse_yaml("flights", data_dir)
-    print(len(flights), "flights")
+    bookings = agenda.travel.parse_yaml("flights", data_dir)
+    for booking in bookings:
+        assert all(flight["airline"] in airlines for flight in booking["flights"])
+    print(len(bookings), "flights")
 
 
 def check_trains() -> None:
@@ -57,31 +59,25 @@ def check_events() -> None:
     print(len(events), "events")
 
 
+def check_coordinates(item: agenda.types.StrDict) -> None:
+    """Check coordinate are valid."""
+    if "latitude" not in item and "longitude" not in item:
+        return
+    assert "latitude" in item and "longitude" in item
+    assert all(isinstance(item[key], (int, float)) for key in ("latitude", "longitude"))
+
+
 def check_accommodation() -> None:
     """Check accommodation."""
     filepath = os.path.join(data_dir, "accommodation.yaml")
     accommodation_list = yaml.safe_load(open(filepath))
 
+    required_fields = ["type", "name", "country", "location", "trip", "from", "to"]
+
     for stay in accommodation_list:
         try:
-            assert all(
-                field in stay
-                for field in (
-                    "type",
-                    "name",
-                    "country",
-                    "location",
-                    "trip",
-                    "from",
-                    "to",
-                )
-            )
-            if "latitude" in stay or "longitude" in stay:
-                assert "latitude" in stay and "longitude" in stay
-                assert all(
-                    isinstance(stay[key], (int, float))
-                    for key in ("latitude", "longitude")
-                )
+            assert all(field in stay for field in required_fields)
+            check_coordinates(stay)
         except AssertionError:
             pprint(stay)
             raise
@@ -109,25 +105,29 @@ def check_stations() -> None:
         assert agenda.get_country(station["country"])
 
 
-def check_airlines() -> None:
+def check_airlines() -> list[agenda.types.StrDict]:
     """Check airlines."""
     airlines = agenda.travel.parse_yaml("airlines", data_dir)
     print(len(airlines), "airlines")
     for airline in airlines:
         assert airline.keys() == {"icao", "iata", "name"}
+        assert len(airline["icao"]) == 3
+        assert len(airline["iata"]) == 2
+
+    return airlines
 
 
 def check() -> None:
     """Validate personal data YAML files."""
+    airlines = check_airlines()
     check_trips()
-    check_flights()
+    check_flights({airline["iata"] for airline in airlines})
     check_trains()
     check_conferences()
     check_events()
     check_accommodation()
     check_airports()
     check_stations()
-    check_airlines()
 
 
 if __name__ == "__main__":