Does the Aircraft have a horn? Yes it does !
They don’t have horns as you are probably thinking of them, as in car, truck or train horns. However, all certified aircraft are equipped with a stall warning device (not engine stalls, but a warning indicating that the critical angle of attack for the wing is being approached, which, if exceeded, results in a stall, or loss of lift, due to separation of airflow over the upper surface of the wing) – some older aircraft use a stall warning light, but most aircraft have a stall warning horn inside the cockpit. In addition, larger aircraft often have warning horns and/or tones for various system failure modes – again, these are audible signals within the cockpit to alert the crew.
The signalling or warning system aboard an aircraft could be compared to a horn. When ground engineers are working in the cockpit, they can contact their colleagues on the ground using a signal. Crew or Engineer Fred to point out the horn, taps on a tiny button marked “GND” on the instrument panel in the cockpit. The button is hard to find, but when it is pressed it sounds as if three steamboats are passing under the plane.
Sometimes engineers on the ground want to contact their colleagues in the cockpit. They can do so via a button located in a little compartment near the nose wheel at the front of the aircraft. By plugging a headset into this compartment, they can communicate directly with the cockpit.
A siren signal in eighth landing gear
The horn is primarily a means of communication, but the aircraft itself can also emit a signal to warn engineers when a system breaks down or when there is fire. This signal sounds like a siren running in eighth gear. This happens, for instance, 90 seconds after any problem arises in the system that cools the aircraft’s other systems, which is much like a computer’s cooling system. The signals vary so that engineers know exactly which system is under threat.the signalling system aboard an aircraft isn’t really used in the same way as the hooters and horns aboard other vehicles and vessels. A pilot can’t hoot at other aircraft. In fact, the horn doesn’t work when the aircraft is in flight, because the signalling system is turned off
Source : Yahoo answer, KLM – jessey de Graaf , KLM blog